Nicole L. Campbell: Core exercises strengthen your core muscles, including the muscles in your abdomen and back. The bridge is a core exercise you can do to work various core muscles in combination. If you're comfortable with the bridge, add a heel dig. This exercise primarily targets the hamstrings, the muscles at the backs of your thighs.

Lie on your back with your legs resting on top of the fitness ball and your arms resting at your sides. When you're ready to begin the exercise, tighten your abdominal muscles and raise your hips and buttocks off of the floor.

Keeping your hips off the floor, dig your heels into the ball while you pull it closer to your buttocks with your feet. Hold for three deep breaths. Then return to the starting position.

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.