2. Muscle Atrophy
In microgravity, the muscles that normally resist gravity—like those in the legs and back—are barely used. This leads to muscle atrophy, where muscle fibers weaken and shrink over time. Astronauts on extended missions can lose up to 20% of their muscle mass, which can impact strength and mobility upon return to Earth. To counteract this, astronauts must follow rigorous daily exercise routines to maintain muscle health and function. [Source: Scientific American]