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.