semimembranous

The semimembranosus is one of the three hamstring muscles of the posterior thigh, lying medially with the semitendinosus. Its name reflects its broad, membranous tendon. It works in concert with the other hamstrings to extend the hip and to flex the knee, with medial rotation of the leg when the knee is flexed.

Anatomy and attachments: The muscle originates from the ischial tuberosity, often sharing a common tendon with the long head of the biceps femoris and the semitendinosus. Its tendon inserts on the posterior aspect of the medial tibial condyle, with fibers giving off expansions that contribute to the oblique popliteal ligament and, in part, to the posteromedial knee capsule. The tendon also has connections in the popliteal region and may send slips to nearby fascial structures.

Innervation and vessels: The semimembranosus is innervated by the tibial part of the sciatic nerve (L5–S2). Blood supply comes from perforating branches of the profunda femoris artery, with contributions from branches of the popliteal artery.

Function: By extending the hip and flexing the knee, the semimembranosus helps slow or control leg movement. When the knee is flexed, it medially rotates the leg and provides dynamic stabilization to the knee joint.

Clinical relevance: Strains or tears of the semimembranosus are common in athletes, particularly with rapid acceleration or sprinting. Proximal tears near the ischial origin may accompany avulsion injuries in adolescents. Knowledge of its anatomy assists in interpreting knee injuries and planning surgical or rehabilitative approaches.