Begin on the floor laying on your back with your feet on top of the ball. Position the ball so that when your legs are extended your ankles are on top of the ball. This will be your starting position. Raise your hips off of the ground, keeping your weight on the shoulder blades and your feet. Flex the knees, pulling the ball as close to you as you can, contracting the hamstrings. After a brief pause, return to the starting position.