Structural and functional adaptations of skeletal muscle to weightlessness.

This brief review examines the effects of weightlessness on the structure and function of human and rat skeletal muscle. Data collected from spaceflights or Earth-based models suggest that a rapid atrophy (5-8 days) occurs in lower limb muscles associated with impairments in muscle strength, physical work capacity and locomotor coordination. The reduction in muscle cross-sectional area cannot entirely explain the loss of strength in postural muscles suggesting a reduced neural drive. Muscle atrophy is accompanied by a general shift in the contractile and enzymatic profiles of a slow-twitch oxidative muscle toward that of a fast twitch glycolytic muscle. In humans, limited structural and functional data collected in astronauts after short periods of microgravity are qualitatively similar to those observed in rats after real or simulated microgravity suggesting that animal models are relevant to muscle research in space. A preventive prescription i.e. exercise or pharmacological treatment, cannot be proposed until future research has better defined the basic mechanisms of muscle plasticity during long duration spaceflights.