I am absolutely, completely FOR exercise. I'm a bicycler and a swimmer (in the summertime). Bicycling is very painful for my bad knee, but overall it's one of the least damaging types of exercise one with bad knees can do. Keeping the weight off is definitely good for the knees. So even if it weren't something that I loved for its own sake, I would exercise just to try to keep a little musculoskeletal health.

dig