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. Let's try the abdominal crunch.

Sit on the fitness ball with your feet resting on the floor, about hip-width apart. Keep your back straight, and cross your arms on your chest.

To begin the exercise, tighten your abdominal muscles. Lean back until you feel your abdominal muscles contract, and hold for three deep breaths. Then return to the starting position.

If you're just starting out, 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.