The knee joint consists of the femur (thigh bone), tibia and fibula 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 (semitendinosus, semimembranosus, and biceps femoris).
The Semimembranosus muscle is the most medial (inside) of the three hamstring muscles and along with the semitendinosus muscle provides dynamic stability to the knee joint.
Vastus Medialis Muscle
Vastus Medialis muscle is the most medially located of the quadriceps muscles and is thought to be important in stabilizing the knee cap or patella.
When running the hamstrings act eccentrically to slow down the knee extension motion. Hamstring strains are common in individuals with chronically tight hamstrings or who do not warm-up thoroughly.
Vastus Intermedius Muscle
Vastus Intermedius is located deep in the Rectus Femoris muscle and is part of the quadriceps muscle group.
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).
Vastus Lateralis Muscle
Vastus Lateralis is the most lateral (outside) of the four quadriceps muscles. It is thought to be a major contributor to patella tracking injuries.
Popliteus muscle is a small muscle that 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.