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Having strong abdominal muscles not only looks good, it's a hallmark for good health. But if you want to get six-pack abs, your daily 100-crunch routine may not only hinder your progress, it could hurt your lower back by putting stress on its weakest part.

We're always told to bend at the waist and lift with the legs when picking up something heavy to prevent a back injury. Well, a crunch is the same kind of motion, only you're lying down instead of standing up. Repeatedly bending over during crunch sessions slowly breaks down spinal disks, creating a disk bulge or herniation, which can cause back pain, weakness and tingling. "There are only so many bends or a 'fatigue life,' in your spinal disks," Stuart M. McGill, a professor of spine biomechanics at the University of Waterloo, told Newsweek. "When people are doing curl up over gym balls and sit-ups, and this kind of thing, they are replicating a very potent injury mechanism on their back," says McGill. "Every time they bend it they are one repetition closer to damaging the disk."

Back pain aside, crunches aren't even the best method to flatten abs. In fact, if you strictly strengthen the rectus abdominus muscles by doing crunches, you could cause your stomach to become abnormally distended instead of flat. "When performing crunches some people add extra resistance, and the primary focus is for strength and to increase the strength of the abs. We're usually talking about adding sheer size to the muscle," says Jessica Matthews, a certified trainer and spokesman for the American Council on Exercise.

Doing exercises that stabilize the core and keep the spine straight, such as push-ups, planks and side bridges, can help prevent back injuries and combat overworking one abdominal muscle group.

Just to be clear, crunches do have some merit if they're done moderately and correctly. People often run into problems only when they lose proper form after doing 50 or more crunches in one set. "There is benefit to [the crunch]," says Matthews, who cited a 2001 ACE study on the effectiveness of the crunch. "Out of 13 exercises, the traditional crunch came in eleventh. It did activate the rectus abdominals and the obliques, but there are quite a number of exercises that are ranked as more effective." The top five exercises for activating both muscle groups were the bicycle maneuver, captain's chair, the exercise ball crunch, vertical leg crunch and reverse crunch. For a complete list of all the exercises that were studied and which were most effective, click here.