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 plank.

Lie on the fitness ball on your stomach, with your feet touching the floor behind the ball. 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. Your feet will be suspended above the floor, and you'll be balancing on your hands and the ball. Keep your shoulders directly above your hands.

Tighten your abdominal muscles, and hold for three deep breaths or as long as you can maintain your balance and form. Then return to the starting position.

If you're just starting out, repeat the exercise five times. To make the exercise more challenging, work the ball down to your shins.

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.