The semimembranosus muscle is flattened and broad. Innervation: Tibial part of the sciatic nerve.Medially rotates the thigh at the hip joint and the leg at the knee joint. Actions: Flexion of the leg at the knee joint.Attachments: It originates from the ischial tuberosity of the pelvis, and attaches to the medial surface of the tibia.It lies medially to the biceps femoris, and covers the majority of the semimembranosus. The semitendinosus is a largely tendinous muscle. Innervation: Long head innervated by the tibial part of the sciatic nerve, whereas the short head is innervated by the common fibular part of the sciatic nerve.It also extends the thigh at the hip, and laterally rotates at the hip and knee. Actions: Main action is flexion at the knee.Together, the heads form a tendon, which inserts into the head of the fibula. The short head originates from the linea aspera on posterior surface of the femur. Attachments: The long head originates from the ischial tuberosity of the pelvis.It is the most lateral of the muscles in the posterior thigh – the common tendon of the two heads can be felt laterally at the posterior knee. Like the biceps brachii in the arm, the biceps femoris muscle has two heads – a long head and a short head. Note: The hamstring portion of the adductor magnus has a similar action to these muscles, but is located in the medial thigh. The muscles located within the posterior compartment of the thigh are the biceps femoris, semitendinosus and semimembranosus.
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