There was a time when the term sit-up meant supine hip flexion--sitting up from the hips while your lower body stayed stationary. This exercise worked your hip flexors more than your abs. Nowadays, the term "sit-up" is synonymous with the more appropriate crunch, which does, in fact, work your abs. Exercise balls make sit-ups even more challenging by forcing your abs to not just lift your body against gravity but stabilize it against the ball's constant threat of instability.
Sizing
The exercise ball will only put an appropriate strain on your abs if it's the right size. The American Council on Exercise recommends that you size the ball according to your height. If you are under 5 feet tall, you should use a 45cm ball; between 5-foot-1 and 5-foot-7, a 55cm ball is appropriate. A 65cm ball will suit you if you are 5-feet-8 to 6-feet-2, while if you are over 6-feet-2, you can use a 75cm ball.
Effects
Exercise ball sit-ups primarily work the rectus abdominus, a long strap of muscle that runs from your ribs down across the front of your pelvis. You can also do oblique sit-ups or crunches on the exercise ball; these work your inner and outer obliques, which are responsible for twisting your torso as it flexes.
Function
To use an exercise ball for sit-ups, sit down on the ball and walk your hips forward as you lean back until the center of your back rests on the ball. Squeeze your lower ribs down toward your pelvis, just as with a standard crunch. Your shoulders will lift up slightly and you may feel your lower back pressing down against the ball, but otherwise the proper crunch motion is fairly subtle; at no point should you flex at the hip as if you were doing a standard sit-up on the ground. You'd fall right off the ball if you did.
Expert Insight
According to a San Diego State University study commissioned by the American Council on Exercise, exercise ball crunches are arguably the best overall ab exercise because they recruit the hip flexors the least. In other words, exercise ball crunches put the most strain on your abs of any other versions of the crunch included in this study, including traditional crunches and various crunch-related exercise machines like the Ab Roller and Torso Track.
History
The Swiss ball--another name for exercise balls--was originally created as a toy in the 1960s and marketed under the product name "Gymnastik." Dr. Susan Klein-Vogelbach, a Swiss physical therapist, pioneered the use of exercise balls in a clinical setting. Since the 1980s, the exercise ball has slowly been working its way from rehabilitation settings into gyms and other athletic settings. It is now an accepted, even expected, piece of standard gym equipment.
Modifications
You can make your exercise ball sit-ups more challenging by extending your arms straight overhead, lengthening the lever your abs are working to lift. Once this is no longer a challenge, hold a dumbbell or weight plate against your chest, then work up to straightening your arms so that the weight is directly over your shoulders throughout the range of motion--this challenges your core muscles to keep your body stable on the ball. You can also lift one leg slightly off the ground, leaving yourself only one leg to balance on as you do your sit-ups.
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