Semimembranosus Anatomy

Semimembranosus Muscle

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

Agonists:

  • Gluteus maximus
  • Semitendinosus
  • Biceps femoris (long head)
  • Adductor magnus (posterior part)

Antagonists:

  • Psoas major
  • Iliacus

2. Flexion of the leg at the knee

Agonists:

  • Biceps femoris
  • Semitendinosus

Antagonists:

  • Vastus lateralis
  • Vastus medialis
  • Vastus intermedius
  • Rectus femoris

Gracilissartoriuspopliteusgastrocnemius, and plantaris assist with flexion of the knee.

3. Internal rotation of the knee when the knee is flexed

Agonists:

  • Popliteus
  • Semitendinosus

Antagonists:

  • Biceps Femoris

Sartorius and gracilis assist with internal rotation of the knee when the knee is flexed.

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