Space is a harsh environment that affects the body in many ways. In microgravity, bone loss occurs at a rate of 1 to 1.5 percent a month, leading to an acceleration of age-related changes similar to osteoporosis. Decreases in bone density and strength are more pronounced in some skeletal regions, such as the pelvis, although much of the loss is reversible upon return to Earth. Prolonged exposure to weightlessness also increases the risks of kidney stones and bone fractures, which are both associated with bone demineralization. In addition, studies suggest that microgravity alters the ability of bones to heal after fractures.
Long stays in space also impact muscles. There is loss of muscle mass, strength and endurance, especially in the lower extremities. Changes in muscle performance, coupled with the effects of microgravity on connective tissues and the demands of activities of varying intensities, place astronauts at risk of fatigue and injury.