The hamstring consists of three large muscles in the back of the thigh: the biceps femoris, the semitendinosus, and the semimembranosus.

A person with a hamstring strain has stretching or tearing of the hamstring muscles in the back of the thigh. A hamstring strain is usually caused by an injury. Most hamstring strains are partial tears of the muscle. Severe hamstring injuries may cause a complete tear, called a hamstring rupture.