Physiological effects in space

Even before humans began to venture into space, serious and reasonable concerns were expressed about exposure of humans to the microgravity of space due to the potential systemic effects on terrestrially evolved life-forms adapted to Earth gravity. Unloading of skeletal muscle, both on Earth via bed-rest experiments and during spaceflight, result in remodeling of muscle (atrophic response). As a result, decrements occur in skeletal-muscle strength, fatigue resistance, motor performance, and connective-tissue integrity. In addition, weightlessness causes cardiopulmonary and vascular changes, including a significant decrease in red blood cell mass, that affect skeletal muscle function. Normal adaptive response to the microgravity environment 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 upon return to Earth.

In the US 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. Collective knowledge gained from these missions has aided in the evolution of exercise hardware and protocols designed to minimize muscle atrophy and the concomitant deficits in skeletal muscle function. 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.