All NASA astronauts face weightlessness in space. The long-term effect of prolonged time spent in a gravity-free environment is that it weakens the body. Space flights cause muscles to atrophy and bones to decrease in density. To prevent that, or at least limit the damage, NASA has athletic trainers and physical rehabilitation technicians on staff to get astronauts in shape for space flights. Trainers also teach astronauts how to keep their bodies in ideal shape during the flight and then work with them to restore their physical condition once they've returned to Earth.
Trainers have developed various exercise and strength training regimens for each stage of the mission. They focus predominantly on multiple-joint, weight-bearing and lower-body exercises to address the parts of the body that are typically most weakened by the lack of gravity. Muscle atrophy and bone density loss is most likely to occur in the lower body, which is used less than the arms and torso in space due to weightlessness. Activities like standing and walking give our muscles and bones a constant workout on Earth; but in space, they hardly get used at all.
NASA research and development of training programs and machinery to prepare, maintain and rehabilitate astronauts has contributed substantially to the general body of knowledge on physiology (exercise science). It has also resulted in commercially available, NASA-inspired exercise and physical therapy equipment.
Studies conducted by NASA on the effects of space travel on the body -- studies that began in the early days of the space program, before anyone had even left the Earth yet -- demonstrated that prolonged exposure to the low-gravity environment of space led to a decreased amount of calcium in bones, misshapen blood cells and an extra layer of fat around the heart (related to the decreased level of physical activity). NASA doctors conducted post-flight examinations of astronauts, all of whom experienced bone and muscle density loss. The agency decided that it would have to develop exercise programs and equipment specific to the particular needs of astronauts.