Semitendinosus Anatomy: Origin, Insertion, Action, Innervation

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.

1. Extension of the thigh at the hip

Agonists:

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

Antagonists:

• Psoas major
• Iliacus

2. Flexion of the leg at the knee

Agonists:

• Biceps femoris
• Semimembranosus

Antagonists:

• Vastus Lateralis
• Vastus Medialis
• Vastus Intermedius
• Rectus Femoris

Gracilis, sartorius, popliteus, gastrocnemius, and plantaris assist with flexion of the knee
3. Internal rotation of the leg at the knee

Agonists:

• Semimembranosus
• Popliteus

Antagonists:

• Biceps Femoris
• Iliacus

Sartorius and gracilis assist with internal rotation of knee