It’s well-known that spending long periods in microgravity can adversely affect astronaut health and physiology. According to decades of research performed aboard the International Space Station (ISS), like NASA’s much-popularized Twins Study, these effects include the loss of muscle mass and bone density, as well as changes to cardiovascular health, eyesight, organ function, and gene expression. There’s even the possibility that astronauts will experience mood swings and psychological problems while in space or during recovery here on Earth.

According to a recent study by a team of Japanese researchers, one of the lesser-studied effects is how long periods spent in microgravity can damage the skeletal muscles that are important to maintaining our posture. This group of muscles – located mostly in our limbs, back, and neck – are rightly known as our “anti-gravity” muscles because they are load-bearing and allow us to stand upright and move against the force of gravity. This research and the countermeasures they propose could have significant implications for astronauts returning from long-term stays in space.

The research was led by Dr. Yoshinobu Ohira, a Professor of space medicine at Doshisha University’s Research Center for Space and Medical Sciences (RCSMC) in Kyoto, Japan. He was joined by fellow researchers with the RCSMC, the Organization for Research Initiatives and Development (ORID) at Doshisha University, the Kindai University Faculty of Medicine (Osaka), Matsumoto University (Nagano), and Toyohashi SOZO University (Aichi). The paper that describes their findings recently appeared in the journal Neuroscience & Biobehavioral Reviews.

Expedition 38/39 astronaut Koichi Wakata (JAXA) uses the advanced Resistive Exercise Device (aRED) in the Tranquility module of the International Space Station in February 2014. Credit: NASA

As they indicated in their study, the team reviewed the neuromuscular properties of the soleus and adductor longus muscles (and their respective dorsal ganglia) in astronauts who had spent extended periods in space. These muscles are located in the calf and inner thigh (respectively) and are responsible for load-bearing and helping us remain upright in normal gravity. When subjected to microgravity, these muscles are “unloaded” and have nothing to work against, leading to the gradual atrophy of their fibers and nerves.

Their study considered how the neuromuscular system’s morphological, functional, and metabolic properties respond to gravitational unloading in microgravity and lower gravity environments. This consisted of running human and rodent simulation models and examining how motoneuron signals between the skeletal muscle and central nervous system (afferent and efferent activity, respectively) regulated neuromuscular properties. Their research appeared in a special issue titled “Space Neurosciences” that commemorates the Moon Landing (NASA’s Apollo 11 mission).

Their analysis confirmed that these signals play a key role in regulating muscle properties and brain activity. This consists of a decrease in the structural unit of muscles (sarcomeres), resulting in a decrease in their development, eventually leading to muscular atrophy – as seen in the soleus and the adductor longus muscles. Their results indicate that exposure to low-gravity environments affects the muscles and nerves, leading to the deterioration of motor control.

This is consistent with symptoms reported by astronauts upon returning to Earth, where they experienced difficulty walking despite regular exercise aboard the ISS – using treadmills, cycles, and resistance training. Their work also showed that additional challenges might arise when astronauts are exposed to microgravity for six months or more, such as they would be while in transit to Mars. In summary, their review indicates that existing countermeasures are inadequate when combatting the effects of gravitational unloading.