Can a type of footwear once associated with hippies be the new key to toned bodies?

If numbers tell us anything, then there are a lot of men and women who are banking on the FitFlop, dubbed by the manufacturer as "the fastest-selling fitness shoe in history," and as a potential saving grace for busy women and men, looking to tone up and trim down.

FitFlops have sold more than 1 1/2 million pairs since they debuted in the U.S. and the United Kingdom a year ago, where they were met by salivating mobs of women. At around $50 a pop and higher, it looks like their makers have hit on something big.

"I'm from a beauty background, so I know that women want something they can easily use that fits into their schedule," said Marcia Kilgore, founder of Bliss Spa and FitFlops.

Kilgore said she got the idea for the sandals while talking to some of her friends.

"I thought, well, 'What about a flip-flop with a gym built-in? A flip-flop that tones your legs when you walk around in it,'" Kilgore said.

So, with the help of two scientists from London's South Bank University, her dream fitness shoe arrived, and in good time. Increasingly, fitness classes that utilize balancing during weight exercises, as well as barefoot exercises to better activate the leg muscles, are becoming more and more common in gyms across the country. Companies, such as Nike, even have shoes that simulate walking barefoot. According to Kilgore, her sandals tap into that idea.

"When you step in the center section, which is quite soft, the eversion rate of your foot, which is how fast your foot rolls over, slows down on this midsole because there's a little bit of wobbling going on," Kilgore said. She claims that by causing the foot to step that way, the FitFlop activates the buttocks, hamstring and the thigh muscles, giving you a little workout with every trip out of the house.

Big Claims About Fitness

This stylish fitness shoe for women claims to tone thighs, burn calories and erase cellulite. They were an instant hit, receiving buzz on television shows across the U.K. and the United States. (Don't worry guys, men's versions will be available in the U.S. later this summer.) Recently they landed on one of pop culture's most coveted lists, Oprah Winfrey's "Top Picks for Summer."

But, are these shoes more hype than science? ABC News brought the FitFlops to a running coach and a sports medicine doctor for their evaluation.

ABC also enlisted the help of two very busy pavement-pounding New Yorkers.

"I'm going to be able to really notice, 'Is this different? Do I feel a difference?'" said Story von Holzhausen, a personal trainer at gyms, like Equinox, who is in tune with her body. She's also a mom to twins.

Von Holzhausen walks five miles a day. Between training sessions, fitness classes and, of course, the needs of two 7-year-olds, and she has even worked as a consultant evaluating other althetic shoes.

Our other evaluator was Minnie Dent, an aspiring singer and waitress at O'Neals restaurant in Manhattan, who regularly spends eight to 12 hours a day on her feet.

We brought the two to Tip Top Shoes in New York to get them fitted, and asked them to wear the sandals for a week straight. We got two very different reactions.

Two different reactions, but what do professionals say? We had correspondent Bill Ritter take his pair of FitFlops to running coach Mindy Solkin for a biomechanical analysis at the Running Center to see what the shoes did to his body when he walked around. Ritter also brought the shoes to a sports medicine doctor, Lisa Callahan, at New York's Hospital for Special Surgery, to evaluate the shoes. Their opinions?

Tune into "20/20," Friday, July 11 to find out.