Kate Dailey of Newsweek exposes the secret of crunches:
Everyone knows that the road to flat, tight abs is paved with crunches. Lots and lots and lots of excruciating crunches. Or is it? As it turns out, the exercises synonymous with strong, attractive abs may not be the best way to train your core—and may be doing damage to your back. “We stopped teaching people to do crunches a long, long time ago,” says Dr. Richard Guyer, president of the Texas Back Institute. That’s because the “full flex” movement—the actual “crunch” part of crunches – puts an unhealthy strain on your back at its weakest point. The section with the most nerves (and most potential for nerve damage) is in the back of the spine, which is the very part that bends and strains during a sit-up. – from Newsweek