After a long space flight, a young and fit astronaut's bones and muscles might be weaker than your grandmother's. Muscles get smaller and weaker in space. In a gravity-free environment, bones stop producing proteins that tell the bones to replace old, worn-out bone cells. So astronauts' bones become less dense and more rickety, and they suffer osteoporosis-like effects. Astronauts can lose at least 10 times as much bone density per year than a person on Earth [source: ACSM]. Once back on Earth, astronauts don't replace bone quickly, and they're at greater risk for fractures.
Workouts help maintain bone and muscular strength and fill the astronauts' schedules -- as many as four hours out of a 16-hour workday. The astronauts aren't doing Pilates, though. To keep up their muscle and bone strength, astronauts turn to the gym classics: treadmills, bikes and weights -- with a twist. They're designed with weightlessness in mind -- the astronauts need to be able get a good workout without worrying about the gym equipment floating away and smacking other astronauts in the head. On the International Space Station, for instance, the treadmill isn't attached to the station -- it floats in air just like the astronauts do -- to keep the craft from shaking as astronauts jog. Exercise bikes are bolted to the station floor, and the astronauts strap their shoes to the pedals and buckle up as they ride. And the weights aren't dumbbells but resistance machines that mimic gravity. In space, it takes ingenuity to stay in shape.
Many astronauts aboard the International Space Station have made staying fit part of their personal goals and have tried to promote fitness back on Earth as well. In 2007, astronaut Sunita Williams ran the Boston marathon on a treadmill. It took her four hours and 24 minutes to run the race at the same time that competitors ran through the streets of Boston [source: Guardian]. Maintaining physical fitness also is important to mental and social well-being of astronauts, who can be confined in small spaces for long durations. Research into the effects of isolation and confinement on neurological and cognitive processes may help improve future health, safety and mission performance [source: ESA].
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