Comprehensive Physiology Wiley Online Library

Neuromuscular Adaptations to Actual and Simulated Spaceflight

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Abstract

The sections in this article are:

1 Plasticity of Skeletal Muscle
2 Variations in Flight and Simulated Flight Experimental Designs
3 Muscle and Muscle Fiber Morphology
3.1 Hindlimb Suspension
3.2 Spaceflight
4 Muscle Fiber Type and Contractile Proteins
4.1 Hindlimb Suspension
4.2 Spaceflight
5 Adaptations in the Metabolic Pathways
5.1 Hindlimb Suspension
5.2 Spaceflight
6 Adaptations in Mechanical Properties
6.1 Hindlimb Suspension
6.2 Spaceflight
7 Neuromuscular Activity
7.1 Hindlimb Suspension
7.2 Spaceflight
8 Countermeasures
8.1 Hindlimb Suspension
8.2 Spaceflight
9 Neural Tissue Adaptations
10 Posture, Locomotor Performance, and Movement Control
10.1 Movement Perception
10.2 Posture and Locomotion
10.3 Maximal Torque Velocity
10.4 Tendon Stretch Reflexes
10.5 Hoffmann‐reflex (H‐reflex)
10.6 Vibration Sensitivity
10.7 Muscle Transverse Stiffness
11 Exercise as a Countermeasure for Adverse Effects of Spaceflight on the Motor System
11.1 Countermeasures for Motor Deficits
11.2 Some Basic Physiological Principles for Developing Exercise Countermeasures
12 Summary and Conclusions
12.1 Muscle Fiber Size Plasticity
12.2 Metabolic Adaptations
12.3 Fatigability of Muscle and Motor Units
12.4 Regulation of Myosin Isoforms
12.5 Integrative Strategies for Adaptations
Figure 1. Figure 1.

Performance of individual crew members on postural tests consisting of standing for 50 s for two trials on a 3.2 and 5.7 cm rail. A score of 100 s is a perfect score. Note the drop in performance on the narrow rail on the first day of flight even with the eyes open. With the eyes closed, successful standing could be achieved after flight only on the wider rail. Note that the deficits in postural control were evident for at least 4 days after flight. The performance of two subjects was unaffected with the eyes open, while all subjects were affected with the eyes closed. The scale on the abscissa is nonlinear.

Data obtained from ref.
Figure 2. Figure 2.

EMG amplitudes of the soleus and tibialis anterior muscles of a crew member on a 7 day shuttle flight (STS 51‐G) when standing with the shoes attached to the floor. The subject was asked to maintain a normal terrestrial standing posture or to lean 4° forward. This task was performed with normal vision, occluded vision, and stabilized vision. Because there were no qualitative differences among the three conditions, only the results from the normal vision tests are shown. In the relaxed upright posture position, the soleus EMG dropped slightly during flight relative to preflight and increased substantially immediately after flight. The EMG returned to preflight levels by 3 days postflight. The tibialis anterior EMG progressively increased during flight and was below preflight levels immediately after flight. These low levels persisted for 3 days postflight. Similar results were obtained from three other crew members from the same flight, although the data were not as complete as for the subject shown in this Figure. Similar results were evident in the 4° lean forward position.

Adapted from Clement et al.
Figure 3. Figure 3.

Probability density distributions of soleus (Sol) and medial gastrocnemius (MG) EMG amplitudes of a Rhesus monkey preflight and 5 (10–2), 10 (10–7), and 12 (10–9) days after flight (Cosmos 2044). The numbers in parentheses refer to the month and day of the recordings. For each time bin of a recording, the mean amplitudes of the EMG of the two muscles are plotted. The incidences of the paired amplitudes are converted to a probability and plotted on the vertical axis on a logarithmic scale. The numbers to the right of each peak indicate the peak probability. Each distribution is viewed looking towards the origin which is obscured by data. Typically, the peak of the distribution occurs at the origin, representing the majority of the data at zero amplitude for each muscle and corresponding to negligible EMG activity between trials. The axes labels for the postflight data are the same for each figure. Note the different scales on the EMG axes for the preflight and postflight figures. Each postflight graph has the same scale for EMG amplitudes. Note that 5 days after flight (10–2), the highest incidences of amplitudes were shifted toward the MG axis relative to preflight and by 10–7 the distribution was similar to preflight. The significance of the lower absolute amplitudes at 10–7 and 10–9 is unclear. (See ref. )

Figure 4. Figure 4.

The Achilles tendon was tapped with a device instrumented to record the impact force and the peak EMG response of the gastrocnemius‐soleus complex was recorded. Each slope was based on the average of 22 tendon taps for each of four conditions for one subject after a prolonged flight: (A) preflight and 2 days postflight, both ankles relaxed; and (B) preflight and 2 days postflight, when the contralateral calf muscles were voluntarily contracted. The tendon reflex threshold and gain were lower after flight than before flight. The relationships observed 2 days after flight were similar after 5 days (data not shown). Note in (B) that both prior to and after flight the gain of the relationship was reduced during contraction of the contralateral calf muscles, suggesting a contralateral inhibitory effect.

Adapted from Kozlovskaya et al.
Figure 5. Figure 5.

Maximum EMG response from a patellar tendon tap of the right and left legs of two cosmonauts (AGN and VIS) before and after an 18 day flight (Soyuz‐9). Note that compared to preflight values the reflex amplitude is clearly higher 2 days after flight, but not thereafter.

Data from ref.
Figure 6. Figure 6.

The peak Hoffman‐reflex amplitude from the soleus muscle induced by stimulating the popliteal nerve with a subcutaneous needle electrode (cathode) and recorded from three subjects during a drop from a 15 cm stool before flight, on flight days 1 and/or 6, and from 1 to 6 days postflight. The data are expressed as a percentage change from a “hanging position.” Two different percentage change scales are shown. Note the absence of a change in the reflex on the first day of the mission, the drop seen in two subjects on flight day 6, and the elevated response postflight. The day of landing is 0 postflight day.

Adapted from Reschke et al.
Figure 7. Figure 7.

Cosmonauts on Salyut‐6 and 7 and on the Mir station stayed in space for 60–366 days. Upon return to 1G, a series of motor tests (2–4 days postflight) were administered and a cumulative rank of the motor effects based on these tests was calculated for each of the 24 cosmonauts. A rank of 1 represents the cosmonaut showing the least effects of spaceflight, that is, the best performance. These tests have been described in detail previously . Briefly, the tests included the following: (1) maximal torque‐velocity output and a fatigue test during which the subjects worked isokinetically at 120°/s for a specific number of repetitions . These tests were conducted on the knee and ankle musculature; (2) posture tests in which EMG was measured during a series of standing tasks under varying conditions, for example, eyes opened or closed, Rhomberg standing, push response; and (3) reflex testing, principally the Achilles tendon reflex. All tests were performed using an anti‐G suit to minimize the potential complication of orthostatic tolerance in the performance and interpretation of the motor tasks. In the lower graph, the relative volume of exercise performed during the flight is associated with the flight duration.

Adapted from Koslovskaya et al.
Figure 8. Figure 8.

Maximal voluntary torques during ankle plantarflexion produced by five cosmonauts 2–4 days after flights of 45–366 days. The torques represent the maximum effort during a constant velocity (isokinetic) movement. d, duration of flight in days.

Data from ref.


Figure 1.

Performance of individual crew members on postural tests consisting of standing for 50 s for two trials on a 3.2 and 5.7 cm rail. A score of 100 s is a perfect score. Note the drop in performance on the narrow rail on the first day of flight even with the eyes open. With the eyes closed, successful standing could be achieved after flight only on the wider rail. Note that the deficits in postural control were evident for at least 4 days after flight. The performance of two subjects was unaffected with the eyes open, while all subjects were affected with the eyes closed. The scale on the abscissa is nonlinear.

Data obtained from ref.


Figure 2.

EMG amplitudes of the soleus and tibialis anterior muscles of a crew member on a 7 day shuttle flight (STS 51‐G) when standing with the shoes attached to the floor. The subject was asked to maintain a normal terrestrial standing posture or to lean 4° forward. This task was performed with normal vision, occluded vision, and stabilized vision. Because there were no qualitative differences among the three conditions, only the results from the normal vision tests are shown. In the relaxed upright posture position, the soleus EMG dropped slightly during flight relative to preflight and increased substantially immediately after flight. The EMG returned to preflight levels by 3 days postflight. The tibialis anterior EMG progressively increased during flight and was below preflight levels immediately after flight. These low levels persisted for 3 days postflight. Similar results were obtained from three other crew members from the same flight, although the data were not as complete as for the subject shown in this Figure. Similar results were evident in the 4° lean forward position.

Adapted from Clement et al.


Figure 3.

Probability density distributions of soleus (Sol) and medial gastrocnemius (MG) EMG amplitudes of a Rhesus monkey preflight and 5 (10–2), 10 (10–7), and 12 (10–9) days after flight (Cosmos 2044). The numbers in parentheses refer to the month and day of the recordings. For each time bin of a recording, the mean amplitudes of the EMG of the two muscles are plotted. The incidences of the paired amplitudes are converted to a probability and plotted on the vertical axis on a logarithmic scale. The numbers to the right of each peak indicate the peak probability. Each distribution is viewed looking towards the origin which is obscured by data. Typically, the peak of the distribution occurs at the origin, representing the majority of the data at zero amplitude for each muscle and corresponding to negligible EMG activity between trials. The axes labels for the postflight data are the same for each figure. Note the different scales on the EMG axes for the preflight and postflight figures. Each postflight graph has the same scale for EMG amplitudes. Note that 5 days after flight (10–2), the highest incidences of amplitudes were shifted toward the MG axis relative to preflight and by 10–7 the distribution was similar to preflight. The significance of the lower absolute amplitudes at 10–7 and 10–9 is unclear. (See ref. )



Figure 4.

The Achilles tendon was tapped with a device instrumented to record the impact force and the peak EMG response of the gastrocnemius‐soleus complex was recorded. Each slope was based on the average of 22 tendon taps for each of four conditions for one subject after a prolonged flight: (A) preflight and 2 days postflight, both ankles relaxed; and (B) preflight and 2 days postflight, when the contralateral calf muscles were voluntarily contracted. The tendon reflex threshold and gain were lower after flight than before flight. The relationships observed 2 days after flight were similar after 5 days (data not shown). Note in (B) that both prior to and after flight the gain of the relationship was reduced during contraction of the contralateral calf muscles, suggesting a contralateral inhibitory effect.

Adapted from Kozlovskaya et al.


Figure 5.

Maximum EMG response from a patellar tendon tap of the right and left legs of two cosmonauts (AGN and VIS) before and after an 18 day flight (Soyuz‐9). Note that compared to preflight values the reflex amplitude is clearly higher 2 days after flight, but not thereafter.

Data from ref.


Figure 6.

The peak Hoffman‐reflex amplitude from the soleus muscle induced by stimulating the popliteal nerve with a subcutaneous needle electrode (cathode) and recorded from three subjects during a drop from a 15 cm stool before flight, on flight days 1 and/or 6, and from 1 to 6 days postflight. The data are expressed as a percentage change from a “hanging position.” Two different percentage change scales are shown. Note the absence of a change in the reflex on the first day of the mission, the drop seen in two subjects on flight day 6, and the elevated response postflight. The day of landing is 0 postflight day.

Adapted from Reschke et al.


Figure 7.

Cosmonauts on Salyut‐6 and 7 and on the Mir station stayed in space for 60–366 days. Upon return to 1G, a series of motor tests (2–4 days postflight) were administered and a cumulative rank of the motor effects based on these tests was calculated for each of the 24 cosmonauts. A rank of 1 represents the cosmonaut showing the least effects of spaceflight, that is, the best performance. These tests have been described in detail previously . Briefly, the tests included the following: (1) maximal torque‐velocity output and a fatigue test during which the subjects worked isokinetically at 120°/s for a specific number of repetitions . These tests were conducted on the knee and ankle musculature; (2) posture tests in which EMG was measured during a series of standing tasks under varying conditions, for example, eyes opened or closed, Rhomberg standing, push response; and (3) reflex testing, principally the Achilles tendon reflex. All tests were performed using an anti‐G suit to minimize the potential complication of orthostatic tolerance in the performance and interpretation of the motor tasks. In the lower graph, the relative volume of exercise performed during the flight is associated with the flight duration.

Adapted from Koslovskaya et al.


Figure 8.

Maximal voluntary torques during ankle plantarflexion produced by five cosmonauts 2–4 days after flights of 45–366 days. The torques represent the maximum effort during a constant velocity (isokinetic) movement. d, duration of flight in days.

Data from ref.
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V. Reggie Edgerton, Roland R. Roy. Neuromuscular Adaptations to Actual and Simulated Spaceflight. Compr Physiol 2011, Supplement 14: Handbook of Physiology, Environmental Physiology: 721-763. First published in print 1996. doi: 10.1002/cphy.cp040132