Astronaut fitness in orbit. Astronaut Blaine Hammond exercising on a rowing machine on the mid- deck of Shuttle Discovery during Mission STS-64. Exercise is vital to astronauts during the longer Shuttle flights, as extended periods in microgravity ('weightlessness') can lead to loss of muscle tone and bone mass. This is particularly true of the legs, so rowing machines and treadmills are normally provided. Mission STS-64 flew on 9-20 September 1994.
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