Semimembranosus muscle

Origin: Ischial tuberosity

Insertion: Medial surface of tibia

Artery: Profunda femoris, gluteal artery

Nerve: Sciatic nerve

Action: Hip extension, Knee flexion

Antagonist: Quadriceps muscle

Description:
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 join.
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.