The hamstring muscle is located directly at the back of your thigh and works in unison with your quadriceps as well as your glutes and calves. This promotes proper movement of the entire thigh muscles. It is actually composed of three posterior thigh muscles of the semitendinosus, semimembranosus and the biceps femoris.
The hamstrings muscles play a large role in flexibility of the lower body. Most effective exercises generally are curls at the knee as well a dead-lift type movements. This large muscle group should not be neglected as it will improve mobility and long term flexibility.