Decades of research aboard the International Space Station (ISS) and other spacecraft in Low Earth Orbit (LEO) have shown that long-duration stays in microgravity will take a toll on human physiology. Among the most notable effects are muscle atrophy and bone density loss and effects on eyesight, blood flow, and cardiovascular health. However, as research like NASA’s Twin Study showed, the effects extend to organ function, psychological effects, and gene expression. Mitigating these effects is vital for future missions to the Moon, Mars, and other deep-space destinations.
To reduce the impact of microgravity, astronauts aboard the ISS rely on a strict regiment of resistance training, proper diet, and cardiovascular exercise to engage their muscles, bones, and other connective tissues that comprise their musculoskeletal systems. Unfortunately, the machines aboard the ISS are too large and heavy to bring aboard spacecraft for long-duration spaceflights, where space and mass requirements are limited. To address this, NASA is investigating whether exercise regimens that rely on minimal or no equipment could provide adequate physical activity.
For every month in space, astronauts’ weight-bearing bones become roughly 1% less dense if they don’t take precautions to counter this loss, while muscles atrophy due to severely reduced loads. On Earth, these symptoms are associated with the aging process, sedentary lifestyles, and degenerative diseases. This has serious implications for astronaut health since missions to deep space require that astronauts be exposed to microgravity for several months. Upon arrival, they will be expected to conduct surface operations that require them to be hale and hearty. Otherwise, they could suffer serious injuries.
A Long Tradition
For decades, astronauts have used stationary bikes and treadmills to get their exercise. The Soviet Salyut program, which operated between 1971 and 1986, carried out multiple studies on astronaut health. To test possible “countermeasures,” these stations included a treadmill, a gravity simulation suit for long wear, a bicycle with an ergometer, drugs, and an anti-gravity suit to be worn immediately post-flight. Exercise regiments were divided into two one-hour shifts in the morning and afternoon between work cycles.
The Soviet/Russian space station Mir had two treadmills (with bungee cords to anchor the cosmonauts) and a stationary bicycle. Each cosmonaut was required to cycle the equivalent of 10 kilometers (6.2 mi) and run the equivalent of 5 kilometers (3.1 mi) per day. NASA followed a similar regimen, as astronauts aboard Skylab were required to perform 90 minutes of exercise a day using equipment that included a stationary bicycle and treadmill-like device, and astronauts found that they could jog around the water tank.
After the ISS became operational in 2001, one of the first exercise systems delivered was the Treadmill with Vibration Isolation Stabilization System (TVIS), which uses a harness to keep users tethered to the machine while adding extra resistance. There’s also the Cycle Ergometer with Vibration Isolation and Stabilization System (CERVIS), an exercise bike contributed by Danish Aerospace. Astronauts also have the Advanced Resistive Exercise Device (ARED) that uses vacuum cylinders and pistons to create resistance, letting astronauts simulate weightlifting in microgravity.
While medical science understands the broad causes of atrophy, researchers continue to investigate the fundamental mechanisms and contributing factors to look for solutions to microgravity-induced atrophy. Much of this research is focused on determining the right combination of diet, exercise, and medication to keep astronauts healthy in space and during missions on the Moon or Mars and to assist with the transition when they return to Earth. For example, the Zero T2 experiment involves astronauts not using the treadmill and focusing instead on aerobic and resistance exercises.
Once the experiment is complete, research teams will compare the participants’ muscle performance and recovery to those of their crewmates who used the treadmill. Another experiment, VR for Exercise, aims to create an immersive virtual reality environment astronauts can enjoy while using the station’s exercise bike. There’s also research that involves “tissue chips,” which are small devices that imitate complex functions of specific tissues and organs.
One such experiment, Human Muscle-on-Chip, used a 3D model of muscle fibers created from muscle cells taken from younger and older adults. The experiment consisted of administering electrical pulses to the tissues to make them contract while looking for changes in function attributed to microgravity. The researchers found that for muscle cells exposed to microgravity, there was decreased expression of genes related to muscle growth and metabolism related to age.
In addition to testing different exercise regimes, researchers are also studying how the entire musculoskeletal system experiences exercise in microgravity. This is the purpose of the ARED Kinematics human physiology experiment supported by the Italian Space Agency (ASI) and the ESA. This system aims to quantify the joint torque, muscle forces, and bone stresses that occur during exercise in microgravity, as well as the adaptations in performance that may occur over time.
Addressing bone density loss, there’s the Vertebral Strength experiment, where detailed scans were taken of astronauts before and after they went to space. These scans examined the bones and muscles supporting the vertebral column, providing researchers with information about how spaceflight affects overall musculoskeletal strength. This and other research into bone density loss and musculoskeletal health overlap with research into osteoporosis here on Earth and could lead to mutually beneficial applications.
Similarly, drugs that fall into the class of myostatin inhibitors have a proven track record on Earth in the treatment of osteoporosis. These drugs suppress myostatin, a human growth factor that prevents excessive muscle growth, which helps reduce bone density loss and prevent fractures in patients. The Rodent Research 19 (RR-19) experiment recently tested this drug on a group of mice during spaceflight, which indicated that the drug could be an effective treatment for astronauts and people with degenerative diseases here on Earth.
Of course, no research into the effects of microgravity on human health would be complete without considering the psychological effects that long periods spent in space can have. This is the purpose of the Complement of Integrated Protocols for Human Exploration Research (CIPHER), an integrated experiment that arrived on the ISS earlier this year. For this experiment, astronauts will participate in 14 studies sponsored by NASA and international partners that will measure the physiological and psychological changes in crew members on missions lasting for a few weeks, 3.5 to 8 months, or up to one year in space.
These research studies will monitor the health of astronauts before, during, and after their missions. By conducting the same research over missions of different durations, scientists can extrapolate the results for multi-year missions – such as a three-year round trip to Mars. As CIPHER project scientist Cherie Oubre explained:
“CIPHER is the first study to integrate multiple physiological and psychological measures, giving us a chance to assess the whole human response to time spent in space. As more astronauts head to space through Artemis and other programs, we hope to learn more about how the various systems of the body, such as the heart, muscles, bones, and eyes, adapt to long-term spaceflight.”
Understanding the effects of prolonged exposure to microgravity and developing countermeasures is particularly important as NASA plans future missions that will take astronauts far beyond LEO. For long-duration missions on the Moon, Mars, and beyond, astronauts will spend extended periods in microgravity or low gravity. Once they arrive, they may need to perform strenuous activity and be in optimal health. Possible solutions currently under study include exercise, diet, and drugs and simulating gravity using rotating modules.
In all likelihood, astronauts bound for Mars or other deep-space destinations in the future will be relying on an “all of the above” approach.
Further Reading: NASA
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