In human anatomy, a hamstring is one of the three posterior thigh muscles (from medial to lateral: semimembranosus, semitendinosus and biceps femoris).

The hamstrings cross and act upon two joints – the hip and the knee.

Semitendinosus and semimembranosus extend the hip when the trunk is fixed; they also flex the knee and medially (inwardly) rotate the lower leg when the knee is bent.

The long head of the biceps femoris extends the hip, as when beginning to walk; both short and long heads flex the knee and laterally (outwardly) rotate the lower leg when the knee is bent.

The hamstrings play a crucial role in many daily activities such as walking, running, jumping, and controlling some movement in the trunk. In walking, they are most important as an antagonist to the quadriceps in the deceleration of knee extension.