My gf and I had a disagreement on this. I have read in several fitness publications that walking is too low impact to have any benefits on the skeletal system, therefore it will not strengthen bones. She thinks it does. This is the only form of exercise she does, and while I applaud her on the fact that she's exercising and keeping her weight down, I don't think she's doing enough to prevent bone loss. I do know that it was widely believed at one time that walking prevented osteoporosis, but haven't recent studies disproved that?