Even if you don’t have an ounce of fat, you could be soft in the middle. Watch the video to see one of the best tests of core of strength, courtesy of Gray Cook, P.T., author of Movement: Functional Movement Systems.

It’s a simple way to determine if your midsection isn’t as strong as it should be.

Warning: The fact that you regularly do ab exercises doesn’t guarantee you’ll pass this test. “You can do lots of crunches and situps and still have a weak core,” says Mike Wunsch, C.S.C.S., director of training at Results Fitness in Newhall, California, “I see that all the time.”

Related: THE 21-DAY METASHRED—an At-Home Body-Shredding Program That Strips Away Fat and Reveals Rock-Hard Muscle

The reason: Classic ab moves like crunches and situps work the muscles that allow you to flex (that is, round) your lower spine. True core exercises, on the other hand, train the muscles that prevent your spine from rounding.

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The bottom line: If you can’t pass this test, you need to upgrade your ab workout. The perfect solution: Check out this 15-Minute Core-Carving Complex which is designed to give you rock-hard strength while melting away belly-fat.