Fall's waist-cinching belts shine a spotlight on meticulously whittled middles. For those of us currently sporting abs of a more convex variety, Michele Olson, Ph.D., who presented her latest ab research at this year's American College of Sports Medicine conference, suggests getting on the ball for the most efficient ab workout: Lie on your back and slide a small core ball (or soccer ball) snugly beneath you, so its apex is at the back's curvature. Sit up, hold, lower, and repeat 10 times. Do three sets. For even stronger, trimmer abs, enhance your routine with pilates, kickboxing (which strengthens as you work up a sweat), and planks.

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