How do you know if an infomercial "ab device" is a ripoff?

Simple: If you answer "yes" to any of the three questions below, consider it a red flag. And if you answer "yes" to all three questions? That's a dead giveaway.

Question #1: Are you being sold a complicated-looking (and expensive!) piece of equipment to do a simple exercise?

What you need to know: Typically, most infomercial products offer a "better" way to do the basic crunch or sit-up. These upgrades usually involve increasing your range of motion, so that you work more muscle. But when researchers at Slippery Rock University put several ab gadgets -- including the Ab Rocket, Ab Lounge, Bender Ball, and The Bean -- to the test, they discovered that the infomercial products didn't activate any more abdominal muscle than the classic sit-up. Here's a quick rundown of how each product fared.

Ab Rocket
This device actually edged out the sit-up in regard to upper-adominal activation. However, the sit-up fared better on lower-abdominal activation. Interesting note: The makers of this abdominal rocking chair tout its neck- and back-supporting design. Yet study co-author Jeff Lynn, Ph.D., says users reported neck strain and thought the motion felt "unnatural."

Ab Lounge
Participants enjoyed this exercise; their backs, however, did not. "It allows you to hyperextend your back," Lynn says. "This puts potentially damaging pressure on the low spine." For an at-home routine can help prevent and even alleviate back pain, check out The Best Exercises for Your Lower Back.

Bender Ball
Standard sit-ups produced 25 percent greater lower-ab activation than the Bender Ball. What's more, a rolled-up towel can serve the similar purpose of increasing your range of motion. So can a Swiss ball, which you can pick up for about $20, and is more versatile. That's because the Swiss ball allows you to perform a much wider range of exercises -- not just for your core, but for your upper and lower body as well.

The Bean
Banish this blow-up chair to the kiddy pool. "The Bean was the worst of the lot," Lynn says. "It actually facilitates movement forward, so you do less work."

Question #2: Does the ad claim that the machine makes the exercise effortless?

What you need to know: You won't sculpt your abs without some work. That should be a given. After all, if it were so easy to achieve a washboard stomach, anyone with a credit card would have abs like Georges St. Pierre. (For comparison’s sake, click here to see The Georges St. Pierre Workout.)

Question #3: Does the commercial suggest that simply using the device will transform a fat gut into flat abs?

What you need to know: If you aren't lean enough to see your midsection muscles already, you need to eliminate the fat that covers your abs. And simply doing ab exercises -- even a lot of them -- isn't going to be very useful. In fact, a new study from Southern Illinois University shows just how futile this approach can be. The researchers had people do two sets of 10 reps of seven different ab exercises -- sit-ups, dumbbell side bends, leg lifts, oblique crunches, Swiss-ball crunches, Swiss-ball twists, and regular crunches -- five days a week for six weeks. That's a total of 140 reps per workout.

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The result: The participants didn't lose belly fat or shrink their waist circumference. The take-home message: If you want to burn blubber, you need to work all of your muscles, not just your abs. (P.S. You also need to watch what you eat. Make sure to avoid these 11 Terrible Breakfasts.)

Of course, some ab-training tools are worth buying. For example, the aforementioned Swiss ball is a great choice, as are inexpensive but portable options such as the Core Slider and Valslides. (Read: These easy-to-stash products won’t clutter up your living room.) Plus, the TRX Suspension Trainer -- while pricier -- is actually a terrific value, since it allows you to do dozens of exercises for your every part of your body, including your core.

The bottom line: Any ab exercise can help make your midsection stronger. But if you want really want to work your core, you should first master the plank. This is a basic stability exercise that you'll see demonstrated in almost every issue of Men’s Health. The plank is one of the very best exercises for building core strength and endurance. And it's free.

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-- Reporting by Laura Roberson