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 ...

With each year that passes, I really don't notice any "noticeable" decline in my movement efficiency or strength. I cruise through most weeks with three or four workouts and pretty much keep trucking.

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." ...