http://well.blogs.nytimes.com/2009/06/17/core-myths/
“There’s so much mythology out there about the core,” maintains Stuart McGill, a highly regarded professor of spine biomechanics at the University of Waterloo in Canada and a back-pain clinician who has been crusading against ab exercises that require hollowing your belly. “The idea has reached trainers and through them the public that the core means only the abs. There’s no science behind that idea.”
Oh no. I've been focusing on my abs lately. Maybe I should follow the advice from this paragraph:
Instead, he suggests, a core exercise program should emphasize all of the major muscles that girdle the spine, including but not concentrating on the abs. Side plank (lie on your side and raise your upper body) and the “bird dog” (in which, from all fours, you raise an alternate arm and leg) exercise the important muscles embedded along the back and sides of the core. As for the abdominals, no sit-ups, McGill said; they place devastating loads on the disks. An approved crunch begins with you lying down, one knee bent, and hands positioned beneath your lower back for support. “Do not hollow your stomach or press your back against the floor,” McGill says. Gently lift your head and shoulders, hold briefly and relax back down. These three exercises, done regularly, McGill said, can provide well-rounded, thorough core stability. And they avoid the pitfalls of the all-abs core routine. “I see too many people,” McGill told me with a sigh, “who have six-pack abs and a ruined back.”