Performing crunches on the stability ball is a great way to tone your abdomen. Taking yourself off the floor and onto the ball brings an added challenge because it makes the surface less stable. By having to stabilize yourself, the muscles that are responsible for stabilizing your hips and trunk will kick in. It is important to keep these muscles strong to help prevent back pain and potential injuries. Exhale on the contraction and draw your navel toward your spine. If your belly bulges as you lift your upper body off the ball, that is incorrect.
Also be mindful of the positioning of your tongue. Be sure to keep it on the roof of your mouth (the position it is in when you swallow). This will ensure that muscles deep in your neck kick in. If they lay dormant as you lift your head, then you are encouraging a weakened neck and forward head lean (when your shoulders roll forward and your chin sticks out).
Step A: Lie with your back resting on the stability ball, and cross your arms over your chest or behind your head without supporting its weight.