To become an astronaut, you have to be in good shape. And once you arrive in space, your workouts continue.
The lack of gravity causes a loss of bone density and muscle mass, and can easily undo astronauts' hard years of training.
In the video above, Sunita Williams of NASA shows off the machines astronauts use to stay in shape. Residents at the International Space Station orbital lab use a bike, a treadmill and a weight-lifting machine to work out in outer space.
To see more about the ISS visit NASA's website.