what are the muscles located behind the knee and what would make them hurt
The popliteus is a small muscle that runs behind the knee. Due to its small size compared to the hamstrings above and the calf below, it is often overlooked in knee and lower leg injuries. It also isn't a very strong muscle, which adds to its relative obscurity. It is, however, of paramount importance to athletes. The popliteus has three basic motions.
In running this would occur when the foot of that leg is in the air, powering toward the next foot strike. Shortness of this muscle would lead to an improper foot plant and excess pronation. Injury occurs in two ways: Any acute sprain/strain to the knee will involve the popliteus muscle.