The hamstring muscles are a group of 4 posterior thigh muscles. The muscles are semitendinosus, semimembranosus, and the biceps femoris, which are considered two muscles due to their two distinct boney attachments. All muscles attach proximally to the ischial tuberosity except for the ‘short head’ of the bicep femoris, which attach to the lateral femur bone. The semitendinosus & semimembranosus muscles have a distal attachment to the superior medial tibia (bottom of the inner knee area) and the biceps femoris muscles attach to the head of the fibula (bottom of the outer knee area).
All the hamstring muscles are innervated by the sciatic nerve and are considered to be hip extensors when walking (the gluteus medius laterally stabilize) and when contracted they extend the leg, flex the knee and laterally rotate the thigh. Posturally, it is interesting to note that when the thigh and leg are fixed, the hamstrings assist the trunk in extension at the hip joint.