Semitendinosus

Introduction

Introduction to Semitendinosus

The semitendinosus is one of the three hamstring muscles occupying the posterior compartment of the thigh. It is a long, fusiform muscle with a particularly long distal tendon, from which it derives its name (semi = half, tendinosus = tendinous). It originates from the ischial tuberosity, sharing a common conjoint tendon with the long head of biceps femoris, and inserts onto the medial surface of the proximal tibia as part of the pes anserinus (goose foot). The semitendinosus is a powerful flexor of the knee, an extensor of the hip, and a medial rotator of the leg when the knee is flexed. Its tendon is commonly harvested for anterior cruciate ligament (ACL) reconstruction and other ligament reconstructions, making it one of the most surgically important muscles of the lower limb.

Study

1. Origin and Insertion

The semitendinosus originates from the upper medial facet of the ischial tuberosity, via a conjoint tendon with the long head of biceps femoris. The muscle belly is fusiform and descends along the posterior thigh. Approximately halfway down the thigh, a distinctive round tendon begins to form and continues distally. This long, cord‑like tendon passes posterior to the medial femoral condyle and inserts onto the upper part of the medial surface of the tibia, as part of the pes anserinus, along with the tendons of the sartorius and gracilis.

2. Relations

The semitendinosus lies superficial in the posterior thigh, covered only by the deep fascia and skin. Its proximal portion is medial to the biceps femoris and posterior to the semimembranosus. Distally, its tendon forms part of the superomedial border of the popliteal fossa. The sciatic nerve runs deep to the muscle, lying on the adductor magnus. The semimembranosus muscle is located deep (anterior) to the semitendinosus throughout most of its course.

3. Nerve Supply

The semitendinosus is innervated by the tibial division of the sciatic nerve (L5, S1, S2). The nerve branches enter the muscle on its deep surface in the proximal thigh. Like the other hamstrings (except the short head of biceps femoris), it is supplied by the tibial portion of the sciatic nerve.

4. Blood Supply

The muscle receives its arterial supply from the perforating branches of the profunda femoris artery, the inferior gluteal artery (proximal part), and the popliteal artery (distal part). Venous drainage follows the corresponding veins.

5. Actions

The semitendinosus flexes the knee and medially rotates the leg when the knee is flexed. It also extends the hip joint, acting synergistically with the other hamstrings. During the stance phase of gait, it assists in hip extension, and during the swing phase, it helps decelerate knee extension before heel strike.

6. Clinical Relevance

The semitendinosus tendon is a commonly used autograft for ACL reconstruction. Combined with the gracilis tendon, it forms a quadrupled hamstring graft. The tendon regenerates partially after harvest, but the muscle belly may show some atrophy. Hamstring strains frequently involve the semitendinosus, often at the myotendinous junction. Proximal hamstring tendinopathy and avulsion fractures of the ischial tuberosity can involve the semitendinosus origin. Pes anserinus bursitis or tendinopathy involves the distal insertion. In patients with cerebral palsy or spasticity, the semitendinosus may be lengthened or released to correct knee flexion contractures.

Summary

Summary of Semitendinosus

The semitendinosus is a hamstring muscle that originates from the ischial tuberosity and inserts on the medial tibia via the pes anserinus. It is innervated by the tibial division of the sciatic nerve (L5–S2) and receives blood from the profunda femoris perforators and popliteal artery. It flexes and medially rotates the knee, and extends the hip. Its long tendon is a workhorse graft for ligament reconstruction, particularly the ACL.