There is almost no data on the effects of low gravity on humans; the great mass of data cited in these cases is all derived from microgravity environments, particularly freefall in orbit. This cannot be extrapolated to Martian gravity.
Bed rest experiments might give some useful data, but short of actually going to Mars, only a rotating environment in Earth Orbit would give an accurate picture of the effects.
The Mars gravity proposal Loglo mentioned is a good way of looking into this
http://www.marsgravity.org/main/mission.html