I have knee problems, warn away cartilage or tendentious or something from playing sports in school. When I do squats and plies, my bones feeling like they are grinding together or are going to pop. If I take a couple mile walk, I am hobbling like an old man by the time I get home. I really want to be able to work out my legs muscles, especially my thighs, but the exercises I know do more bad than good. Any suggestions?