Origin: Posterior surface of the ischial tuberosity, deep to the conjoined tendon of the semitendinosus and the long head of the biceps femoris.
Insertion: Posterior aspect of the medial condyle of tibia, sending fibers into the fascia over popliteal ligament.
Actions: Extension of the thigh at the hip, flexion of the leg at the knee, internal rotation of the knee when the knee is flexed
Innervation: Tibial portion of the sciatic nerve (L5, S1, S2)
Blood Supply: Branches from the internal iliac, popliteal, and profunda femoris arteries.
Primary Actions of the Semimembranosus
1. Extension of the thigh at the hip
- Gluteus maximus
- Biceps femoris (long head)
- Adductor magnus (posterior part)
- Psoas major
2. Flexion of the leg at the knee
- Biceps femoris
- Vastus lateralis
- Vastus medialis
- Vastus intermedius
- Rectus femoris
3. Internal rotation of the knee when the knee is flexed
- Biceps Femoris
Muscle Pain Information: The semimembranosus muscle contributes to pain just below the buttock that descends into the back of the thigh and knee.