Bent-over twist -- The person bends forward at the waist at a 90-degree angle and then twists from one side to the other. The spine and ligaments do most of the work in this exercise, so excessive use of this exercise can put unnecessary strain on both those parts.

In addition to these five exercises, a number of abdominal machines are ineffective and potentially dangerous. The electronic ab stimulator does not really do anything. The ab rocker is about 80 percent less effective than the traditional crunch done properly. {relatedarticles}The ab wheel and TorsoTrack machines can be useful in maintaining great abs for someone who has the upper body strength required to use the machine. If you do not have the upper body strength already, then the ab wheel basically just works the arm and shoulder muscles. The Nautilus machine is effective but can be dangerous if not used properly.

The bottom line for great abs is that they take time, but you can help yourself by making sure the exercises you are doing actually target the inner core muscles without damaging the lower back.