A torn ligament behind the kneecap is one of the most common sports injuries. Athletes whose sports involve pivoting of the knee -- such as tennis and basketball -- are especially prone to this injury. Some athletes unknowingly increase their risk of this injury by training their quadriceps (muscles in front of the thigh) more than the hamstrings (back of the thigh). To help protect the knees, it's important to include exercises for both quads and hamstrings in a strength-training program. (i.e. leg extensions for quads and leg curls for hamstrings)