My Knee at the gym

I had an appointment today at the gym. We did back and bi. Jason asked if I felt I could do any abs. I told him maybe leg raises and sit ups on the ball. He had me do leg raises with a ball held between my legs and sit ups on the ball. After we were done I wanted to do some cardio, I knew I couldn't do bicycle I would bend my knee to much. So I walked/ jogged on the treadmill. I felt just fine I think the doctor was just trying to scare me.