Semimembranosus muscle

The semimembranosus muscle is one of the three hamstring muscles of the posterior compartment of the thigh. It lies deep to the semitendinosus and medial to the biceps femoris. The muscle is named for its broad, flattened (membranous) proximal tendon, which distinguishes it from the more cord-like semitendinosus.

Origin

• The semimembranosus muscle arises from the superolateral impression of the ischial tuberosity

• Originates by a broad, flattened tendon located superolateral to the conjoined tendon of the semitendinosus and long head of biceps femoris

Course

• Descends vertically and medially in the posterior thigh

• Lies deep to the semitendinosus throughout most of its course

• Near the knee, the muscle expands and gives rise to multiple distal expansions

Insertion

The distal tendon divides into three main expansions:

• Direct insertion: Into the posterior part of the medial condyle of the tibia

• Reflected (oblique) popliteal ligament: A strong fibrous expansion that passes superolaterally to reinforce the posterior capsule of the knee joint

• Popliteus fascia expansion: Fibres blend with the fascia over the popliteus muscle

Nerve Supply

• Tibial division of the sciatic nerve

• Roots: L5, S1, S2

Actions

Relations

• Superficial: Semitendinosus

• Deep: Adductor magnus (hamstring part)

• Medial: Gracilis (distally)

• Lateral: Biceps femoris

• Inferiorly: Forms the superomedial boundary of the popliteal fossa