A hamstring strain or a pulled hamstring is a tear in one of the hamstrings muscles. It often results from an overload of the muscles or trying to move the muscles too fast. If you have taken an impact in the back of the leg it must be treated as a contusion. Improper stretching and overuse can result in a hamstring strain. Chronic injuries arise from not properly rehabilitating acute hamstring injuries, but may also occur from small amounts of repeated trauma over a period of time. Poor flexibility of the hamstring increases the likelihood of small tears, which causes the muscle to shorten and get tighter.