The reverse crunch is commonly used to target the lower abs. Sadly, this is a myth. It is impossible to isolate specific areas of the rectus abdominis. Your midsection always contracts as one muscle group. With that said, the reverse crunch is still a fairly decent abdominal exercise. The only potential downside is that it does not completely isolate the abs, due to the involvement of the hip flexors. The hip flexors are the muscles of the upper leg the allow you to kick your leg forward (e.g. as in running).

The purpose of the workout ball is to add resistance to the exercise. The ball also forces you to use your hamstring muscles to secure the ball under the legs. So there are some advantages to using it. Listed below are instructions for performing the reverse crunch with a workout ball:

Lie down on an exercise mat. Secure a workout ball under your legs. Place your hands flat on the ground next to your buttocks. Lift the ball toward your chest. As your knees approach your chest, try to elevate your hips off the ground. This will enhance the abdominal contraction. Be sure to keep your head down on the mat at all times.