Comprehensive Physiology Wiley Online Library

Muscle, the Motor

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Representation of Muscle Properties
1.1 Dimensions Used
1.2 Models
1.3 Real Motor Systems
1.4 Summary
2 Properties of the Contractile Unit
2.1 Passive Mechanical Contributions
2.2 Response to Neural Signals
2.3 Interaction Between Neural and Other Inputs
2.4 Functional Variations
2.5 Summary
3 Multiple Units of Muscle
3.1 The Unit of Muscle Function
3.2 Relationships in Multiunit Function
3.3 Summary
4 Some Measures of Muscle Function
4.1 Summary
5 Motor Functions of Muscles
6 Final Summary
7 Epilogue
8 Appendix
8.1 Units for Measure of Motor Function
Figure 1. Figure 1.

Examples of physiological actions of various muscles. A: twitch response to excitation of a few motor nerve fibers of human muscle obtained as ensemble average of output force from whole arm. Upper record, response during weak contraction of whole muscle; lower record, response from same motor units stimulated when remainder of muscle was strongly contracted. In the twitch during strong contractions, averaged force contribution rose to an appreciably lower peak than in the other series; then the average decreased to a value less than background level. In this measure both the primary or direct effect of excitation of the test unit and any consistent secondary effects that modify the activity of other units are combined in the total recorded response. B: two voluntary movements of human arm. Thickness of horizontal bars, periods of high, low, and no electromyograph (EMG) activity in agonist and antagonist muscles. First record made during a relaxed movement, second with a stiff arm. Periods of agonist and antagonist activities in first record show alternation with complete separation. In 2nd record the periods are completely overlapping with a rapid cycling of intensity of EMG. In spite of differences in electrical activity, however, the two movements follow similar time courses of position. C: sound produced by vibration of locust tympanum by tymbale muscle when artificially stimulated at 22.5 pulses/s. Stimulus points shown on lower record. Compare Fig. B, which shows isometric response of a similar muscle stimulated at 22 pulses/s. D: measures of triceps sura muscle action in 2 steps of a uniformly walking cat . All of the data originated in measurements (*) of length, force, and EMG by Yager . Continuous approximations of length, velocity, and acceleration were derived by applying spline functions to 128 photographically determined point values of length. Power calculated as product of velocity and force (measured by transducer on tendon). EMG is shown as rectified and filtered interpretation of direct record. Ratio of force to acceleration does not represent changing inertia of load, in part because system is not free of external forces and also because acceleration is not measured in an inertial reference frame. For 1st step (ending at arrowheads) force, velocity, and length are plotted also in a pseudo‐3‐dimensional form, starting in swing phase and progressing continuously in direction of time arrow; pickets mark 10‐ms intervals and eliminate ambiguity of 4‐dimensional points as graphed on a plane. This trajectory over the phase plane (length vs. velocity) shows that in swing phase large velocities and displacements are associated with small forces, while largest forces occur at midlengths and lengthening velocities during stance phase of step. Rest length, l = 1, is near long end of lengths involved in walking, as also described in ref. . The EMG trajectory for this same step can be seen in ref. . For separated soleus and gastrocnemius records of force in walking cat, see refs. and . Note that 2 steps shown in this figure to have similar length patterns have appreciably more variation in acceleration, velocity, and EMG . As is most apparent on phase plane graph, when muscle returns to the starting length for 1st step, 2nd step is initiated at a different velocity and therefore cannot follow a course identical to the first.

A: from Milner‐Brown et al. , with permission of Cambridge Univ. Press. B: from Wacholder . C: from Pringle , with permission of Cambridge Univ. Press. D: from Partridge , with permission of Raven Press
Figure 2. Figure 2.

Conventional nomenclature of successive structural subdivisions of vertebrate skeletal muscle down to sarcomere level. Functional motor unit does not correspond to any of these divisions. (For examples of diffuse distribution of a motor unit within a muscle, see Fig. in the chapter by Burke in this Handbook and refs. . It also should be noted that individual fibers may cross from one fasciculus to another or end within a fasciculus . Furthermore, the densely packed organization of fibers seen in histological section and in this schematic diagram of a fasciculus may represent a shift of extracellular fluid after loss of blood pressure . Normally, adjacent fibers may float separately within the muscle.

Adapted from Bloom and Fawcett
Figure 3. Figure 3.

Details of structure in gross muscles. A: primitive vertebrate muscle arrangement as seen in a single myomere in cartilagenous fish. Longitudinal and cross‐sectional views show pattern that may be approximated as alternating concentric cones made up of these segmentally innervated muscle units. B: regional organization of major innervation divisions (arbitrarily numbered) as projected on two surface views of anterior tibial muscle of dog, showing mixtures of series and parallel relationships. C: conformation change in simple pennate frog muscle as result of total contraction . Table summarizes measured length and angle changes associated with A' fasciculus produced by contraction of whole muscle. The 40% shortening contribution by muscle fasciculus exceeded the 32% shortening of that fasciculus itself and includes effect of increase in angle of these active fibers. An added 13% decrease of total length resulted from the changing angle of connective tissue. Due to changing angles, as well as fiber shortening, total shortening of 7.7 mm is greater, not less, than 5.5‐mm shortening of involved fibers in spite of angled orientation of fibers. D: organization of fasciculi (dashed lines) in the frequently studied frog gastrocnemius, as seen from superficial and deep surfaces and from a longitudinal section. Solid lines and locations Ap, Z, D, and P all mark major connective tissue structures. Sample fasciculus lengths, overall length and width, and a few pennation angles are marked.

A: from Alexander . B: from Wilder et al. . C and D: from Beritoff
Figure 4. Figure 4.

Effects of load modify muscle responses. A: twitches of muscle from different initial lengths in frog gastrocnemius muscle operating on nearly isometric load (left), compliant spring load (middle), and isotonic load (right). These loads determine force‐length slope through course of any contraction and thereby influence the pattern of maxima reached in the twitches. Passive length‐force relationship (right border, each record) also differs slightly between stretching sequence (upper passive curve, each record) and release sequence. Load‐dependent length‐tension curves for twitch maxima are joined by dashed lines. B: cat triceps sura muscle responses to modulated stimulus. Cyclic modulation of rate of stimulus used for both tests is shown as middle graph. Upper graph, isometric force, measured in newtons. Lower graph, force from the identical muscle, subjected to the same stimulus sequence, but attached to an inertial load. Response was measured with an accelerometer in gravity units g from which the newton scale N was calculated. Mean force and length were similar in both cases, and isometric force changed smoothly with period of stimulus. Inertial load was moved against gravity with the same period, but change in force producing that load movement does not have a simple period. Actually only a small fraction of maximum isometric force (and that delivered intermittently) is required to move physiological‐sized loads at moderate frequencies. C: calculated stretch of “series elastic component” with changes of the load on rat gracilis anticus muscles. Compliance (slope of curve) decreases as force increases. Bars, standard deviation of values measured from 5 muscles. This stretch represents the part of an overall length change that is considered not to occur in the contractile components. D: energy exchanges occurring with complex load on a 2‐joint muscle (human hamstrings) through part of a running step. The 2‐joint load allows separation of muscle length change (right graph) from that due to individual joint movement (two curves on left graph). Contributions to muscle length deviations, produced separately by hip movement and by knee movement, are plotted against force, as is total resulting change in muscle length. Numbered points identify correspondence of time on the 3 lines. Energy exchange at individual joint need not represent just energy produced or absorbed by the muscle. Area under curves is work done (integral of force with respect to distance). From 1–10 at knee, work is done on muscle by the load, whereas work is done by muscle at hip. Right: combined effect or net exchange in muscle energy for same points in time. Area under right curve from 1st to 6th point shows energy actually absorbed by muscle, and area under curve from 6th to 10th point shows mechanical energy delivered by muscle (part of delivered energy probably was from series elastic structures that shortened as tensile force was decreasing; if lengthening parallel elastic elements were under tension in this period they would have been absorbing energy). Comparison of areas under 3 curves shows that energy is conserved by transfer through muscle from knee to hip joint.

A: from Blix . B: from Partridge . C: from Bahler . D: from Elftman
Figure 5. Figure 5.

Static and dynamic properties of unstimulated muscles. A: variations in relationships between muscle length and measured force in unstimulated muscles of several species. Abscissa lengths are normalized at maximum body lengths. In some muscles no force is generated passively within lengths attainable in situ, but for other muscles passive tension in normal operating range of lengths exceeds active contribution (see also Fig. B). Curve A, bumblebee flight muscle; curve B, locust flight muscle; curve C, frog sartorius; curve D, clam (mytilus) anterior byssal retractor; curve E, snail (Helix) pharynx retractor muscle. Other muscle curves can also be seen in part B and Figs. A; C and D; and B. B: length‐tension relationship in an unstimulated frog gastrocnemius muscle measured at different rates of loading and unloading, using mercury to produce smooth force changes. Less stretch occurred with force increase, and particularly with fast increase, than with unloading. Force vs. displacement graphs were recorded directly using a special mechanical linkage. Dynamic aspect of passive response of muscle represented by difference between stretch and shortening curves commonly is ignored in dealing with muscle properties (compare Fig. .). C: “stress relaxation” after a step change in length of an unstimulated ileotibialis muscle of tortoise. Unbroken curve, force change in period immediately after stretch; dashed line, level at which force finally stabilized. D: three different time expansions of a record of “creep” of length after a step change in load on a single unstimulated frog fiber. Time marks in curve III, 10 s; time marks in curves II and I, 0.1 s and 0.01 s, also show initial oscillatory transient details as well as creep. Stretch scaled in percentage of equilibrium length (just taut).

A: from Hanson and Lowy . B: from Blix . C: from Hill . D: from Buchthal and Kaiser
Figure 6. Figure 6.

Various responses of muscles to single stimulus pulses. A: first published record of muscle twitch . Frog gastrocnemius was attached to a complex inertial and elastic load, and record was made by a steel point scratching on a lightly smoked, moving glass plate. Oscillatory detail is characteristic of the particular combination of load and muscle and is avoided in most modern muscle experiments by reducing both inertia and elastic compliance of loads. B: Simultaneous force and length records through a series of twitches with different isotonic loads. Frog gastrocnemius muscle was afterloaded; that is, load was mechanically supported at a fixed position before and after muscle tension exceeded force required to lift the load. Characteristically the latent period for load movement increased with load size (weight marked in g), and movement period decreased. The shorter latent period for force development was unchanged. (The same 1893–1895 ref. contains a large number of other records of twitches made with different types of load and load transients; both time‐based records and force vs. length records are included.) C: effect of preload on muscle twitch (weight in g marked on lines). Weights were hung near the fulcrum to minimize effect of inertia, thus approximating an isotonic load. Stretch of loaded muscle before stimulus as well as changes in response can be seen. D: volume change in frog sartorius during isometric twitch. Major divisions on abscissa, 10 ms. Detail of volume change is highly sensitive to load conditions . Peak volume decrease is about 5 × 10–3% of total muscle volume.

A: from Helmholtz . B and C: from Blix . D: from Baskin
Figure 7. Figure 7.

Time graphs of muscle response to 2 or more stimuli in succession. A: traditional student laboratory demonstration (1905) of wave summation for 2 stimuli in nearly isotonic contractions of frog gastrocnemius. Stationary ordinates mark stimulus points. Lower line, time at 50 periods/s. Upper 2 records, response to 2 stimuli superimposed on record produced following a single stimulus. B: typical isometric response of striated muscle to trains of stimulus pulses delivered at 0.9, 22, 46, and 100 pulses/s. At 100 pulses/s response approaches a fused tetanus. The muscle used for this record was tymbal muscle of locust. Compare to physiological response of a similar muscle when attached to its normal load in Fig. C. C: isometric records of wave summation. Lower records, from cat soleus with 2 stimuli delivered at a 70 ms‐interval at 37°C. Area added to record (mechanical impulse) by second stimulus is greater than that produced by 1st stimulus. Upper record, from frog sartorius with stimulus interval of 120 ms. (Time scale of frog muscle record is 77% as fast as that shown for cat record.) Impulse added by 2nd stimulus in this case is less than that contributed by 1st. In both records time from stimulus to peak of contributed force is longer for 2nd twitch than for 1st. D: successive contributions of individual stimuli in a sequence. Upper graphs, superimposed records of responses with different total number of stimuli in trains. Lower graphs, contribution of individual stimuli calculated by subtracting responses to trains differing by one pulse. Left graphs: theoretical linear summation, each impulse producing same contribution. Right graphs: actual frog sartorius responses, 5 pulses/s. Contribution and rate of rise of successive pulses decreased markedly, especially at higher rate. See Fig. C for a measure of isotonic contribution in displacement time product resulting from a pulse as a function of stimulus rate at which it is added.

A: from Beddard et al. . B: from Pringle , with permission of Cambridge Univ. Press. C: from Ranatunga . D: from Gilson et al.
Figure 8. Figure 8.

Effects of stimulus rate on muscle response. A: several different ways of representing the same results. For these graphs a single fiber from frog semitendinosis was used, and isometric response was measured from periods of steady stimulus. Curve a, force differences in individual responses; curve b, contraction remainder at low point in response between stimuli; curve c, ratio of contraction remainder to stimulus rate; curve d, force peak (a + b); curve e, mean tension over time. All curves are plotted as functions of stimulus rate. B: rate of rise of tension (right ordinate and × values) at beginning of a stimulus train continues to increase through appreciably higher stimulus rates than does contractile tension (left ordinate and • values). This is an additional example of measures of muscle response that are not equally sensitive to stimulus rate. C: family of nearly isotonic responses of cat triceps sura muscle to slowly increasing stimulus rate (1 pulse/s2). Separate lines are made with different loads. At low stimulus rates and with small loads the responses to individual stimuli are particularly apparent (compare twitches in Fig. C). Lower graphs show derivative of average (negative) length with respect to stimulus rate (–d//dR), which was calculated using spline functions on mean (smoothed) values of the upper graphs. These superimposed curves indicate that the principal sensitivity to stimulus rate for this muscle falls in the range of 8–25 pulses/s over a variety of loads. Separate peaks could represent populations of fast and slow muscle units within this muscle.

A: from Buchthal . B: from Machin and Pringle . C: from Partridge
Figure 9. Figure 9.

Dynamic effects of modulated stimulation. A: time graphs of response of load moving by cat triceps sura muscle driven with a sinusoidally modulated stimulus rate, 5–25 pulses/s, and at modulation frequencies marked on graphs. Amplitude of modulation was constant, but response amplitude changed with frequency of modulation. B: change in movement amplitude with frequency of modulation cycle. All stimulus cycles ranged from 5 to 35 pulses/s. Although the gravity part of loads was constant, each line represents a different load inertia, spanning a 28‐fold variation within the physiological range . Periodic force required for sinusoidally moving an inertial load increases proportional to inertia and proportional to the square of the frequency or, for this range 28 × (6.3/0.04)2 = 7 × 105, which is the ratio of force cycles for a constant amplitude movement. Considering the proportionality of force cycle to movement amplitude, this graph represents an actual 8 × 103 range of amplitudes of response cycles of force produced by a constant amplitude of modulation of stimulus rate. C: temporal displacement between stimulus cycle and response cycle in isometric records of cat triceps sura force as a function of cyclic frequency. Upper graph, lag of response in ms; lower graph, same data in fractions of a 360° cycle. D: representation of muscle properties combining both nonlinear and dynamic aspects. Each set of 3 coefficients (G0, fn, and ζ) define one 2nd‐order differential equation approximating data similar to that shown in parts in C and B. These coefficients when inserted in the equation specify dynamic properties of the muscle in one operating range of stimulus rates. Changes of coefficients with firing rate range represent some of the nonlinearities. •, ○, Values obtained from different muscles.

A and B: from Partridge . C: from Partridge . D: from Mannard and Stein , with permission of Cambridge Univ. Press
Figure 10. Figure 10.

Relations between muscle actions and temperature. A: twitch variations in frog muscle at 3 different temperatures. Time reference, 100 Hz. B: effect of temperature on tortoise muscle stiffness. Lower lines, static stiffness; upper lines, dynamic stiffness. Pairs of points are measured from one sequence of increasing and one sequence of decreasing temperature. C: height of frog muscle twitches at a sequence of temperatures between 0°C and 36°C. D: effect of temperature on duration of twitch. Temperature ranges between 5°C and 41°C, at which point response failed completely. E: change in temperature of body core, rt, and of muscle, mt, as a result of muscle activity in human. Crossing of the two curves shows a reversal of direction of heat transfer between muscle and relatively constant body core. Externally observable motor performance also changed in speed with temperature in this study, which may be related to “warm up” before athletic activity.

A: from Handbook of Physiology, by W. D. Halliburton, © 1911, used with permission of McGraw‐Hill Book Co. B: from Fowler and Crowe . C and D: from Howell . E: from Asmussen and Bøje
Figure 11. Figure 11.

Muscle contraction as modified by length. A: relationship between stress and strain in 12 tetanically stimulated glossopharyngeal muscles of frog. Data are from tables made by Weber in 1847 . Eleven muscles, •, were loaded before stimulus. One muscle, ▽, was stimulated before loading. Normalization to stress and strain from length and tension, as described by Weber, allows comparison of muscles of quite different sizes. (We have crudely confirmed Weber's description of the very long range of lengths over which this muscle can respond.) B: comparison of active contribution to length‐tension effect in several different muscles. (For passive contributions of the same muscles, see Fig. A.) A, bee flight muscle; B, locust flight muscle; C, frog sartorius; D, anterior byssal retractor of clam. Ordinates are not normalized to compensate for muscle size differences. C: conventional measurement of active contribution to length‐tension effect. Passive response and maximal voluntary force were measured directly in human forearm flexor muscle with cineplastic tunnel prepared to allow external action of the tendon. Developed tension is calculated as difference between passive and maximal forces with no correction for result of local stresses within muscle and tendon. See Fig. A for similar data from twitches. D: family of length‐tension curves for single fiber of frog semimembranous muscle. Parameter distinguishing individual curves is stimulus rate. Note nonuniform increments of stimulus rate and compare Fig. . Values were calculated as mean of 2 values measured with increasing and with decreasing stimulus rate.

B: from Hanson and Lowry . C: from Ralston et al. . D: from Buchthal
Figure 12. Figure 12.

Effect of velocity on muscle response, measured at constant velocities while passing through a test length. All sets of curves are drawn on same abscissa. A: Force‐velocity effect at several different stimulus rates in cat soleus muscle using rotation of stimulus among 5 nerve branches. (Also see ref. for differences introduced by slightly different test conditions.) B: rate of exchange of energy between muscle and load, or power, is equal to product of force F and velocity V. Negative power indicates output of energy by the muscle. (Energy delivered by each impulse, i.e., power ÷ pulse rate, varies with stimulus rate but is nearly independent of velocity over an appreciable range.) C: slope ∂F/∂V calculated from force‐velocity curves in A, a measure of sensitivity of force to velocity, or mechanical impedance (as in ref. ). Negative values are proportional to a viscosity‐like measure. The value of this measure is greatest at low shortening velocities with high stimulus rates. Graphs B and C were calculated for this review from data points on force‐velocity graph (A) using spline interpolation . Spline fit lines pass through same data points, but they differ slightly from lines joining points in A.

A: from Joyce and Rack , with permission of Cambridge Univ. Press
Figure 13. Figure 13.

Expected 3‐dimensional mechanical relationship in response of muscle. Muscle force as related to length, velocity, and excitation drawn by combining usual descriptions (for related experimental data, see Figs. C, B, , D, A). Arrows on velocity axes show relationship between velocity sign and change of length (angle). Arrows on torque axis in F relate torque to corresponding direction of change of velocity. A: one 3‐dimensional description of response properties of a single muscle at a particular moment. Excitation of the muscle defines length‐tension‐velocity relationships in that muscle. At a single length this relationship is described as a force‐velocity curve, and at a single velocity it is a length‐tension curve. For length and velocity at some moment, instantaneous force falls at intersection of 2 such curves in length‐velocity‐force space. Red and green lines might correspond to 2 curves from a stimulus rate family, such as shown in Fig. D for length axis and Fig. A along velocity axis. At any other moment force output could be defined in terms of intersection of an appropriate stimulus‐determined pair of intersecting length‐tension and force‐velocity curves, at least a 4‐dimensional relationship. (Note: No dynamic component is represented in the passive response, although one is known to exist; see Fig. BD.) B: the 26 different red graphs of active length‐tension curves intersect in this diagram with each of the 26 red force‐velocity curves to define 676 of the points to which a particular excitation level might bring a muscle. Same number of intersections in green represent passive muscle. (The different individual curves were derived from assumption that at any length the nonlinear velocity effect is proportional to isometric active force at that length.) Inclusion of all possible points between samples drawn here would produce a “response surface”. If red surface designates the possible responses of maximally active muscle, then neural control would operate in space between red and green surfaces. (see ref. for an experimentally derived half‐surface drawn with a different orientation of the axes.) C: another variation of a response surface differing from B by a simple shift in relationship between passive and active length effects. Different muscles have properties that range between and beyond these types. D: representation of a sinusoidal movement as a trajectory over a response surface. A sinusoidal change in length, l = A sin ωt, defines a cyclic velocity, V = A ω cos ωt, that varies in amplitude with frequency. Such pairs of curves trace eliptic paths on phase (base) plane and define complex closed loop trajectories over response surface. Time‐varying force traced by such trajectories is periodic but not sinusoidal. This transformation from movement into force is inferred by the accepted length and velocity properties of maximally and constantly activated muscle. Response cycle has a phase‐shift based on shape of response surface; it changes with movement frequency, amplitude, and bias. (Nonsinusoidal patterns of movement similarly can be graphed over this theoretical surface. As illustrated in Figs. and , more complex stimulus and movement patterns tend to produce deviations from response surface predicted by the usual muscle description and represented by these graphs.) E: time graphs for one movement over response surface of D. Two cycles of sinusoidal length change centered around rest length, Lo, are shown by black curve at top of figure. Corresponding but phase‐shifted force cycle for a maximally activated muscle is shown by the periodic, nonsinusoidal, red curve marked Fmax. In some stretching parts of the cycle that force exceeds maximum isometric force, 100% Fi. At the specified lengths passive forces are all near 0 (0% Fi), as shown by line marked Fmin. Any constant force (e.g., 40% of Fi) falls at different relative positions with respect to minimum and maximum force for velocity‐length combinations as values change through the movement. If such a constant force can be produced through a movement, then a variation of activation of the muscle would be required. Here activation A is defined for any particular force as the relative position of that force between Fmax and the passive force for existing physical conditions at the moment. In this example in order to maintain the constant 40% Fi force through the described movement, the neural drive would have to provide a time‐varying activation (from 33% to 80% of maximal), as plotted on line marked % A. F: theoretical response surfaces for a pair of matched antagonist muscles. Green lines represent surface expected while both muscles are unexcited. Red lines represent the response surface expected when both muscles are maximally active. This surface does not show a simple cancellation of force; i.e., coactive surface is not a horizontal plane equal to passive surface. For the slopes, ∂T/∂θ relates to stiffnesslike properties, and ∂T/∂V relates to a viscosity‐like property. Lengths, velocities, and forces that describe linear action of a single muscle are replaced here by angles, angular velocities, and torques, which describe combined action of antagonist muscles on a hinge joint.

Adapted from Partridge
Figure 14. Figure 14.

Effects of excitation history on dynamics of muscle response. A: sequence of afterload frog muscle twitches recorded as fatigue develops. Increased height over first few trials (treppe) is followed by decreased amplitude (fatigue) as well as by increased latent period, increased time to peak, and slowed relaxation. This latter effect is known as contracture. B: change of response in 2 cat limb muscles as a result of surgically crossed innervation. Upper two graphs, normal muscle twitches in crureus and gracilis muscles. Lower records, altered response under influence of reinnervation by crossed nerves—slow muscle becoming faster and fast muscle becoming slower. Line below twitch record shows a digital measure of contraction time in ms arranged in series of dots, each column with a maximum of 10 1‐ms points. C: adaptation of response of anterior tibial muscle of rabbit to long‐term activity pattern. Two isometric responses to stimulus trains of 25 pulses/s. Lower record, response of normal muscle; upper record, muscle modified by stimulation at 10 pulses/s over a 5‐month period.

A: from Handbook of Physiology by W. D. Halliburton, © 1911. Used with permission of McGraw‐Hill Book Co. B: from Buller et al. , with permission of Cambridge Univ. Press. C: from Salmon
Figure 15. Figure 15.

Deviations of muscle response to stimulus from that of a simple, response surface model with lag. Examples show that muscle responses vary with stimulus history and past activity in other units and that mechanical effects are variable with stimulus level. A: hysteresis in cat triceps sura muscle response to rate modulation of stimulus. Insert, time sequence of stimulus. In graph of length vs. stimulus rate, time progresses in a counterclockwise direction. After a high stimulus rate, contraction is sustained even at appreciably lower stimulus rate. Similar hysteresis patterns are also found in isometric responses. Hysteresis did not occur in cycle with lowest maximum rate. B: difference in relationships between stimulus rate and force in lengthening and in isometric muscle. At high stimulus rates lengthening muscle, •, produced more force than isometric muscle, ×, a relationship represented in the response surface diagrams of Fig. . At low stimulus rates, however, the relationship is reversed, suggesting a locally reversed slope for the force‐velocity curve at such stimulus rates. C: hysteresis in muscle response to recruiting type of stimulus modulation in human peroneus muscle at constant rate of 50 pulses/s. Ust, time graphs of stimulus intensity; Miso, isometric responses to stimulus. Lowest graph, force vs. stimulus, derived by combining data of type shown in the 2 time graphs and as labeled for point A and point B.

A: from Partridge . B: from Joyce et al. , with permission of Cambridge Univ. Press. C: from Trnkoczy
Figure 16. Figure 16.

Responses to mechanical inputs that deviate from response prediction implied by a simple version of the type of model illustrated in Fig. . A: response of stimulated triceps sura muscle to stretch showing mechanical hysteresis. In both lines stimulus was 120 pulses/s and velocity 0 at the same final lengths. In response marked by stimulus was started with muscle at a shorter length followed by a quick stretch to test length, producing a persistent increase of force over that in other response, with test length used throughout stimulus period. (See also Fig. A.) B: length‐tension relationship in flight muscle of water beetle. Lower curve, passive muscle; upper curve, active muscle; loop, trajectory plotted on work plane with tetanized muscle subjected to cyclic stretch. Response travels around loop in an energy‐delivering, counterclockwise direction, in contrast to energy‐absorbing, clockwise loops ordinarily described with other muscles. In this case, unlike the diagramed force surfaces, force at shortening velocities is greater than those at the same length and lengthening velocities. This relationship allows these muscles to drive an oscillatory movement without a corresponding oscillating excitation pattern. C: length of a tetanically stimulated muscle before, during, and after a transition from one force to a larger force. In each line, muscle shortened at beginning of stimulus, stretched as load increased, then lengthened again at the end of stimulus. Initial and final forces were the same for each load increase, but rate of change of force varied, increasing from line to line . Effect of different rates of loading persisted in terminal periods of constant and equal load. Greatest stretch occurred with fastest loading. D: disruption of fusion of tetanus during mechanical movement. Left part of record shows a stretch; right part shows a subsequent release of cat soleus while stimulated at 8.5 pulses/s. Upper lines, force; lower lines, muscle length.

A: from Walker . B: from Machin and Pringle . C: from Katz , with permission of Cambridge Univ. Press. D: from Joyce et al. , with permission of Cambridge Univ. Press
Figure 17. Figure 17.

Combinations of independent units of muscle. (See also Figs. B, F, and C.) A: recruitment of 3 separate respiratory motor units and accompanying increase of firing in individual units (measured from unit electromyogram (EMG) records) as total response intensity increases. Measured motor response is tidal air moved as it increased with rebreathing. B: smoothing of soleus isometric response by alternation of excitation among groups of motor units. Muscle nerve was divided into 5 branches, and each was provided with separate stimulus electrodes. Responses to individual stimuli can be distinguished in record made with synchronous stimulus to all branches. Rotating of branch stimulus (with each division stimulated in turn at same rate) produced response that was not only smoother but greater in magnitude than average of synchronous response. (Normal recruitment produces asynchronous instead of rotating excitation, presumably a less effective means of smoothing.) C: combination of activity of several muscles during bicycling. Weak and strong EMG activity are shown by single and broad lines, respectively. Note variable overlap between muscle activity periods, as well as two separated phases of strong activity in gracilis, Gr. D: recruitment of 3 types of motor units in 2 muscles of lion as speed of locomotion increases. Discontinuities in graphs represent change in type of gait. As seen in histochemical evidence, relative intensity of activity of different types of units does not change in the same order through all gaits and muscles.

A: from Gesell et al. . B: from Rack and Westbury , with permission of Cambridge Univ. Press. C: from Houtz and Fischer . D: from Armstrong et al.
Figure 18. Figure 18.

Relationships between several measures of muscle action. Broad arrows represents the multiple signals carried in the signal pathway within the biological system. Mechanical and electrical impedances through which biological signals are coupled to recorder system are marked Z. Electromyogram (EMG) signal may be further processed to some form of pulse signal or to an averaged EMG. Functional output may be measured as any of a variety of motor actions, as well as in form of direct muscle responses. A: nerve activity (ENG) and motor action as measured from phrenic nerve and as tracheal pressure show examples of entry of nerve signal into a muscle system and a motor consequence. B: direct EMG, averaged EMG (AEMG), muscle force, and length—measured during one step of walking, from cat triceps sura muscle. Non‐phase‐shifting digital filter was used to average EMG signal. Force was sampled from tendon in situ and length measured from slow motion movie films. These signals differ as a result of transformations that have occurred in instrumentation as well as in biological components. C: muscle sound pulses recorded from auricularis superior muscle. Myosonogram (MSG) pulses are related one to one to EMG pulses of same unit. Little systematic information is available about how effects of length, velocity, and temperature are represented in the sonogram. D: 3 pulse‐based measures from a unit of respiratory muscle. Bottom line, time of visually identified unit pulses; points on graph, pulse‐by‐pulse calculation of firing rates (inverse interval); solid line, smoothed pattern of firing rate. Accepted methods include variations in derivation of each of these measures of muscle activity, and these variations introduce noticeable differences in the way a particular action is displayed.

A: from Gesell . B: from Yager . C: from Gordon and Holbourn , with permission of Cambridge Univ. Press. D: from Gesell et al.
Figure 19. Figure 19.

Variation of relations between measures of physiological muscle activity with type of measurement and mechanical conditions. A: relationship between muscle force and integrated electromyogram (EMG). Each measurement on upper line made from the muscle while shortening; measurements on lower line made during lengthening of same muscle, showing velocity effect on force‐EMG relationship. B: effect of muscle length on force‐EMG relation in human soleus. Separate lines, lengths determined by ankle position in 0.5‐in. increments of muscle length. Mechanical action of gastrocnemius was avoided by bending the knee. C: different nonlinear functions relating force to integrated EMG and to action potential count from same EMG (scaled to match at high and low ends of samples). D: effect of different electrode arrangements on relationship between force and EMG recorded in same tests. Monopolar and bipolar electrodes were used, and spike count was measured. Electrode placements also affects voltage and frequency distributions of EMG .

A: from Bigland and Lippold , with permission of Cambridge Univ. Press. B: from Close et al. . C and D: from Close


Figure 1.

Examples of physiological actions of various muscles. A: twitch response to excitation of a few motor nerve fibers of human muscle obtained as ensemble average of output force from whole arm. Upper record, response during weak contraction of whole muscle; lower record, response from same motor units stimulated when remainder of muscle was strongly contracted. In the twitch during strong contractions, averaged force contribution rose to an appreciably lower peak than in the other series; then the average decreased to a value less than background level. In this measure both the primary or direct effect of excitation of the test unit and any consistent secondary effects that modify the activity of other units are combined in the total recorded response. B: two voluntary movements of human arm. Thickness of horizontal bars, periods of high, low, and no electromyograph (EMG) activity in agonist and antagonist muscles. First record made during a relaxed movement, second with a stiff arm. Periods of agonist and antagonist activities in first record show alternation with complete separation. In 2nd record the periods are completely overlapping with a rapid cycling of intensity of EMG. In spite of differences in electrical activity, however, the two movements follow similar time courses of position. C: sound produced by vibration of locust tympanum by tymbale muscle when artificially stimulated at 22.5 pulses/s. Stimulus points shown on lower record. Compare Fig. B, which shows isometric response of a similar muscle stimulated at 22 pulses/s. D: measures of triceps sura muscle action in 2 steps of a uniformly walking cat . All of the data originated in measurements (*) of length, force, and EMG by Yager . Continuous approximations of length, velocity, and acceleration were derived by applying spline functions to 128 photographically determined point values of length. Power calculated as product of velocity and force (measured by transducer on tendon). EMG is shown as rectified and filtered interpretation of direct record. Ratio of force to acceleration does not represent changing inertia of load, in part because system is not free of external forces and also because acceleration is not measured in an inertial reference frame. For 1st step (ending at arrowheads) force, velocity, and length are plotted also in a pseudo‐3‐dimensional form, starting in swing phase and progressing continuously in direction of time arrow; pickets mark 10‐ms intervals and eliminate ambiguity of 4‐dimensional points as graphed on a plane. This trajectory over the phase plane (length vs. velocity) shows that in swing phase large velocities and displacements are associated with small forces, while largest forces occur at midlengths and lengthening velocities during stance phase of step. Rest length, l = 1, is near long end of lengths involved in walking, as also described in ref. . The EMG trajectory for this same step can be seen in ref. . For separated soleus and gastrocnemius records of force in walking cat, see refs. and . Note that 2 steps shown in this figure to have similar length patterns have appreciably more variation in acceleration, velocity, and EMG . As is most apparent on phase plane graph, when muscle returns to the starting length for 1st step, 2nd step is initiated at a different velocity and therefore cannot follow a course identical to the first.

A: from Milner‐Brown et al. , with permission of Cambridge Univ. Press. B: from Wacholder . C: from Pringle , with permission of Cambridge Univ. Press. D: from Partridge , with permission of Raven Press


Figure 2.

Conventional nomenclature of successive structural subdivisions of vertebrate skeletal muscle down to sarcomere level. Functional motor unit does not correspond to any of these divisions. (For examples of diffuse distribution of a motor unit within a muscle, see Fig. in the chapter by Burke in this Handbook and refs. . It also should be noted that individual fibers may cross from one fasciculus to another or end within a fasciculus . Furthermore, the densely packed organization of fibers seen in histological section and in this schematic diagram of a fasciculus may represent a shift of extracellular fluid after loss of blood pressure . Normally, adjacent fibers may float separately within the muscle.

Adapted from Bloom and Fawcett


Figure 3.

Details of structure in gross muscles. A: primitive vertebrate muscle arrangement as seen in a single myomere in cartilagenous fish. Longitudinal and cross‐sectional views show pattern that may be approximated as alternating concentric cones made up of these segmentally innervated muscle units. B: regional organization of major innervation divisions (arbitrarily numbered) as projected on two surface views of anterior tibial muscle of dog, showing mixtures of series and parallel relationships. C: conformation change in simple pennate frog muscle as result of total contraction . Table summarizes measured length and angle changes associated with A' fasciculus produced by contraction of whole muscle. The 40% shortening contribution by muscle fasciculus exceeded the 32% shortening of that fasciculus itself and includes effect of increase in angle of these active fibers. An added 13% decrease of total length resulted from the changing angle of connective tissue. Due to changing angles, as well as fiber shortening, total shortening of 7.7 mm is greater, not less, than 5.5‐mm shortening of involved fibers in spite of angled orientation of fibers. D: organization of fasciculi (dashed lines) in the frequently studied frog gastrocnemius, as seen from superficial and deep surfaces and from a longitudinal section. Solid lines and locations Ap, Z, D, and P all mark major connective tissue structures. Sample fasciculus lengths, overall length and width, and a few pennation angles are marked.

A: from Alexander . B: from Wilder et al. . C and D: from Beritoff


Figure 4.

Effects of load modify muscle responses. A: twitches of muscle from different initial lengths in frog gastrocnemius muscle operating on nearly isometric load (left), compliant spring load (middle), and isotonic load (right). These loads determine force‐length slope through course of any contraction and thereby influence the pattern of maxima reached in the twitches. Passive length‐force relationship (right border, each record) also differs slightly between stretching sequence (upper passive curve, each record) and release sequence. Load‐dependent length‐tension curves for twitch maxima are joined by dashed lines. B: cat triceps sura muscle responses to modulated stimulus. Cyclic modulation of rate of stimulus used for both tests is shown as middle graph. Upper graph, isometric force, measured in newtons. Lower graph, force from the identical muscle, subjected to the same stimulus sequence, but attached to an inertial load. Response was measured with an accelerometer in gravity units g from which the newton scale N was calculated. Mean force and length were similar in both cases, and isometric force changed smoothly with period of stimulus. Inertial load was moved against gravity with the same period, but change in force producing that load movement does not have a simple period. Actually only a small fraction of maximum isometric force (and that delivered intermittently) is required to move physiological‐sized loads at moderate frequencies. C: calculated stretch of “series elastic component” with changes of the load on rat gracilis anticus muscles. Compliance (slope of curve) decreases as force increases. Bars, standard deviation of values measured from 5 muscles. This stretch represents the part of an overall length change that is considered not to occur in the contractile components. D: energy exchanges occurring with complex load on a 2‐joint muscle (human hamstrings) through part of a running step. The 2‐joint load allows separation of muscle length change (right graph) from that due to individual joint movement (two curves on left graph). Contributions to muscle length deviations, produced separately by hip movement and by knee movement, are plotted against force, as is total resulting change in muscle length. Numbered points identify correspondence of time on the 3 lines. Energy exchange at individual joint need not represent just energy produced or absorbed by the muscle. Area under curves is work done (integral of force with respect to distance). From 1–10 at knee, work is done on muscle by the load, whereas work is done by muscle at hip. Right: combined effect or net exchange in muscle energy for same points in time. Area under right curve from 1st to 6th point shows energy actually absorbed by muscle, and area under curve from 6th to 10th point shows mechanical energy delivered by muscle (part of delivered energy probably was from series elastic structures that shortened as tensile force was decreasing; if lengthening parallel elastic elements were under tension in this period they would have been absorbing energy). Comparison of areas under 3 curves shows that energy is conserved by transfer through muscle from knee to hip joint.

A: from Blix . B: from Partridge . C: from Bahler . D: from Elftman


Figure 5.

Static and dynamic properties of unstimulated muscles. A: variations in relationships between muscle length and measured force in unstimulated muscles of several species. Abscissa lengths are normalized at maximum body lengths. In some muscles no force is generated passively within lengths attainable in situ, but for other muscles passive tension in normal operating range of lengths exceeds active contribution (see also Fig. B). Curve A, bumblebee flight muscle; curve B, locust flight muscle; curve C, frog sartorius; curve D, clam (mytilus) anterior byssal retractor; curve E, snail (Helix) pharynx retractor muscle. Other muscle curves can also be seen in part B and Figs. A; C and D; and B. B: length‐tension relationship in an unstimulated frog gastrocnemius muscle measured at different rates of loading and unloading, using mercury to produce smooth force changes. Less stretch occurred with force increase, and particularly with fast increase, than with unloading. Force vs. displacement graphs were recorded directly using a special mechanical linkage. Dynamic aspect of passive response of muscle represented by difference between stretch and shortening curves commonly is ignored in dealing with muscle properties (compare Fig. .). C: “stress relaxation” after a step change in length of an unstimulated ileotibialis muscle of tortoise. Unbroken curve, force change in period immediately after stretch; dashed line, level at which force finally stabilized. D: three different time expansions of a record of “creep” of length after a step change in load on a single unstimulated frog fiber. Time marks in curve III, 10 s; time marks in curves II and I, 0.1 s and 0.01 s, also show initial oscillatory transient details as well as creep. Stretch scaled in percentage of equilibrium length (just taut).

A: from Hanson and Lowy . B: from Blix . C: from Hill . D: from Buchthal and Kaiser


Figure 6.

Various responses of muscles to single stimulus pulses. A: first published record of muscle twitch . Frog gastrocnemius was attached to a complex inertial and elastic load, and record was made by a steel point scratching on a lightly smoked, moving glass plate. Oscillatory detail is characteristic of the particular combination of load and muscle and is avoided in most modern muscle experiments by reducing both inertia and elastic compliance of loads. B: Simultaneous force and length records through a series of twitches with different isotonic loads. Frog gastrocnemius muscle was afterloaded; that is, load was mechanically supported at a fixed position before and after muscle tension exceeded force required to lift the load. Characteristically the latent period for load movement increased with load size (weight marked in g), and movement period decreased. The shorter latent period for force development was unchanged. (The same 1893–1895 ref. contains a large number of other records of twitches made with different types of load and load transients; both time‐based records and force vs. length records are included.) C: effect of preload on muscle twitch (weight in g marked on lines). Weights were hung near the fulcrum to minimize effect of inertia, thus approximating an isotonic load. Stretch of loaded muscle before stimulus as well as changes in response can be seen. D: volume change in frog sartorius during isometric twitch. Major divisions on abscissa, 10 ms. Detail of volume change is highly sensitive to load conditions . Peak volume decrease is about 5 × 10–3% of total muscle volume.

A: from Helmholtz . B and C: from Blix . D: from Baskin


Figure 7.

Time graphs of muscle response to 2 or more stimuli in succession. A: traditional student laboratory demonstration (1905) of wave summation for 2 stimuli in nearly isotonic contractions of frog gastrocnemius. Stationary ordinates mark stimulus points. Lower line, time at 50 periods/s. Upper 2 records, response to 2 stimuli superimposed on record produced following a single stimulus. B: typical isometric response of striated muscle to trains of stimulus pulses delivered at 0.9, 22, 46, and 100 pulses/s. At 100 pulses/s response approaches a fused tetanus. The muscle used for this record was tymbal muscle of locust. Compare to physiological response of a similar muscle when attached to its normal load in Fig. C. C: isometric records of wave summation. Lower records, from cat soleus with 2 stimuli delivered at a 70 ms‐interval at 37°C. Area added to record (mechanical impulse) by second stimulus is greater than that produced by 1st stimulus. Upper record, from frog sartorius with stimulus interval of 120 ms. (Time scale of frog muscle record is 77% as fast as that shown for cat record.) Impulse added by 2nd stimulus in this case is less than that contributed by 1st. In both records time from stimulus to peak of contributed force is longer for 2nd twitch than for 1st. D: successive contributions of individual stimuli in a sequence. Upper graphs, superimposed records of responses with different total number of stimuli in trains. Lower graphs, contribution of individual stimuli calculated by subtracting responses to trains differing by one pulse. Left graphs: theoretical linear summation, each impulse producing same contribution. Right graphs: actual frog sartorius responses, 5 pulses/s. Contribution and rate of rise of successive pulses decreased markedly, especially at higher rate. See Fig. C for a measure of isotonic contribution in displacement time product resulting from a pulse as a function of stimulus rate at which it is added.

A: from Beddard et al. . B: from Pringle , with permission of Cambridge Univ. Press. C: from Ranatunga . D: from Gilson et al.


Figure 8.

Effects of stimulus rate on muscle response. A: several different ways of representing the same results. For these graphs a single fiber from frog semitendinosis was used, and isometric response was measured from periods of steady stimulus. Curve a, force differences in individual responses; curve b, contraction remainder at low point in response between stimuli; curve c, ratio of contraction remainder to stimulus rate; curve d, force peak (a + b); curve e, mean tension over time. All curves are plotted as functions of stimulus rate. B: rate of rise of tension (right ordinate and × values) at beginning of a stimulus train continues to increase through appreciably higher stimulus rates than does contractile tension (left ordinate and • values). This is an additional example of measures of muscle response that are not equally sensitive to stimulus rate. C: family of nearly isotonic responses of cat triceps sura muscle to slowly increasing stimulus rate (1 pulse/s2). Separate lines are made with different loads. At low stimulus rates and with small loads the responses to individual stimuli are particularly apparent (compare twitches in Fig. C). Lower graphs show derivative of average (negative) length with respect to stimulus rate (–d//dR), which was calculated using spline functions on mean (smoothed) values of the upper graphs. These superimposed curves indicate that the principal sensitivity to stimulus rate for this muscle falls in the range of 8–25 pulses/s over a variety of loads. Separate peaks could represent populations of fast and slow muscle units within this muscle.

A: from Buchthal . B: from Machin and Pringle . C: from Partridge


Figure 9.

Dynamic effects of modulated stimulation. A: time graphs of response of load moving by cat triceps sura muscle driven with a sinusoidally modulated stimulus rate, 5–25 pulses/s, and at modulation frequencies marked on graphs. Amplitude of modulation was constant, but response amplitude changed with frequency of modulation. B: change in movement amplitude with frequency of modulation cycle. All stimulus cycles ranged from 5 to 35 pulses/s. Although the gravity part of loads was constant, each line represents a different load inertia, spanning a 28‐fold variation within the physiological range . Periodic force required for sinusoidally moving an inertial load increases proportional to inertia and proportional to the square of the frequency or, for this range 28 × (6.3/0.04)2 = 7 × 105, which is the ratio of force cycles for a constant amplitude movement. Considering the proportionality of force cycle to movement amplitude, this graph represents an actual 8 × 103 range of amplitudes of response cycles of force produced by a constant amplitude of modulation of stimulus rate. C: temporal displacement between stimulus cycle and response cycle in isometric records of cat triceps sura force as a function of cyclic frequency. Upper graph, lag of response in ms; lower graph, same data in fractions of a 360° cycle. D: representation of muscle properties combining both nonlinear and dynamic aspects. Each set of 3 coefficients (G0, fn, and ζ) define one 2nd‐order differential equation approximating data similar to that shown in parts in C and B. These coefficients when inserted in the equation specify dynamic properties of the muscle in one operating range of stimulus rates. Changes of coefficients with firing rate range represent some of the nonlinearities. •, ○, Values obtained from different muscles.

A and B: from Partridge . C: from Partridge . D: from Mannard and Stein , with permission of Cambridge Univ. Press


Figure 10.

Relations between muscle actions and temperature. A: twitch variations in frog muscle at 3 different temperatures. Time reference, 100 Hz. B: effect of temperature on tortoise muscle stiffness. Lower lines, static stiffness; upper lines, dynamic stiffness. Pairs of points are measured from one sequence of increasing and one sequence of decreasing temperature. C: height of frog muscle twitches at a sequence of temperatures between 0°C and 36°C. D: effect of temperature on duration of twitch. Temperature ranges between 5°C and 41°C, at which point response failed completely. E: change in temperature of body core, rt, and of muscle, mt, as a result of muscle activity in human. Crossing of the two curves shows a reversal of direction of heat transfer between muscle and relatively constant body core. Externally observable motor performance also changed in speed with temperature in this study, which may be related to “warm up” before athletic activity.

A: from Handbook of Physiology, by W. D. Halliburton, © 1911, used with permission of McGraw‐Hill Book Co. B: from Fowler and Crowe . C and D: from Howell . E: from Asmussen and Bøje


Figure 11.

Muscle contraction as modified by length. A: relationship between stress and strain in 12 tetanically stimulated glossopharyngeal muscles of frog. Data are from tables made by Weber in 1847 . Eleven muscles, •, were loaded before stimulus. One muscle, ▽, was stimulated before loading. Normalization to stress and strain from length and tension, as described by Weber, allows comparison of muscles of quite different sizes. (We have crudely confirmed Weber's description of the very long range of lengths over which this muscle can respond.) B: comparison of active contribution to length‐tension effect in several different muscles. (For passive contributions of the same muscles, see Fig. A.) A, bee flight muscle; B, locust flight muscle; C, frog sartorius; D, anterior byssal retractor of clam. Ordinates are not normalized to compensate for muscle size differences. C: conventional measurement of active contribution to length‐tension effect. Passive response and maximal voluntary force were measured directly in human forearm flexor muscle with cineplastic tunnel prepared to allow external action of the tendon. Developed tension is calculated as difference between passive and maximal forces with no correction for result of local stresses within muscle and tendon. See Fig. A for similar data from twitches. D: family of length‐tension curves for single fiber of frog semimembranous muscle. Parameter distinguishing individual curves is stimulus rate. Note nonuniform increments of stimulus rate and compare Fig. . Values were calculated as mean of 2 values measured with increasing and with decreasing stimulus rate.

B: from Hanson and Lowry . C: from Ralston et al. . D: from Buchthal


Figure 12.

Effect of velocity on muscle response, measured at constant velocities while passing through a test length. All sets of curves are drawn on same abscissa. A: Force‐velocity effect at several different stimulus rates in cat soleus muscle using rotation of stimulus among 5 nerve branches. (Also see ref. for differences introduced by slightly different test conditions.) B: rate of exchange of energy between muscle and load, or power, is equal to product of force F and velocity V. Negative power indicates output of energy by the muscle. (Energy delivered by each impulse, i.e., power ÷ pulse rate, varies with stimulus rate but is nearly independent of velocity over an appreciable range.) C: slope ∂F/∂V calculated from force‐velocity curves in A, a measure of sensitivity of force to velocity, or mechanical impedance (as in ref. ). Negative values are proportional to a viscosity‐like measure. The value of this measure is greatest at low shortening velocities with high stimulus rates. Graphs B and C were calculated for this review from data points on force‐velocity graph (A) using spline interpolation . Spline fit lines pass through same data points, but they differ slightly from lines joining points in A.

A: from Joyce and Rack , with permission of Cambridge Univ. Press


Figure 13.

Expected 3‐dimensional mechanical relationship in response of muscle. Muscle force as related to length, velocity, and excitation drawn by combining usual descriptions (for related experimental data, see Figs. C, B, , D, A). Arrows on velocity axes show relationship between velocity sign and change of length (angle). Arrows on torque axis in F relate torque to corresponding direction of change of velocity. A: one 3‐dimensional description of response properties of a single muscle at a particular moment. Excitation of the muscle defines length‐tension‐velocity relationships in that muscle. At a single length this relationship is described as a force‐velocity curve, and at a single velocity it is a length‐tension curve. For length and velocity at some moment, instantaneous force falls at intersection of 2 such curves in length‐velocity‐force space. Red and green lines might correspond to 2 curves from a stimulus rate family, such as shown in Fig. D for length axis and Fig. A along velocity axis. At any other moment force output could be defined in terms of intersection of an appropriate stimulus‐determined pair of intersecting length‐tension and force‐velocity curves, at least a 4‐dimensional relationship. (Note: No dynamic component is represented in the passive response, although one is known to exist; see Fig. BD.) B: the 26 different red graphs of active length‐tension curves intersect in this diagram with each of the 26 red force‐velocity curves to define 676 of the points to which a particular excitation level might bring a muscle. Same number of intersections in green represent passive muscle. (The different individual curves were derived from assumption that at any length the nonlinear velocity effect is proportional to isometric active force at that length.) Inclusion of all possible points between samples drawn here would produce a “response surface”. If red surface designates the possible responses of maximally active muscle, then neural control would operate in space between red and green surfaces. (see ref. for an experimentally derived half‐surface drawn with a different orientation of the axes.) C: another variation of a response surface differing from B by a simple shift in relationship between passive and active length effects. Different muscles have properties that range between and beyond these types. D: representation of a sinusoidal movement as a trajectory over a response surface. A sinusoidal change in length, l = A sin ωt, defines a cyclic velocity, V = A ω cos ωt, that varies in amplitude with frequency. Such pairs of curves trace eliptic paths on phase (base) plane and define complex closed loop trajectories over response surface. Time‐varying force traced by such trajectories is periodic but not sinusoidal. This transformation from movement into force is inferred by the accepted length and velocity properties of maximally and constantly activated muscle. Response cycle has a phase‐shift based on shape of response surface; it changes with movement frequency, amplitude, and bias. (Nonsinusoidal patterns of movement similarly can be graphed over this theoretical surface. As illustrated in Figs. and , more complex stimulus and movement patterns tend to produce deviations from response surface predicted by the usual muscle description and represented by these graphs.) E: time graphs for one movement over response surface of D. Two cycles of sinusoidal length change centered around rest length, Lo, are shown by black curve at top of figure. Corresponding but phase‐shifted force cycle for a maximally activated muscle is shown by the periodic, nonsinusoidal, red curve marked Fmax. In some stretching parts of the cycle that force exceeds maximum isometric force, 100% Fi. At the specified lengths passive forces are all near 0 (0% Fi), as shown by line marked Fmin. Any constant force (e.g., 40% of Fi) falls at different relative positions with respect to minimum and maximum force for velocity‐length combinations as values change through the movement. If such a constant force can be produced through a movement, then a variation of activation of the muscle would be required. Here activation A is defined for any particular force as the relative position of that force between Fmax and the passive force for existing physical conditions at the moment. In this example in order to maintain the constant 40% Fi force through the described movement, the neural drive would have to provide a time‐varying activation (from 33% to 80% of maximal), as plotted on line marked % A. F: theoretical response surfaces for a pair of matched antagonist muscles. Green lines represent surface expected while both muscles are unexcited. Red lines represent the response surface expected when both muscles are maximally active. This surface does not show a simple cancellation of force; i.e., coactive surface is not a horizontal plane equal to passive surface. For the slopes, ∂T/∂θ relates to stiffnesslike properties, and ∂T/∂V relates to a viscosity‐like property. Lengths, velocities, and forces that describe linear action of a single muscle are replaced here by angles, angular velocities, and torques, which describe combined action of antagonist muscles on a hinge joint.

Adapted from Partridge


Figure 14.

Effects of excitation history on dynamics of muscle response. A: sequence of afterload frog muscle twitches recorded as fatigue develops. Increased height over first few trials (treppe) is followed by decreased amplitude (fatigue) as well as by increased latent period, increased time to peak, and slowed relaxation. This latter effect is known as contracture. B: change of response in 2 cat limb muscles as a result of surgically crossed innervation. Upper two graphs, normal muscle twitches in crureus and gracilis muscles. Lower records, altered response under influence of reinnervation by crossed nerves—slow muscle becoming faster and fast muscle becoming slower. Line below twitch record shows a digital measure of contraction time in ms arranged in series of dots, each column with a maximum of 10 1‐ms points. C: adaptation of response of anterior tibial muscle of rabbit to long‐term activity pattern. Two isometric responses to stimulus trains of 25 pulses/s. Lower record, response of normal muscle; upper record, muscle modified by stimulation at 10 pulses/s over a 5‐month period.

A: from Handbook of Physiology by W. D. Halliburton, © 1911. Used with permission of McGraw‐Hill Book Co. B: from Buller et al. , with permission of Cambridge Univ. Press. C: from Salmon


Figure 15.

Deviations of muscle response to stimulus from that of a simple, response surface model with lag. Examples show that muscle responses vary with stimulus history and past activity in other units and that mechanical effects are variable with stimulus level. A: hysteresis in cat triceps sura muscle response to rate modulation of stimulus. Insert, time sequence of stimulus. In graph of length vs. stimulus rate, time progresses in a counterclockwise direction. After a high stimulus rate, contraction is sustained even at appreciably lower stimulus rate. Similar hysteresis patterns are also found in isometric responses. Hysteresis did not occur in cycle with lowest maximum rate. B: difference in relationships between stimulus rate and force in lengthening and in isometric muscle. At high stimulus rates lengthening muscle, •, produced more force than isometric muscle, ×, a relationship represented in the response surface diagrams of Fig. . At low stimulus rates, however, the relationship is reversed, suggesting a locally reversed slope for the force‐velocity curve at such stimulus rates. C: hysteresis in muscle response to recruiting type of stimulus modulation in human peroneus muscle at constant rate of 50 pulses/s. Ust, time graphs of stimulus intensity; Miso, isometric responses to stimulus. Lowest graph, force vs. stimulus, derived by combining data of type shown in the 2 time graphs and as labeled for point A and point B.

A: from Partridge . B: from Joyce et al. , with permission of Cambridge Univ. Press. C: from Trnkoczy


Figure 16.

Responses to mechanical inputs that deviate from response prediction implied by a simple version of the type of model illustrated in Fig. . A: response of stimulated triceps sura muscle to stretch showing mechanical hysteresis. In both lines stimulus was 120 pulses/s and velocity 0 at the same final lengths. In response marked by stimulus was started with muscle at a shorter length followed by a quick stretch to test length, producing a persistent increase of force over that in other response, with test length used throughout stimulus period. (See also Fig. A.) B: length‐tension relationship in flight muscle of water beetle. Lower curve, passive muscle; upper curve, active muscle; loop, trajectory plotted on work plane with tetanized muscle subjected to cyclic stretch. Response travels around loop in an energy‐delivering, counterclockwise direction, in contrast to energy‐absorbing, clockwise loops ordinarily described with other muscles. In this case, unlike the diagramed force surfaces, force at shortening velocities is greater than those at the same length and lengthening velocities. This relationship allows these muscles to drive an oscillatory movement without a corresponding oscillating excitation pattern. C: length of a tetanically stimulated muscle before, during, and after a transition from one force to a larger force. In each line, muscle shortened at beginning of stimulus, stretched as load increased, then lengthened again at the end of stimulus. Initial and final forces were the same for each load increase, but rate of change of force varied, increasing from line to line . Effect of different rates of loading persisted in terminal periods of constant and equal load. Greatest stretch occurred with fastest loading. D: disruption of fusion of tetanus during mechanical movement. Left part of record shows a stretch; right part shows a subsequent release of cat soleus while stimulated at 8.5 pulses/s. Upper lines, force; lower lines, muscle length.

A: from Walker . B: from Machin and Pringle . C: from Katz , with permission of Cambridge Univ. Press. D: from Joyce et al. , with permission of Cambridge Univ. Press


Figure 17.

Combinations of independent units of muscle. (See also Figs. B, F, and C.) A: recruitment of 3 separate respiratory motor units and accompanying increase of firing in individual units (measured from unit electromyogram (EMG) records) as total response intensity increases. Measured motor response is tidal air moved as it increased with rebreathing. B: smoothing of soleus isometric response by alternation of excitation among groups of motor units. Muscle nerve was divided into 5 branches, and each was provided with separate stimulus electrodes. Responses to individual stimuli can be distinguished in record made with synchronous stimulus to all branches. Rotating of branch stimulus (with each division stimulated in turn at same rate) produced response that was not only smoother but greater in magnitude than average of synchronous response. (Normal recruitment produces asynchronous instead of rotating excitation, presumably a less effective means of smoothing.) C: combination of activity of several muscles during bicycling. Weak and strong EMG activity are shown by single and broad lines, respectively. Note variable overlap between muscle activity periods, as well as two separated phases of strong activity in gracilis, Gr. D: recruitment of 3 types of motor units in 2 muscles of lion as speed of locomotion increases. Discontinuities in graphs represent change in type of gait. As seen in histochemical evidence, relative intensity of activity of different types of units does not change in the same order through all gaits and muscles.

A: from Gesell et al. . B: from Rack and Westbury , with permission of Cambridge Univ. Press. C: from Houtz and Fischer . D: from Armstrong et al.


Figure 18.

Relationships between several measures of muscle action. Broad arrows represents the multiple signals carried in the signal pathway within the biological system. Mechanical and electrical impedances through which biological signals are coupled to recorder system are marked Z. Electromyogram (EMG) signal may be further processed to some form of pulse signal or to an averaged EMG. Functional output may be measured as any of a variety of motor actions, as well as in form of direct muscle responses. A: nerve activity (ENG) and motor action as measured from phrenic nerve and as tracheal pressure show examples of entry of nerve signal into a muscle system and a motor consequence. B: direct EMG, averaged EMG (AEMG), muscle force, and length—measured during one step of walking, from cat triceps sura muscle. Non‐phase‐shifting digital filter was used to average EMG signal. Force was sampled from tendon in situ and length measured from slow motion movie films. These signals differ as a result of transformations that have occurred in instrumentation as well as in biological components. C: muscle sound pulses recorded from auricularis superior muscle. Myosonogram (MSG) pulses are related one to one to EMG pulses of same unit. Little systematic information is available about how effects of length, velocity, and temperature are represented in the sonogram. D: 3 pulse‐based measures from a unit of respiratory muscle. Bottom line, time of visually identified unit pulses; points on graph, pulse‐by‐pulse calculation of firing rates (inverse interval); solid line, smoothed pattern of firing rate. Accepted methods include variations in derivation of each of these measures of muscle activity, and these variations introduce noticeable differences in the way a particular action is displayed.

A: from Gesell . B: from Yager . C: from Gordon and Holbourn , with permission of Cambridge Univ. Press. D: from Gesell et al.


Figure 19.

Variation of relations between measures of physiological muscle activity with type of measurement and mechanical conditions. A: relationship between muscle force and integrated electromyogram (EMG). Each measurement on upper line made from the muscle while shortening; measurements on lower line made during lengthening of same muscle, showing velocity effect on force‐EMG relationship. B: effect of muscle length on force‐EMG relation in human soleus. Separate lines, lengths determined by ankle position in 0.5‐in. increments of muscle length. Mechanical action of gastrocnemius was avoided by bending the knee. C: different nonlinear functions relating force to integrated EMG and to action potential count from same EMG (scaled to match at high and low ends of samples). D: effect of different electrode arrangements on relationship between force and EMG recorded in same tests. Monopolar and bipolar electrodes were used, and spike count was measured. Electrode placements also affects voltage and frequency distributions of EMG .

A: from Bigland and Lippold , with permission of Cambridge Univ. Press. B: from Close et al. . C and D: from Close
References
 1. Abbott, B. C., and X. M. Aubert. The force exerted by active striated muscle during and after change of length. J. Physiol. London 117: 77–86, 1952.
 2. Abbott, B. C., B. Bigland, and J. M. Ritchie. The physiological cost of negative work. J. Physiol. London 117: 380–390, 1952.
 3. Abbott, B. C., and J. M. Ritchie. Early tension relaxation during a muscle twitch. J. Physiol. London 113: 330–335, 1951.
 4. Abbott, B. C., and D. R. Wilkie. The relation between velocity of shortening and the tension‐length curve of skeletal muscle. J. Physiol. London 120: 214–223, 1953.
 5. Adams, R. D. Diseases of Muscle: A Study in Pathology (3rd ed.). Hagerstown, MD: Harper & Row, 1975.
 6. Adrian, E. D. The spread of activity in the tenuissimus muscle of the cat and in other complex muscles. J. Physiol. London 60: 301–315, 1925.
 7. Adrian, E. D. The Mechanism of Nervous Action: Electrical Studies of the Neurone. Philadelphia: Univ. of Pennsylvania Press, 1932.
 8. Adrian, E. D., and D. W. Bronk. The discharge of impulses in motor nerve fibres. Part 1. Impulses in single fibres of the phrenic nerve. J. Physiol. London 66: 81–101, 1928.
 9. Adrian, E. D., and D. W. Bronk. The discharge of impulses in motor nerve fibres. Part 2. The frequency of discharge in reflex and voluntary contraction. J. Physiol. London 67: 119–151, 1929.
 10. Alexander, R. McN. The orientation of muscle fibres in the myomeres of fishes. J. Mar. Biol. Assoc. UK 49: 263–290, 1964.
 11. Alexander, R. McN. Muscle performance in locomotion and other strenuous activities. In: Comparative Physiology, edited by I. Bolis, K. Schmidt‐Nielsen, and S. H. P. Maldrell. Amsterdam: North Holland, 1973, p. 1–20.
 12. Alexander, R. McN., and A. Vernon. The mechanics of hopping by kangaroos (Macropodidas). J. Zool. 117: 265–303, 1975.
 13. Alexander, R. S. Viscoelastic determinants of muscle contractility and “cardiac tone.” Federation Proc. 21: 1001–1005, 1962.
 14. American College of Sports Medicine. Encyclopedia of Sport Sciences and Medicine. New York: Macmillan, 1971.
 15. American Society for Testing and Materials. Standard for Metric Practice. Philadelphia, PA: Am. Soc. Testing Materials, 1976. Document ANSI/ASTM E 380–76. (Also available as Institute of Electrical and Electronics Engineers document IEEE Std 268–1979. New York: Inst. Elec. Electron. Eng., 1979.).
 16. Andersson‐Cedergren, E. Ultrastructure of motor end plate and sarcoplasmic components of mouse skeletal muscle fibers as revealed by three dimensional reconstructions from serial sections. J. Ultrastruct. Res. Suppl. 1: 1–191, 1959.
 17. Armstrong, R. B., P. Marum, C. W. Saubert IV, H. J. Seeherman, and C. R. Taylor. Muscle fiber activity as a function of speed and gait. J. Appl. Physiol:. Respirat. Environ. Exercise Physiol. 43: 672–677, 1977.
 18. Asmussen, E., and O. Bøje. Body temperature and capacity for work. Acta Physiol. Scand. 10: 1–22, 1945.
 19. Asmussen, E., F. Bonde‐Petersen, and K. Jørgensen. Mechano‐elastic properties of human muscles at different temperatures. Acta Physiol. Scand. 96: 83–93, 1976.
 20. Aubert, X. La relation entre Ia force et Ia vitesse d'allongement et de raccorucissement du muscle strié. Arch. Int. Physiol. Biochim. 64: 121–122, 1956.
 21. Badoux, D. M. An introduction to biomechanical principles in primate locomotion and structure. In: Primate Locomotion, edited by F. A. Jenkins, Jr. New York: Academic, 1974. p. 1–43.
 22. Bagust, J., S. Knott, D. M. Lewis, J. C. Luck, and R. A. Westerman. Isometric contractions of motor units in a fast twitch muscle of the cat. J. Physiol. London 231: 87–104, 1973.
 23. Bagust, J., D. M. Lewis, and R. A. Westerman. Polyneural innervation of kitten skeletal muscle. J. Physiol. London 229: 241–255, 1973.
 24. Bahler, A. S. Series elastic component of mammalian skeletal muscle. Am. J. Physiol. 213: 1560–1564, 1967.
 25. Bahler, A. S. Modeling of mammalian skeletal muscle. IEEE Trans. Biomed. Eng. 15: 249–257, 1968. (Correction 16: 159, 1969.).
 26. Bahler, A. S. Mechanical properties of relaxing frog skeletal muscle. Am. J. Physiol. 220: 1983–1990, 1971.
 27. Bahler, A. S., and J. T. Fales. Dynamics of mammalian muscle contractile component. Physiologist 9: 133, 1966.
 28. Bahler, A. S., J. T. Fales, and K. L. Zierler. The active state of mammalian skeletal muscle. J. Gen. Physiol. 50: 2239–2253, 1967.
 29. Bahler, A. S., J. T. Fales, and K. L. Zierler. The dynamic properties of mammalian skeletal muscle. J. Gen. Physiol. 51: 369–384, 1968.
 30. Banus, M. G., and A. M. Zetlin. The relation of isometric tension to length in skeletal muscle. J. Cell. Comp. Physiol. 12: 403–420, 1938.
 31. Bárány, M. ATPase activity of myosin correlated with speed of muscle shortening. J. Gen. Physiol. 50: Suppl.: 197–216, 1967.
 32. Bardeen, C. R. Variation in the internal architecture of the m. obliquus abdominis externus in certain mammals. Anat. Am. 23: 241–249, 1903.
 33. Barrett, B. The length and mode of termination of individual muscle fibers in the human sartorius and posterior femoral muscles. Acta Anat. 48: 242–257, 1962.
 34. Baskin, R. J. Changes of volume in striated muscle. Am. Zool. 7: 593–601, 1967.
 35. Basmajian, J. V. Muscles Alive: Their Functions Revealed by Electromyography (4th ed.). Baltimore, MD: Williams & Wilkins, 1978.
 36. Basmajian, J. V., and M. A. MacConnall. Muscles and Movements: A Basis for Human Kinesiology (2nd ed.). Huntington, NY: Krieger, 1977.
 37. Bawa, P., A. Mannard, and R. B. Stein. Predictions and experimental tests of a visco‐elastic muscle using elastic and inertial loads. Biol. Cybern. 22: 139–145, 1976.
 38. Bawa, P., and R. B. Stein. Frequency response of human soleus muscle. J. Neurophysiol. 39: 788–793, 1976.
 39. Beaunis, H. Recherches physiologiques sur Ia contraction simultanée des muscles antagonistes. Arch. Physiol. Norm. Pathol. Ser. 5 1: 55–69, 1889.
 40. Becker, R. O. The electrical response of human skeletal muscle to passive stretch. J. Bone Jt. Surg. 42A: 1091–1103, 1960.
 41. Beckett, S. D., R. S. Hudson, D. F. Walker, T. M. Reynolds, and R. I. Vachon. Blood pressure and penile muscle activity in the stallion during coitus. Am. J. Physiol. 225: 1072–1075, 1973.
 42. Beddard, A. P., J. S. Edkins, L. Hill, J. J. R. Macleod, and M. S. Pembrey. Practical Physiology. London: Arnold, 1905.
 43. Beevor, C. E. Croonian lectures on muscular movements and their representations in the central nervous system. Br. Med. J. 1: 1357–1360, 1417–1421, 1480–1484; 1903.
 44. Br. Med. J. 2: 12–16, 1903.
 45. Bekey, G. A., C‐W. Chang, J. Perry, and M. M. Hoffer. Pattern recognition of multiple EEG signals applied to the description of human gait. Proc. IEEE 65: 674–681, 1977.
 46. Bennet‐Clark, H. C. The energetics of the jump of the locust, Schistocerca gregaria. J. Exp. Biol. 63: 53–83, 1975.
 47. Bergström, R. M. The relationship between number of impulses and the integrated electrical activity in electromyogram. Acta Physiol. Scand. 45: 97–101, 1959.
 48. Beritoff, J. Über die Kontraktionsfähigkeit der Skelettmuskeln. IV. Über die physiologische Bedeutung des gefiederten Baues der Muskeln. Pfluegers Arch. Ges. Physiol. 209: 763–778, 1925.
 49. Bigland, B., and O. C. J. Lippold. Motor unit activity in the voluntary contraction of human muscle. J. Physiol. London 125: 322–335, 1954.
 50. Bigland, B., and O. C. J. Lippold. The relation between force, velocity and integrated electrical activity in human muscles. J. Physiol. London 123: 214–224, 1954.
 51. Binder, M. D., W. E. Cameron, and D. G. Stuart. Speed‐force relations in the motor units of the cat tibialis posterior muscle. Am. J. Phys. Med. 57: 57–65, 1978.
 52. Binder, M. D., J. S. Kroin, G. P. Moore, and D. G. Stuart. The response of Golgi tendon organs to single motor unit contractions. J. Physiol. London 271: 337–349, 1977.
 53. Biró, G., and L. D. Partridge. Analysis of multiunit spike records. J. Appl. Physiol. 30: 521–526, 1971.
 54. Bishop, G. H., and A. S. Gilson, Jr. Action potentials accompanying the contractile process in skeletal muscle. Am. J. Physiol. 82: 478–495, 1927.
 55. Blix, M. Die Länge und Spannung des Muskels. Skand. Arch. Physiol. 3: 295–318, 1892;
 56. Die Länge und Spannung des Muskels. Skand. Arch. Physiol. 4: 399–409, 1893;
 57. Die Länge und Spannung des Muskels. Skand. Arch. Physiol. 5: 150–172, 173–206, 1895.
 58. Bloom, W., and D. W. Fawcett. A Textbook of Histology (10th ed.). Philadelphia, PA: Saunders, 1975.
 59. Borejdo, J., and M. F. Morales. Fluctuations in tension during contraction of single muscle fibers. Biophys. J. 20: 315–334, 1977.
 60. Bornhorst, W. J., and J. E. Minardi. A phenomenological theory of muscular contraction. II. Generalized length variations. Biophys. J. 10: 155–171, 1970.
 61. Boudet (de Paris). Recherches sur le bruit musculaire. C. R. Soc. Biol. 32: 40–44, 1880.
 62. Bouisset, S., and F. Goubel. Relation entre l'activité électromyographique intégrée et Ia vitesse d'exécution de mouvements monoarticulaires simples. J. Physiol. Paris 59: Suppl. 19: 359, 1967.
 63. Bouisset, S., and B Maton. Comparaison des activités électromyographiques globale élémentaire au cours du mouvement volontaire. Rev. Neurol. 122: 427–429, 1969.
 64. Bouman, H. D., and G. Van Rijnberk. Die akustischen Erscheinungen der Muskeln (Muskelgeräusch, Muskelton). Arch. Néerl. Physiol. 23: 441–507, 1938.
 65. Bouman, H. D., and G. Van Rijnberk. On muscle sound produced during voluntary contraction in man: an experimental study. Arch. Néerl. Physiol. 23: 34–55, 1938.
 66. Boyd, O. C., P. O. Lawrence, and P. J. A. Dratty. On modeling the single motor unit action potential. IEEE Trans. Biomed. Eng. 25: 236–243, 1978.
 67. Boyle, P. J., E. J. Conway, F. Kane, and H. L. O'Reilly. Volume of interfibre spaces in frog muscle and the calculation of concentrations in the fibre water. J. Physiol. London 99: 401–414, 1941.
 68. Bozler, E. Mechanical control of the rising phase of contraction of frog skeletal and cardiac muscle. J. Gen. Physiol. 70: 697–705, 1977.
 69. Braune, C. W., and O. Fisher. Der Gang des Menschen. I. Versuche unbelasten und belasten Menschen. Abh. Math. Phys. Kl. Koenigl. Saeschs. Ges. Wiss. 21: 153–322, 1895.
 70. Brokaw, C. J. Computer simulation of movement‐generating cross bridges. Biophys. J. 16: 1013–1041, 1976.
 71. Bronk, D. W. The energy expended in maintaining a muscular contraction. J. Physiol. London 69: 306–315, 1930.
 72. Brooke, M. H. A Clinician's View of Neuromuscular Diseases. Baltimore, MD: Williams & Wilkins, 1977.
 73. Brooks, V. B. Motor programs revisited. In: Posture and Movement, edited by R. E. Talbott and D. R. Humphrey. New York: Raven, 1979, p. 13–49.
 74. Brooks, V. B., and S. D. Stoney, Jr. Motor mechanisms: the role of the pyramidal system in motor control. Annu. Rev. Physiol. 33: 337–392, 1971.
 75. Brown, A. C. Analysis of the Myotatic Reflex (Ph.D. thesis). Seattle: Univ. of Washington, 1959. (Abstr. in Diss. Abstr. 20: 60–4277, 1960.).
 76. Brown, G. L., and U. S. von Euler. The after effects of a tetanus on mammalian muscle. J. Physiol. London 93: 39–60, 1938.
 77. Brown, M. C., and P. B. C. Matthews. An investigation into the possible existence of polyneuronal innervation of individual skeletal muscle fibers in certain hind‐limb muscles of the cat. J. Physiol. London 151: 436–457, 1960.
 78. Brumlik, J. The Quantitation of Muscle Tone in the Spastic State: an Analysis of Increased Muscle Tone, Comparing Spasticity and Rigidity (Ph.D. thesis). Evanston, IL: Northwestern Univ., 1961. (Abstr. in Diss. Abstr. 22: 61–5296, 1961.).
 79. Buchthal, F. The mechanical properties of the single striated muscle fibre at rest and during contraction and their structural interpretation. Dan. Biol. Medd. Kbh. 17: 1–140, 1942.
 80. Buchthal, F. The rheology of the cross striated muscle fibre and its minute structural interpretation. Pubbl. Staz. Zool. Napoli 23: 115–146, 1951.
 81. Buchthal, F., and L. Engbeck. Refractory period and conduction velocity of striated muscle fiber. Acta Physiol. Scand. 59: 199–220, 1963.
 82. Buchthal, F., F. Erminio, and P. Rosenfalck. Motor unit territory in different human muscles. Acta Physiol. Scand. 45: 72–87, 1959.
 83. Buchthal, F., C. Guld, and P. Rosenfalck. Multielectrode study of the territory of a motor unit. Acta Physiol. Scand. 39: 83–104, 1957.
 84. Buchthal, F., C. Guld, and P. Rosenfalck. Volume conduction of the spike of the motor unit potential investigated with a new type of multielectrode. Acta Physiol. Scand. 38: 331–354, 1957.
 85. Buchthal, F., and E. Kaiser. The rheology of the cross striated muscle fiber with particular reference to isotonic conditions. Dan. Biol. Medd. Kbh. 21: 1–318, 1951.
 86. Buchthal, F., and P. Rosenfalck. Elastic properties of striated muscle. In: Tissue Elasticity, edited by J. W. Remington. Washington, DC: Am. Physiol. Soc, 1957, p. 73–97.
 87. Buchthal, F., and P. Rosenfalck. On the structure of motor units. In: New Developments in Electromyography and Clinical Neurophysiology, edited by J. E. Desmedt. Basel, Switzerland: Karger, 1973, vol. 1, p. 71–85.
 88. Buchthal, F., and H. Schmalbruch. Contraction times and fibre types in intact human muscle. Acta Physiol. Scand. 79: 435–452, 1970.
 89. Buller, A. J. The physiology of skeletal muscle. In: Neurophysiology, edited by C. Hunt. Baltimore, MD: Univ. Park, 1975, p. 279–302. (Int. Rev. Physiol. Ser., vol. 3.).
 90. Buller, A. J., J. C. Eccles, and R. M. Eccles. Interactions between motoneurones and muscles in respect of the characteristic speeds of their responses. J. Physiol. London 150: 417–439, 1960.
 91. Buller, A. J., and D. M. Lewis. The rate of tension development in isometric tetanic contractions of mammalian fast and slow skeletal muscle. J. Physiol. London 176: 337–354, 1965.
 92. Burke, R. E. Motor unit types of cat triceps surae muscle. J. Physiol. London 193: 141–160, 1967.
 93. Burke, R. E., D. N. Levine, P. Tsairis, and F. E. Zajac III. Physiological types and histochemical profiles in motor units of the cat gastrocnemius. J. Physiol. London 234: 723–748, 1973.
 94. Burke, R. E., D. N. Levine, F. E. Zajac III, P. Tsairis, and W. K. Engel. Mammalian motor units: physiological‐histochemical correlation in three types in cat gastrocnemius. Science 174: 709–712, 1971.
 95. Burke, R. E., P. Rudomin, and F. E. Zajac III. The effect of activation history on tension production by individual muscle units. Brain Res. 109: 515–529, 1976.
 96. Burke, R. E., W. Z. Rymer, and J. V. Walsh, Jr. Relative strength of synaptic input from short‐latency pathways to motor units of defined type in cat medial gastrocnemius. J. Neurophysiol. 39: 447–458, 1976.
 97. Burke, R. E., and P. Tsairis. Anatomy and innervation ratios in motor units of cat gastrocnemius. J. Physiol. London 234: 749–765, 1973.
 98. Calvert, T. W., and A. E. Chapman. The relationship between the surface emg and force transients in muscle: simulation and experimental studies. Proc. IEEE 65: 682–689, 1977.
 99. Carlsöö, S. Eine electromyographische Untersuchung der Muskelactivität im Musculus deltoideus. Acta Morphol. Neert. Scand. 2: 346–352, 1959.
 100. Cathcart, E. P., D. T. Richardson, and W. Campbell. Studies in muscle activity. II. The influence of speed on the mechanical efficiency. J. Physiol. London 58: 355–361, 1924.
 101. Cavagna, G. M. Storage and utilization of elastic energy in skeletal muscle. Exercise Sport Sci. Rev. 5: 89–129, 1977.
 102. Cavagna, G. A., H. Thys, and A. Zamboni. The source of external work in level walking and running. J. Physiol. London 262: 639–657, 1976.
 103. Chao, E. Y. Experimental methods for biomechanical measurements of joint kinematics. In: CRC Handbook of Engineering in Medicine and Biology, edited by B. N. Feinberg and D. G. Fleming. West Palm Beach, FL: CRC Press, 1978, vol. 1B, p. 385–411.
 104. Chaplain, R. A. On the contractile mechanism of insect fibrillar flight muscle. IV. A quantitative chemo‐mechanical model. Biol. Cybern. 18: 137–153, 1975.
 105. Chauveau, A. De l'enervation partielle des muscles. Arch. Physiol. Norm. Pathol. Ser. 5 1: 124–140, 1889.
 106. Clamann, H. P. Statistical analysis of motor unit firing patterns in a human skeletal muscle. Biophys. J. 9: 1233–1251, 1969.
 107. Clamann, H. P. Activity of single motor units during isometric tension. Neurology 20: 254–260, 1970.
 108. Clark, J. W. Jr., E. C. Greco, and T. L. Harman. Experience with a Fourier method for determining the extracellular potential fields of excitable cells with cylindrical geometry. CRC Crit. Rev. Bioeng. 3: 1–22, 1978.
 109. Clark, R. W., and E. S. Luschei. Short latency jaw movement produced by low intensity intracortical microstimulation of the precentral face area in monkeys. Brain Res. 70: 144–147, 1974.
 110. Clarke, R. S. J., R. F. Hellen, and A. R. Lind. The duration of sustained contractions of the human forearm at different muscle temperatures. J. Physiol. London 143: 454–473, 1952.
 111. Clemente, C. D. Anatomy: A Regional Atlas of the Human Body. Philadelphia, PA: Lea & Febiger, 1975.
 112. Close, J. R. Functional Anatomy of the Extremities: Some Electronic and Kinematic Methods of Study. Springfield, IL: Thomas, 1973.
 113. Close, J. R., E. D. Nickel, and F. N. Todd. Motor‐unit action‐potential counts (their significance in isometric and isotonic contractions). J. Bone Jt. Surg. 42A: 1207–1222, 1960.
 114. Close, R. The relation between intrinsic speed of shortening and duration of the active state of muscle. J. Physiol. London 180: 542–559, 1965.
 115. Close, R. I. Dynamic properties of mammalian skeletal muscles. Physiol. Rev. 52: 129–197, 1972.
 116. Cnockaert, J. C. Comparaison électromyographique du travail en allongement et en raccourcissement au cours de mouvements de va‐et‐vient. Electromyogr. Clin. Neurophysiol. 15: 477–489, 1975.
 117. Cnockaert, J. C., and F. Goubel. Rôle de l'énergie potentielle élastique dans le travail musculaire. Eur. J. Appl. Physiol. Occup. Physiol. 34: 131–140, 1975.
 118. Cnockaert, J. C., and E. Pertuzon. Sur Ia géometrie musculo‐squelettique du triceps brachii: application à Ia détermination dynamique de sa compliance. Eur. J. Appl. Physiol. Occup. Physiol. 32: 149–158, 1974.
 119. Coggshall, J. C., and G. A. Bekey. EMG‐force dynamics in human skeletal muscle. Med. Biol. Eng. 8: 265–270, 1970.
 120. Cohen, A. H., and C. Gans. Muscle activity in rat locomotion: movement analysis and electromyography of the flexors and extensors of the elbow. J. Morphol. 146: 177–196, 1975.
 121. Connolly, R., W. Gough, and S. Winegrad. Characteristics of the isometric twitch of skeletal muscle immediately after tetanus. J. Gen. Physiol. 57: 697–709, 1971.
 122. Cooper, S., and J. C. Eccles. The isometric responses of mammalian muscles. J. Physiol. London 69: 377–385, 1930.
 123. Costantin, L. L. Activation in striated muscle. In: Handbook of Physiology. The Nervous System, edited by J. M. Brookhart and V. B. Mountcastle. Bethesda, MD: Am. Physiol. Soc, 1977, sect. 1, vol. I, pt. 1, chapt. 7, p. 215–259.
 124. Coulter, N. A. Jr., and J. C. West. Nonlinear passive mechanical properties of skeletal muscle. Wright Air Dev. Div. Tech. Rep. 60–636: 1–7, 1960.
 125. Craig, E. S. Morphology of Human Skeletal Muscle Cells: Ultrastructural Observations Which Challenge the Concept of Stability in the Definitive State of These Cells (Ph.D. thesis). Memphis: Univ. of Tennessee, 1970. (Abstr. in Diss. Abstr. 32B: 673, 1971.).
 126. Creese, R., N. W. Scholes, and W. J. Whalen. Resting potentials of diaphragm muscle after prolonged anoxia. J. Physiol. London 140: 301–317, 1958.
 127. Crowe, A. A mechanical model of muscle and its application to the intrafusal fibers of the mammalian muscle spindle. J. Biomech. 3: 583–592, 1970.
 128. Crowninshield, R. D., and R. A. Brand. Kinematics and kinetics of gait. In: CRC Handbook of Engineering in Medicine and Biology, edited by B. N. Feinberg and D. G. Fleming. West Palm Beach, FL: CRC Press, 1978, p. 413–425.
 129. Cull‐Candy, S. G., H. Lundh, and S. Thesleff. Effects of botulinum toxin on neuromuscular transmission in the rat. J. Physiol. London 260: 177–203, 1976.
 130. Cullen, T. S., and M. Brödel. Lesions of the rectus abdominus muscle simulating an acute intra‐abdominal condition. I. Anatomy of the rectus muscle Bull. Johns Hopkins Hosp. 61: 295–316, 1937.
 131. Dahlbäck, O., D. Elmqvist, T. R. Johns, S. Radner, and S. Thesleff. An electrophysiologic study of the neuromuscular junction in myasthenia gravis. J. Physiol. London 156: 336–343, 1961.
 132. D'Arsonval, A. Sur un appareil destiné à measurer Ia conductibilité des tissus vivants pour le son. C. R. Soc. Biol. 38: 103–104, 1886.
 133. D'Arsonval, A. Production de chaleur dans les muscles. C. R. Soc. Biol. 38: 124–125, 1886.
 134. Davies, R. E. A molecular theory of muscle contraction: calcium‐dependent contractions with hydrogen bond formation plus ATP‐dependent extensions of part of the myosin‐actin cross‐bridges. Nature 199: 1068–1074, 1963.
 135. DeLuca, C. J. Physiology and mathematics of myoelectric signals. IEEE Trans. Biomed. Eng. 26: 313–325, 1979.
 136. DeDuca [sic], C. J., and W. J. Forrest. Force analysis of individual muscles acting simultaneously on the shoulder joint during isometric abduction. J. Biomech. 6: 385–393, 1973.
 137. Demiéville, H. N. Existence and Nature of Mechanical Interactions Between Motor Units: Experimental and Theoretical Evidence (Ph.D. thesis). Memphis: Univ. of Tennessee, 1973. (Abstr. in Diss. Abstr. 34B: 4001–4002, 1973.).
 138. Demiéville, H. N., and L. D. Partridge. Probability of peripheral interaction between motor units and implications for motor control. Am. J. Physiol.: 238: (Regulatory, Integrative Comp. Physiol. 7): R119–R137, 1980.
 139. Dempster, W. T., and J. C. Finerty. Relative activity of wrist moving muscles in static support of the wrist joint: an electromyographic study. Am. J. Physiol. 150: 596–606, 1947.
 140. Denny‐Brown, D. Interpretation of the electromyogram. Arch. Neurol. Psychiatry Chicago 61: 99–128, 1949.
 141. Denny‐Brown, D., and J. B. Pennybacker. Fibrillation and fasciculation in voluntary muscle. Brain 61: 311–334, 1938.
 142. Desmedt, J. E., and S. Borenstein. Collateral innervation of muscle fibres by motor axons of dystrophic motor units. Nature London 246: 500–501, 1973.
 143. Desmedt, J. E., B. Emeryk, H. Hainaut, H. Reinhold, and S. Borenstein. Muscular dystrophy and myasthenia gravis. In: New Developments in Electromyography and Clinical Neurophysiology, edited by J. E. Desmedt. Basel, Switzerland: Karger, 1973, vol. 1, p. 380–399.
 144. Desmedt, J. E., and E. Godaux. Critical evaluation of the size principle of human motoneurons recruitment in ballistic movements and in vibration‐induced inhibition or potentiation. Trans. Am. Neurol. Assoc. 102: 104–108, 1977.
 145. Desmedt, J. E., and E. Godaux. Fast motor units are not preferentially activated in rapid voluntary contractions in man. Nature London 267: 717–719, 1977.
 146. Desmedt, J. E., and K. Hainaut. Kinetics of myofilament activation in potentiated contraction: staircase phenomenon in human skeletal muscle Nature London 217: 529–532, 1968.
 147. Devanandan, M. S., R. M. Eccles, and R. A. Westerman. Single motor units of mammalian muscle. J. Physiol. London 178: 359–367, 1965.
 148. Dijkstra, S., and J. J. Denier van der Gon. An analog computer study of fast, isolated movements. Kybernetik 12: 102–110, 1973.
 149. Dijkstra, S., J. J. Denier van der Gon, T. Blangé, J. M. Karemaker, and A. E. J. L. Kromer. A simplified sliding‐filament muscle model for simulation purposes. Kybernetik 12: 94–101, 1973.
 150. Downey, J. A., and R. C. Darling. Physiological Basis of Rehabilitative Medicine. Philadelphia, PA: Saunders, 1971.
 151. Doyle, A. M., and R. F. Mayer. Studies of the motor unit in the cat. Bull. Univ. Md. Sch. Med. 54: 11–17, 1969.
 152. Draper, N. R., and H. Smith. Applied Regression Analysis. New York: Wiley, 1966.
 153. Drazil, J. V. Dictionary of Quantities and Units. Cleveland, OH: CRC Press, 1971.
 154. Dubuisson, M. Muscular Contraction. Springfield, IL: Thomas, 1954.
 155. Duchenne, G. B. Physiology of Motion, transl. by E. B. Kaplan. Philadelphia, PA: Saunders, 1959. (From Physiologie des Mouvements, Paris, 1867.).
 156. Eccles, J. C., and C. S. Sherrington. Numbers and contraction‐values of individual motor‐units examined in some muscles of the limb. Proc. R. Soc. London Ser. B 106: 326–357, 1930.
 157. Edman, K. A. P., L. A. Mulieri, and B. Scubon‐Mulieri. Non‐hyperbolic force‐velocity relationship in single muscle fibers. Acta Physiol. Scand. 98: 143–156, 1976.
 158. Edström, L., and E. Kugelberg. Histochemical composition, distribution of fibers and fatigueability of single motor units. Anterior tibial muscle of the rat. J. Neurol. Neurosurg. Psychiatry 31: 424–433, 1968.
 159. Elftman, H. The function of muscles in locomotion. Am. J. Physiol. 125: 357–366, 1939.
 160. Elftman, H. The transverse tarsal joint and its control. Clin. Orthop. 16: 41–46, 1960.
 161. Elftman, H. Biomechanics of muscle with particular application to studies of gait. J. Bone Jt. Surg. 48A: 363–377, 1966.
 162. Elftman, H. Dynamic structure of the human foot. Artificial Limbs 13: 49–58, 1969.
 163. Engberg, I., and A. Lundberg. An electromyographic analysis of muscular activity in the hindlimb of the cat during unrestrained locomotion. Acta Physiol. Scand. 75: 614–630, 1969.
 164. Ernst, E. Biophysics of the Striated Muscle. Budapest: Hungarian Acad. Sci., 1963.
 165. Ernst, E., K. Kovács, G. Metzger‐Török, and C. Trombitás. Longitudinal structure of the striated fibril. Acta Biochim. Biophys. Acad. Sci. Hung. 4: 177–186, 1969.
 166. Ernst, E., K. Kovács, G. Metzger‐Török, and C. Trombitás. Transverse structure of the striated fibril. Acta Biochim. Biophys. Acad. Sci. Hung. 4: 187–194, 1969.
 167. Evans, C. L., and A. V. Hill. The relation of length to tension development and heat production on contraction in muscle. J. Physiol. London 49: 10–16, 1914.
 168. Eykhoff, P. System Identification: Parameter and State Estimation. London: Wiley, 1974.
 169. Faaborg‐Andersen, K. Electromyographic investigation of intrinsic laryngeal muscles in humans. Acta Physiol. Scand. Suppl. 140: 1–149, 1957.
 170. Feinstein, B., B. Lindegård, E. Nyman, and G. Wohlfart. Morphologic studies of motor units in normal human muscles. Acta Anat. 23: 127–142, 1955.
 171. Feng, T. P., L‐Y. Lee, C‐W. Meng, and S‐C. Wang. Studies on the neuromuscular junction. IX. The after effects of tetanization on N‐M transmission in cat. Chinese J. Physiol. 13: 79–108, 1938.
 172. Fenn, W. O. Contractility. In: Physical Chemistry of Cells and Tissues, edited by R. Höber. Philadelphia, PA: Blackiston, 1945, sect. 7, p. 445–522.
 173. Fenn, W. O., and P. H. Garvey. The measurement of elasticity and viscosity of skeletal muscle in normal and pathological cases; a study of so‐called “muscle tonus”. J. Clin. Invest. 13: 383–397, 1934.
 174. Fenn, W. O., and B. S. Marsh. Muscular force at different speeds of shortening. J. Physiol. London 85: 277–297, 1935.
 175. Fick, A. Untersuchungen über Muskel‐Arbeit. Basel, Switzerland: H. Georg, 1867.
 176. Fick, A. Specielle Bewegunugslehre. In: Handbuch der Physiologic, edited by L. Hermann. Leipzig, Germany: 1879, vol. I, pt. 2, p. 239–346.
 177. Fields, R. W., and J. J. Faber. Biophysical analysis of the mechanical properties of the sarcolemma. Can. J. Physiol. Pharmacol. 48: 394–404, 1970.
 178. Finerty, J. C., and R. Gesell. The effect of cH on humoral stimulation of striated muscle and its application to the chemical control of breathing. Am. J. Physiol. 145: 1–15, 1945.
 179. Fixler, D. E., J. M. Atkins, J. H. Mitchell, and L. D. Horwitz. Blood flow to respiratory, cardiac, and limb muscles in dogs during graded exercise. Am. J. Physiol. 231: 1515–1519, 1976.
 180. Fletcher, W. M. The relation of oxygen to the survival metabolism of muscle. J. Physiol. London 28: 474–498, 1902.
 181. Folk, G. E. Textbook of Environmental Physiology (2nd ed.). Philadelphia, PA: Lea & Febiger, 1974.
 182. Forbes, A., L. R. Whitaker, and J. F. Fulton. The effect of reflex excitation and inhibition on the response of a muscle to stimulation through its motor nerve. Am. J. Physiol. 82: 693–716, 1927.
 183. Fowler, W. S., and A. Crowe. Effect of temperature on resistance to stretch of tortoise muscle. Am. J. Physiol. 231: 1349–1355, 1976.
 184. French, A. P. Newtonian Mechanics. New York: Norton, 1971.
 185. Fulton, J. F. Muscular Contraction and the Reflex Control of Movement. Baltimore, MD: Williams & Wilkins, 1926.
 186. Fulton, J. F., and L. G. Wilson. Selected Readings in the History of Physiology (2nd ed.). Springfield, IL: Thomas, 1966.
 187. Galvani, L. Commentary on the Effects of Electricity on Muscular Motion (transl. by M. G. Foley). Norwalk, CT: Burndy Library, 1954. (De Viribus Electricitatis in Motu Musculari Commentarius. Bologna: 1791.).
 188. Gambaryan, P. P. How Mammals Run: Anatomical Adaptations [transl. from Russian]. New York: Wiley, 1974. (Leningrad: Mlekopitaiuschikh‐Prisposobitel'nye, 1972.).
 189. Gasser, H. S., and A. V. Hill. The dynamics of muscular contraction. Proc. R. Soc. London. Ser. B 96: 398–437, 1924.
 190. Gesell, R. A neurophysiological interpretation of the respiratory act. Ergeb. Physiol. Biol. Chem. Exp. Pharmakol. 42: 477–639, 1940.
 191. Gesell, R., A. K. Atkinson, and R. C. Brown. The gradation of intensity of inspiratory contractions. Am. J. Physiol. 131: 659–673, 1941.
 192. Gesell, R., C. S. Magee, and J. W. Bricker. Activity patterns of the respiratory neurons and muscles. Am. J. Physiol. 128: 615–628, 1940.
 193. Gesink, J. W. Transfer Characteristics of Load Moving Skeletal Muscle (Ph.D. thesis). Ann Arbor: Univ. of Michigan, 1973. (Abstr. in Diss. Abstr. 34B: 3764–3765, 1973.).
 194. Gilson, A. S. Jr., S. M. Walker, and G. M. Schoepfle. The forms of the isometric twitch and isometric tetanus curves recorded from the frog's sartorius muscle. J. Cell. Comp. Physiol. 24: 185–199, 1944.
 195. Goldberg, A. L., J. D. Etlinger, D. F. Goldspink, and C. Jablecki. Mechanism of work‐induced hypertrophy of skeletal muscle. Med. Sci. Sports 7: 248–261, 1975.
 196. Goldberg, S. J., G. Lennerstrand, and C. D. Hull. Motor unit responses in the lateral rectus muscle of the cat: intracellular current injection of abducens nucleus neurons. Acta Physiol. Scand. 96: 58–63, 1976.
 197. Goldspink, G. Design of muscles in relation to locomotion. In: Mechanics and Energetics of Animal Locomotion, edited by R. McN. Alexander and G. Goldspink. London: Chapman & Hall, 1977. p. 1–22.
 198. Goldstein, H. Classical Mechanics. Reading, MA.: Addison‐Wesley, 1950.
 199. Goodall, M. C. Kinetics of muscular contraction. Yale J. Biol. Med. 30: 224–243, 1957.
 200. Goodgold, J. Anatomical Correlates of Clinical Electromyography. Baltimore, MD: Williams & Wilkins, 1974.
 201. Goodgold, J., and A. Eberstein. Electrodiagnosis of Neuromuscular Diseases (2nd ed.). Baltimore, MD: Williams & Wükins, 1978.
 202. Goodwin, G. M., and E. S. Luschei. Discharge of spindle afferents from jaw‐closing muscles during chewing in alert monkeys. J. Neurophysiol. 38: 560–571, 1975.
 203. Gordon, A. M., A. F. Huxley, and F. J. Julian. The length‐tension diagram of single vertebrate striated muscle fibers. J. Physiol. London 171: 28P–30P, 1964.
 204. Gordon, A. M., A. F. Huxley, and F. J. Julian. The variation of isometric tension with sarcomere length in vertebrate muscle fibres. J. Physiol. London 184: 170–192, 1966.
 205. Gordon, G., and A. H. S. Holbourn. A simultaneous study of the action potentials and movements of single motor units in the tonic stretch reflex. J. Physiol. London 107: 18P, 1948.
 206. Gordon, G., and A. H. S. Holbourn. The sounds from single motor units in a contracting muscle. J. Physiol. London 107: 456–464, 1948.
 207. Gordon, G., and A. H. S. Holbourn. The mechanical activity of single motor units in reflex contractions of skeletal muscle. J. Physiol. London 110: 26–35, 1949.
 208. Goslow, G. E., Jr, R. M. Reinking, and D. G. Stuart. Physiological extent, range and rate of muscle stretch for soleus, medial gastrocnemius and tibialis anterior. Pfluegers Arch. 341: 77–86, 1973.
 209. Goslow, G. E. Jr., E. K. Stauffer, W. C. Nemeth, and D. G. Stuart. Digit flexor muscles in the cat: their action and motor units. J. Morphol. 137: 335–352, 1972.
 210. Goss, R. J. The Physiology of Growth. New York: Academic, 1978.
 211. Gotch, F. The submaximal electrical response of nerve to a single stimulus. J. Physiol. London 28: 395–416, 1902.
 212. Goubel, F. Évaluation de l'énergie élastique emmagasinée par le muscle lors d'une contraction isométrique. J. Physiol. Paris 65: 238A, 1972.
 213. Goubel, F., F. Lestienne, and S. Bouisset. Détermination dynamique de Ia compliance musculaire in situ. J. Physiol. Paris 60: 255, 1968.
 214. Graham, D., and D. McRuer. Analysis of Nonlinear Control Systems. New York: Wiley, 1961.
 215. Granit, R. Neuromuscular interaction in postural tone of the cat's isometric soleus muscle. J. Physiol. London 143: 387–402, 1958.
 216. Grant, P. G. Biomechanical significance of the instantaneous center of rotation: the human temporomandibular joint. J. Biomech. 6: 109–113, 1973.
 217. Gray, D. E. (editor). American Institute of Physics Handbook (3rd ed.). New York: McGraw‐Hill, 1972.
 218. Gray, J. Studies in the mechanics of the tetrapod skeleton. J. Exp. Biol. 20: 88–116, 1944.
 219. Gray, J. How Animals Move. Cambridge, England: Cambridge Univ. Press, 1960.
 220. Greene, P. H. Problems of organization of motor systems. Quart. Rep., Inst. Computer Res., Univ. of Chicago, 1971, ser. II C., no. 29, p. 1–66.
 221. Grillner, S. The role of muscle stiffness in meeting the changing postural and locomotor requirements for force development by the ankle extensors. Acta Physiol. Scand. 86: 92–108, 1972.
 222. Grillner, S. On the generation of locomotion in the spinal dogfish. Exp. Brain Res. 20: 459–470, 1974.
 223. Grillner, S., and S. Kashin. On the generation and performance of swimming in fish. In: Neural Control of Locomotion, edited by R. M. Herman, S. Grillner, P. S. G. Stein, and D. G. Stuart. New York: Plenum, 1976, p. 181–201.
 224. Gurfinkel', V. S., and Yu, S. Levik. Dependence of contraction of the muscle on the sequence of stimulating pulses. Biophysics USSR 18: 121–127, 1973.
 225. Guttman, S. A., R. G. Horton, and D. T. Wilber. Enhancement of muscle contraction after tetanus. Am. J. Physiol. 119: 463–473, 1937.
 226. Gydikov, A. Microstructure of the Voluntary Movements in Man [Engl. summary and legends]. Sofia: Bulgarian Acad. Sci., 1970.
 227. Hall, V. M. D. The role of force or power in Liebig's physiological chemistry. Med. Hist. 24: 20–59, 1980.
 228. Halliburton, W. D. Handbook of Physiology (10th ed.). Philadelphia, PA: Blakistons, 1911.
 229. Halpern, W., and N. R. Alpert. A stochastic signal method for measuring dynamic mechanical properties of muscle. J. Appl. Physiol. 31: 913–925, 1971.
 230. Hamm, T. M. Identification of the Mechanical Properties of Skeletal Muscle (Ph.D. thesis). Memphis: Univ. of Tennessee, 1979. (Abstr. in Diss. Abstr. 40: 5556B, 1980.).
 231. Hammond, P. H., P. A. Merton, and G. G. Sutton. Nervous gradation of muscular contraction. Br. Med. Bull. 12: 214–218, 1956.
 232. Hanson, J., and J. Lowy. Structure and function of the contractile apparatus in the muscles of invertebrate animals. In: Structure and Function of Muscle, edited by G. H. Bourne. New York: Academic, 1960, vol. I, p. 265–335.
 233. Harris, A. J. Inductive functions of the nervous system. Annu. Rev. Physiol. 36: 251–305, 1974.
 234. Hartree, W., and A. V. Hill. The nature of the isometric twitch. J. Physiol. London 55: 389–411, 1921.
 235. Hatze, H. A myocybernetic control model of skeletal muscle. Biol. Cybern. 25: 103–119, 1977.
 236. Hatze, H. A complete set of control equations for the human musculo‐skeletal system. J. Biomech. 10: 799–805, 1977.
 237. Hatze, H. A general myocybernetic control model of skeletal muscle. Biol. Cybern. 28: 143–157, 1978.
 238. Haycraft, J. B. Upon the production of rapid voluntary movements. J. Physiol. London 23: 1–9, 1898.
 239. Heitler, W. J., and M. Burrows. The locust jump. I. The motor programme. J. Exp. Biol. 66: 203–219, 1977.
 240. Helmholtz, H. von. Ueber die Wärmeentwickelung bei der Muskelaction. Arch. Anat. Physiol. 144–164, 1848.
 241. Helmholtz, H. von. Messungen über den zeitlichen Verlauf der Zuckung animalischer Muskeln und die Fortpflanzungsgeschwindigkeit der Reizung in den Nerven. Arch. Anat. Physiol. 276–364, 1850.
 242. Helmholtz, H. von. Verlaütige, Bericht über die Fortpflanzungsgeschwindigkeit der Reizung. Arch. Anat. Physiol. 71–73, 1850.
 243. Helmholtz, H. von. Messungen über Fortpflanzungsgeschwindigkeit der Reizung in den Nerven. Arch. Anat. Physiol. 199–216, 1852.
 244. Helmholtz, H. von. Monatsber. d Berliner Acad. P 328, 1854. Cited in Handbuch der Physiologie, by L. Hermann. Leipzig, Germany: 1879, vol. 1.
 245. Henneman, E. Peripheral mechanisms involved in the control of muscle. In: Medical Physiology (13th ed.), edited by V. B. Mountcastle. St. Louis, MO: Mosby, 1974, p. 617–635.
 246. Henneman, E., and C. B. Olson. Relations between structure and function in the design of skeletal muscles. J. Neurophysiol. 28: 581–598, 1965.
 247. Henneman, E., G. Somjen, and D. O. Carpenter. Functional significance of cell size in spinal motoneurons. J. Neurophysiol. 28: 560–580, 1965.
 248. Herman, R. The myotatic reflex: clinico‐physiological aspects of spasticity and contracture. Brain 93: 273–312, 1970.
 249. Herman, R., H. Schaumberg, and S. Reiner. A rotational joint apparatus: a device for study of tension‐length relations of human muscle. Med. Res. Eng. 6: 18–20, 1967.
 250. Hermann, L. Allgemeine Muskelphysik. In: Herman's Handbuch der Physiologie, by L. Hermann. Leipzig, Germany: 1879, vol. I, pt. 1, p. 3–260.
 251. Hertel, H. Structure, Form and Movement (transl. by M. S. Katz). New York: Reinhold, 1966.
 252. Hess, A. Vertebrate slow muscle fibers. Physiol. Rev. 50: 40–62, 1970.
 253. Hill, A. V. The maximum work and mechanical efficiency of human muscles, and their most economical speed. J. Physiol. London 56: 19–41, 1922.
 254. Hill, A. V. The heat of shortening and the dynamic constants of muscle. Proc. R. Soc. London Ser. B 126: 136–195, 1938.
 255. Hill, A. V. The pressure developed in muscle during contraction. J. Physiol. London 107: 518–526, 1948.
 256. Hill, A. V. The abrupt transition from rest to activity in muscle. Proc. R. Soc. London Ser. B 136: 399–420, 1949.
 257. Hill, A. V. The development of the active state of muscle during the latent period. Proc. R. Soc. London Ser. B 137: 320–329, 1950.
 258. Hill, A. V. The effect of series compliance on the tension developed in a muscle twitch. Proc. R. Soc. London Ser. B 138: 325–329, 1951.
 259. Hill, A. V. The mechanics of active muscle. Proc. R. Soc. London Ser. B 141: 104–117, 1953.
 260. Hill, A. V. Production and absorption of work by muscle. Science 131: 897–903, 1960.
 261. Hill, A. V. Trails and Trials in Physiology. Baltimore, MD: Williams & Wilkins, 1965.
 262. Hill, A. V. First and Last Experiments in Muscle Mechanics. London: Cambridge Univ. Press, 1970.
 263. Houk, J., and E. Henneman. Feedback control of skeletal muscles. Brain Res. 5: 433–451, 1967.
 264. Houk, J. C., Jr. A study of human postural control. NEREM Rec. 138–139, 1963.
 265. Houtz, S. J., and F. J. Fischer. An analysis of muscle action and joint excursion during exercise on a stationary bicycle. J. Bone Jt. Surg. 41A: 123–131, 1959.
 266. Howell, W. H. A Text‐Book of Physiology (5th ed.). Philadelphia, PA: Saunders, 1913.
 267. Hoyle, G. Comparative Physiology of the Nervous Control of Muscular Contraction. London: Cambridge Univ. Press, 1957.
 268. Hoyle, G. Diversity of striated muscle. Am. Zool. 7: 435–449, 1967.
 269. Hoyle, G., and J. Lowy. The paradox of Mytilus muscle. A new interpretation. J. Exp. Biol. 33: 295–310, 1956.
 270. Huber, E. Über das Muskelgebiet des N. facialis bei Katze und Hund nebst allgemeinen Bemerkungen über die Fascialismuskulatur der Säuger. Anat. Anz. 51: 1–17, 1918.
 271. Huber, G. C. On the form and arrangement in fasciculi of striated voluntary muscle fibers. Anat. Rec. 11: 149–168, 1917.
 272. Hudlická, O. Muscle Blood Flow: Its Relation to Muscle Metabolism and Function. Amsterdam: Swets & Zeitlinger, 1973.
 273. Huxley, A. F. Interpretation of muscle striation: evidence from visible light microscopy. Br. Med. Bull. 12: 167–170, 1956.
 274. Huxley, A. F. Muscle structure and theories of contraction. Prog. Biophys. Biophys. Chem. 7: 255–318, 1957.
 275. Huxley, A. F., and L. D. Peachey. The maximum length for contraction in vertebrate striated muscle. J. Physiol. London 156: 150–165, 1961.
 276. Huxley, A. F., and R. M. Simmons. Proposed mechanism of force generation in striated muscle. Nature London 233: 533–538, 1971.
 277. Huxley, A. F., and R. M. Simmons. Mechanical transients and the origin of muscular force. Cold Spring Harbor Symp. Quant. Biol. 37: 669–680, 1972.
 278. Huxley, H. E. The ultra‐structure of striated muscle. Br. Med. Bull. 12: 171–173, 1956.
 279. Huxley, H. E. Muscle cells. In: The Cell, edited by J. Brachet and A. E. Mirsky. New York: Academic, 1960, vol. 4, 365–481.
 280. Huxley, H. E. Structural organization and the contraction mechanism in striated muscle. In: Biology and the Physical Sciences, edited by S. Devons. New York: Columbia Univ. Press, 1969, p. 114–138.
 281. Hyndman, R. W. Jr., and R. K. Beach. The transient response of the human operator. IRE Trans. Med. Electron. 12: 67–71, 1958.
 282. Ikai, M., K. Yabe, and K. Ischii. Muskelkraft und muskuläre Ermüdung bei willkürlicher Anspannung und elektrischer Reizung des Muskels. Sportarzt Sportmed. 5: 197–204, 1967.
 283. Inbar, G. F., and D. Adam. Estimation of muscle active state. Biol. Cybern. 23: 61–72, 1976.
 284. Inman, V. T., H. J. Ralston, J. B. de C. M. Saunders, B. Feinstein, and E. W. Wright, Jr. Relation of human electromyogram to muscular tension. Electroencephalogr. Clin. Neurophysiol. 4: 187–194, 1952.
 285. Jansen, J. K. S., and P. M. H. Rack. The reflex response to sinusoidal stretching of soleus in the decerebrate cat. J. Physiol. London 183: 15–36, 1966.
 286. Jarcho, L. W., C. Eyzaguirre, B. Berman, and J. L. Lilien‐Thal, Jr. Spread of excitation in skeletal muscle: some factors contributing to the form of the electromyogram. Am. J. Physiol. 168: 446–457, 1952.
 287. Jarcho, L. W., C. L. Vera, C. G. McCarthy, and P. B. Williams. The form of motor unit and fibrillation potentials. Electroencephalogr. Clin. Neurophysiol. 10: 527–540, 1958.
 288. Jedwab, J. D. Telemetry of Force, Length and EMG From Individual Muscles of the Triceps Surae During Unrestrained Locomotion (Thesis). Clayton, Victoria, Australia: Monash Univ., 1977.
 289. Jedwab, J. D., R. A. Westerman, and S. P. Ziccone. Telemetry of force, length and EMG in the soleus muscle of the freely moving cat. Aust. Physiol. Pharmacol. Soc. Proc. 8: 181P, 1977.
 290. Jewell, B. R. The nature of the phasic and the tonic responses of the anterior byssal retractor muscle of Mytilus. J. Physiol. London 149: 154–177, 1959.
 291. Jewell, B. R., and D. R. Wilkie. An analysis of the mechanical components in frog's striated muscle. J. Physiol. London 143: 515–540, 1958.
 292. Jewell, B. R., and D. R. Wilkie. The mechanical properties of relaxing muscle. J. Physiol. London 152: 30–47, 1960.
 293. Ji, S. A general theory of ATP synthesis and utilization. Ann. NY Acad. Sci. 227: 211–226, 1974.
 294. Jímenez‐Pabón, E., and R. A. Nelson. Quantitative measurements of muscle tone in cats. Neurology 15: 1120–1126, 1965.
 295. Joseph, J. Electromyographic studies on muscle tone and the erect posture in man. Br. J. Surg. 51: 616–622, 1964.
 296. Journal of Physiology. Suggestions to authors. J. Physiol. London 182: 1–33, 1966.
 297. Joyce, G. C., and P. M. H. Rack. Isotonic lengthening and shortening movements of cat soleus muscle. J. Physiol. London 204: 475–491, 1969.
 298. Joyce, G. C., P. M. H. Rack, and H. F. Ross. The forces generated at the human elbow joint in response to imposed sinusoidal movements of the forearm. J. Physiol. London 240: 351–374, 1974.
 299. Joyce, G. C., P. M. H. Rack, and D. R. Westbury. The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements. J. Physiol. London 204: 461–474, 1969.
 300. Julian F. J. Activation in a skeletal muscle contraction model with a modification for insect fibrillar muscle. Biophys. J. 9: 547–570, 1969.
 301. Julian, F. J., K. R. Sollins, and M. R. Sollins. A model for the transient and steady‐state mechanical behavior of contracting muscle. Biophys. J. 14: 546–562, 1974.
 302. Julian, F. J., and M. R. Sollins. Variation of muscle stiffness with force at increasing speeds of shortening. J. Gen. Physiol. 66: 287–302, 1975.
 303. Kadefors, R., I. Petersén, and H. Broman. Spectral analysis of events in the electromyogram. In: New Developments in Electromyography and Clinical Neurophysiology, edited by J. E. Desmedt. Basel, Switzerland: Karger, 1973, vol. 1, p. 628–637.
 304. Kaiser, K. Ueber die Elasticität des thätigen muskels. Z. Biol. Munich 38: 1–15, 1899.
 305. Kamada, T. The supernormal phase in muscular contraction. J. Physiol. London 76: 187–192, 1932.
 306. Karpovich, P. V., P. H. Cohan, and M. Ikai. Study of endurance of various muscle groups. Res. Q. Am. Assoc. Health Phys. Educ. Recreat. 35: 393–397, 1964.
 307. Katz, B. The relation between force and speed in muscular contraction. J. Physiol. London 96: 45–64, 1939.
 308. Katz, B. The role of the cell membrane in muscular activity. Br. Med. Bull. 12: 210–213, 1956.
 309. Katz, B., and S. W. Kuffler. Multiple motor innervation of the frog's sartorius muscle. J. Neurophysiol. 4: 209–223, 1941.
 310. Keatinge, W. R. Survival in Cold Water. Oxford, England: Blackwell, 1969.
 311. Kennedy, D., and W. J. Davis. Organization of invertebrate motor systems. In: Handbook of Physiology. The Nervous System, edited by J. M. Brookhart and V. B. Mountcastle. Bethesda, MD: Am. Physiol. Soc, 1977, sect. 1, vol. I, pt. 2, chapt. 27, p. 1023–1087.
 312. Kent, B. E. Functional anatomy of the shoulder complex: a review. Phys. Ther. 51: 867–887, 1971.
 313. Kernell, D., A. Ducati, and H. Sjöholm. Properties of motor units in the first deep lumbrical muscle of the cat's foot. Brain Res. 98: 37–55, 1975.
 314. Kernell, D., and H. Sjöholm. Recruitment and firing rate modulation of motor unit tension in a small muscle of the cat's foot. Brain Res. 98: 57–72, 1975.
 315. Kleeman, F. J., L. D. Partridge, and G. H. Glaser. Resting potential and distribution of muscle fibers in living mammalian muscle. Am. J. Phys. Med. 40: 183–191, 1961.
 316. Knotts, D., M. Lewis, and J. C. Luck. Motor unit areas in a cat limb muscle. Exp. Neurol. 30: 475–483, 1971.
 317. Knowlton, F. P., and C. J. Campbell. Observations on peripheral inhibition in arthropods. Am. J. Physiol. 91: 19–26, 1929.
 318. Kolman, B. Elementary Linear Algebra. Toronto, Canada: Macmillan, 1970.
 319. Komi, P. V., and J. H. T. Viitasalo. Signal characteristics of EMG at different levels of muscle tension. Acta Physiol. Scand. 96: 267–276, 1976.
 320. Krogh, A. The supply of oxygen to the tissues and the regulation of the capillary circulation. J. Physiol. London 52: 457–474, 1919.
 321. Kronecker, H., and W. Stirling. Die Genesis des Tetanus. Arch. Anat. Physiol. Physiol. Abt. Leipzig 2: 1–40, 1878.
 322. Kugelberg, E. Properties of the rat hind‐limb motor units. In: New Developments in Electromyography and Clinical Neurophysiology, edited by J. E. Desmedt. Basel, Switzerland: Karger, 1973.
 323. Kugelberg, E., and C. R. Skoglund. Natural and artificial activation of motor units—a comparison. J. Neurophysiol. 9: 399–412, 1946.
 324. Lamarre, Y., and J. P. Lund. Load compensation in human masseter muscles. J. Physiol. London 253: 21–35, 1975.
 325. Lamb, D. R. Androgens and exercise. Med. Sci. Sports 7: 1–5, 1975.
 326. Landowne, M., and R. W. Stacy. Glossary of terms. In: Tissue Elasticity, edited by J. W. Remington. Washington, D.C.: Am. Physiol. Soc., 1957, p. 191–203.
 327. Lapedes, D. N. (editor). Dictionary of Scientific and Technical Terms. New York: McGraw‐Hill, 1974.
 328. Lee, Y. W., and M. Schetzen. Measurement of the Wiener kernels of a non‐linear system by cross‐correlation. Int. J. Control 2: 237–254, 1965.
 329. Leeuwenhoek, A. van. Letter to Royal Society of London, 7 September, 1688. In: Collected Letters, edited by J. J. Swart. Amsterdam: Swets & Zeitlinger, 1967, vol. 8, no. 110, p. 2–57.
 330. Leonardo da Vinci. On the Human Body (facsimile and transl. by C. D. O'Malley and J. B. de C. M. Saunders). New York: Schuman, 1952.
 331. Lestienne, F. Rôle des forces d'origine visco‐élastique dans l'activité freinatrice exercée par le groupe musculaire antagoniste. J. Physiol. Paris 65: 263A, 1972.
 332. Lestienne, F., and S. Bouisset. Pattern temporel de Ia mise en jeu d'un agoniste et d'un antagoniste en fonction de Ia tension de l'agoniste. Rev. Neurol. 118: 550–554, 1968.
 333. Lestienne, F., and S. Bouisset. Quantification of the bicepstriceps synergy in simple voluntary movements. In: Visual Information Processing and Control of Motor Activity Symposium. Sofia: Bulgarian Acad. Sci., 1969, p. 445–449.
 334. Levin, A., and J. Wyman. The viscous elastic properties of muscle. Proc. R. Soc. London Ser. B. 101: 218–243, 1927.
 335. Lewis, D. M., J. C. Luck, and S. Knott. A comparison of isometric contractions of the whole muscle with those of motor units in a fast‐twitch muscle of the cat. Exp. Neurol. 37: 68–85, 1972.
 336. Licht, S. Electrodiagnosis and Electromyography (3rd ed.). New Haven, CT: Licht, 1971.
 337. Liddell, E. G. T., and C. S. Sherrington. A comparison between certain features of the spinal flexor reflex and of the decerebrate extensor reflex respectively. Proc. R. Soc. London Ser. B. 95: 299–339, 1924.
 338. Liddell, E. G. T., and C. S. Sherrington. Recruitment and some other features of reflex inhibition. Proc. R. Soc. London Ser. B 97: 488–518, 1925.
 339. Liley, A. W., and K. A. K. North. An electrical investigation of effects of repetitive stimulation on mammalian neuromuscular junction. J. Neurophysiol. 16: 509–527, 1953.
 340. Lindström, L. H., and R. I. Magnusson. Interpretation of myoelectric power spectra: a model and its applications. Proc. IEEE 65: 653–662, 1977.
 341. Lippold, O. C. J. The relative between integrated action potentials in a human muscle and its isometric tension. J. Physiol. London 117: 492–499, 1952.
 342. Lippold, O. C. J., J. W. T. Redfearn, and J. Vučo. The rhythmical activity of groups of motor units in the voluntary contraction of muscle. J. Physiol. London 137: 473–487, 1957.
 343. Little, R. C., and W. B. Wead. Diastolic viscoelastic properties of active and quiescent cardiac muscle. Am. J. Physiol. 221: 1120–1125, 1971.
 344. Lombard, W. P., and F. M. Abbott. The mechanical effects produced by the contraction of individual muscles of the thigh of the frog. Am. J. Physiol. 20: 1–60, 1907.
 345. Loofbourrow, G. N. Electrographic evaluation of mechanical response in mammalian skeletal muscle in different conditions. J. Neurophysiol. 11: 153–167, 1948.
 346. Lorente de Nó, R. A study of neurophysiology, Part II. Studies from the Rockefeller Institute. 132: 1946.
 347. Lowe, D. A. A Guide to International Recommendations on Names and Symbols for Quantities and on Units of Measurement. Geneva, Switzerland: World Health Organiz., 1975.
 348. Lowy, J. Contraction and relaxation in the adductor muscles of pecten maximus. J. Physiol. London 124: 100–105, 1954.
 349. Lowy J., and B. M. Millman. Contraction and relaxation in smooth muscle of lamellibranch molluscs. Nature London 183: 1730–1731, 1959.
 350. Lucas, K. On the gradation of activity in a skeletal muscle fibre. J. Physiol. London 33: 125–137, 1905.
 351. Lucas, K. The “all or none” contraction of the amphibian skeletal muscle fibre. J. Physiol. London 38: 113–133, 1909.
 352. Luff, A. R., and U. Proske. Properties of motor units of the frog iliofibularis muscle. Am. J. Physiol. 236 (Cell Physiol. 5): C35–C40, 1979.
 353. Lupton, H. An analysis of the effects of speed on the mechanical efficiency of human muscular movement, (with Appendix by A. V. Hill). J. Physiol. London 57: 336–353, 1923.
 354. Luschei, E. S., and G. M. Goodwin. Patterns of mandibular movement and jaw muscle activity during mastication in the monkey. J. Neurophysiol. 37: 954–966, 1974.
 355. Machin, K. E., and J. W. S. Pringle. The physiology of insect fibrillar muscle. II. Mechanical properties of a beetle flight muscle. Proc. R. Soc. London Ser. B 151: 204–225, 1959.
 356. Macklen, P. T. Respiratory mechanics. Annu. Rev. Physiol. 40: 157–184, 1978.
 357. Mannard, A., and R. B. Stein. Determination of the frequency response of isometric soleus muscle in the cat using random nerve stimulation. J. Physiol. London 229: 275–296, 1973.
 358. Manter, J. T. The dynamics of quadrupedal walking. J. Exp. Biol. 15: 522–540, 1938.
 359. Marey, E. J. De Ia locomotion terrestre chez les bipèdes et les quadrupèdes. J. Anat. Physiol. 9: 42–80, 1873.
 360. Marey, E. J. La Machine Animate. Paris: F. Alcan, 1886.
 361. Marinacci, A. A. Applied Electromyography. Philadelphia, PA: Lea & Febiger, 1968.
 362. Markee, J. E., W. W. Thompson, and S. O. Thorne, Jr. The relation of separately innervated areas of muscle to amount of shortening and strength of contraction. Anat. Rec. 97: 355, 1947.
 363. Marmarelis, P. Z., and V. Z. Marmarelis. Analysis of Physiological Systems: The White‐Noise Approach. New York: Plenum, 1978.
 364. Marshall, J., and E. G. Walsh. Physiological tremor. J. Neurol. Neurosurg. Psychiatry 19: 260–267, 1956.
 365. Mashima, H., K. Akazawa, H. Kushima, and K. Fujii. The force‐load‐velocity relation and the viscous‐like force in the frog skeletal muscle. Jpn. J. Physiol. 22: 103–120, 1972.
 366. Matthews, P. B. C. The dependence of tension upon extension in the stretch reflex of the soleus muscle of the decerebrate cat. J. Physiol. London 147: 521–546, 1959.
 367. Matyushkin, D. P. Motor innervation of tonic muscle fibers of the oculomotor system. Federation Proc. 22: T728–T731, 1963. (Transl. from Fiziol. Zh. SSSR Im. I. M. Sechenova 48: 534, 1962.).
 368. McArdle, B. Familial periodic paralysis. Br. Med. Bull. 12: 226–229, 1956.
 369. McPhedran, A. M., R. B. Wuerker, and E. Henneman. Properties of motor units in a homogeneous red muscle (soleus) of the cat. J. Neurophysiol. 28: 71–84, 1965.
 370. Merton, P. A. Speculations on the servo‐control of movement. In: The Spinal Cord, edited by J. L. Malcolm, J. A. B. Gray, and G. E. W. Wolstenholme. Boston, MA: Little, Brown, 1953, p. 247–260. (Ciba Found. Symp.).
 371. Merton, P. A. Interaction between muscle fibers in a twitch. J. Physiol. London 124: 311–324, 1954.
 372. Meyer‐Lohmann, J., W. Riebold, and D. Robrecht. Mechanical influence of the extrafusal muscle on the static behaviour of deefferented primary muscle spindle endings in cat. Pfluegers Arch. 352: 267–278, 1974.
 373. Miledi, R. The acetylcholine sensitivity of frog muscle fibres after complete or partial denervation. J. Physiol. London 151: 1–23, 1960.
 374. Millman, B. M. Mechanism of contraction in molluscan muscle. Am. Zool. 7: 583–591, 1967.
 375. Milner‐Brown, H. S., R. B. Stein, and R. Yemm. Changes in firing rate of human motor units during linearly changing voluntary contractions. J. Physiol. London 230: 371–390, 1973.
 376. Milner‐Brown, H. S., R. B. Stein, and R. Yemm. The contractile properties of human motor units during voluntary isometric contractions. J. Physiol. London 228: 285–306, 1973.
 377. Mommaerts, W. F. H. M. The molecular transformation of actin. Parts I, II, III. J. Biol. Chem. 198: 445–457, 459–467, 469–475, 1952.
 378. Monod, H. How muscles are used in the body. In: The Structure and Function of Muscle (2nd ed.), edited by G. H. Bourne. New York: Academic, 1972, vol. 1, p. 23–74.
 379. Moore, A. D. Synthesized EMG waves and their implications. Am. J. Phys. Med. 46: 1302–1316, 1967.
 380. Morrison, J. B. The mechanics of muscle function in locomotion. J. Biomech. 3: 431–451, 1970.
 381. Morrison, J. B. The mechanics of the knee joint in relation to normal walking. J. Biomech. 3: 51–61, 1970.
 382. Mosher, C. G., R. L. Gerlach, and D. G. Stuart. Soleus and anterior tibial motor units of the cat. Brain Res. 44: 1–11, 1972.
 383. Moss, R. L., and W. Halpern. Elastic and viscous properties of resting frog skeletal muscle. Biophys. J. 17: 213–228, 1977.
 384. Muybridge, E. The Human Figure in Motion [1887]. (Reprint.) New York: Dover, 1955.
 385. Muybridge, E. Animals in Motion [1887]. (Reprint, edited by L. S. Brown.) New York: Dover, 1957.
 386. Myrhage, R. Microvascular supply of skeletal muscle fibers. Acta Orthop. Scand. Suppl. 168: 1–46, 1977.
 387. Needham, D. M. Machina Carnis: The Biochemistry of Muscular Contraction in Its Historical Development. Cambridge, England: Cambridge Univ. Press, 1971.
 388. Newman, H. F., J. D. Northrup, and J. Devlin. Mechanism of human penile erection. Invest. Urol. 1: 350–353, 1964.
 389. Newson‐Davis, J., A. J. Pinching, A. Vincent, and S. G. Wilson. Function of circulating antibody to acetylcholine receptor in myasthenia gravis: investigation by plasma exchange. Neurology 28: 266–272, 1978.
 390. Nichols, T. R. Reflex and non‐reflex stiffness of soleus muscle in the cat. In: Control of Posture and Locomotion, edited by R. B. Stein, K. G. Pearson, R. S. Smith, and J. B. Redford. New York: Plenum, 1974, p. 407–410. (Adv. in Behav. Biol., vol. 7.).
 391. Nichols, T. R., and J. C. Houk. Reflex compensation for variation in the mechanical properties of a muscle. Science 181: 182–184, 1973.
 392. Noble, M. I. M., and G. H. Pollack. Molecular mechanisms of contraction. Circ. Res. 40: 333–342, 1977.
 393. Norris, F. H. Jr., and E. L. Gasteiger. Action potentials of single motor units in normal muscle. Electroencephalogr. Clin. Neurophysiol. 7: 115–126, 1955.
 394. Pantin, C. F. A. Comparative physiology of muscle. Br. Med. Bull. 12: 199–202, 1956.
 395. Partridge, L. D. Motor control and the myotatic reflex. Am. J. Phys. Med. 40: 96–103, 1961.
 396. Partridge, L. D. A simulation approach to analysis of muscle control reflexes. Digest Int. Cong. on Med. Electron., New York, 1961, p. 105.
 397. Partridge, L. D. Modifications of neural output signals by muscles: a frequency response study. J. Appl. Physiol. 20: 150–156, 1965.
 398. Partridge, L. D. Signal‐handling characteristics of load‐moving skeletal muscle. Am. J. Physiol. 210: 1178–1191, 1966.
 399. Partridge, L. D. Intrinsic feedback factors producing inertial compensation in muscle. Biophys. J. 7: 853–863, 1967.
 400. Partridge, L. D. Interrelationships studied in a semibiological “reflex.” Am. J. Physiol. 223: 144–158, 1972.
 401. Partridge, L. D. Some consequences of nonlinear properties of a motor system. Wiss. Z. Karl Marx Univ. Leipzig 21: 156–165, 1972.; also
 402. Biocybernetics 4: 156–165, 1972.
 403. Partridge, L. D. Integration in the central nervous system. In: Engineering Principles in Physiology, edited by J. H. U. Brown and D. S. Gann. New York: Academic, 1973, vol. 1 p. 47–98.
 404. Partridge, L. D. Muscle properties: a problem for the motor controller physiologist. In: Posture and Movement: Perspective for Integrating Sensory and Motor Research on the Mammalian Nervous System, edited by R. E. Talbott and D. R. Humphrey. New York: Raven, 1979, p. 189–229.
 405. Partridge, L. D., and G. H. Glaser. Adaptation in regulation of movement and posture. A study of stretch responses in spastic animals. J. Neurophysiol. 23: 257–268, 1960.
 406. Peachey, L. D., and R. H. Adrian. Electrical properties of the transverse tubular system. In: The Structure and Function of Muscle (2nd ed.), edited by G. H. Bourne. New York: Academic, 1973, vol. 3, p. 1–30.
 407. Pedotti, A. A study of motor coordination and neuromuscular activities in human locomotion. Biol. Cybern. 26: 53–62, 1977.
 408. Pertuzon, E., and G. Comyn. Étude, sur un modèle muscle‐movement, de Ia forme du signal de commande du muscle. J. Physiol. Paris 65: Suppl.: 284A, Oct. 1972.
 409. Pierce, D. S., and I. H. Wagman. A method of recording from single muscle fibers or motor units in human skeletal muscle. J. Appl. Physiol. 19: 366–368, 1964.
 410. Piper, H. Über die Rhythmik der Innervationsimpulse bei willkürlichen Muskelcontraktionen und über verschiedene Arten der künstlichen Tetanisierung menschlicher Muskeln. Z. Biol. Munich 53: 140–156, 1910.
 411. Podolsky, R. J., A. C. Nolan, and S. A. Zaveler. Cross‐bridge properties derived from muscle isotonic velocity transients. Proc. Natl. Acad. Sci. USA 64: 504–511, 1969.
 412. Polissar, M. J. Physical chemistry of contractile process in muscle, I‐IV. Am. J. Physiol. 168: 766–781, 782–792, 793–804, 805–811, 1952.
 413. Pollard, J. H. A Handbook of Numerical and Statistical Techniques. London: Cambridge Univ. Press, 1977.
 414. Portzehl von Hildegard, H. Der Arbeitszyklus geordneter Aktomosinsysteme (Muskel und Muskelmodelle). Z. Naturforsch. Teil B 7: 1–10, 1952.
 415. Pratt, F. H. The all‐or‐none principle in graded response of skeletal muscle. Am. J. Physiol. 44: 517–542, 1917.
 416. Pringle, J. W. S. The mechanism of the myogenic rhythm of certain insect striated muscles. J. Physiol. London 124: 269–291, 1954.
 417. Pringle, J. W. S. Models of muscle. Symp. Soc. Exp. Biol. 14: 41–68, 1960.
 418. Pringle, J. W. S. The contractile mechanism of insect fibrillar muscle. Prog. Biophys. Mol. Biol. 17: 1–60, 1967.
 419. Pringle, J. W. S. Evidence from insect fibrillar muscle about the elementary contractile process. J. Gen. Physiol. Suppl. 50: 139–156, 1967.
 420. Prochazka, V. J., K. Tate, R. A. Westerman, and S. P. Ziccone. Remote monitoring of muscle length and EMG in unrestrained cats. Electroencephalogr. Clin. Neurophysiol. 37: 649–653, 1974.
 421. Prochazka, A., R. A. Westerman, and S. P. Ziccone. Discharges of single hindlimb afferents in the freely moving cat. J. Neurophysiol. 39: 1090–1104, 1976.
 422. Proske, U., and P. M. H. Rack. Short‐range stiffness of slow fibers and twitch fibers in reptilian muscle. Am. J. Physiol. 231: 449–453, 1976.
 423. Pryor, M. G. M. Mechanical properties of fibres and muscles. In: Progress in Biophysics and Biophysical Chemistry, edited by J. A. V. Butler and J. T. Randall. New York, Academic, 1950, vol. 1, p. 216–268. Also in: Deformation and Flow in Biological Systems, edited by A. Frey‐Wyssling. Amsterdam: North‐Holland, 1950, p. 157–193.
 424. Purohit, R. C., and S. D. Beckett. Penile pressure and muscle activity associated with erection and ejaculation in the dog. Am. J. Physiol. 231: 1343–1348, 1976.
 425. Rack, P. M. H. The behavior of a mammalian muscle during sinusoidal stretching. J. Physiol. London 183: 1–14, 1966.
 426. Rack, P. M. H., and D. R. Westbury. The effects of length and stimulus rate on tension in the isometric cat soleus muscle. J. Physiol. London 204: 443–460, 1969.
 427. Rack, P. M. H., and D. R. Westbury. The short range stiffness of active mammalian muscle and its effect on mechanical properties. J. Physiol. London 240: 331–350, 1974.
 428. Ralston, H. J., V. T. Inman, L. A. Strait, and M. D. Shaffrath. Mechanics of human isolated voluntary muscle. Am. J. Physiol. 151: 612–620, 1947.
 429. Ramsey, R. W., and S. F. Street. The isometric length‐tension diagram of isolated skeletal muscle fibers of the frog. J. Cell Comp. Physiol. 15: 11–34, 1940.
 430. Ranatunga, K. W. Influence of temperature on the characteristics of summation of isometric mechanical responses of mammalian skeletal muscle. Exp. Neurol. 54: 513–532, 1977.
 431. Ranvier, L. De quelques faits relatifs à l'histologie et à Ia physiologie des muscles striés. Arch. Physiol. Norm. Pathol. Ser. 2, 1: 1–15, 1874.
 432. Rasch, P. J., and R. K. Burke. Kinesiology and Applied Anatomy (3rd ed.). Philadelphia, PA: Lea & Febiger, 1967.
 433. Reighard, J., and H. S. Jennings. Anatomy of the Cat (2nd ed.). New York: Holt, 1901.
 434. Reinking, R. M., J. A. Stephens, and D. G. Stuart. The motor units of cat medial gastrocnemius: problem of their categorisation on the basis of mechanical properties. Exp. Brain Res. 23: 301–313, 1975.
 435. Reinking, R. M., J. A. Stephens, and D. G. Stuart. The tendon organs of cat medial gastrocnemius: significance of motor unit type and size for the activation of Ib afferents. J. Physiol. London 250: 491–512, 1975.
 436. Remington, J. W. Introduction to muscle mechanics, with a glossary of terms. Federation Proc. 21: 954–963, 1962.
 437. Richmond, F. J. R., and V. C. Abrahams. Morphology and enzyme histochemistry of dorsal muscles of the cat neck. J. Neurophysiol. 38: 1312–1321, 1975.
 438. Ritchie, J. M. The duration of the plateau of full activity in frog muscle. J. Physiol. London 124: 605–612, 1954.
 439. Roberts, T. D. M. Rhythmic excitation of a stretch reflex, revealing (a) hysteresis, (b) difference between the responses to pulling and to stretching. Q. J. Exp. Physiol. Cogn. Med. Sci. 48: 328–345, 1963.
 440. Robinson, D. A. The mechanics of human saccadic eye movement. J. Physiol. London 174: 245–264, 1964.
 441. Robinson, D. A. A quantitative analysis of extraocular muscle cooperation and squint. Invest. Ophthalmol. 14: 801–825, 1975.
 442. Robinson, M. The temporomandibular joint: theory of reflex controlled nonlever action of the mandible. J. Am. Dent. Assoc. 33: 1260–1271, 1946.
 443. Rohmert, W. Ermittlung von Erholungspausen fur statische Arbeit des Menschen. Int. Z. Angew. Physiol. Einschl. Arbeitsphysiol. 18: 123–164, 1960.
 444. Roos, J. The latent period of skeletal muscle. J. Physiol. London 74: 17–33, 1932.
 445. Rosemann, R. Landois' Lehrbuch der Physiologie des Menschen (17th ed.). Berlin: Urban‐Schwarzenberg, 1921.
 446. Rosenblueth, A. The Transmission of Nerve Impulses at Neuroeffector Junctions and Peripheral Synapses. New York: Wiley, 1950.
 447. Rosenthal, J. Trophic interactions of neurons. In: Handbook of Physiology. The Nervous System, edited by J. M. Brookhart and V. B. Mountcastle. Bethesda, MD: Am. Physiol. Soc., 1977, sect. 1, vol. 1, pt. 2, chapt. 21, p. 775–801.
 448. Rosenthal, N. P., T. A. McKean, W. J. Roberts, and C. A. Terzuolo. Frequency analysis of stretch reflex and its main subsystems in triceps surae muscles of the cat. J. Neurophysiol. 33: 713–749, 1970.
 449. Rothschuh, K. E. History of Physiology (transl. and edited by G. B. Risse). Huntington, NY: Krieger, 1973.
 450. Ruch, T. C., and H. D. Patton. Physiology and Biophysics. Philadelphia, PA: Saunders, 1965.
 451. Rushton, W. A. H. Nerve supply to Lucas's alpha substance. J. Physiol. London 74: 231–261, 1932.
 452. Salmons, S. Letter to Editor. Med. Biol. Eng. 13: 608–609, 1975.
 453. Samojloff, A., and W. Wassiljewa. Zur Frage der plurisegmentellen Innervation. IV. Mitteilung. Pfluegers Arch. Gesamte Physiol. Menschen Tiere 213: 723–734, 1926.
 454. Sandow, A. Studies on the latent period of muscular contraction. Method. General properties of latency relaxation. J. Cell. Comp. Physiol. 24: 221–256, 1944.
 455. Sandow, A. Skeletal muscle. Annu. Rev. Physiol. 32: 87–138, 1970.
 456. Santesson, C. G. Studien über die allgemeine Mechanik des Muskels. Scand. Arch. Physiol. 3: 381–436, 1892;
 457. Scand. Arch. Physiol. 4: 46–97, 98–134, 135–193, 1893.
 458. Sassa, K., and C. S. Sherrington. On the myogram of the flexor‐reflex evoked by a single break‐shock. Proc. R. Soc. London Ser. B 92: 108–117, 1921.
 459. Savant, C. J. Basic Feedback Control System Design. New York: McGraw‐Hill, 1958.
 460. Schäfer, E. A., H. E. L. Canney, and J. O. Tunstall. On the rhythm of muscular response to volitional impulses in man. J. Physiol. London 7: 111–117, 1886.
 461. Schäfer, S. S., and S. Kijewski. The dependence of the acceleration response of primary muscle spindle endings on the mechanical properties of the muscle. Pfluegers Arch. 350: 101–122, 1974.
 462. Schäffer, B., and J. Örkényi. Time‐relations of initial volume decrease and contraction in frog muscles. Acta Biochim. Biophys. Acad. Sci. Hung. 7: 255–261, 1972.
 463. Schiller, H. H., and E. Strålberg. Human botulism studied with single‐fiber electromography. Arch. Neurol. Chicago 35: 346–349, 1978.
 464. Schoenberg, M., J. B. Wells, and R. J. Podolsky. Muscle compliance and the longitudinal transmission of mechanical impulses. J. Gen. Physiol. 64: 623–642, 1974.
 465. Schultz, D. G., and J. L. Melsa. State Function and Linear Control Systems. New York: McGraw‐Hill, 1967.
 466. Schwarzacher, H. G. Über die Länge und Anordnung der Muskelfasern in menschlichen Skeletmuskeln. Acta Anat. 37: 217–231, 1959.
 467. Seireg, A. Leonardo da Vinci—The bio‐mechanician. In: Biomechanics, edited by D. Bootzin and H. C. Muffley. New York: Plenum, 1969, p. 65–74.
 468. Seireg, A., and R. J. Arvikar. A mathematical model for evaluation of forces in lower extremities of the musculo‐skeletal system. J. Biomech. 6: 313–326, 1973.
 469. Severin, F. V., M. L. Shik, and G. N. Orlovskii. Work of the muscles and single motor neurons during controlled locomotion. Biophysics USSR 12: 762–772, 1967. (Transl. from Biofizika 12: 660–668, 1967.).
 470. Seyffarth, H. The behavior of motor units in healthy and paretic muscles in man. Parts I and II. Acta Psychiat. Neurol. 16: 79–109, 261–278, 1941.
 471. Sherrington, C. S. Experiments in examination of the peripheral distribution of the fibres of the posterior roots of some spinal nerves. Part II. Phil. Trans. R. Soc. London Ser. B 190: 45–186, 1898.
 472. Sherrington, C. S. Some functional problems attached to convergence. Proc. R. Soc. London Ser. B 105: 332–362, 1930.
 473. Shiavi, R. G. Control of and Interaction Between Motor Units in a Human Skeletal Muscle During Isometric Contractions (Ph.D. thesis). Philadelphia, PA: Drexel Univ., 1972. (Abstr. in Diss. Abstr. 33: 72–24–563, 1972.).
 474. Shiavi, R., and M. Negin. Multivariate analysis of simultaneously active motor units in human skeletal muscle. Biol. Cybern. 20: 9–16, 1975.
 475. Sichel, F. J. M. The elasticity of isolated resting skeletal muscle fibres. J. Cell. Comp. Physiol. 5: 21–42, 1934.
 476. Sichel, F. J. M. The relative elasticity of the sarcolemma and of the entire skeletal muscle fiber. Am. J. Physiol. 133: P446–P447, 1941.
 477. Simpson, J. A. Terminology of electromyography. Electroencephalogr. Clin. Neurophysiol. 26: 224–226, 1969.
 478. Sinclair, D. C. Muscles and fasciae. In: Cunningham's Textbook of Anatomy (11th ed.), edited by G. J. Romanes. London: Oxford Univ. Press, 1972, p. 259–397.
 479. Smidt, G. L. Biomechanical analysis of knee flexion and extension. J. Biomech. 6: 79–92, 1973.
 480. Smith, D. O. Central nervous control of excitatory and inhibitory neurons of opener muscle of the crayfish claw. J. Neurophysiol. 37: 108–118, 1974.
 481. Smith, J. L., B. Betts, V. R. Edgerton, and R. F. Zernicke. Rapid ankle extension during paw shakes: selective recruitment of fast ankle extensors. J. Neurophysiol. 43: 612–620, 1980.
 482. Soechting, J. F., and W. J. Roberts. Transfer characteristics between EMG activity and muscle tension under isometric conditions in man. J. Physiol. Paris 70: 779–793, 1975.
 483. Sotavalta, O. Some studies on the flying tones of insects and the determination of the frequency of the wing strokes. Ann. Entomol. Fenn. Suom. Hyonteistiet Aikak. 7: 32–52, 1941. (Abstr. in Biol. Abstr. 21: 23712, 1947.).
 484. Sréter, F. A., A. R. Luff, and J. Gergely. Effect of cross‐reinnervation on physiological parameters and on properties of myosin and sarcoplasmic reticulum of fast and slow muscles of the rabbit. J. Gen. Physiol. 66: 811–821, 1975.
 485. Stark, L. Neurological Control Systems: Studies in Bioengineering. New York: Plenum, 1968.
 486. Stein, R. B. Peripheral control of movement. Physiol. Rev. 54: 215–243, 1974.
 487. Stein, R. B., A. S. French, A. Mannard, and R. Yemm. New methods for analysing motor function in man and animals. Brain Res. 40: 187–192, 1972.
 488. Steinberger, W. W., and E. M. Smith. Maintenance of denervated rabbit muscle with direct electrostimulation. Arch. Phys. Med. Rehabil. 49: 573–577, 1968.
 489. Stephens, J. A., R. M. Reinking, and D. G. Stuart. The motor unit of cat medial gastrocnemius: electrical and mechanical properties as a function of muscle length. J. Morphol. 146: 495–512, 1975.
 490. Stephens, J. A., and D. G. Stuart. The motor units of cat medial gastrocnemius: speed‐size relations and their significance for the recruitment order of motor neurons. Brain Res. 91: 177–195, 1975.
 491. Stephens, J. A., and D. G. Stuart. The motor units of cat medial gastrocnemius: twitch potentiation and twitch‐tetanus ratio. Pfluegers Arch. 356: 359–372, 1975.
 492. Stetson, R. H., and H. D. Bouman. The coordination of simple skilled movements. Arch. Neerl. Physiol. 20: 177–254, 1935.
 493. Suzuki, J‐I., and B. Cohen. Integration of semicircular canal activity. J. Neurophysiol. 29: 981–995, 1966.
 494. Takeuchi, A., and N. Takeuchi. A study of the inhibitory action of γ‐amino‐butyric acid on neuromuscular transmission in the crayfish. J. Physiol. London 183: 418–432, 1966.
 495. Tardieu, C., P. Colbeau‐Justin, M. D. Bret, A. Lespargot, E. Huet de Ia Tour, and G. Tardieu. An apparatus and a method for measuring the relationship of triceps surae torques to tibiotarsal angles in man. Eur. J. Appl. Physiol. Occup. Physiol. 35: 11–20, 1976.
 496. Tashiro, N. Mechanical properties of the longitudinal and circular muscle in the earthworm. J. Exp. Biol. 54: 101–110, 1971.
 497. Tashiro, N., and T. Yamamoto. The phasic and tonic contraction in the longitudinal muscle of the earthworm. J. Exp. Biol. 55: 111–122, 1971.
 498. Taylor, A. Grouping of action potentials in voluntary muscle. J. Physiol. London 157: 55P–56P, 1961.
 499. Taylor, A. The significance of grouping of motor unit activity. J. Physiol. London 162: 259–269, 1962.
 500. Taylor, C. P. S. Isometric muscle contraction and the active state: an analog computer study. Biophys. J. 9: 759–780, 1969.
 501. Teig, E. Force and contraction velocity of the middle ear muscles in the cat and the rabbit. Acta Physiol. Scand. 84: 1–10, 1972.
 502. Teig, E. Tension and contraction time of motor units of the middle ear muscles in the cat. Acta Physiol. Scand. 84: 11–21, 1972.
 503. Terzuolo, C. A., J. F. Soechting, and R. Palminteri. Studies on the control of some simple motor tasks. III. Comparison of the EMG pattern during ballistically initiated movements in man and squirrel monkev. Brain Res. 62: 242–246, 1973.
 504. Terzuolo, C. A., J. F. Soechting, and P. Viviani. Studies on the control of some simple motor tasks. I. Relations between parameters of movement and EMG activities. Brain Res. 58: 212–216, 1973.
 505. Thesleff, S. Supersensitivity of skeletal muscle produced by botulinum toxin. J. Physiol. London 151: 598–607, 1960.
 506. Thomas, J. G. The torque angle transfer function of the human eye. Kybernetik 3: 254–263, 1967.
 507. Thomson, J. D. Dimensional and dynamic features of mammalian gastrocnemius muscle. Am. J. Physiol. 200: 951–954, 1961.
 508. Thorson, J., and D. C. S. White. Distributed representations for actin‐myosin interaction in the oscillatory contraction of muscle. Biophys. J. 9: 360–390, 1969.
 509. Thorson, J., and D. C. S. White. Dynamic force measurement at the microgram level, with application to myofibrils of striated muscle. IEEE Trans. Bio.‐Med. Eng. 22: 293–299, 1975.
 510. Timoshenko, S. P., and J. M. Gere. Mechanics of Materials. New York: Van Nostrand, 1972.
 511. Tokumasu, K., K. Goto, and B. Cohen. Eye movements produced by the superior oblique muscle. Arch. Ophthalmol. 73: 851–862, 1965.
 512. Townsend, M. A. A relationship between muscle performance when producing and absorbing work. J. Biomech. 6: 261–265, 1973.
 513. Townsend, M. A., S. P. Lainhart, R. Shiavi, and J. Caylor. Variability and biomechanics of synergy patterns of some lower‐limb muscles during ascending and descending stairs and level walking. Med. Biol. Eng. 16: 681–688, 1978.
 514. Trnkoczy, A. Static hysteresis loop of electrically stimulated muscles. Med. Biol. Eng. 12: 182–187, 1974.
 515. Trueman, E. R., and A. Packard. Motor performances of some cephalopods. J. Exp. Biol. 49: 495–507, 1968.
 516. Truong, X. T. Extensional wave‐propagation characteristics in striated muscle. J. Acoust. Soc. Am. 51: 1352–1356, 1972.
 517. Truong, X. T. Visco‐elastic propagation of longitudinal waves in skeletal muscle. J. Biomech. 5: 1–10, 1972.
 518. Truong, X. T. Viscoelastic wave propagation and rheologic properties of skeletal muscle. Am. J. Physiol. 226: 256–264, 1974.
 519. Twarog, B. M. The regulation of catch in molluscan muscle. J. Gen. Physiol. 50: (Suppl.) 157–168, 1967.
 520. Tyrer, N. M. Innervation of the abdominal intersegmental muscles in the grass‐hopper. I, II. J. Exp. Biol. 55: 305–314, 315–324, 1971.
 521. Usherwood, P. N. R. Insect neuromuscular mechanisms. Am. Zool. 7: 553–582, 1967.
 522. Usherwood, P. N. R., and H. Grundfest. Peripheral inhibition in skeletal muscle of insects. J. Neurophysiol. 28: 497–518, 1965.
 523. Van Harreveld, A. On the force and size of motor units in the rabbit's sartorius muscle. Am. J. Physiol. 151: 96–106, 1947.
 524. Viviani, P., J. F. Soechting, and C. A. Terzuolo. Influence of mechanical properties on the relation between EMG activity and torque. J. Physiol. Paris 72: 45–58, 1976.
 525. Volkmann, A. W. Versuche und Betrachtungen über Muskel‐contractilität. Arch. Anat. Physiol. Wiss. Med. 215–288, 1858.
 526. Von Buddenbrock, W. Vergleichende Physiologie. Physiologic der Erfolgsorgane. Basel, Switzerland: Birkhäuser, 1961, vol. V.
 527. Wacholder, K. Willkürliche Haltung und Bewegung inbesondere im Lichte elektrophysiologisher Untersuchungen. Ergeb. Physiol. Biol. Chem. Exp. Pharmakol. 26: 568–775, 1928.
 528. Wagner, R. Muskeltonus und Aktionsstrom im Umklammerungsreflex. Z. Biol. Munich 82: 21–26, 1924.
 529. Wagner, R. Arbeitsdiagramme bei der Willkürbewegung I and II. Z. Biol. Munich 86: 367–396, 397–426, 1927.
 530. Wainwright, S. A., F. Vosburgh, and J. H. Hebrank. Shark skin: function in locomotion. Science 202: 747–749, 1978.
 531. Walker, L. B., Jr. Multiple motor innervation of individual muscle fibers in the m. tibialis anterior of the dog. Anat. Rec. 139: 1–11, 1961.
 532. Walker, S. M. Failure of potentiation in successive, post‐tetanic, and summated twitches in cooled skeletal muscle of the rat. Am. J. Physiol. 166: 480–484, 1951.
 533. Walker, S. M. Tension and extensibility changes in muscle suddenly stretched during tetanus. Am. J. Physiol. 172: 37–41, 1953.
 534. Walker, S. M. The relation of stretch and of temperature to contraction of skeletal muscle. Parts I, II. Am. J. Phys. Med. 39: 191–215, 234–258, 1960.
 535. Walker, S. M. Lengthening contraction and interpretations of active state tension in the isometric twitch response of skeletal muscle. Am. J. Phys. Med. 55: 192–204, 1976.
 536. Walker, S. M., and A. G. Thomas. Changes in twitch tension induced by quick stretch and stress relaxation. Am. J. Physiol. 198: 523–527, 1960.
 537. Walmsley, B., J. A. Hodgson, and R. E. Burke. The forces produced by hind limb muscles in freely moving cats. Soc. Neurosci. Abstr. 3: 280, 1978.
 538. Walmsley, B., J. A. Hodgson, and R. E. Burke. Forces produced by medial gastrocnemius and soleus muscles during locomotion in freely moving cats. J. Neurophysiol. 41: 1203–1216, 1978.
 539. Walton, J. N. (editor). Disorders of Voluntary Muscle (3rd ed.). London: Churchill Livingstone, 1974.
 540. Ward, M. Mountain Medicine, a Clinical Study of Cold and High Altitude. New York: Van Nostrand, 1975.
 541. Watkins, B. O. Introduction to Control Systems. New York: MacMillan, 1969.
 542. Weast, R. C. (editor). Handbook of Chemistry and Physics (56th ed.). Cleveland, OH: CRC Press, 1975.
 543. Weber, E. Muskelbewegung. In: Handwörterbuch der Physiologie, edited by R. Wagner. Brunswick, Germany: Bieweg, 1846, vol. III, pt. 2, p. 1–122.
 544. Weber, E. H. Ueber Eduard Weber's Entdeckungen in der Lehre von der Muskelcontraction. Arch. Anat. Physiol. Wiss. Med. 483–527, 1846.
 545. Webster, D. D. A method of measuring the dynamic characteristics of muscle rigidity, strength, and tremor in the upper extremity. IRE Trans. Med. Electron. 6: 159–164, 1959.
 546. Weijs, W. A., and R. Damtuma. Electromyography and mechanics of mastication in the albino rat. J. Morphol. 146: 1–34, 1975.
 547. Wells, J. B. Comparison of mechanical properties between slow and fast mammalian muscles. J. Physiol. London 178: 252–269, 1965.
 548. Wells, J. B. Relationship between elastic and contractile components in mammalian skeletal muscle. Nature 214: 198–199, 1967.
 549. Weltman, G., H. Groth, and J. Lyman. A myoelectric system for training functional dissociation of muscles. Arch. Phys. Med. Rehabil. 43: 534–537, 1962.
 550. White, D. C. S., and J. Thorson. The kinetics of muscle contraction. Prog. Biophys. Mol. Biol. 27: 173–255, 1973.
 551. White, H. J., and S. Tauber. Systems Analysis. Philadelphia, PA: Saunders, 1969.
 552. Wiener, N. Response of nonlinear device to noise. MIT Radiation Lab Report 5: 16S , 1942.
 553. Wiener, N. Cybernetics: Control and Communications in the Animal and the Machine. New York: Wiley, 1948.
 554. Wilder, B. J., T. C. Kenaston, P. A. Mabe Jr., T. L. Dulin, J. A. Gergan, F. R. Hook Jr., M. Williams, and J. E. Markee. Observations on fatigue patterns of anterior tibial muscles. Am. J. Phys. Med. 32: 331–337, 1953.
 555. Wilkie, D. R. The relation between force and velocity in human muscle. J. Physiol. London 110: 249–280, 1950.
 556. Wilkie, D. R. The mechanical properties of muscle. Br. Med. Bull. 12: 177–182, 1956.
 557. Wilkie, D. R. The contractile system. In: Starling's Principles of Human Physiology (14th ed.), edited by H. Davson and M. G. Eggleton. London: Lea & Febiger, 1968, p. 812–831.
 558. Wills, J. H. Speed of responses of various muscles of cats. Am. J. Physiol. 136: 623–628, 1942.
 559. Wilson, D. M. Bifunctional muscles in the thorax of grasshoppers. J. Exp. Biol. 39: 669–677, 1962.
 560. Wilson, D. M., and J. L. Larimer. The catch property of ordinary muscle. Proc. Natl. Acad. Sci. USA 61: 909–916, 1968.
 561. Wilson, D. M., D. O. Smith, and P. Dempster. Length and tension hysteresis during sinusoidal and step function stimulation of arthropod muscle. Am. J. Physiol. 218: 916–922, 1970.
 562. Winter, D. A., R. K. Greenlaw, and D. A. Hobson. Television‐computer analyses of kinematics of human gait. Comput. Biomed. Res. 5: 498–504, 1972.
 563. Wisnes, A., and A. Kirkeø. Regional distribution of blood flow in calf muscles of rat during passive stretch and sustained contraction. Acta Physiol. Scand. 96: 256–266, 1976.
 564. Woledge, R. C. The energetics of tortoise muscle. J. Physiol. London 197: 685–707, 1968.
 565. Wood, G. A., and L. S. Jennings. On the use of the spline function for data smoothing. J. Biomech. 12: 477–479, 1979.
 566. Woodbury, J. W., A. M. Gordon, and J. T. Conrad. Muscle. In: Physiology and Biophysics (19th ed.), edited by T. C. Ruch and H. D. Patton. Philadelphia, PA: Saunders, 1965, p. 113–152.
 567. Wuerker, R. B., A. M. McPhedran, and E. Henneman. Properties of motor units in a heterogeneous pale muscle (m. gastrocnemius) of the cat. J. Neurophysiol. 28: 85–99, 1965.
 568. Wyman, J., Jr. Studies on the relation of work and heat in tortoise muscle. J. Physiol. London 61: 337–352, 1926.
 569. Yager, J. G. The Electromyogram as a Predictor of Muscle Mechanical Response in Locomotion (Ph.D. thesis). Memphis, Univ. of Tennessee, 1972. (Abstr. in Diss Abstr. 33B: 3909–3910, 1972.).
 570. Yamamoto, T. Electrical and mechanical properties of the red and white muscles in the silver carp. J. Exp. Biol. 57: 551–567, 1972.
 571. Young, M. Molecular basis of muscle contraction. Annu. Rev. Biochem. 38: 913–950, 1969.
 572. Zachar, J. Electrogenesis and Contractility in Skeletal Muscle Cells. Bratislava, Czechoslovakia: Slovak Acad. Sci., 1971.
 573. Zachar, J., and D. Zacharová. The length‐tension diagram of single muscle fibres of the crayfish. Experientia 22: 451–452, 1966.
 574. Zahalak, G. I., J. Duffy, P. A. Stewart, H. M. Litchman, R. H. Hawley, and P. R. Paslay. Force‐velocity‐EMG data for the skeletal muscle of athletes. National Science Found. Report GK 40010X‐32, 1973.
 575. Zierler, K. L. Some aspects of the biophysics of muscle. In: The Structure and Function of Muscle (2nd ed.), edited by G. H. Bourne. New York: Academic, 1973, vol. 3, p. 117–183.
 576. Zierler, K. I. Mechanism of muscle contraction and its energetics. In: Medical Physiology (13th ed.), edited by V. B. Mountcastle. St. Louis, MO: Mosby, 1974, vol. 1, p. 77–120.
 577. Zimkin, N. V., and T. G. Pakhomova. The interrelationship between hardness, viscosity, strength, and bioelectric activity in human muscles [in Russian]. Fiziol. Zh. SSSR im. I. M. Sechenova 58: 1099–1108, 1972. (Abstr. in Biol. Abstr. 56: 50058, 1973.).
 578. Zimkin, N. V., V. G. Panov, and V. T. Raikov. Fast and slow fibers in human muscles. Neurosci. Behav. Physiol. 6: 1–8, 1973. (Transl. from Fiziol. Zh. SSSR im. I. M. Sechenova 57: 1259–1266, 1971.).
 579. Zuber, B. L. Eye movement dynamics in the cat: the final motor pathway. Exp. Neurol. 20: 255–260, 1968.
 580. Zuniga, E. N., and D. G. Simons. Nonlinear relationship between averaged electromyogram potential and muscle tension in normal subjects. Arch. Phys. Med. Rehabil. 50: 613–620, 1969.

Contact Editor

Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite

L. D. Partridge, L. A. Benton. Muscle, the Motor. Compr Physiol 2011, Supplement 2: Handbook of Physiology, The Nervous System, Motor Control: 43-106. First published in print 1981. doi: 10.1002/cphy.cp010203