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Author(s): Joanne R. Lupton, PhD
Showing Results for: astronauts Return to Presentation

Knee Extensor Muscle Size in Response to Different States of Unloading

This slide helps to answer the question raised at the end of the last slide: “What do you think the effects will be on muscle strength if one lives a lifestyle described as a couch potato?” The graph illustrates the relationship between knee extensor muscle size decrements (i.e., the extent that the muscle gets smaller due to the different unloading conditions, and the resulting change in strength- or force-generating capacity, such as needed for lifting a weight). Note the loss in strength as a function of the decrease in muscle size (e.g., muscle atrophy). This important relationship highlights the need to maintain muscle mass, which is critical in assessing the physical fitness of an astronaut to perform a variety of tasks while exposed to varying gravity states.

Do you think performance would be compromised further when living in the space suits, as shown in the figure above? If so, how could the loss of muscle mass and strength be minimized?

Suggested Reading:
Adams, G.R, Caiozzo, V.J., & Baldwin, K.M. (2003). Skeletal muscle unweighting: spaceflight and ground based models. J. Appl. Physiol. (Invited Review) 95: 2185-2201.

Our muscles get smaller under the conditions of unloading. And what are the consequence of this lost muscle size? The graph shows a decreasing size of the knee extensor (thigh) muscles. As those thigh muscles decrease in size, note that the strength of these muscles, as depicted on the Y axis, is now decreasing, as well. So there is a direct relationship: the smaller the muscle, the less strength it can generate. Conversely, bigger individuals with bigger muscles generally are stronger than smaller individuals with smaller muscles. That is why athletes, for example, or football players, do a lot of strength training in their legs and in their arms to build up strength, so that they can be more effective.