Nicole L. Campbell: Core exercises strengthen your core muscles, including the muscles in your abdomen and your back. You can do many core exercises with a fitness ball. Let's try the squat and reach.

Hold the fitness ball in front of you and bend your knees into a squat position. Keep your back straight and your arms parallel to the floor. Don't let your knees extend beyond your feet.

To begin the exercise, tighten your abdominal muscles. Keeping your knees bent, rotate your trunk and reach with the ball to the left. Hold for three deep breaths. Return to the starting position and repeat to your right.

To vary the exercise, you can hold the fitness ball in a downward position, like this, or in an upward position, like this.

If you're just starting out, repeat the exercise five times on each side. Remember to breathe freely and deeply throughout your exercise, and keep the abdominal muscles nice and tight. As you get stronger, gradually increase to 10 to 15 repetitions on each side.