Exercise Advice: Lie flat on your back with your lower legs resting on an exercise ball with your knees bent at a 90 degree angle. Place your feet three to four inches apart and point your toes inward so they touch. Cross your arms across your chest. Begin by pushing the small of your back down to the floor to isolate your ab muscles. Begin to roll your shoulders off the floor. Continue to push down with your lower back. Your shoulders should come up off the floor only about four inches, and your lower back should always remain on the floor.