A nice, flat stomach with a six-pack of muscles. We all want to know the best way to flatten our tummies. The Ab Slide claims to be a complete upper body workout, in just minutes a day. But does it really shape, strengthen and tone like it promises?

Look who we found working out at Health South, when we went to shoot this story! Our own Kathy Sweeney. It took some persuading, but Kathy finally agreed to help us test the Ab Slide.

"Rolling backwards first will damage the mechanism," Kathy reads the directions out loud. "All right, so you have to roll forward."

The Ab Slide is one of those roller wheels that promises big results in one easy exercise. But before we watch Kathy, a word of caution from a Health South physical therapist. "Certainly someone with a low back problem would want to be screened by a health care provider before they used an ab roller," says Wendy Nall, of Health South. "Or the same for somebody with should problems, knee problems, wrist problems." In other words, ab wheels are hard on you, especially your back!

"Arch your back..." Kathy reads as she starts her first exercise. Kathy is already in good shape and works out regularly. But she quickly discovers why Wendy warned it's not for beginners. "That hurt my back, just that first one!" she describes.

A few more rolls, and Kathy still feels pain in her back. "What am I doing wrong?" Kathy asks the physical therapist. "Am I going out too far? Okay."

And here's the kicker. Kathy is in too good a shape to shorten the movement -- yet her back won't let her extend all the way. Plus, she was concentrating on her back, arms and knees more than her stomach. "After a while, I felt it in my lower abs," says Kathy. "But it's not that immediate, burning feeling you get when you do crunches, to let you know you're getting somewhere."

Amy Jacquin admits she's not as fit as Kathy, and really notices the overall stress the ab wheel puts on her body. If you do insist on trying something like this, make sure you don't extend very far. "Go to about where you feel the top of your head is even with the front of the Ab Slide," Wendy instructs. "Not very far. It's all a beginner needs to do."

Ab wheels do offer short and long muscle contractions -- and they do also work your arms, back, and even hips. "Economically, they're one of the cheaper things you can buy to accelerate your program," add Wendy. But keep in mind floor exercises can give you the same results.

"I do about 500 crunches a day," says Mac Downs, who also works out at Health South. Mac is in very good shape. He has a version of an ab wheel at home, and says they're easy for him. "I feel it in my upper and lower abs," he describes when using the ab wheel. But despite the fact he says the wheel works, "I'll stick with my regular crunches!"

Nothing will get rid of your outer layer of fat without aerobic exercise and a good diet. Ab wheels can tone your muscles when used properly. But there's a high risk of back injury. And you can get the same benefits from a variety of crunches. So the $30 Ab Slide -- and all ab wheels in general -- get a C-.