Abstract
Long-term manned spaceflight requires that flight crews be exposed to extended periods of unweighting of antigravity skeletal muscles. This exposure will result in adaptations in these muscles that have the potential to debilitate crew members on return to increased gravity environments. Therefore, the development of countermeasures to prevent these unwanted adaptations is an important requirement. The limited access to microgravity environments for the purpose of studying muscle adaptation and evaluating countermeasure programs has necessitated the use of ground-based models to conduct both basic and applied muscle physiology research. In this review, the published results from ground-based models of muscle unweighting are presented and compared with the results from related spaceflight research. The models of skeletal muscle unweighting with a sufficient body of literature included bed rest, cast immobilization, and unilateral lower limb suspension. Comparisons of changes in muscle strength and size between these models in the context of the limited results available from spaceflight suggest that each model may be useful for the investigation of certain aspects of the skeletal muscle unweighting that occur in microgravity.
Highlights
The results reported by LeBlanc et al [60] were obtained by using MRI, suggesting that methodological differences could account for these divergent reports
It should be noted that flight crews on various missions have participated in widely varying types and amounts of physical activity, including exercise intended as a countermeasure to muscle loss and general deconditioning
As a result of the high costs of conducting in-flight research during the space shuttle era, relatively few missions have had a substantial component devoted to human physiological measurements focused on skeletal muscle
Summary
In general, the results from bed rest studies of shorter duration indicate that, across both muscle and fiber type, the impact of unweighting on fiber size appears to be similar in both ST and FT myofibers (Fig. 4) [6, 10, 25, 40, 99, 103]. These findings suggest that the mechanisms that underlie bed rest-induced alterations in skeletal muscle size and performance are most likely similar to those that mediate the spaceflight response.
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