But the studies don’t show whether more engagement leads to meaningful changes in muscle tone or appearance over time. Nor is it clear whether the high level of engagement continues once the walker becomes accustomed to the shoe.

Reebok’s EasyTone has made the biggest splash in the muscle-shoe market, especially with its advertising. In one commercial, the camera drifts away from the woman’s face and zooms in on her backside. Another advertisement claims that the leg and butt-toning effects of EasyTone will “make your boobs jealous.”

The advertisements, aimed at younger women, have appeared in magazines and online, and a big television campaign is under way: 3,000 commercial slots have been scheduled on network and cable in November and December.

But the claim that the shoes offer muscle toning is backed by a single study involving just five people, not published in a peer-reviewed academic journal. In that study, done at the University of Delaware, five women walked on a treadmill for 500 steps wearing either the EasyTone or another Reebok walking shoe, and while barefoot. Using sensors that measure muscle activity, the researchers showed that wearing the EasyTone worked gluteal muscles an average of 28 percent more than regular walking shoes. Hamstring and calf muscles worked 11 percent harder.

Reebok’s head of advanced innovation, Bill McInnis, said the size of the study was adequate to determine the effect of the shoe and added that exercise studies of this nature commonly used small numbers of participants.

The EasyTone is the brainchild of Mr. McInnis, a former NASA engineer, who said he was interested in the stability balls used in gym workouts and wanted to translate the technology to a shoe. In particular, he was intrigued by the Bosu ball, a small half-sphere that exercisers stand on during workouts as a way to engage leg and core muscles better.

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In designing the EasyTone, Mr. McInnis and his team sought to mimic that concept by adding “balance pods” to the toe and heel of the shoe. As the person walks, the air pushes back and forth between toe and heel, and the person sinks into the shoe. The effect is similar to that of walking on a sandy beach — which requires more work, balance and muscle engagement than walking on a flat surface.

John Lynch, head of United States brand marketing for Reebok, said the company’s market research showed that four out of five women were especially interested in products that toned their leg and gluteal muscles. Mr. Lynch added that retailers were reporting brisk sales of the shoe; one Los Angeles sporting goods store reported that its Reebok sales more than doubled in November.

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Reebok says it has collected 15,000 hours’ worth of wear-test data from shoe users who say they notice the difference. “They definitely feel something in their muscles after they’ve walked in the product,” Mr. McInnis said.

One of them is Carol Vanner, 51, an executive assistant in Atlanta who had tried the larger-soled FitFlop shoe and was skeptical she would notice much difference with the EasyTone.

“I thought there was no way they would work, but I tried them and I felt like I had worked out,” she said. “Do I look like I’m 20? No, but I feel like when I wear them for periods of time that I have exercised and worked those muscles.”

Shay Gipson, 31, an apparel product manager in New York City, said she tried the shoes after hearing a friend rave about them. She immediately felt the balancing effect, she said, and she likes walking in the shoe.

“I can definitely feel the muscle groups in my legs working more than I would in regular shoes,” she said. “I feel more toned.”

But it remains to be seen whether such effects will make a difference over time. In a July 2008 study of instability boards and balls, Canadian researchers found that among experienced exercisers, moderate instability balls like the Bosu had little effect on muscle activation.

The shoes are designed only for walking, and because of the instability design, wearers are discouraged from running, jumping and engaging in other athletic activities while wearing them. So the real effect may come from simple awareness that they are wearing a muscle-activating shoe, causing them to walk more briskly and with purpose.

“I think buying them with this in mind is likely to increase mindfulness, which is good for health,” said Ellen J. Langer, a Harvard psychologist who has studied the connections between mindfulness, exercise and health. “It will probably result in even more walking, with the implicit and explicit virtues endemic to exercise.”