For the past few days I have been having sharp pains in my left knee. It started on Saturday so I rested it on till Tuesday. I went to have a hit with my friends when i started to have the pain again. Its a sharp pain on the top of my knee and it only hurts when it is bend and has weight on it its perfectly fine when I walk but when I go down my stairs it starts to hurt. What could be the problem. I have in decent shape I run and play tennis a few hours most every day. I did tae kwn do as a kid so I think my knee's are in fairly good shape too.