Forget for a moment that the shape of your midsection largely determines how good you'll look on the beach this summer (and in the bedroom this weekend). We'll get back to that in a minute. The pursuit of abs goes deeper. Most people strive for a six-pack as if their lives depended on it, and science proves that it does.

At a meeting of the North American Association for the Study of Obesity, researchers reported that waist circumference is more conclusive than either weight or body-mass index (BMI) as a measure of disease risk. Unlike fat directly under the skin, belly fat, which adheres to organs, is associated with increases in C-reactive protein (CRP) and other markers of inflammation that can lead to heart disease.

Motivated yet? Good. But don’t think for a minute that the path to a six-pack is lined with crunches. A recent study by the Biomechanics Laboratory at San Diego State University found that this classic move didn’t even rank in the top 10 abs exercises. The reason: Crunches don’t require much abdominal stabilization or body rotation—two key ingredients for sculpting a rock-solid six-pack.

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