Effects of weightlessness on muscle and bone

Now we're going to talk about going to outer space! Yay!

Maintaining Muscle and Bone

When you work out, your muscles get stronger. The best type of exercise for this purpose is resistance exercise, which requires you to work against gravity. As I mentioned in a previous post, stretching and movement can activate MAPKs, which are important in growth and repair. Furthermore, exercise causes hormones such as IGF-1 to be released. (IGF-1, or insulin growth factor 1, is the most important hormone for muscle growth). The flipside of this is that if you don't have gravity to work against, these pathways are no longer activated and your muscles get weaker.

Bone mass can be determined by measuring bone mineral density (BMD), and just like muscle mass, it requires a constant load in order to be maintained. I've written about the regular processes involved in maintenance of bone here.

Space

Astronauts in spaceships and so forth are subject to "microgravity," or a feeling of weightlessness. This is because space vessels that orbit the Earth are essentially in constant freefall, giving that feeling of weightlessness. Unfortunately, this feeling of weightlessness means that muscles and bones lose their mass due to the reduced loads.

As already discussed, without the effects of gravity, muscles lose mass and therefore strength and speed as well. The muscles that are most significantly affected by space flight are postural "anti-gravity" muscles. In space, muscle fibres become smaller, and data from rats suggests that type I fibres may change phenotype to type II. Fibres may also be damaged and their contractile proteins moved out of alignment. Another effect of space flight on muscle is that muscles can become more prone to fatigue.

Without gravity, bones also lose mass, predisposing to osteoporosis. Just like with muscles, the bones that are most greatly affected are those that are "load bearing," like the legs and the lumbar spine. There is evidence to show that more calcium is lost in the urine during space flight as compared to before the flight. This suggests that bone loss may be due to excessive resorption by osteoclasts.

Preventing Muscle and Bone Loss

Initially, astronauts were given treadmills and bicycles to work out, but these machines do not provide the right loads required to prevent muscle wasting. Newer contraptions are being developed, such as bungee cords and "penguin suits" to provide resistance with every step the astronaut takes. So far, however, our efforts have been inadequate, so maybe we need to find better forms of resistance exercise in order to prevent muscle wastage. Other possible solutions include dietary supplementation with protein, and drug/hormone supplementation with steroids, IGF and so on. This latter suggestion is unlikely to be effective as there is no evidence to suggest that hormone levels change during space flight.

Bone loss is also a problem in space. If we can find better forms of resistance exercise, this might also help with bone loss. Another possibility are oral bisphosphonates, which inhibit osteoclasts and bone breakdown.

Aaaaand that concludes this section on skeletal muscle! Next up we'll be learning about mechanobiology and cell signalling!