Hamstrings are a group of strong muscles that line the back portion of the thigh, extending from the lower pelvis to the shin bone. They are responsible for flexibility of the knee joint and stretching of the hip. There are three primary types of hamstring muscles, namely, semitendinosus, semimembranosus, and biceps femoris. These muscles function in coordination for sprinting and carrying out the leg movements. Injury to any of these muscles is referred to as a pulled hamstring.