The semimembranosus has the most complex insertion of any hamstring, with expansions reinforcing the posteromedial knee. Its broad membranous proximal tendon (giving it its name) expands at the ischial tuberosity. A semimembranosus bursa between it and the medial gastrocnemius head is a primary source of Baker cysts.
| Origin | Superolateral facet of the ischial tuberosity (separate from the conjoined semitendinosus and biceps femoris long head origin) |
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| Insertion | Posteromedial tibial condyle (primary); with expansions to the posterior capsule forming the oblique popliteal ligament, the popliteus fascia, and the medial collateral ligament |
| Nerve Supply | Tibial division of the sciatic nerve (L5, S1, S2) |
| Blood Supply | Inferior gluteal and perforating arteries |
| Actions | Hip extension; Knee flexion; Internal rotation of the tibia when the knee is flexed; Tightens the posterior knee capsule via the oblique popliteal ligament expansion |
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The semimembranosus contribution to posteromedial knee stability through its capsular expansion (oblique popliteal ligament) makes it a dynamic ligamentous support — damage to this expansion contributes to posteromedial rotational instability.
The semimembranosus-gastrocnemius bursa communicates with the knee joint in most adults via a posterior capsular hiatus, allowing joint effusions to decompress posteriorly forming Baker cysts. Semimembranosus tendinopathy at the posteromedial knee is a cause of medial knee pain reproduced by resisted knee flexion from the extended position.
The semimembranosus is palpable as the medial hamstring cord in the posterior thigh, distinguishable from the semitendinosus by its broader, flatter tendon at the knee.
Semimembranosus-gastrocnemius bursa distension from knee joint effusion producing a posterior knee soft swelling, managed by treating the underlying joint pathology.
