(Above) A 1969 station concept. The station was to rotate on its central axis to produce artificial gravity. The majority of early space station concepts created artificial gravity one way or another in order to simulate a more natural or familiar environment for the health of the astronauts. This station was to be assembled on-orbit from spent Apollo program stages.
After returning from a micro-gravity environment, astronauts find their muscles weak because they have not been using them. Long-term exposure to micro-gravity could generate long-term health problems for astronauts who do not utilize their muscles. This is why there are exercise machines on the Shuttle and the International Space Station.