An exercise ball really is not necessary when it comes to doing crunches but if you doing crunches on the floor or even a mat causes you paint, then the exercise ball crunch could solve your discomfort problems completely. Some people experience pain or chaffed skin on their back or around the back of their hips during crunches which is caused by your body thumping against the ground during even repetition. Add in a sweaty t-shirt rubbing against the skin at the same time and the problem can get pretty nasty.
An exercise ball crunch gets you up off the floor and provides a cushion for your back while you perform the exercise. Get in the starting position for you exercise ball crunch by sitting on the ball with your feet on the floor and your knees bent naturally at a comfortable angle. If you cannot sit on the ball comfortably then it is probably too large or too small for you and you should try to find a more appropriately sized exercise ball for you.
Walk your feet out just one or two steps so that you are leaning back on the ball. The ball will be about at your lower back and your shoulder should not be resting on the ball. Now you can lace your hands behind your neck or cross them on top of your chest to get ready for your first exercise ball crunch.
Hold your abdominal muscles in tight as if you are trying to suck in your stomach and then lift your upper body straight up off the ball. You should feel the movement in your abs rather than in your neck or back. You don't have to sit up so far that you feel like you are going to fall over to do a proper exercise ball crunch just focus on contracting your abs and raising your upper body at least a few inches. As you become more comfortable with the movement your exercise ball crunch form will improve.
Lower yourself back down onto the ball and repeat the movement to perform another exercise ball crunch but do not bounce on the ball to perform the crunches quickly. You need to pause at the top and bottom of the movement so that it actually works your abdominal muscles instead of just using your momentum to bounce up and down. Bouncing may be fun but you can't call it an exercise ball crunch and you may just end up back on the floor when you bounce yourself right off the exercise ball.