By Dorene Internicola - (Reuters Life!) - Crunches, curls and sit-ups may be standard workout fare in gyms, basements and living rooms across the land. But the authors of a new book suggest people get plenty of that movement in their daily lives. They say to get a really strong midsection the back of the body needs to be worked. "Sitting at desks, working on computers, waiting in traffic, we are continually contracting our abs, throwing our shoulders forward and, ultimately, shutting down the back of the body, said Dr. Eric Goodman, co-author with Peter Park of "Foundation: Redefine Your Core, Conquer Back Pain, and Move with Confidence." "If we're going to keep our posture and our spines strong, it has to be done