Nicole L. Campbell: A fitness ball looks like a large beach ball. You can do exercises for your abdomen and other core muscles on a fitness ball. You can also use a fitness ball to improve your flexibility and balance. You can even use a fitness ball as a chair.

Fitness balls come in various sizes, depending on the ball's diameter when it's inflated. For most exercises, you'll want a ball that allows your knees to be at a right angle when you sit on the ball with your feet on the floor.

If you're between 5'1" (155 centimeters) and 5'7" (170 centimeters), a 55-centimeter ball is a good choice. If you're between 5'8" (173 centimeters) and 6'1" (185 centimeters), try a 65-centimeter ball. If you're 6'2" (188 centimeters) or taller, try a 75-centimeter ball.

Air pressure counts, too. The firmer the fitness ball, the more difficult the exercise will be. If you're just starting out, you might want to slightly deflate the ball. You can inflate the ball as you gain strength and confidence.