They need time to get used to the Earth's gravity after spending long periods in weightlessness.
It's one of the challenges that needs to be overcome if people were to visit/colonise Mars due to the long travel time.
Attach a spinning 1g environment module to the space station in the future.Maybe they can periodically visit orbiting exercise stations that generate one standard G via centripetal force.
Maybe they can periodically visit orbiting exercise stations that generate one standard G via centripetal force.