Comment by taconacho

The symptoms match health effects caused by a sedentary lifestyle: https://en.wikipedia.org/wiki/Sedentary_lifestyle#Health_eff... . In a gravitational environment, just moving around is a form of exercise since one has to move about the entire weight of one's body. In space, where everything is weightless, it takes much less effort to move around and you see the same effects (even with rigorous exercise routines). Imagine exercising for an hour or two and then sitting down for the other 22 hours. You'd end up with similar health effects.