Semimembranosus muscle

At a glance

English

Semimembranosus muscle

Latin (TA2)

Musculus semimembranosus

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the semimembranosus muscle

Semimembranosus lies furthest medially of the three hamstrings at the back of the thigh. It takes its name from the flat, sheet-like tendon at its upper end. It lies deep to semitendinosus, which runs in a groove on its surface, and it is broader and flatter than its partner. Lower down it bulges on either side of semitendinosus and can be felt at the inner back of the knee.

Its origin is a thick tendon from the upper outer impression on the ischial tuberosity, above and lateral to the shared tendon of semitendinosus and biceps. This tendon widens into an aponeurosis on the front of the upper muscle. Fleshy fibres pass from it to a second aponeurosis on the back of the lower muscle, which narrows into the tendon of insertion. The main attachment is a horizontal groove on the back of the tibia's medial condyle.

From this tendon several expansions spread out. One runs upwards and outwards behind the knee to the lateral femoral condyle, forming the oblique popliteal ligament that strengthens the back of the joint capsule. Another passes down onto the fascia covering popliteus, and some fibres reach the medial collateral ligament and the medial meniscus. Through these links the muscle helps tension the back of the knee and draws the medial meniscus back as the joint bends.

Semimembranosus extends the hip and flexes the knee. With the knee bent it turns the tibia medially, and with the hip straight it can rotate the thigh medially; it also checks forward tilting of the pelvis when bending at the hips. It is supplied by the tibial division of the sciatic nerve, like the other true hamstrings, and by perforating branches of the deep femoral artery and branches of the popliteal artery. A bursa between its tendon and the medial head of gastrocnemius may swell to form a popliteal (Baker's) cyst behind the knee.

Attachments, action and supply

Origin

Superolateral impression on the ischial tuberosity.

Insertion

Posterior groove on the tibia's medial condyle, with expansions forming the oblique popliteal ligament and reaching the popliteus fascia.

Action

Extends the hip and flexes the knee; medially rotates the flexed leg and, with the hip extended, the thigh.

Nerve supply

Tibial division of the sciatic nerve (L5, S1, S2).

Blood supply

Perforating branches of the deep femoral artery, the inferior gluteal artery and branches of the popliteal artery.