Semimembranosus

Introduction

Introduction to Semimembranosus

The semimembranosus is the largest and most medial of the three hamstring muscles occupying the posterior compartment of the thigh. It is named for the broad, flattened, membranous tendon that forms its proximal portion. The semimembranosus originates from the ischial tuberosity, deep to the semitendinosus and biceps femoris, and inserts primarily on the posterior aspect of the medial tibial condyle. It is a powerful flexor of the knee, an extensor of the hip, and a medial rotator of the leg when the knee is flexed. Through its extensive distal attachments, which include contributions to the oblique popliteal ligament and the posterior capsule of the knee joint, the semimembranosus plays a crucial role in stabilising the posteromedial corner of the knee. Its proximal tendon is frequently involved in hamstring tendinopathy and avulsion injuries.

Study

1. Origin and Insertion

The semimembranosus originates from the upper lateral facet of the ischial tuberosity, deep (anterior) to the common origin of the semitendinosus and long head of biceps femoris. The proximal tendon is broad, flat, and membranous. The muscle belly descends medial to the biceps femoris and deep to the semitendinosus. Distally, the semimembranosus gives off five distinct tendinous expansions that insert into: (1) a groove on the posterior aspect of the medial tibial condyle (main direct insertion); (2) the oblique popliteal ligament (which reinforces the posterior knee capsule); (3) the fascia overlying the popliteus muscle; (4) the medial meniscus via the posterior oblique ligament; and (5) the posterior capsule and the medial collateral ligament.

2. Relations

The semimembranosus lies deep to the semitendinosus for most of its course. Proximally, it is covered by the gluteus maximus. Laterally, it is related to the biceps femoris and the sciatic nerve. The sciatic nerve lies directly on the posterior surface of the adductor magnus, which is deep to the semimembranosus. Distally, its tendon forms the superomedial boundary of the popliteal fossa. The semimembranosus tendon curves around the medial femoral condyle and is separated from the medial head of gastrocnemius by a bursa—the semimembranosus bursa. This bursa often communicates with the knee joint and is the most common site for a Baker's (popliteal) cyst.

3. Nerve Supply

The semimembranosus is innervated by the tibial division of the sciatic nerve (L5, S1, S2). The nerve branches enter the muscle on its deep surface in the proximal thigh, often arising from the same branch that supplies the semitendinosus.

4. Blood Supply

The muscle receives its arterial supply from the perforating branches of the profunda femoris artery, the inferior gluteal artery (proximal part), and the popliteal artery (distal part). Venous drainage follows the corresponding veins into the profunda femoris vein and the popliteal vein.

5. Actions

The semimembranosus flexes the knee joint and medially rotates the leg when the knee is flexed. It is also a powerful extensor of the hip joint. Through its attachments to the oblique popliteal ligament, the posterior capsule, and the meniscus, the semimembranosus actively tightens the posterior capsule during knee flexion and extension, preventing capsular impingement. It also contributes to posteromedial stability of the knee, resisting valgus and external rotation forces.

6. Clinical Relevance

Proximal hamstring tendinopathy and avulsion fractures of the ischial tuberosity frequently involve the semimembranosus origin. Deep gluteal and posterior thigh pain worsened by sitting is characteristic. Baker's cyst (popliteal cyst) often arises from the semimembranosus‑gastrocnemius bursa and can cause posterior knee pain and swelling. Injuries to the posteromedial corner of the knee, including the semimembranosus attachments, lead to rotatory knee instability. The semimembranosus tendon is also used as a graft for ligament reconstructions around the knee, though less commonly than the semitendinosus. In spastic cerebral palsy, semimembranosus spasticity contributes to knee flexion contractures and crouch gait, and may be addressed by surgical lengthening.

Summary

Summary of Semimembranosus

The semimembranosus is the largest hamstring muscle, with a broad membranous proximal tendon from the ischial tuberosity, a deep belly, and multiple distal attachments to the medial tibia, oblique popliteal ligament, and knee capsule. It is innervated by the tibial nerve (L5–S2) and supplied by the profunda femoris and popliteal arteries. It flexes and medially rotates the knee, extends the hip, and stabilises the posteromedial knee. It is clinically significant in hamstring tendinopathy, Baker's cysts, and posteromedial corner injuries.