Semimembranosus muscle

The semimembranosus is a muscle in the back of the thigh. It is the most medial of the three hamstring muscles.

Structure
The semimembranosus, so called from its membranous tendon of origin, is situated at the back and medial side of the thigh.

It arises by a thick tendon from the upper and outer impression on the tuberosity of the ischium, above and lateral to the biceps femoris and semitendinosus.

The tendon of origin expands into an aponeurosis, which covers the upper part of the anterior surface of the muscle; from this aponeurosis muscular fibers arise, and converge to another aponeurosis which covers the lower part of the posterior surface of the muscle and contracts into the tendon of insertion.

It is inserted mainly into the horizontal groove on the posterior medial aspect of the medial condyle of the tibia.

The tendon of insertion gives off certain fibrous expansions: one, of considerable size, passes upward and lateralward to be inserted into the back part of the lateral condyle of the femur, forming part of the oblique popliteal ligament of the knee-joint; a second is continued downward to the fascia which covers the Popliteus muscle; while a few fibers join the tibial collateral ligament of the joint and the fascia of the leg.

The muscle overlaps the upper part of the popliteal vessels.

Innervation
The semimembranosus is innervated by the tibial part of the sciatic nerve.

Actions
The semimembranosus helps to extend (straighten) the hip joint and flex (bend) the knee joint. It also helps medially rotate the knee.

Variations
It may be reduced or absent, or double, arising mainly from the sacrotuberous ligament and giving a slip to the femur or adductor magnus.