Nicole Krupa: Core exercises strengthen your core muscles, including the muscles in your abdomen and back. You can do many core exercises with a fitness ball. To work various core muscles in combination, let's try a bridge.

Lie on your back with your legs resting on top of the fitness ball.

When you're ready to begin the exercise, tighten your abdominal muscles and raise your hips and buttocks off the floor. Besides your core muscles, you'll feel your gluteal and hamstring muscles contract to keep you in place. Hold for three deep breaths. Then return to the starting position.

If you're just starting out, repeat the exercise five times. To make the exercise more challenging, raise your right leg a few inches off of the fitness ball, and repeat with your left leg.

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.