A hamstring (/ˈhæmstrɪŋ/) is any one of the three posterior thigh muscles in human anatomy between the hip and the knee: from medial to lateral, the semimembranosus, semitendinosus and biceps femoris.

The short head of the biceps femoris crosses only one joint (knee) and is therefore not involved in hip extension.

[7] The hamstrings cross and act upon two joints – the hip and the knee – and as such they are termed biarticular muscles.

The hamstrings play a crucial role in many daily activities such as walking, running, jumping, and controlling some movement in the gluteus.

A common running injury in several sports, excessive stretch of a hamstring results from extensive hip flexion while the knee is extended.

[4][8] During sprinting, a hamstring injury may occur from excessive muscle strain during eccentric contraction late in the leg swing phase.