The hamstring curl, also called a leg curl, is an exercise that strengthens the hamstrings. It involves bending your knees and moving your heels toward your butt while the rest of your body stays still. This machine exercise primarily works the hamstrings, which consists of four muscles: the long head of the biceps femoris, the short head of the biceps femoris, the semimembranosus, and the semitendinosus.