"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.Repeated compression and strain on your spine can lead to herniated disks. Unlike the more regenerative parts of your body, your spine has a shelf life and putting unnecessary strain on it is a sure way to end up with eventual injury. What can you do if sit-ups are a no go?
Consider the pushup. Not usually thought of as a great ab move, the pushup forces you to work several muscles at once: it forces your core muscles to stabilize your trunk as your arms and back work to move the body up and down.
"Do you see how a pushup is a full body challenge?" says McGill. "It challenges abdominals, front of your legs, your arms and your back. That is how you use those muscles in real life."Source: lifehacker.com