The traditional crunch (sit-up) -- The person lies flat on his back with knees bent, then he sits up to touch his knees. Not only is this exercise ineffective in working the abdominal muscles, but it also puts a great deal of strain on the neck and spine.

The straight-leg sit-up -- This is a variation of the basic sit-up in which the person's legs are straight instead of bent; this often involves another person holding the feet stabilized. This exercise puts pressure on the lower spine.

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The Roman chair sit-up -- This variation begins with the person in a sitting position, with legs flat on the floor; he then lowers his back from a 90-degree angle to about a 120-degree angle. This exercise causes a compression of the spine that can be dangerous. In addition, the hips do most of the work, so the abs are not necessarily being worked at all.

Side bend with weights -- The person stands with arms outstretched (holding weights) and bends to one side and then the other. While the side bend can be useful in working the abdominal muscles, the use of weights actually negates the resistance by creating momentum when the person moves from side to side. Not only that, but because this exercise does not diminish fat at all, it can actually increase the size of the waist by creating a layer of muscle under the fat.