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"Oprah hated these when she started doing them ten years ago," Greene recalls. "But now she can do them at the toughest incline. She has perfect form. Notice that she's got her chin up; the mistake that most people make is rolling their chin down to their chest, which creates the risk of neck injury."
Abdominal muscles respond well to daily training, according to Greene. "When they're done properly," he says, "crunches can help align the spine."