Semimembranosus
Musculus semimembranosus
Also known as: medial hamstring
The deep medial hamstring, broad and flat with a membranous origin (hence "half-membrane"). It inserts on the posteromedial tibial condyle and reinforces the back of the knee capsule.
Overview
Semimembranosus arises by a long, flat membranous tendon from the superolateral impression on the ischial tuberosity, deep to the semitendinosus–biceps tendon. Its thick belly, the bulkiest of the hamstrings, lies deep to semitendinosus in the lower thigh. It inserts mainly on the posterior surface of the medial tibial condyle (in a groove), with expansions that form the oblique popliteal ligament of the knee capsule and that reach the medial meniscus and the fascia over popliteus.
It extends the hip, flexes the knee and medially rotates the flexed leg. Through its capsular expansion it pulls the medial meniscus backward during knee flexion and tightens the posteromedial capsule, stabilising the knee. It forms the superomedial border of the popliteal fossa with semitendinosus.
Anatomy
- Origin
Ischial tuberosity (superolateral impression).
- Insertion
Posterior part of the medial condyle of the tibia; the reflected expansion forms the oblique popliteal ligament.
- Action
Extension of the thigh; flexion of the leg; medial rotation of the flexed knee.
- Innervation
Tibial division of the sciatic nerve.
- Nerve roots
L5–S2
- Arterial supply
Perforating branches of the profunda femoris; inferior gluteal artery; popliteal artery (medial genicular branches).
- Relations
Deep to semitendinosus. The semimembranosus–medial gastrocnemius bursa lies between its tendon and the medial head of gastrocnemius.
Function
- Extends the hip
- Flexes the knee and medially rotates the flexed leg
- Stabilises the posteromedial knee (oblique popliteal ligament)
Clinical significance
Baker's (popliteal) cyst
Distension of the semimembranosus–gastrocnemius bursa, which often communicates with the knee joint, by synovial fluid in arthritis or meniscal tears. It forms a swelling in the medial popliteal fossa. Rupture mimics deep vein thrombosis (pain and swelling of the calf).
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.