The knee joint consists of the femur (thigh bone), tibia and fiblua bones of the lower leg and the patella or kneecap. The muscles which flex and extend (bend and straighten) the joint are the quadriceps muscles (rectus femoris, vastus lateralis, vastus medialis) and the hamstring muscles at the back of the thigh (semitendinosis, semimembranosus and semitendinosis).
Biceps Femoris is one of the three muscles which form the hamstring group. The muscle is often described as having a long head (the attachment from the ischium) and a short head (attached to the femur).
Popliteus muscle is a small muscle which is often described as the key of the knee joint. It unlocks the knee joint by rotating the femur at the beginning of knee flexion to allow full knee flexion to occur.