Dr. Mercer, a professor of biomechanics at the University of Nevada, Las Vegas, was talking with a friend who runs an athletic shoe store. The friend told him that customers were coming in and requesting toning shoes, which are soft sneakers, often with a rocker-shaped sole, that promise to exercise and tighten muscles in the calves, thighs and buttocks. (“Your boobs will be jealous,” a refined advertising campaign for one of the shoes declared.) Many manufacturers make them: Reebok, New Balance, MBT, FitFlops and Crocs, among others.
The store owner carried various models of the toning shoes. But, he told Dr. Mercer, he was uncomfortable recommending them to his customers, because he didn’t know if they actually functioned as claimed.
Dr. Mercer didn’t know, either. So he recruited a group of healthy young female students (toning shoes are marketed almost exclusively to women) and had them walk on a treadmill for 10 minutes at a time while wearing, alternately, a walking shoe or a toning shoe — in this case, the Skechers Shape-ups. He and his colleagues attached sensors to the women’s legs to measure the electrical impulses generated as their muscles contracted. They also determined the women’s oxygen consumption, to see if they worked harder and burned more calories with one shoe rather than the other.
But as it turned out, according to results presented in June at the annual meeting of the American College of Sports Medicine, muscle activation and oxygen consumption were almost identical whether the women wore walking shoes or Shape-ups. The finding “was a little surprising,” Dr. Mercer said, since his volunteers commented that the toning shoes, with their bowed, unstable bottoms, felt different underfoot from the walking shoes. But that difference didn’t change how they moved in the various models, he said.
Dr. Mercer’s study joins a small but growing body of science about toning shoes, much of which does not support the makers’ claims. A study conducted last year by exercise physiologists at the University of Wisconsin-La Crosse, for instance, found that muscle activation and calorie burning did not change whether people wore ordinary athletic shoes or any of three different models of toning shoes. “There is simply no evidence to support the claims that these shoes will help wearers exercise more intensely, burn more calories or improve muscle strength and tone,” the authors concluded.
Other results have been a bit more equivocal. A 2009 study showed greater muscle activation when women wore the Reebok toning shoe, but it involved only five women and was financed by the shoe company, as my colleague Tara Parker-Pope reported at the time. A newer experiment presented in June at the sports medicine meeting showed that when someone walks in a rocker-style toning shoe, forces generated by the foot striking the ground move up the leg differently than if that person wears a walking shoe. But this shifting of forces had little discernible effect on muscle activation.
“We know that the force value changed,” said Heidi A. Orloff, a professor at the University of Puget Sound who oversaw the study. “We can’t say whether there are benefits to that or not.”
Meanwhile, in perhaps the most telling and longest-term study of the shoes, Canadian researchers at the University of Calgary last year had volunteers wear a rocker shoe throughout the day for six weeks. In the beginning, the volunteers wobbled in the unstable shoes, activating and strengthening small, underused muscles in the feet and ankles that stabilize balance. But after six weeks, the swaying had diminished and those stabilizing muscles were not being exercised to the same extent. The toning shoes, in other words, had provided benefits, but for a limited time and not to the big, showy muscles in the wearers’ calves and buttocks.
(Both the University of Puget Sound and the University of Calgary studies were financed in part by shoe companies.)
Disappointment with the performance of toning shoes has begun to percolate into the wider world. This year lawsuits were filed against several makers of toning shoes, claiming that the shoes had not fulfilled their promises or had caused injury.
Little scientific evidence exists yet about injury risk from the shoes, and Dr. Mercer said there was no obvious biomechanical reason why the design of the shoes would contribute to such problems. (His work was not financed by shoe companies.)
Meanwhile, those finding fault with the shoes may be missing a broader lesson, he continued. The subtext of his and other studies, he said, is that the human body is endlessly ingenious and utterly indolent. “Humans are quite lazy, from a physiological standpoint,” he said. “Our bodies will try to do the least work possible in any situation.” So in even as short a time as the 10-minute walks that his subjects completed, people’s large leg muscles adjusted to the rounded soles of the toning shoes and expended no more energy than in everyday shoes.
That finding does not mean that toning shoes have no utility, he added, which is the message that he ultimately passed along to his store-owning friend. Toning shoes, in his study, might not have functioned as advertised, but “some people love how they feel,” Dr. Mercer said, “and if that’s enough to get those people out and moving, then, in my opinion, the shoes are working fine.”