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There is a growing research database which suggests that skeletal muscles, particularly postural muscles of the lower limb, undergo atrophy and structural and metabolic alterations during space flight. However, the relationships between in-flight exercise, muscle changes and performance are not well understood. Efforts should be made to try to understand the current status of in-flight and post-flight exercise performance capacity and what the goals/target areas for protection are with the current in flight exercise program. ==Introduction== From the very beginning of the U.S. human space program, serious and reasonable concern has been expressed about exposure of humans to the microgravity of space due to the potential systemic effects on terrestrially-evolved life forms that are so suitably adapted to Earth gravity. Humans in the microgravity environment of space, within our current space vehicles, are exposed to various mission-specific periods of skeletal muscle unloading (unweighting). Unloading of skeletal muscle, both on Earth and during spaceflight, results in remodeling of muscle (atrophic response) as an adaptation to the reduced loads placed upon it. As a result, decrements occur in skeletal muscle strength, fatigue resistance, motor performance, and connective tissue integrity. In addition, there are cardiopulmonary and vascular changes, including a significant decrease in red blood cell mass, that have an impact on skeletal muscle function. This normal adaptive response to the microgravity environment is, for the most part, of little consequence within the space vehicle ''per se'', but may become a liability resulting in increased risk of an inability or decreased efficiency in crewmember performance of physically demanding tasks during extravehicular activity (EVA) or abrupt transitions to environments of increased gravity (return to Earth, landing on the surface of another planetary body). In the U.S. human space program, the only in-flight countermeasure to skeletal muscle functional deficits that has been utilized thus far is physical exercise. In-flight exercise hardware and protocols have varied from mission to mission, somewhat dependent on mission duration and the volume of the spacecraft available for performing countermeasures. Collective knowledge gained from these mission has aided in the evolution of exercise hardware and protocols in attempts to refine the approach to prevention of spaceflight-induced muscle atrophy and the concomitant deficits in skeletal muscle function. Long duration missions and exploration missions with several transitions between gravitational environments present the greatest challenges to risk mitigation and to development of countermeasures of proven efficacy. Russian scientists have utilized a variety of exercise hardware and in-flight exercise protocols during long-duration spaceflight (up to and beyond one year) aboard the Mir space station. On the International Space Station (ISS), a combination of resistive and aerobic exercise has been used. Outcomes have been acceptable according to current expectations for crewmember performance on return to Earth. However, for missions to the Moon, establishment of a lunar base, and interplanetary travel to Mars, the functional requirements for human performance during each specific phase of these missions have not been sufficiently defined to determine whether currently developed countermeasures are adequate to meet physical performance requirements. Access to human crewmembers during both short- and long-duration mission for the study of skeletal muscle adaptation to microgravity and the efficacy of countermeasures has been, and continues to be, limited. Consequently, a more complete understanding of physiologic models for conduct of both fundamental and applied skeletal muscle research. Various models for which sufficient data have been collected have been concisely reviewed. Such models include horizontal or head-down bed rest, dry immersion bed rest, limb immobilization, and unilateral lower-limb suspension. While none of these ground-based analogs provides a perfect simulation of human microgravity exposure during spaceflight, each is useful for study of particular aspects of muscle unloading as well as for investigation of sensorimotor alterations. Due to limitations in the number of spaceflights and crewmembers in which novel countermeasures can be tested, future development, evaluation and validation of new countermeasures to the effects of skeletal muscle unloading will likely employ variations of these same basic ground-based models. Prospective countermeasures may include pharmacologic and/or dietary interventions, innovative exercise hardware providing improved loading modalities, locomotor training devices, passive exercise devices, and artificial gravity either as an integral component of the spacecraft or as a discrete device contained within it. With respect to the latter, the hemodynamic and metabolic responses to increased loading provided by a human-powered centrifuge have been described recently. Even more recently, an approach to provide both aerobic and resistive exercise by incorporating a cage-like platform into the design has been developed by the same investigator group. Animal studies, conducted both during spaceflight and in ground-based simulations of the skeletal muscle unloading associated with spaceflight, have contributed to the scientific knowledge base in a manner not totally achievable by means of human spaceflight and ground-based analog studies alone. This is because many of the variables present with human subject investigations can be more tightly controlled in animal studies, and the much larger number of animals typical of such experiments contributes to a greater statistical power to detect differences. A major advantage in use of rodent models is that the adaptive changes to both spaceflight and hind-limb suspension occur in a much shorter time frame than they do in humans (hours to days versus days to weeks). This enables prediction of long-term changes in human skeletal muscle based on the shorter absolute time frame of the rodent investigations. Additionally, it is possible to perform a highly controlled, straightforward experiment in rodents without a requirement to provide some type of countermeasure intervention that introduces a confounding variable. In human studies, it is not possible on ethical grounds to withhold countermeasures known to have some degree of effectiveness to provide a population of true control subjects, in which only the effects of spaceflight are seen, for comparison to subjects utilizing countermeasures modalities. Animal studies do not suffer from such restrictions. Further work is needed to provide a better understanding of the problem, which will allow novel approaches to countering loss of skeletal muscle function associated with spaceflight in humans. Relevant animal spaceflight studies, as well as investigations using muscle unloading paradigms that contribute to our current knowledge base, are presented. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Reduced muscle mass, strength and performance in space」の詳細全文を読む スポンサード リンク
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