Why do astronauts exercise so much?

In weightlessness, no one carries their own weight anymore. The muscles of the legs, trunk and back barely receive any stimulus, and the load-bearing bones of the hips and spine lose mineral mass, in a way similar to several weeks of bed rest, but faster. The circulatory system also readjusts: the heart no longer has to pump blood upward against gravity, fluid shifts toward the chest and head, and blood volume decreases. Without countermeasures, an astronaut could barely stand up on her own after half a year in orbit.

Weights are useless up there, because nothing has weight. The station makes do with substitute forces: a treadmill holds the crew down against the surface with harnesses, a cycle ergometer demands effort from the heart and circulation, and a resistance device generates resistance through vacuum cylinders and flywheels. With them, squats, deadlifts or bench presses can be reproduced — precisely the exercises that work the large muscle groups and the bones of the legs, pelvis and spine.

Exercise appears in the daily schedule as just another experiment or repair job, with its own setup and teardown, pulse and performance measurement, and medical evaluation on the ground. The programs are tailored to each individual, because every body reacts differently. The data is also research: what slows bone loss in orbit is also of interest to osteoporosis medicine and rehabilitation after long bed rest. Despite all the training, a recovery phase follows landing until the sense of balance, circulation and muscles adapt again to Earth's gravity.