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Postural Orientation and Equilibrium

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Abstract

The sections in this article are:

1 Neural Control of Postural Orientation and Equilibrium
1.1 Behavioral Goals
1.2 Biomechanical Principles
1.3 Postural Strategies
2 Postural Orientation
2.1 Stiffness and Tonic Muscle Activation
2.2 Controlling Postural Orientation
2.3 Internal Representation of Postural Orientation
3 Coordination of Postural Equilibrium
3.1 Triggered Reactions to External Disturbances
3.2 Anticipatory Postural Adjustments for Voluntary Movement
3.3 Modeling of Postural Coordination
4 Sensory Control of Postural Orientation and Equilibrium
4.1 Sensory Integration
4.2 Somatosensory System
4.3 Vestibular System
4.4 Visual System
5 Central Neural Control of Posture
5.1 Spinal Cord and Brainstem
5.2 Basal Ganglia
5.3 Cerebellum
5.4 Cerebral Cortex
6 Concluding Remarks
Figure 1. Figure 1.

Equilibrium in different postural orientations. The center of mass (CoM) of the body (indicated by the X) may lie inside (A and C) or outside (B) the body limits. The net ground reaction force takes origin within the base of support and must pass through the CoM if the body is in equilibrium.

Figure 2. Figure 2.

Stick figures illustrating postural orientation in the cat. Changes in stance distance (A) or surface tilt (B) in the cat are accommodated primarily through pivoting of the limbs at the most proximal and distal joints. The trunk remains parallel to the support surface and intralimb geometry is relatively conserved.

A adapted from Fung and Macpherson , B adapted from Lacquaniti et al.
Figure 3. Figure 3.

Stabilization of the head in space during dynamic motor activities, A, Example of subject locomoting while maintaining the Frankfort plane at a constant angle with respect to vertical. Berthoz and Pozzo created these drawings by tracing and superimposing time‐lapse photos from Muybridge . B, Tracing from a cine film of a Macaca mulatta juvenile in the wild, leaping between branches.

Drawing courtesy of Dr. D. C. Dunbar
Figure 4. Figure 4.

Control system model for spatial orientation. See text for details.

Adapted from Merfeld et al.
Figure 5. Figure 5.

Ankle‐to‐hip strategy continuum for dynamic equilibrium in standing humans. The plot of ankle angle vs. hip angle shows examples of trajectories in sagittal plane angle–angle space for the ankle, the hip, and mixed ankle and hip strategies. The initial, erect position is at the origin of the graph. The perturbation, a backward translation, induces forward sway primarily about the ankle joints to the dot on the upward‐going ankle axis. The ankle strategy brings the subject back to the erect equilibrium position, whereas the hip strategy brings the subject to a nonerect equilibrium position. Mixed strategies can follow a trajectory anywhere within the shaded region. Insets show the patterns of EMG activity in the neck, trunk and leg for the various strategies. TRAP, trapezius; SCM, sternocleidomastoid; PARA, paraspinals; ABD, abdominals; HAM, hamstrings; QUAD, quadriceps; GAS, gastrocnemius; TIB, tibialis anterior.

Adapted from Horak and Nashner
Figure 6. Figure 6.

Characteristics of postural responses of the cat to linear translation in many directions in the horizontal plane. A, Active horizontal plane force vectors exerted by each limb in response to 16 directions of translation. Arrows illustrate the mean vector directions for each cluster. LE, RF, left and right forelimb; LH, RH, left and right hindlimb. Inset shows coordinate system in degrees. B, Probability of response (% of responsive trials) for two selected muscles, lateral gastrocnemius (LG) and soleus (Sol) as a function of the direction of translation. Note the monotonic nature of the response profiles, and the fact that even for those directions that a muscle is activated, it may never be recruited for 100% of the trials (e.g., Sol). C, EMG tuning curves: activity evoked in several muscles during translation plotted in polar coordinates of direction of translation (degrees) vs. amplitude of normalized evoked activity. D, Peak change in torque during response to translation. The circles represent 0.4 Nm. F, flexor; E, extensor; GLUT, gluteus medius; ILIO, iliopsoas; Med BF, medial bicepts femoris, SART, anterior sartorius; STEN, semitendinosus; VMED, vastus medialis; SOL, soleus; TIBA, tibialis anterior.

Figure 7. Figure 7.

Ankle muscle activation in the two lower limbs in response to backward translation of the support surface under two instructions, “remain standing” (thin line, hatched fill) and “step as soon as the perturbation is detected” (thick line, no fill). The response to translation alone consists of bilateral activation of soleus and gastrocnemius. When the instruction is to step, the postural response is significantly attenuated, particularly in the stance limb.

Adapted from Burleigh, Horak, and Malouin
Figure 8. Figure 8.

A, The anticipatory postural response in the lower limb muscles, gastrocnemius (Gastroc) and hamstrings (Ham), precedes the onset of activity in biceps brachii (Biceps), the agonist for the arm‐pull task. B, When the subject is firmly supported at the shoulder, the same arm‐pull task illustrated in A does not elicit an anticipatory postural response in the leg muscles (compare gastrocnemius in A and B. C, In rocking forward on the toes, the first change in EMG is an inhibition of tonic activity in soleus (arrow), which allows the CoM to move forward toward the toes. This is followed by the focal response of activation of tibialis (Tib), soleus (Sol), and quadriceps (Quad).

A and B adapted from Cordo and Nashner , C adapted from Nardone and Schieppati
Figure 9. Figure 9.

Optimal control model of human postural coordination predicts the ankle and hip strategy for control of sagittal sway. A, The filled region represents the feasible acceleration set for ankle, knee, and hip movement in the sagittal plane, given the constraint of keeping the feet on the ground in stance. The slice through the region represents the plane of hip and ankle accelerations when the knee is constrained to remaining straight. There is only a narrow region in which the ankle strategy (horizontal extent of shaded area) may be used without the toes or heels lifting off. Note that the ankle acceleration scale has been expanded compared to the hip. B, Block diagram of the optimal control model (see text for details). C, Ankle/hip angle trajectories predicted by the model for two different cost functions.

Adapted from Kuo
Figure 10. Figure 10.

Mean sway over 20 s of stance under six different sensory conditions in normal subjects (open bars) and in patients with vestibular loss (top), and a sensory organization deficit (bottom). Anterior/posterior peak‐to‐peak sway at the hips is normalized for each subject's height such that 100% represents a “fall.” Sensory conditions include blindfolding (conditions 2 and 5), sway‐referencing the visual surround (conditions 3 and 6), and sway‐referencing the support surface (conditions 4, 5 and 6).

Adapted from Black et al.
Figure 11. Figure 11.

Context‐dependency of changes in postural orientation induced by vibration. A and B, Subjects are instructed to hold the forearm horizontal when freely standing (A) or stabilized at the trunk (B). C through E, Backward sway is induced by triceps brachii vibration when subjects are instructed to orient the finger to a visual target (C), a tactile target (D), or an imagined target in space (E).

Adapted from Quoniam et al.
Figure 12. Figure 12.

Relationship between the head and trunk position and direction of body sway elicited by galvanic vestibular stimulation. A, Head rotation alone. B, Head and trunk rotated together. C, The face is kept forward by rotating the head and trunk in opposite directions.

Adapted from Lund and Broberg
Figure 13. Figure 13.

A, Early activation of eye, arm, and leg muscles in a human subject during an unexpected drop from a height of 20 cm B, Gastrocnemius activity in the cat during a sudden drop from a height of 45–50 cm onto a foam pad. Note that the early burst of activity (indicated by arrow) was unaffected by the inactivation of the semicircular canals through plugging, whereas this burst was absent following total bilateral labyrinthectomy, indicating that the early response is mediated by the otoliths. C, Response to backward platform translation inducing forward body sway. Subjects with loss of vestibular function respond with a postural response that has similar timing but larger amplitude than that of healthy subjects. Dashed line indicates onset of platform movement. D, Active force response in the cat to translations in each of eight directions in the horizontal plane, before and after total bilateral labyrinthectomy. Note the increase in response amplitude after lesion, not only for each limb, but also for the resultant force vector (sum of the forces from each limb). Coordinate system as in Figure . E, Head acceleration (HACC) and EMG activity in a healthy subject and a patient with loss of vestibular function evoked by a perturbation to the head/neck at time 0. Note the absence of early neck and leg responses in the vestibular absent subject. TRAP, trapezius; HAM, hamstrings; GAS, gastrocnemius; PAR, paraspinals.

Adapted from Greenwood and Hopkins . Adapted from Watt . Adapted from Inglis and Macpherson . C and E adapted from Horak et al.
Figure 14. Figure 14.

Displacement of perceived vertical during rollvection induced by large disk rotating around subject's line of sight. Clockwise visual rotation causes an illusion of counterclockwise tilt of the observer's body that is “corrected” by lateral body tilt in the same direction as the visual rotation .

Adapted from Brandt, Paulus, and Straube
Figure 15. Figure 15.

Response of the gastrocnemius to forward sway induced by backward translation of the support surface. “Normal Vision” is the control response with eyes open. The “Eyes Closed” response shows no change in the evoked EMG activity with visual deprivation. “Vision Stabilized” is the response when the visual surround is stabilized with respect to the subject, showing a marked attenuation of the early burst of activity (at 100 ms).

Adapted from Vidal, Berthoz, and Millanvoye
Figure 16. Figure 16.

Schematic representation of three separate postural systems for regulation of background postural tone, centrally initiated postural adjustments, and peripherally triggered postural reactions. Separate pathways show how basal ganglia disruption from parkinsonism could affect all three postural systems but dopamine replacement therapy improves only background postural tone and centrally initiated postural adjustments.

Adapted from Horak and Frank
Figure 17. Figure 17.

A, Patient with anterior lobe cerebellar atrophy does not scale the initial gastrocnemius (GAS) response to varying amplitudes of platform displacement (indicated in centimeters), but uses later tibialis (TIB) antagonist activation to counteract the hypermetric GAS response. B, Mean regressions (N = 10 subjects) showing relationship between displacement amplitude and initial torque response in normal and cerebellar subjects. When trials are blocked in groups of like amplitudes, normal, but not cerebellar, subjects scale their initial response to the expected amplitude through the mechanism of central set. When trials are randomized for amplitude, no correlation is seen with initial response.

Adapted from Horak and Diener


Figure 1.

Equilibrium in different postural orientations. The center of mass (CoM) of the body (indicated by the X) may lie inside (A and C) or outside (B) the body limits. The net ground reaction force takes origin within the base of support and must pass through the CoM if the body is in equilibrium.



Figure 2.

Stick figures illustrating postural orientation in the cat. Changes in stance distance (A) or surface tilt (B) in the cat are accommodated primarily through pivoting of the limbs at the most proximal and distal joints. The trunk remains parallel to the support surface and intralimb geometry is relatively conserved.

A adapted from Fung and Macpherson , B adapted from Lacquaniti et al.


Figure 3.

Stabilization of the head in space during dynamic motor activities, A, Example of subject locomoting while maintaining the Frankfort plane at a constant angle with respect to vertical. Berthoz and Pozzo created these drawings by tracing and superimposing time‐lapse photos from Muybridge . B, Tracing from a cine film of a Macaca mulatta juvenile in the wild, leaping between branches.

Drawing courtesy of Dr. D. C. Dunbar


Figure 4.

Control system model for spatial orientation. See text for details.

Adapted from Merfeld et al.


Figure 5.

Ankle‐to‐hip strategy continuum for dynamic equilibrium in standing humans. The plot of ankle angle vs. hip angle shows examples of trajectories in sagittal plane angle–angle space for the ankle, the hip, and mixed ankle and hip strategies. The initial, erect position is at the origin of the graph. The perturbation, a backward translation, induces forward sway primarily about the ankle joints to the dot on the upward‐going ankle axis. The ankle strategy brings the subject back to the erect equilibrium position, whereas the hip strategy brings the subject to a nonerect equilibrium position. Mixed strategies can follow a trajectory anywhere within the shaded region. Insets show the patterns of EMG activity in the neck, trunk and leg for the various strategies. TRAP, trapezius; SCM, sternocleidomastoid; PARA, paraspinals; ABD, abdominals; HAM, hamstrings; QUAD, quadriceps; GAS, gastrocnemius; TIB, tibialis anterior.

Adapted from Horak and Nashner


Figure 6.

Characteristics of postural responses of the cat to linear translation in many directions in the horizontal plane. A, Active horizontal plane force vectors exerted by each limb in response to 16 directions of translation. Arrows illustrate the mean vector directions for each cluster. LE, RF, left and right forelimb; LH, RH, left and right hindlimb. Inset shows coordinate system in degrees. B, Probability of response (% of responsive trials) for two selected muscles, lateral gastrocnemius (LG) and soleus (Sol) as a function of the direction of translation. Note the monotonic nature of the response profiles, and the fact that even for those directions that a muscle is activated, it may never be recruited for 100% of the trials (e.g., Sol). C, EMG tuning curves: activity evoked in several muscles during translation plotted in polar coordinates of direction of translation (degrees) vs. amplitude of normalized evoked activity. D, Peak change in torque during response to translation. The circles represent 0.4 Nm. F, flexor; E, extensor; GLUT, gluteus medius; ILIO, iliopsoas; Med BF, medial bicepts femoris, SART, anterior sartorius; STEN, semitendinosus; VMED, vastus medialis; SOL, soleus; TIBA, tibialis anterior.



Figure 7.

Ankle muscle activation in the two lower limbs in response to backward translation of the support surface under two instructions, “remain standing” (thin line, hatched fill) and “step as soon as the perturbation is detected” (thick line, no fill). The response to translation alone consists of bilateral activation of soleus and gastrocnemius. When the instruction is to step, the postural response is significantly attenuated, particularly in the stance limb.

Adapted from Burleigh, Horak, and Malouin


Figure 8.

A, The anticipatory postural response in the lower limb muscles, gastrocnemius (Gastroc) and hamstrings (Ham), precedes the onset of activity in biceps brachii (Biceps), the agonist for the arm‐pull task. B, When the subject is firmly supported at the shoulder, the same arm‐pull task illustrated in A does not elicit an anticipatory postural response in the leg muscles (compare gastrocnemius in A and B. C, In rocking forward on the toes, the first change in EMG is an inhibition of tonic activity in soleus (arrow), which allows the CoM to move forward toward the toes. This is followed by the focal response of activation of tibialis (Tib), soleus (Sol), and quadriceps (Quad).

A and B adapted from Cordo and Nashner , C adapted from Nardone and Schieppati


Figure 9.

Optimal control model of human postural coordination predicts the ankle and hip strategy for control of sagittal sway. A, The filled region represents the feasible acceleration set for ankle, knee, and hip movement in the sagittal plane, given the constraint of keeping the feet on the ground in stance. The slice through the region represents the plane of hip and ankle accelerations when the knee is constrained to remaining straight. There is only a narrow region in which the ankle strategy (horizontal extent of shaded area) may be used without the toes or heels lifting off. Note that the ankle acceleration scale has been expanded compared to the hip. B, Block diagram of the optimal control model (see text for details). C, Ankle/hip angle trajectories predicted by the model for two different cost functions.

Adapted from Kuo


Figure 10.

Mean sway over 20 s of stance under six different sensory conditions in normal subjects (open bars) and in patients with vestibular loss (top), and a sensory organization deficit (bottom). Anterior/posterior peak‐to‐peak sway at the hips is normalized for each subject's height such that 100% represents a “fall.” Sensory conditions include blindfolding (conditions 2 and 5), sway‐referencing the visual surround (conditions 3 and 6), and sway‐referencing the support surface (conditions 4, 5 and 6).

Adapted from Black et al.


Figure 11.

Context‐dependency of changes in postural orientation induced by vibration. A and B, Subjects are instructed to hold the forearm horizontal when freely standing (A) or stabilized at the trunk (B). C through E, Backward sway is induced by triceps brachii vibration when subjects are instructed to orient the finger to a visual target (C), a tactile target (D), or an imagined target in space (E).

Adapted from Quoniam et al.


Figure 12.

Relationship between the head and trunk position and direction of body sway elicited by galvanic vestibular stimulation. A, Head rotation alone. B, Head and trunk rotated together. C, The face is kept forward by rotating the head and trunk in opposite directions.

Adapted from Lund and Broberg


Figure 13.

A, Early activation of eye, arm, and leg muscles in a human subject during an unexpected drop from a height of 20 cm B, Gastrocnemius activity in the cat during a sudden drop from a height of 45–50 cm onto a foam pad. Note that the early burst of activity (indicated by arrow) was unaffected by the inactivation of the semicircular canals through plugging, whereas this burst was absent following total bilateral labyrinthectomy, indicating that the early response is mediated by the otoliths. C, Response to backward platform translation inducing forward body sway. Subjects with loss of vestibular function respond with a postural response that has similar timing but larger amplitude than that of healthy subjects. Dashed line indicates onset of platform movement. D, Active force response in the cat to translations in each of eight directions in the horizontal plane, before and after total bilateral labyrinthectomy. Note the increase in response amplitude after lesion, not only for each limb, but also for the resultant force vector (sum of the forces from each limb). Coordinate system as in Figure . E, Head acceleration (HACC) and EMG activity in a healthy subject and a patient with loss of vestibular function evoked by a perturbation to the head/neck at time 0. Note the absence of early neck and leg responses in the vestibular absent subject. TRAP, trapezius; HAM, hamstrings; GAS, gastrocnemius; PAR, paraspinals.

Adapted from Greenwood and Hopkins . Adapted from Watt . Adapted from Inglis and Macpherson . C and E adapted from Horak et al.


Figure 14.

Displacement of perceived vertical during rollvection induced by large disk rotating around subject's line of sight. Clockwise visual rotation causes an illusion of counterclockwise tilt of the observer's body that is “corrected” by lateral body tilt in the same direction as the visual rotation .

Adapted from Brandt, Paulus, and Straube


Figure 15.

Response of the gastrocnemius to forward sway induced by backward translation of the support surface. “Normal Vision” is the control response with eyes open. The “Eyes Closed” response shows no change in the evoked EMG activity with visual deprivation. “Vision Stabilized” is the response when the visual surround is stabilized with respect to the subject, showing a marked attenuation of the early burst of activity (at 100 ms).

Adapted from Vidal, Berthoz, and Millanvoye


Figure 16.

Schematic representation of three separate postural systems for regulation of background postural tone, centrally initiated postural adjustments, and peripherally triggered postural reactions. Separate pathways show how basal ganglia disruption from parkinsonism could affect all three postural systems but dopamine replacement therapy improves only background postural tone and centrally initiated postural adjustments.

Adapted from Horak and Frank


Figure 17.

A, Patient with anterior lobe cerebellar atrophy does not scale the initial gastrocnemius (GAS) response to varying amplitudes of platform displacement (indicated in centimeters), but uses later tibialis (TIB) antagonist activation to counteract the hypermetric GAS response. B, Mean regressions (N = 10 subjects) showing relationship between displacement amplitude and initial torque response in normal and cerebellar subjects. When trials are blocked in groups of like amplitudes, normal, but not cerebellar, subjects scale their initial response to the expected amplitude through the mechanism of central set. When trials are randomized for amplitude, no correlation is seen with initial response.

Adapted from Horak and Diener
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Fay B. Horak, Jane M. Macpherson. Postural Orientation and Equilibrium. Compr Physiol 2011, Supplement 29: Handbook of Physiology, Exercise: Regulation and Integration of Multiple Systems: 255-292. First published in print 1996. doi: 10.1002/cphy.cp120107