Nicole L. Campbell: Core exercises strengthen your core muscles, including the muscles in your abdomen and back. You can do many core exercises with a fitness ball. If you're comfortable using a fitness ball and want to try an advanced core exercise, consider the reverse crunch.
Lie on the fitness ball on your stomach, with your feet touching the floor behind you. Lean forward until you touch the floor with your hands. Walk your hands away from the ball until you feel the ball reach your upper thighs. Keep your shoulders directly above your hands.
Tighten your abdominal muscles and press your knees into the ball. Use your abdominal muscles to bring your knees toward your chest. Hold for three deep breaths. Then return to the starting position.
Repeat the exercise five times. Remember to breathe freely and deeply throughout the exercise, and keep your abdominal muscles nice and tight. As you get stronger, gradually increase to 10 to 15 repetitions.