Among the many functions performed by skeletal muscles, an important one is maintaining our posture. If it weren’t for these muscles, Earth’s gravitational pull could prevent us from standing and walking. The group of muscles – mainly found in our limbs, back and neck – which are responsible for maintaining our posture and allowing us to move against the force of gravity are aptly called the “anti-gravity” muscles.
But what happens to these muscles when there is no gravity (or gravitational “unloading”) against which they can work? The question may seem ridiculous to some, but not to an astronaut aboard the International Space Station (ISS)! In outer space, where gravity is minimal, our muscles (especially anti-gravity ones) are not stressed as much, which can lead to their atrophy and changes in their structure and properties. In fact, human calf muscles are known to shrink during space flight.
So how can astronauts avoid these neuromuscular problems?
A team of Japanese researchers led by Dr Yoshinobu Ohira of Doshisha University, Japan set out to find the answer. The team also included Dr. Takashi Ohira, who works with Doshisha University and Kindai University, Japan; Dr. Fuminori Kawano, associated with Doshisha University and Matsumoto University, Japan; Dr. Katsumasa Goto, who works with Doshisha University and Toyohashi SOZO University, Japan; and Dr. Hiroshi Kaji of Kindai University. They were recently able to study the responses of neuromuscular properties to gravitational discharge and share research-based information on how astronauts can avoid neuromuscular problems during extended spaceflight. This review, online March 10, 2022 and published in volume 136 of Neuroscience and Biobehavioral Reviews in May 2022 – was written in response to an invitation asking authors to contribute to a special issue. This issue, titled “Space Neurosciences”, was intended to commemorate the first human landing on the Moon, as part of NASA’s Apollo 11 lunar mission.
The team examined how the morphological, functional and metabolic properties of the neuromuscular system respond to reduced anti-gravitational activities. They first looked at human and rodent simulation models and also saw how the activity of afferent and efferent motor neurons regulate neuromuscular properties. Their review suggests that afferent neural activity (which involves signals sent from skeletal muscle to the central nervous system during muscle activity) plays a key role in regulating muscle properties and brain activity.
Inhibition of anti-gravitational muscle activities leads to remodeling of sarcomeres (which are the structural unit of muscles), leading to a decrease in their number, further leading to a decrease in strength development ultimately leading to muscle atrophy. A reduction in the amplitude of the electromyograms in the anti-gravitational muscles, namely the soleus and the long adductor, is also observed. This indicates that exposure to low gravity environments affects not only muscles, but also nerves.
Gravitational discharge results in deterioration of motor control, seen as impaired coordination of antagonistic muscles and impaired mechanics. Difficulty walking has also been observed in crews after spaceflight, despite exercising regularly on the ISS. Astronauts aboard the ISS must use treadmills, cycle ergometers and resistance training equipment to counter the effect of reduced gravity on the neuromuscular system and protect their physical health. However, these exercise-based countermeasures are not always effective in preventing certain undesirable neuromuscular changes.
Additional challenges can arise when astronauts are exposed to a microgravity environment for six months or more; for example, en route to or from the planet Mars. This review therefore has major implications in the field of space research, with particular emphasis on the well-being of astronauts (whose recommendations are mentioned by the authors).
Changes in muscle properties due to gravitational discharge may be related to decreased neuronal activity, as well as contraction- and/or stretch-dependent mechanical stress. Adequate stimulation of the soleus muscle appears to reduce the risk of atrophy. So, during the exercise, astronauts should walk or run slowly with a back-strike landing (using a bungee cord would also help). Periodic passive stretching of the soleus also appears to be effective. Thus, information from a unique perspective, as discussed in this review, may play an important role in the development of appropriate countermeasures against neuromuscular problems for future long-duration human space exploration missions. ISS astronauts will thank the research team for sharing this important information. In the meantime, we wish the researchers good luck for their next mission!
About Dr. Yoshinobu Ohira from Doshisha University, Japan
Dr. Yoshinobu Ohira is currently a visiting professor at the Research Center for Space and Medical Sciences and the Organization for Research and Development Initiatives at Doshisha University. His research interests focus on muscle regeneration, neuromuscular adaptation and the spatial environment. He works mainly in the field of life sciences and sports sciences. Dr. Ohira has conducted original research at several reputable institutes around the world. He has more than 400 publications and 3 patents to his credit. Dr. Ohira has also won several prestigious awards, including the NASA Cosmos Biosatellite Group Achievement Award.
This study was supported, in part, by Doshisha University’s Space-DREAM Project at YO and Japan Society for the Promotion of Science (JSPS) KAKENHI, grant numbers JP19K07291 to YO and JP21K21000 to TO. Funders have no role in any aspect of this manuscript.