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Exercise Countermeasures to Neuromuscular Deconditioning in Spaceflight

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Abstract

The mechanical unloading of spaceflight elicits a host of physiological adaptations including reductions in muscle mass, muscle strength, and muscle function and alterations in central interpretation of visual, vestibular, and proprioceptive information. Upon return to a terrestrial, gravitational environment, these result in reduced function and performance, the potential consequences of which will be exacerbated during exploration missions to austere and distant destinations such as the moon and Mars. Exercise is a potent countermeasure to unloading‐induced physiological maladaptations and has been employed since the early days of spaceflight. In‐flight exercise hardware has evolved from rudimentary and largely ineffective devices to the current suite onboard the International Space Station (ISS) comprised of a cycle ergometer, treadmill, and resistance exercise device; these contemporary devices have either fully protected or significantly attenuated neuromuscular degradation in spaceflight. However, unlike current microgravity operations on the ISS, future exploration missions will include surface operations in partial gravity environments, which will require greater physiological capacity and work output of their crews. For these flights, it is critical to identify physiological thresholds below which task performance will be impaired and to develop exercise countermeasures—both pre‐ and in‐flight—to ensure that crewmembers are able to safely and effectively complete physically demanding mission objectives. © 2020 American Physiological Society. Compr Physiol 10:171‐196, 2020.

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Figure 1. Figure 1. The exercise device used on some Apollo missions was based on the Exer‐Genie (Exer‐Genie, Inc., Fullerton, CA). Within the cylinder, the nylon cords rotate around a shaft, developing controlled resistance. The cords are attached to loop handles. When not in use, the flight device was stored in a cloth bag (inset).
Figure 2. Figure 2. Ground reaction forces during an iRED squat in 1‐g 8.
Figure 3. Figure 3. Ground reaction forces during a free‐weight (Smith machine) squat in 1‐g 8.
Figure 4. Figure 4. (A) The Advanced Resistive Exercise Device (ARED), pictured on the ground; (B) ARED deployed on the International Space Station with a crewmember performing a deadlift.
Figure 5. Figure 5. The second‐generation treadmill (T2) on the International Space Station; the crewmember is loaded via a harness and bungee cord system.
Figure 6. Figure 6. The Cycle Ergometer with Vibration Isolation System (CEVIS) on the International Space Station. The crewmember clips into the pedals via cleated shoes and stabilizes himself using handholds on the frame; there is no seat/saddle.
Figure 7. Figure 7. (A) Scatter plot of time to complete the course (TCC) for 18 long‐duration subjects (6‐month exposure to spaceflight) showing a 48% increase in time to traverse the obstacle course 1 day after landing. (B) Diagram of obstacle course. (C) Recovery of function took an average of 15 days to return to within 95% of their preflight level of performance.
Figure 8. Figure 8. Results from two tests of postural stability (top two rows) along with an assessment of lower body muscle function (bottom row) in long‐duration ISS crewmembers (Spaceflight group) and bed rest subjects who performed exercise while in bed (Exercise group) and those who did not exercise (Control group).
Figure 9. Figure 9. A subset of prominent physiological systems that are (i) altered by the microgravity of spaceflight and (ii) amenable to protection via exercise countermeasures.


Figure 1. The exercise device used on some Apollo missions was based on the Exer‐Genie (Exer‐Genie, Inc., Fullerton, CA). Within the cylinder, the nylon cords rotate around a shaft, developing controlled resistance. The cords are attached to loop handles. When not in use, the flight device was stored in a cloth bag (inset).


Figure 2. Ground reaction forces during an iRED squat in 1‐g 8.


Figure 3. Ground reaction forces during a free‐weight (Smith machine) squat in 1‐g 8.


Figure 4. (A) The Advanced Resistive Exercise Device (ARED), pictured on the ground; (B) ARED deployed on the International Space Station with a crewmember performing a deadlift.


Figure 5. The second‐generation treadmill (T2) on the International Space Station; the crewmember is loaded via a harness and bungee cord system.


Figure 6. The Cycle Ergometer with Vibration Isolation System (CEVIS) on the International Space Station. The crewmember clips into the pedals via cleated shoes and stabilizes himself using handholds on the frame; there is no seat/saddle.


Figure 7. (A) Scatter plot of time to complete the course (TCC) for 18 long‐duration subjects (6‐month exposure to spaceflight) showing a 48% increase in time to traverse the obstacle course 1 day after landing. (B) Diagram of obstacle course. (C) Recovery of function took an average of 15 days to return to within 95% of their preflight level of performance.


Figure 8. Results from two tests of postural stability (top two rows) along with an assessment of lower body muscle function (bottom row) in long‐duration ISS crewmembers (Spaceflight group) and bed rest subjects who performed exercise while in bed (Exercise group) and those who did not exercise (Control group).


Figure 9. A subset of prominent physiological systems that are (i) altered by the microgravity of spaceflight and (ii) amenable to protection via exercise countermeasures.
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Teaching Material

Kirk L. English, Jacob J. Bloomberg, Ajitkumar P. Mulavara, Lori L. Ploutz-Snyder. Exercise Countermeasures to Neuromuscular Deconditioning in Spaceflight. Compr Physiol 10: 2020, 171-196.

Didactic Synopsis

Major Teaching Points:

*The mechanical unloading of spaceflight induces significant alterations in the neuromuscular system including decreased muscle mass, muscle strength, muscle power, functional performance, and insulin sensitivity.

*The sensorimotor system also undergoes adaptation during exposure to the microgravity conditions of spaceflight.

*Integrated aerobic and resistance exercise is a potent countermeasure to the degradation of neuromuscular and cardiovascular function and can help in attenuating impairments in functional performance.

*Novel exercise regimens and other strategies are required to mitigate post-flight postural control dysfunction and maintain functional performance.

*Early exercise countermeasures on the International Space Station (ISS) were only partially effective to attenuate physiologic maladaptations due to hardware shortcomings and low intensity exercise protocols.

*In the last 10 y, new exercise hardware and protocols have led to increasingly effective exercise countermeasures and better protection of pre-flight physiologic status.

*Future exploration missions will necessitate smaller, yet equally robust exercise hardware due to smaller vehicles. However, particularly for exploration missions with terrestrial objectives, the physiologic demands on crewmembers will likely be higher than those for ISS crewmembers. As such, in addition to the current emphasis on in-flight exercise countermeasures, greater focus should be placed on pre-flight fitness for duty with a particular emphasis on physiologic preparation for the demands of the mission.

*Aging exacerbates unloading-induced alterations in skeletal muscle, and thus, adjuncts to exercise such as nutritional supplements and testosterone should be considered for optimal physiologic protection.

Didactic Legends

The following legends to the figures that appear throughout the article are written to be useful for teaching.

Figure 1. Teaching Points: The exercise device used on some Apollo missions was based on the Exer-Genie (Exer-Genie, Inc., Fullerton, CA). Within the cylinder, the nylon cords rotate around a shaft, developing controlled resistance. The cords are attached to loop handles. When not in use, the flight device was stored in a cloth bag (inset).

Figure 2. Teaching Points: Ground reaction forces during an iRED squat in 1-g (8).

Figure 3. Teaching Points: Ground reaction forces during a free weight (Smith machine) squat in 1-g (8).

Figure 4. Teaching Points: a) The Advanced Resistive Exercise Device (ARED), pictured on the ground; b) ARED deployed on the International Space Station with a crewmember performing a deadlift.

Figure 5. Teaching Points: The second generation treadmill (T2) on the International Space Station; the crewmember is loaded via a harness and bungee cord system.

Figure 6. Teaching Points: The Cycle Ergometer with Vibration Isolation System (CEVIS) on the International Space Station. The crewmember clips into the pedals via cleated shoes and stabilizes himself using handholds on the frame; there is no seat/saddle.

Figure 7. Teaching Points: Left: Scatter plot of time to complete the course (TCC) for 18 long-duration subjects (6-months exposure to spaceflight) showing a 48% increase in time to traverse the obstacle course one day after landing. Center: Diagram of obstacle course. Right: Recovery of function took an average of 15 days to return to within 95% of their pre-flight level of performance.

Figure 8. Teaching Points: Results from two tests of postural stability (top two rows) along with an assessment of lower body muscle function (bottom row) in long-duration ISS crewmembers (Spaceflight group) and bed rest subjects who performed exercise while in bed (Exercise group) and those that did not exercise (Control group).

Figure 9. Teaching Points: A subset of prominent physiologic systems that are: 1) altered by the microgravity of spaceflight and 2) amenable to protection via exercise countermeasures.  

 


Related Articles:

Microgravity Stress: Bone and Connective Tissue
Neuromuscular Adaptations to Actual and Simulated Spaceflight
Teaching Material

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How to Cite

Kirk L. English, Jacob J. Bloomberg, Ajitkumar P. Mulavara, Lori L. Ploutz‐Snyder. Exercise Countermeasures to Neuromuscular Deconditioning in Spaceflight. Compr Physiol 2019, 10: 171-196. doi: 10.1002/cphy.c190005