Hamstring

In human anatomy, a hamstring is any of the three tendons contracted by three posterior thigh muscles (semitendinosus, semimembranosus and biceps femoris), and the term is often also used to refer to the muscles themselves. The hamstring tendons make up the borders of the space behind the knee; the muscles are involved in knee flexion and hip extension.

In quadrupeds, the hamstring is the single large tendon found behind the knee or comparable area.

Structure

The three muscles of the posterior thigh (semitendinosus, semimembranosus, biceps femoris long & short head) flex (bend) the knee, while all but the short head of biceps femoris extend (straighten) the hip. The three 'true' hamstrings cross both the hip and the knee joint and are therefore involved in knee flexion and hip extension. The short head of the biceps femoris crosses only one joint (knee) and is therefore not involved in hip extension. With its divergent origin and innervation it is sometimes excluded from the 'hamstring' characterization.