Stability ball exercises can activate more muscle fibers if done correctly.
The stability ball is one of the most versatile and cost effective pieces of exercise equipment. You can take it with you and do exercises at home, work or on vacation. Doing your ab crunches on the stability ball is superior to the traditional floor crunches. According to researchers in the Department of Kinesiology at Occidental College in Los Angeles, crunches performed by 41 test subjects on the stability ball showed significantly greater abdominal muscle activation than the traditional floor crunches.
I recommend doing the ab ball curl up (pictured below) instead of crunches to avoid low back injuries.
The results found heightened activity in the upper rectus abdominis, lower rectus abdominis and external oblique activity by 31%, 38% and 24%, respectively, when compared with the traditional floor crunch. The rectus abdominis muscles are affectionately called the "six-pack" of the abdominal region. There is also another great reason to do your ab curl ups on the stability ball: you achieve a greater range of motion during the exercise because you are able to go back further and extend your trunk along the contours of the ball. When you first try ball curl ups, it will feel very unstable to you. Don't let that stop you---you will adjust to this exercise!