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.

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.

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.”