Space shuttle exercise. Astronaut Robert D. Cabana (mission commander) working out using an exercise bike on the Space Shuttle Endeavour while in Earth orbit. An extensive daily exercise regime is followed by astronauts to prevent muscle atrophy and the loss of bone tissue that can occur in the microgravity environment of space. Cabana was in space for over 280 hours during this mission. Photographed on 8 December 1998, during mission STS-88 to start building the International Space Station (ISS).