They've been dubbed the flip-flop with a gym built in.

Advertisements say by simply wearing them, you can walk your way to smoother thighs.

They are called FitFlops, the miraculous sandals with platform soles that make you exercise the muscles from your bottom to the soles of your feet while you walk.

They made Oprah's coveted "summer favorites" list in May, and have become a favorite among chronic knee-pain and back-pain sufferers.

The manufacturers say despite their being originally designed to help body-conscious women tone their buttocks and legs, stories have flooded in of how people crippled with arthritis or with back pain are suddenly able to walk again.

Launched in May 2007, FitFlops has sold more than a million pairs, mostly in the United States and the United Kingdom. Last summer, they were named among Newsweek magazine's "summer must-haves." This summer, the brand introduced the first men's model -- the Dass.

The shoe was engineered by Dave Cook, a biomechanist from the United Kingdom. Cook says the shoes feature a multidensity midsole to maximize muscle load while you're moving, minimizing the joint force you feel.

In laymen's terms, the FitFlop is said to produce a similar effect to walking barefoot, but with shock absorption. This makes your muscles work harder, creating more toned legs.

Celebrities love them. Hilary Swank, Heidi Klum, Jennifer Garner and Jessica Biel have all been photographed sporting the fitness flip-flop. Olympic long jumper Jade Johnson, who suffers from a painful foot condition, told a U.K. newspaper that she gained the same benefit from wearing FitFlops as she did from exercises recommended by her physiotherapist.

While the benefits of walking the suggested 30 minutes a day in the sandals are yet to be proved, FitFlop company founder Marcia Kilgore says a lot of people with severe and chronic back pain have reported a vast improvement after putting the shoes on.

"They were originally launched as a vanity shoe -- you could walk and help tone your legs. But over and over again, we'd hear things like 'I've had a degenerative spine disease, haven't been off morphine, and all I have to do is put on a pair of FitFlops and suddenly I don't feel pain anymore,' " Kilgore says in a FitFlops news release.

According to Philip Graham-Smith, a consultant biomechanist for UK Athletics and head of the directorate of sport at Salford University in England, the pain-reducing phenomenon is being investigated. He said he believes walking in FitFlops promotes better force alignment and a biomechanically correct gait, reducing stress on the knee.

This is one pair you will have to try for yourself. And that won't be hard, because this shoe seems to be popping up everywhere. I recently spotted them in an Avon catalog.

You can read more testimonials about FitFlops and find out where to buy them by visiting

They cost about $50.