Muscle
Semimembranosus
Deep medial hamstring with an expanded proximal tendon
High-yield
It crosses both hip and knee, so it can extend the hip while flexing the knee. At a flexed knee, it turns the leg inward, complementing the outward pull of biceps femoris.
Overview
Semimembranosus descends from the ischial tuberosity beneath semitendinosus. Its proximal tendon spreads as a sheet along the muscle, while its distal attachment reaches the back of the medial tibial condyle.
Reference facts
- Origin
- Superolateral part of the ischial tuberosity
- Insertion
- Posterior medial tibial condyle, with expansions into nearby capsular and ligamentous tissues
- Action
- Extends the hip, flexes the knee and internally rotates the flexed leg
- Innervation
- Tibial division of the sciatic nerve; L5-S2 is the usual hamstring root summary
- Blood supply
- Profunda femoris perforating branches, supplemented by inferior gluteal and popliteal muscular branches
Clinical correlation
The bursa between its tendon and the medial gastrocnemius can enlarge into a Baker's cyst. This bursa may communicate with the knee joint, but swelling behind the knee has other possible causes and needs appropriate assessment.
Frequently asked questions
What does the semimembranosus do?
The semimembranosus extends the hip, flexes the knee and internally rotates the flexed leg.
What nerve innervates the semimembranosus?
Tibial division of the sciatic nerve; L5-S2 is the usual hamstring root summary.
Where does the semimembranosus attach?
Origin: Superolateral part of the ischial tuberosity. Insertion: Posterior medial tibial condyle, with expansions into nearby capsular and ligamentous tissues.
What is the blood supply of the semimembranosus?
The semimembranosus is supplied by the profunda femoris perforating branches, supplemented by inferior gluteal and popliteal muscular branches.