Contrary to popular belief, sit-ups don't protect your back. Studies show that building static back strength is the key to preventing injury.
Sit-ups and crunches actually compress your spine more than is allowed by occupational-safety standards.
You risk even more damage if you do the move of the moment: a sit-up on a physio ball. Hyperextending your back below its midline is a no-no.
Flexing forward at the waist is also a bad idea. Your abs were designed to be a rigid hoop to support your spine.
Sit-ups are a far less effective abs exercise than planks, which work your six-pack and obliques up to 25 percent more efficiently than sit-ups or crunches.
—By Paul John Scott