In human anatomy, the hamstrings are a group of muscles on the underside (posterior aspect) of the thigh. They are responsible for flexion of the knee, and all attach proximally to the ischial tuberosity.

Because they attach to part of the hip, they also act to extend the lower limb at the hip joint. The hamstrings assist in slowing down passive extension of the knee when walking.

The hamstrings include:

• semitendinosus - attaches distally to the medial tibial condyle.
• semimembranosus - also attaches to the medial tibial condyle.
• biceps femoris - It has a long and a short head, the short head's proximal attachment being the linea aspera of the femur. The long head attaches to the ischial tuberosity.

Hamstring can also be used as a verb. A person that is hamstrung is disabled; the term comes from the fact that hamstring injuries make movement very difficult.