http://AskTheTrainer.com Personal Trainer Mike Behnken, MS, CSCS Use a medicine ball for resistance while you perform a stability ball crunch. Keep the ball directly above your head and do not use it for momentum. If you need to create momentum with the ball you should just get rid of it and do the stability ball crunch by itself. Keep your feet under your thighs and do not perform hamstring curls to pull yourself up. http://AskTheTrainer.com/sets-and-reps.html