Reality-TV shows like MTV's Jersey Shore show how crunches can ripple the abs, but it wasn't until the last decade or so that we've fully understood what these intense sit-ups do to our spines. "They're really hard on the discs," says Dr. Michael R. Bracko, an exercise physiologist and certified strength and conditioning specialist in Alberta, Canada. "Imagine that the discs are like jelly doughnuts stacked in between each vertebra," he says. "If you repeatedly flex the spine by doing crunches, it could cause the disks to rupture." (That's a situation we'd like to avoid.) The best way to work the core is to keep the spine in a neutral position, Bracko says. He recommends planks, which work the rectus abdominis as well as the obliques.
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