You all have given so much good knee advice I decided to post my situation for help. I haven't read about this condition anywhere. When skiing for longer periods on consecutive days in crud or something challenging, the back of my left knee hurts (not much the right knee). (The left knee is the weaker one, anyway.) It doesn't seem to bother me much while skiing, but shows up after, and it will hurt behind my knee to straighten my leg out. It sometimes looks a bit puffy back there. I actually can keep skiing the next day or two, tho it's limiting in that I don't want to push it and it hurts more later. It is better after a week or two of rest. The most I can figure is a ligament gets stretched, and/or my knee is hyperextended, but I really don't know. That said, any clue on what's going on, how it happens, how to prevent it, and how to strengthen and ski better so it won't happen?