With more than a million pairs sold, the FitFlop has hit its stride.

Billed as “the flip-flop with the gym built in,” the sandals promise a workout with every step. Devotees, celebrities among them, report dramatic results: trimmer legs, improved muscle definition, and decreased joint pain. Supermodel Heidi Klum recently touted the FitFlop’s virtues on “Oprah,” and newswoman Diane Sawyer and singer Carly Simon have been photographed wearing these thick-soled shoes.

Launched a year ago, the FitFlop (available in women’s sizes only) has capitalized on the craze in wellness footwear — a category that includes the SpringBoost and Lytess Muscle Trainer athletic shoes, and an ever-expanding array of sneakers and sandals by Masai Barefoot Technology, or MBT. That a shoe could provide benefits beyond that of the average sneaker is something that appeals to American consumers, who are increasingly turning to footwear said to deliver everything from firmer thighs to increased calorie burn to a reduction of cellulite.

The idea behind products such as MBTs or FitFlops, which simulate a sensation similar to walking through sand, is to destabilize a portion of the foot. This forces the body to compensate by activating often-neglected muscles in the lower body. While MBTs do this by positioning the heel at a point lower than the rest of the foot so that each step incorporates a roll forward, FitFlops are outfitted with miniature built-in wobble boards. “You’re forced very quickly into the soft midsection, where your foot wobbles a bit,” a FitFlop spokeswoman, Katie Neiman, said.

Sales of wellness footwear have been on the rise in recent years — thanks to the continued success of the MBT, and the launch of the FitFlop, the director of consumer insights for the spa chain Bliss, Christopher McGrath, said. The FitFlop, he noted, has become one of the company’s “most successful third-party brands” — with an average of 5,000 pairs sold each month through the Bliss catalog and Web site.

Most styles of the FitFlop, which include striped, fringed, and sequined varieties, sell for between $50 and $60. The limited-edition gladiator-style “Aurelia,” born of a design collaboration with the Kirna Zabête boutique in SoHo, retails for $150 to $165. That’s still well below the price point for the more clunky-looking MBTs, which start at around $240.

In New York City, where many residents travel on foot to school, work, or the grocery store, products such as the FitFlop have a particular appeal. The owner of Tip Top Shoes, Lester Wasserman, said his Upper West Side store sells as many as 60 pairs of FitFlops a day. He noted customers are anticipating the introduction this fall of the FitFlop’s first closed-toe model, which will resemble a clog. “In this city, where walking is the no. 1 mode of transportation, people figure, ‘Why not get in a workout just moving about?'” Mr. Wasserman said.

That’s precisely what compelled Sarah Falender, a Manhattan resident who walks 30 minutes to and from work each day, to buy a pair of FitFlops about a month ago. “I thought they were pricey for a pair of flip-flops, but they’re really worth it,” Ms. Falender, 25, told The New York Sun. “My calves are tighter, my jeans are looser, and my knees don’t bother me as much. For me, these shoes have replaced the gym,” she said.

But don’t expect a miracle, a Park Avenue podiatric surgeon, Suzanne Levine, said. “You’re going against a little more resistance, so you’re going to get a little something extra, but these are not shoes that are going to make your thighs look like they did when you were 21,” she said.

Ms. Levine, who owns a pair of FitFlops, said there’s a psychological component to the success of the FitFlop: The idea that these flip-flops improve fitness might inspire the person wearing them to increase the distance and pace of a walking workout — making it difficult to discern how much the shoes, themselves, are doing.

She advises wearing FitFlops for no more than two or three hours at a time. “They’re not for long-distance walking — they don’t have the necessary arch support — and they’re not a substitute for sneakers,” Ms. Levine said, noting that extended wear could lead to foot problems, such as heel spurs. “The idea that these are shoes that you want to live in is not something I endorse. That said, FitFlops are comfortable and they have a fabulous place in a woman’s wardrobe.”