While there are lots of ways to injure a back, the sit-up is an easily preventable one. According to his research, a crunch or traditional sit-up generates at least 3,350 newtons (the equivalent of 340 kg) of compressive force on the spine. The U.S. National Institute for Occupational Safety and Health states that anything above 3,300 newtons is unsafe.

So McGill suggests replacing sit-ups with exercises to strengthen the core while not bending the spine: bridges, planks, leg extensions, bird dogs, and “stir the pot.” The bird dog, for instance, simply involves getting on all fours and, while keeping the core muscles tight, extending the opposite arm and leg, then switching limbs. “Stir the pot” is a more complex movement: moving shoulders in a small circle while in a prone push-up position with forearms balanced on an exercise ball.

Patricia Treble, The man who wants to kill crunches, Macleans.ca, Jan 19th, 2010