SEMIMEMBRANOSUS MUSCLE

Definition: The semimembranosus acts as a flexor and internal rotator of the tibia and, through the oblique popliteal ligament, tightens the posterior capsule.

Origin: Ischial tuberosity

Insertion: The semimembranosus has several attachments at the knee. The posterior oblique ligament courses from the common tendon to reinforce the posteromedial joint capsule and attach to the adductor tubercle. The first is the oblique popliteal ligament, which passes from its origin at the confluence of the capsular arm of the posterior oblique ligament and the oblique popliteal expansion of the common semimembranosis tendon obliquely, and laterally toward its attachment on a cartilagenous or osseous fabella, posterolateral joint capsule, and the plantaris muscle. When the oblique popliteal ligament is contracted, the anteromedial pull tightens the posterior capsule. The second attachment is to the posterior capsule. The direct head attaches to the posteromedial tibia just below the joint line. An anterior arm extends off the direct arm to course deep to the tibial collateral ligament. A distal tibial expansion of the semimembranosus continues distally to form a fibrous expansion with two divisions over the popliteus. The two divisions share a common insertion into the periosteum of the distomedial tibia. In addition, a proximal capsular arm courses just proximal to the OPL to attach to the lateral aspect of the knee.

Innervation: tibial portion of the sciatic nerve

Blood Supply:

1. LaPrade, R.F., Morgan, P.M., Wentorf, F.A., Johansen, S., Engebretsen, L.: Qualitative and quantitative anatomy of the posterior aspect of the knee. Am J. Sports Med., submitted.