semimembranosus /-mem′brənō″səs/ [L, semi + membrana, membrane] , one of three posterior femoral muscles. Situated at the back and medial side of the thigh, it originates in a thick tendon attached to the tuberosity of the ischium and inserts into the horizontal groove on the medial condyle of the tibia. The tendon of insertion passes some fibers laterally and upward to insert on the lateral condyle of the femur and form part of the oblique popliteal ligament behind the knee. The tendon of insertion forms one of the two medial hamstrings. The muscle is innervated by several branches of the tibial part of the sciatic nerve, containing fibers from the fifth lumbar and the first two sacral nerves. The muscle functions to flex the leg, to rotate it medially after flexion, and to extend the thigh. Compare biceps femoris, hamstring muscle, semitendinosus.
