Why Do Skeletal Muscle Fibers Atrophy During Space Flight?
The photos in this slide provide some insight into the important question of why our muscles (and muscle fibers) atrophy during space flight. The photo on the left shows a typical posture of an astronaut in the space module. This individual is considered to be in a state of “free fall”. That is, he is floating around and is essentially weightless. While he can move all of his muscle groups, he does not need to generate much force to float around. The flick of one finger against the wall or floor of the module is all that is needed to propel a person around in this space capsule. In essence, his key muscle groups, especially the lower leg calf muscles, the thigh muscles, and the back/neck muscles, are unloaded. That is, there is no need for these key muscle groups to generate any appreciable force to perform activities and, importantly, to oppose the strong force of gravity that is ever present on Earth.
The astronaut on the right is performing important construction duties outside the space module. This is referred to as extravehicular activity. To perform this task, the astronaut wears a very heavy and cumbersome space suit that would be difficult to wear on Earth because of its weight and restrictiveness on limb and hand movement. Yet the astronaut can tolerate wearing this device for many hours while performing various duties outside of the space capsule.
If astronauts experience this state of unloading, do you think the muscle systems are receiving appropriate stimuli to maintain their structural and functional integrity? What about the motor protein genes that evolved to provide movement diversity?
Edgerton, V. R. & Roy, R. R. (1996). Neuromuscular adaptation to actual and simulated spaceflight. In Handbook of Physiology-Environmental Physiology. Vol 1. Part III. The Gravitational Environment (721-763). Bethesda, MD: Am. Physiol Soc.
Baldwin, K. M., Edgerton, V. R., & Roy, R. R. Muscle Loss in space: physiological consequences. In H. Mark, M. Salkin, & A. Yousef (Eds.), Encyclopedia of Space Sciences and Technology. Vol. 2 (pp. 149–166). Hoboken, NJ: John Wiley & Sons, Inc.
ADDITIONAL NOTES FROM SPEAKER’S TRANSCRIPT (http://www.bioedonline.org/presentations/)
When individuals enter the space environment, say on the space shuttle, they lose the Earth’s force that would be pulling on the body. The individual you see on the left is in the space capsule, in a state of what we call suspension. He is free floating. That individual can sit there for hours and hours and hours without having the muscles or the hands actually pushing against any object. In this state of unloading, if an individual were to move to one side of the capsule by pushing one finger against the capsule wall, that would allow the individual to shoot across to the other side. Individuals can somersault. They can do all kinds of things because the body is essentially unloaded. If we look over to the right side, we see an individual outside the space capsule, wearing a spacesuit that is pressurized, much as if you were to wear scuba gear and dive below the surface in the ocean. But this spacesuit spacesuit weighs about 200 pounds, and that does not feel very heavy to this individual because in this environment, the spacesuit is actually weightless. But it is cumbersome. What you see up at the top of the screen is the umbilical system that attaches the individual to the space shuttle. This individual is performing some routine checks on the space capsule, which we do not see in this particular photo. The other thing I should point out is that that individual is traveling 17,500 miles an hour in this vacuum of weightlessness. So it looks like the astronaut is just sitting there, but in actuality they are moving at this very rapid speed through a vacuum.
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