The hamstrings are three powerful muscles that travel along the back of your thigh. They are the semitendinosus, the semimembranosus and the biceps femoris muscles. They help bend your knee and extend your leg behind your body. Because these muscles handle high loads of stress, they are susceptible to stretching and tearing. This is called a muscle "strain." Hamstring strains are common among runners, dancers, and athletes who play sports that require sudden starts and stops.