Effects of Altered Gravity on Spinal Cord Excitability (H-Reflex) - 08.20.14
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Prolonged weightlessness results in a loss of muscle strength, muscle volume and bone density, particularly in the legs. These conditions can cause reduced spinal cord excitability, which can lead to loss of locomotor function in the legs. Spinal cord excitability was isolated and measured to study possible ways to reverse the process while still in flight. Reversal of this process will result in a healthier crew following long duration space flight.
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Rigorous exercising in space helps counter the loss of muscle and bone caused by spaceflight. However, if weightlessness reduces spinal cord excitability (SCE), a reaction of muscles after electrical nerve stimulation, it would mean more effort is required to produce the same level of exercise. Researchers measured SCE before, during, and after flight and found a significant decrease after five days, lasting for the duration of the flight and up to 10 days after landing. This indicates microgravity does indeed reduce SCE and that muscle mass therefore decreases even with exercise. The reduction was not seen with prolonged bed rest on the ground, suggesting nervous system changes in space may be responsible.
Canadian Space Agency (CSA), Ottawa, Ontario, Canada
Sponsoring Space Agency
Canadian Space Agency (CSA)
ISS Expedition Duration
March 2001 - June 2002
Previous ISS Missions
Related experiments on Skylab, STS-9, STS-41G, STS-61, STS-40, STS-42, STS-52, STS-58, STS-78 and Increments 2, 3 and 4.
- Confirm that spinal cord excitability is gradually diminished during prolonged exposure to microgravity. Spinal cord excitability appears to decrease after approximately 5 days in space (incidental observation from IML). This would make inflight exercise less efficient and/or less effective at maintaining muscle mass and bone calcification.
- Unlike vestibular adaptation to weightlessness, it may be possible to reverse the process while still inflight.
- If spinal cord excitability of nerve cells decreased, then H-Reflex measures the extent of the change, how rapidly it occurs, and how long it continues after returning to the ground.
In the weightlessness of low Earth orbit, the body loses muscle mass and bone density. The only known countermeasure for this atrophy is exercise. However, as astronauts spend longer durations in space, will exercise continue to be an effective countermeasure? Along with changes in muscle and bone, the neurovestibular system (the complex sensory system that maintains posture, balance, and coordination) adapts to changes in gravity. Researchers hypothesize that, as part of this neurovestibular system adaptation, spinal cord excitability decreases and the spinal cord reacts less to stimuli. If this hypothesis is correct, exercise may become less effective the longer astronauts stay in microgravity, and researchers may have to adjust exercise programs accordingly.
H-Reflex tested this hypothesis by measuring muscle response to mild electrical shocks (40-90 volts). Nerves in the leg perceive the electrical shock and send a signal along the spinal cord to the brain. The signal stimulates motor neurons in the brain, which, in turn, send signals that cause leg muscles to contract. The bigger the contraction, the more the neurons are stimulated, indicating the level of spinal cord excitability. Researchers compared measurements taken before, during, and after flight to determine whether the spinal cord’s ability to respond to stimuli changed over time. The H-Reflex equipment recorded the EMG activity in the muscle—the electrical activity that caused the muscle to move—rather than the movement that follows the electrical activity (as a knee-tap test would), allowing researchers to take more precise measurements.
Prolonged weightlessness results in a weakening of the heart, loss of skeletal muscle strength and volume, and decreased bone density. At this time, the only effective treatment for these problems is inflight exercise. The most basic unit of exercise is the contraction of a small group of muscle fibers innervated by the projections of a single nerve cell. If this and other nerve cells in the spinal cord become less excitable during space flight, it would be more difficult to make muscle fibers contract. As a result, more effort would be required to produce the same level of exercise, or if the same apparent effort were maintained, the actual level of exercise would decrease. If present after landing, it would be more difficult to stand and walk. However, depending on the underlying mechanisms, it may be possible to reverse the process while still inflight.
The information gained by this investigation may help researchers develop countermeasures to overcome decreases in spinal excitability. There are many disorders that involve decreases in nerve impulses and loss of sensitivity. Countermeasures that are used on orbit may lead to advances on Earth in treatment of various nerve disorders and spinal injuries.
H-Reflex is designed to minimize crew requirements; it is sufficiently automated so that any individual may act as both subject and operator. Each test session takes only 5 minutes, plus set up time; measurements take 130 minutes for three crew members when performed all on one day. At the end of the session, the computer analyzes and displays the recorded data, allowing the crew member to determine if the test was successfully completed.
No Station power is needed to operate the experiment. When not in use, the H-Reflex kit is stored in the HRF.
A crew member is restrained in a sitting position and his or her legs are fitted with five electrodes: stimulating electrodes are placed behind the knees and recording electrodes are placed over the lower calf muscles. The HRTU then sends mild electrical shocks of varying strength and times to the calf muscles via the stimulating electrodes. The resulting electrical activity in the calf muscles is recorded, and the computer determines the minimum and maximum of spinal cord excitability for each session. Data is saved on the hard drive and downlinked to ground operations as soon as possible. Ground support for H-Reflex is provided by the Canadian Space Agency's Payload Mission Support Centre in Saint-Hubert, Quebec.
H-Reflex will be conducted four times during each expedition's stay at the Station: three early in flight (pre-docked and docked), and once late in the mission. The principal investigator added an additional early session during Expedition 3, after results received during Expedition 2 indicated that an additional session would better define the rate of change in this neurovestibular reflex. For comparison, the experiment is also run three times preflight and four times post-flight.
This study of spinal cord excitability using the Hoffman reflex was completed by a total of eight subjects over ISS Expeditions 2-4. H-Reflex measured how excitable the nerve cells were by applying small electrical shocks behind the knee. Each shock produced a reflex response in the calf muscles (the H-Reflex response); the data collected indicated that this response decreased significantly while in microgravity. The study found that spinal cord excitability decreased by about 35% in weightlessness, and stayed at this new level for the duration of the mission. Although there was notable improvement in the H-Reflex response the day after landing, it took about ten days back on Earth for astronauts to fully recover their muscle strength and spinal cord excitability (Watt and Lefebvre 2001; Watt 2003).
This difference in excitability means that only a portion of muscle fiber units are contracting in response to signals from the nervous system and explains functionally why muscle mass declines in weightlessness, even with exercise. Reduced excitability means that there might be limits on the degree to which heart muscle strength, leg muscle tone, and bone density (for which muscle contraction is an important regulating factor) can be maintained through exercise on long-duration missions. Because this decrease in excitability is only observed on orbit and not during bed rest, an analogue for weightless space travel, the results highlight the possibility that reduced excitability with corresponding loss of muscle and bone might be partly a nervous system response and not simply due to disuse of the legs.
Based on the results of this study, decreased spinal cord excitability could be an issue for long-duration stays in partial-gravity environments such as are found on the moon and Mars. Future designs of exercise equipment that provide feedback on work actually performed would help crewmembers compensate for decreases in exercise efficiency. (Evans et al. 2009)
Watt DG, Lefebvre L. Effects of altered gravity on spinal cord excitability. First Research on the International Space Station. Conference and Exhibit on International Space Station Utilization, Cape Canaveral, FL; 2001
Watt DG. Effects of altered gravity on spinal cord excitability (final results). Bioastronautics Investigators’ Workshop, Galveston, TX; 2003 1 pp.
Ground Based Results Publications
Paquet N, Watt DG, Lefebvre L. Rhythmical eye-head-torso rotation alters fore-aft head stabilization during treadmill locomotion in humans. Journal of Vestibular Research. 2000; 10(1): 41-49.
Rhatigan JL, Robinson JA, Sawin CF. Exploration-related research on ISS: Connecting science results to future mission. 44th Aerospace Sciences Meeting and Exhibit. Reno, NV; 2006 January 9 15 pp.
Watt DG. Pointing at memorized targets during prolonged microgravity. Aviation, Space, and Environmental Medicine. 1997 Feb; 68(2): 99-103. PMID: 9125097.
International Space Station Medical Project (ISSMP)
Life Sciences Data Archive
NASA Fact Sheet
NASA Image: JSC2001E34377 - Expedition 3 Commander Culbertson in the shuttle middeck performing the H-reflex experiment.
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NASA Image: 2785_7180107 - A still frame from a video of astronauts Jim Voss and Susan Helms performing the H-reflex experiment in the shuttle middeck.
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NASA Image: ISS003-330-006 - Cosmonaut Vladimir Dezhurov on ISS Expedition 3 performing H-Reflex activity in the U.S. Lab.
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NASA Image: ISS003-330-011 - Closeup of H-Reflex hardware on the knee of cosmonaut Vladimir Dezhurov during ISS Expedition 3.
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