For astronauts in space, exercise is not a choice. In order to prevent the loss of muscle mass and bone density due to a lack of gravity, working out is a necessity. NASA even mandates that astronauts aboard the International Space Station exercise two hours per day.

But how exactly do astronauts work up a sweat in microgravity? For starters, the equipment they use barely resembles what you'll find at the neighborhood gym.

Below, take a look at five bizarre contraptions that help keep astronauts in tip-top shape. Your treadmill has never looked so mundane.

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PHOTO GALLERY The International Space Station