Check this out: http://blog.newsweek.com/blogs/thehumancondition/archive/2009/06/03/stop-doing-sit-ups-why-crunches-don-t-work.aspx
From the article: “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.
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.”
Like the pushup, the best exercises for back health and a firmer stomach are ones that work your abs while holding your spine straight, like planks or leg drops (done when you lie flat on your back, with your hands at the base of your spine for added support. Raise your legs up at a 90 degree angle, then slowly lower until they’re only inches from the ground. Repeat until your stomach burns and you want to throw up).
Makes sense to me. Any thoughts ?