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Maintaining Muscle Mass in Space

Author(s): Kenneth M. Baldwin, PhD

Skeletal Muscle and Loading State

This slide helps to define what we know about the stimuli necessary to maintain homeostasis (normal structure and function) of the skeletal muscle system. If individuals do not receive daily exposure to weight bearing conditions (i.e., normal standing and movement), the muscles fibers will get smaller, weaker, and more easily fatigued. One does not have to travel in space to illustrate this problem. This phenomenon can be demonstrated by having individuals spend continuous time in the state of bed rest (the subject is not allowed to get out of bed), which has become a ground-based analogue to spaceflight.

Looking at these issues from a different perspective, if we put young individuals (i.e., pre-adolescent) in space, do you think their muscles would grow appropriately and achieve the normal myosin patterns that we have examined in the previous sections of the presentation?

Suggested Reading:
Gallagher, P., Trappe, S., Harger, M. , Creer, A. ,Mazzetti, S., Trappe, T., Alkner, B., & Tesch, P. (2005). Effects of 84-days of bed rest and resistance training on single muscle fiber myosin heavy chain distribution in human vastus lateralis and soleus muscles. Acta. Physiol. Scand. 185: 61-69.

We have come to understand through a series of research studies that skeletal muscle is very sensitive to loading state. If we want to make the muscles bigger, we need to put high levels of mechanical stress on them. And so we say that the skeletal muscles require almost continuous weight bearing stimuli to maintain normal protein expression—the normal amounts of protein, myosin and actin in the contractile element. If you eliminate the everlasting stress (that we call gravity) on the muscles, strange things happen to them. The muscle enters a state of protein imbalance in which the muscle proteins undergo degradation, the speed of which determined by the rate at which the muscle fibers are synthesizing protein. Keep in mind that every muscle fiber undergoes continuous buildup and degradation of protein. And if your muscles stay in a normal size, the degradation process is equaling the synthesis process. However, if the muscles are unloaded, this process changes and the degradation process exceeds the synthesis process. In the schematic on the lower side of the figure, you can see that over time, the nice bulky muscle gets smaller and weaker, and less effective to support movement under loading conditions, as seen during normal gravity. The slow myosin genes are turned off and the fast myosin genes are turned on.

Funded by the following grant(s)

National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.