semimembranosus

The semimembranosus is one of the hamstring muscles located in the posteromedial compartment of the thigh. It originates from the ischial tuberosity, with some fibers blending with the tendon of the semitendinosus. The muscle has a broad, flat tendon that inserts onto the posteromedial aspect of the medial condyle of the tibia, and from this insertion gives rise to an expansion that contributes to the oblique popliteal ligament and to the posterior joint capsule and sometimes to the medial meniscus.

Innervation is by the tibial portion of the sciatic nerve (typically L5 to S2). The blood supply arises from branches of the profunda femoris and the popliteal arteries.

Functionally, the semimembranosus extends the hip and flexes the knee, with medial rotation of the tibia when the knee is flexed. It also helps stabilize the knee joint during movement.

In terms of anatomy, the semimembranosus lies medially in the posterior thigh and is deeper and more wide-ranging in its tendonous expansion than the semitendinosus. Its tendon and expansions participate in the posterior knee structures, including connections to the oblique popliteal ligament and the knee capsule.

Clinical relevance typically centers on hamstring injuries. Proximal strains or avulsion injuries at the ischial tuberosity are common in athletes, whereas distal tears may affect knee function and loading. Variants may include small slips to adjacent structures such as the medial meniscus.