The semitendinosus is the most medial of the three hamstring muscles and has the longest tendon relative to its muscle belly, which forms in the lower third of the thigh before continuing to the pes anserinus insertion on the medial tibia. This long tendon makes it the most commonly harvested hamstring for ACL reconstruction, as it provides a strong and predictable graft with a minimal functional deficit after harvest.
| Origin | Ischial tuberosity via the conjoined tendon shared with the biceps femoris long head |
|---|---|
| Insertion | Upper medial surface of the tibia via the pes anserinus, alongside gracilis and sartorius |
| Nerve Supply | Tibial nerve (L5, S1, S2) |
| Blood Supply | Inferior gluteal artery; Perforating branches of the deep femoral artery |
| Actions | Flexion of the knee; Extension of the hip; Internal rotation of the tibia when the knee is flexed |
|---|
Its internal tibial rotation when the knee is flexed is the functional opposite of the biceps femoris and is important for controlling tibial rotation during cutting movements and the de-screw phase of knee unlocking.
Semitendinosus harvest for ACL reconstruction produces temporary hamstring weakness that resolves as the muscle regenerates from the remaining proximal belly. The regenerated muscle-tendon unit is visible on MRI by 12 months in most patients. Semitendinosus strains at the proximal musculotendinous junction are less common than biceps femoris strains but occur with the same sprinting mechanism.
The semitendinosus tendon is the most posterior and medial of the pes anserinus tendons, palpable as a firm cord just posterior to the gracilis at the medial knee during resisted knee flexion with internal tibial rotation.
Proximal musculotendinous junction tear from sprinting producing posterior thigh pain, less common than biceps femoris strains but managed identically with progressive loading rehabilitation.
