Your favorite ab-building machine at the gym may be causing you more harm than good. That's right—we're talking about the torso rotation machine. This is the Chubby Checker–inspired ab machine you sit ...

Even after three months at the Port Charlotte YMCA, Cara McCarthy hesitates at the torso-rotation machine. "I'm still figuring this one out," she said. "Adjust it and lean back, I'm thinking." ...