Attachments
The three hamstrings share an origin on the ischial tuberosity and then split to either side of the knee.
| Muscle | Origin | Insertion | Nerve |
|---|---|---|---|
| Biceps femoris, long head | Ischial tuberosity, by a common tendon with semitendinosus | Head of the fibula | Tibial part of sciatic |
| Biceps femoris, short head | Lateral lip of the linea aspera and lateral supracondylar line | Joins the long head to the fibular head | Common fibular part of sciatic |
| Semitendinosus | Ischial tuberosity, common tendon with the long head of biceps | Upper medial surface of the tibia (pes anserinus) | Tibial part of sciatic |
| Semimembranosus | Upper lateral impression on the ischial tuberosity | Groove on the back of the medial tibial condyle; expansions form the oblique popliteal ligament | Tibial part of sciatic |
Semitendinosus is named for its long, round tendon, which begins halfway down the thigh. Semimembranosus is named for its flat, membranous upper tendon; its fleshy belly lies deep to semitendinosus in the lower thigh. The short head of biceps is not a true hamstring: it does not cross the hip and has a different nerve supply. The hamstring part of adductor magnus, from the ischial tuberosity to the adductor tubercle, behaves like one and is supplied by the tibial part of the sciatic nerve.
All three extend the hip and flex the knee. With the knee flexed, biceps rotates the leg laterally and the two 'semi' muscles rotate it medially.
Relations: sciatic nerve and vessels
The sciatic nerve (L4–S3) is the key relation of the hamstrings. It enters the thigh from the gluteal region below the lower border of gluteus maximus, having crossed obturator internus with the gemelli and quadratus femoris. In the thigh it lies on adductor magnus and is crossed superficially and obliquely by the long head of biceps femoris, from medial to lateral.
The nerve usually divides at the upper angle of the popliteal fossa into the tibial and common fibular (peroneal) nerves, though a high division, even within the pelvis, is common. It gives all its hamstring branches from the medial side, except the branch to the short head of biceps, which comes from the lateral side.
The posterior cutaneous nerve of the thigh (S1–S3) runs down the middle of the thigh deep to the fascia lata, superficial to the hamstrings, and supplies the skin of the back of the thigh and upper calf. Near the knee, the popliteal artery and vein enter from the adductor hiatus, and the small saphenous vein pierces the deep fascia to join the popliteal vein.
Blood supply and innervation
The hamstrings have no single main artery. The upper parts take branches from the inferior gluteal artery and the medial circumflex femoral artery; the middle parts are supplied by the perforating branches of the profunda femoris, which pierce adductor magnus; the lower parts receive branches from the popliteal artery. The nerve supply is summarised above: tibial part of the sciatic nerve for the long head of biceps, semitendinosus and semimembranosus, and common fibular part for the short head of biceps.
Clinical relevance
Hamstring injuries are among the commonest sports injuries, and the sciatic nerve is at risk in the same area.
- Hamstring strain: usually at the musculotendinous junction of the long head of biceps femoris, during sprinting when the muscle contracts while lengthening.
- Proximal hamstring avulsion: the common tendon tears off the ischial tuberosity during forced hip flexion with the knee straight, as in water-skiing. Complete avulsions with retraction are often repaired, and the sciatic nerve lies close to the surgical field.
- Sciatic nerve injury: from posterior hip dislocation, hip replacement or misplaced gluteal injections. The common fibular part is more often affected, producing foot drop.
- Hamstring graft: the semitendinosus tendon, often with gracilis, is harvested for anterior cruciate reconstruction.
- Ischial bursitis: prolonged sitting on hard surfaces inflames the bursa over the tuberosity.
On the specimen
The station shows the back of the thigh from the buttock to the popliteal fossa, with gluteus maximus reflected. Start at the ischial tuberosity: the common tendon arising from it is shared by semitendinosus and the long head of biceps. Follow biceps laterally to the knee; the short head joins it from the femoral shaft in the lower thigh.
On the medial side, the pencil-like tendon lying on the surface is semitendinosus, and the broad muscle beneath it is semimembranosus. Gracilis is the thin strap more medial still. The large nerve in the midline is the sciatic nerve, dividing into the tibial and common fibular nerves; the fine branch linking the common fibular nerve to the sural nerve is the sural (fibular) communicating nerve. Above the hamstrings, piriformis, quadratus femoris and gluteus maximus mark the gluteal region, and the posterior cutaneous nerve of the thigh runs superficial to the hamstrings.