How should I do crunches using an exercise ball?

To do crunches using an exercise ball, position the ball under the nape of your back. In this video, Heather Carty, of Memorial Hospital Jacksonville's H.E.A.R.T Fitness Center, demonstrates crunches using an exercise ball.

When performing crunches on the ball, begin lying on the ball with your feet hip-width apart and pointed straight ahead.  Be sure to draw-in your belly button and keep your neck in a relaxed position. Place the ball in the middle of your back and slowly extend your back over the ball. Cross your hands over your chest and slowly crunch up making sure the ball does not move. Pause at the top and then slowly lower yourself back down over the ball. To make this exercise more challenging, start with the ball toward the low back/glute area. To make this easier, begin with the ball toward the upper back/shoulder blade region. Perform this exercise for the desired number of repetitions and sets.
 

The exercise ball crunch is one of the most effective abdominal exercises. Elevating while leaning on an exercise ball helps you better accommodate your unique biomechanics and allows your hips, back, and butt the freedom to move unimpeded -- something you can't do when you're pressed against a floor for a traditional crunch.

Lean back against a large exercise ball with your feet flat on the floor. With your hands behind your head, slowly raise your upper body as you would in a floor crunch. Be sure to maintain your balance, and don't worry if you squirm to stabilize yourself -- that means that more of your ab muscles are working! Lower yourself as you would for a floor crunch.

Perform 3 sets of 15 reps.