The muscles of the posterior compartment of the thigh, also called the hamstrings, are a group of three muscles located in the back (posterior) part of the thigh:

• Biceps femoris

• Semitendinosus

• Semimembranosus.

These muscles work together to extend the hip and flex the knee, which are key movements for walking, running, and climbing.

The hamstrings are mainly supplied by branches of the deep femoral artery and inferior gluteal artery, and they receive nerve signals from the sciatic nerve (L4-S3), which divides into the tibial and common fibular branches.

Biceps Femoris

The biceps femoris is the most lateral muscle of the hamstring group and has two heads: a long head and a short head.

• Long head: originates from the ischial tuberosity and the sacrotuberous ligament.

• Short head: originates from the linea aspera and the lateral supracondylar line on the femur.

Both heads join to form a single tendon that attaches to the head of the fibula.

Innervation:

• Long head: tibial division of the sciatic nerve.

• Short head: common fibular division of the sciatic nerve.

It flexes the leg at the knee joint, extends the thigh at the hip (long head), laterally rotates the leg when the knee is flexed, and assists in raising the trunk from a bent position.

Semitendinosus

The semitendinosus is a long, mostly tendinous muscle located on the medial side of the posterior thigh, just above the semimembranosus. 

It flexes the leg at the knee joint, extends the thigh at the hip joint, medially rotates both the thigh and leg when the knee is flexed, and stabilizes the knee, especially during walking or running.

• Origin: The ischial tuberosity of the pelvis.

• Insertion: The medial surface of the tibia, below the knee, as part of the pes anserinus.

• Innervation: The tibial division of the sciatic nerve.

Semimembranosus

The semimembranosus is a broad, flat muscle that lies deep to the semitendinosus, on the medial side of the posterior thigh. 

It flexes the leg at the knee, extends the thigh at the hip, medially rotates the leg when the knee is flexed, helps stabilize the hip and knee joints, and assists in bringing the trunk upright from a bent position.

• Origin: The ischial tuberosity, slightly higher than the semitendinosus and biceps femoris.

• Insertion: The medial condyle of the tibia.

• Innervation: The tibial division of the sciatic nerve.

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