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

The semimembranosus muscle originates by a thick tendon from the superolateral aspect of the ischial tuberosity.

The semimembranosus muscle inserts on the: The tendon of insertion gives off certain fibrous expansions: one, of considerable size, passes upward and laterally to be inserted into the posterior 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 medial collateral ligament of the joint and the fascia of the leg.

[2] The tibial part of the sciatic nerve is also responsible for innervation of semitendinosus and the long head of biceps femoris.

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