Ever since the early 1960’s from the Gemini space missions, the loss of skeletal muscle mass in response to microgravity exposure has been a major physiological concern. As I mentioned in my last post, the absence of gravity – or “loading” as is more routinely referred to – causes the muscles that are responsible for keeping us upright to lose some of their mass. This is termed muscle atrophy and is primarily because they have nothing to interact with; nothing to work against. Every day, our leg, hip and back muscles save us from toppling over when assuming the standing position. In space, there is no such thing as the standing position, because there is no “floor” or anything to stand against.
The calf muscle is believed to be one of the most affected muscles by microgravity unloading due to its postural role in a gravitational environment. Decreases in quadriceps (the big group of muscles at the front of your thigh) volume of 6% have been reported as early as 8 days spaceflight respectively. Longer durations of 90-120 day missions have shown atrophy in quadriceps and calf muscle volume as high as 30%. Muscle mass is a product of cross sectional area (CSA), and fibre type and number; any alterations in these could consequentially affect muscle mass. Changes in both CSA and fibre type (more so than number) have been reported with prolonged microgravity exposure.
So what causes muscles to become smaller in space? Let’s start by explaining how muscles maintain their size normally. Muscle mass is maintained by “protein turnover”, which is the balance between protein synthesis and protein breakdown. Quite simply, protein synthesis is the process by which muscles grow, and protein breakdown is self-explanatory (the opposite). More synthesis than breakdown would indicate growth of lean muscle tissue (also known as an anabolic state) whereas more breakdown than synthesis would indicate a burning of lean muscle tissue (also known as a catabolic state). Data from the last 20 years of spaceflight has suggested that the loss of muscle mass observed in microgravity is the result of a disruption between this protein synthesis and breakdown. Collectively these data suggest that the primary mechanism for muscle atrophy is a decline in protein synthesis, rather than an increase in protein breakdown.
Is it only muscle mass that suffers? Unfortunately not; the function i.e. strength of these muscles also reduce, which is typically a consequence of the atrophy. Having smaller and weaker muscles is not ideal!
Let me make something clear though. Astronauts are not sick; these changes are simply a result of an environment that demands less from them than on Earth. If they were to stay in space, it wouldn’t really be a problem. It’s only when they are faced with the “loaded” environment of Earth that these muscles have to be in tip top condition again in order to work against it. Having said that, if the astronauts had to make an emergency escape, or were faced with any other scenario that required these muscles to work harder than what they had been used to, they may very well struggle. Also, if we wish to send humans to Mars, which involves a 6-9 month weightless journey, and upon arrival they cannot manoeuver properly due to muscle atrophy, we’re in trouble!
Thankfully, there are multiple countermeasures– an action taken to reduce the chances of something bad happening – on board the space station that help to prevent these muscle dysfunctions. I’ll be writing an entire blog piece on countermeasures in due course, so keep your eyes peeled for that one!
Written by Julia Attias
Be sure to check out more of Julia’s work at GlamSci! Promoting STEM subjects for young girls and disadvantaged youngsters.