When I was in high school, back at the dawn of time, we had a football coach who had a trick knee. He would have the team doing wind sprints and he would join in with the kids. Every few weeks or so, his trick knee would give out and he would fall headlong to the grass. He was fine in a short time. I never did learn what a trick knee is. Hang on - I just googled 'trick knee'. http://en.wikipedia.org/wiki/Luxating_patella