The hamstring muscles are actually a group of three muscles that run down the back part of the upper leg. Together, this group of muscles, the biceps femoris, the semitendinosus, and the semimembranosus, bend the leg at the knee and extend the leg at the hip joint. This action allows us to run, walk and jump. Pulled hamstrings typically occur when the knee extends and the muscle is contracted suddenly. Pulled hamstring muscles are a common sports-related injury and can be a result of insufficient warm-up. In a grade-one tear, the muscle is tender, but not bruised. Grade-two tears will show bruising where the muscle fibers tore. A grade-three tear involves bruising and actual separation of the muscle. Rest and application of ice to the affected area are required for proper healing. After healing begins, exercises to strengthen the hamstrings can help prevent recurrent injury.