I posted this on the H&W thread but thought you PE people would know something about it too: I have had the cartilage done in one knee and the other is on the way out. I've got osteo-arthritis behind both kneecaps. Taking weight upon my bent knees e.g. squats and lunges is painful to the point of undoable beyond a certain angle. I can do it a bit. I don't go any further than my bearable limit and I avoid twisting because of the cartilage problem. My knees hurt like hell after an exercise class or gym session and ache in bed. It gets better as the next day goes on. The next time I do a class it hurts all over again and gets better all over again. Do you think this is doing them any actual damage? It's certainly not getting easier with practice so I don't think it's just post-exercise ache (the arms and shoulders are fine now).