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.
Step B: Inhale and expand your belly, then exhale and pull your naval in as you lift your head (with your neck muscles rather then your hands) and shoulders off the ball. Focus on using your abdominal muscles to perform the action rather than your chest and shoulders.
Step C: Slowly reverse the curl. Repeat until you begin to lose your form (that is, crunching without pulling naval in). Rest one minute and repeat the set one to two more times.
Hold a weight plate or dumbbell across your chest as you crunch.
The crunch on the stability ball primarily targets the upper abdominals. It is good to perform these at the end of a workout. If they are sore the following day, rest them until soreness subsides before training.
Note: If your neck muscles get tired before your abdominal muscles do, most likely it is because you have an imbalance of strength between your neck flexors and trunk flexors (the muscles that bend your body forward). If one of them is stronger than the other, those stronger muscles will eventually pull the body into a certain position creating poor posture and vulnerability to injury.