The semimembranosus is the deepest and most medial hamstring, named for the broad flat membranous tendon from which it arises at the ischium. Its multiple distal insertions including the posterior capsule and oblique popliteal ligament make it an important dynamic stabiliser of the posteromedial knee. A semimembranosus bursa at its insertion is one of the most common causes of a posterior knee swelling.
| Origin | Ischial tuberosity via a broad flat tendon (the membranous tendon that gives the muscle its name) |
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| Insertion | Posterior aspect of the medial tibial condyle, with expansions to the posterior capsule, oblique popliteal ligament, and medial collateral ligament |
| Nerve Supply | Tibial nerve (L5, S1, S2) |
| Blood Supply | Inferior gluteal artery; Perforating branches of the deep femoral artery |
| Actions | Flexion of the knee; Extension of the hip; Internal rotation of the tibia when the knee is flexed; Tightens the posterior knee capsule via the oblique popliteal ligament |
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The semimembranosus reinforces the posterior medial knee capsule through its oblique popliteal ligament expansion, providing dynamic resistance to hyperextension and external rotation forces that the static capsuloligamentous structures resist passively.
Semimembranosus tendinopathy at the medial tibial condyle insertion produces posteromedial knee pain reproduced by resisted knee flexion with internal rotation that is distinct from MCL and meniscal pathology in its location and provocation. A semimembranosus bursa between the tendon and the posterior knee capsule can communicate with the knee joint and produce a popliteal cyst (Baker cyst) that fluctuates with knee effusion.
The semimembranosus is the deepest structure in the posteromedial thigh, palpable with firm pressure just medial and anterior to the semitendinosus tendon in the popliteal fossa during resisted knee flexion with internal tibial rotation.
Overuse degeneration at the medial tibial condyle insertion producing posteromedial knee pain in runners and cyclists, reproduced by resisted knee flexion with internal rotation.
Inflammation of the bursa at the semimembranosus insertion producing a posteromedial knee mass that may communicate with the joint and cause a Baker cyst.
