Claims about fitness sandals based on science

Do workout flip-flops really work?

Doubtless you've seen the slew of ads for so-called "toning" sneakers and sandals? The camera focuses on the lower half of a slender model wearing the teeniest of shorts, lingering on her pert rear and flawless legs, then traveling down to the miracle shoe on her foot that helped her get cellulite-free, jiggle-free, wiggle-free thighs.

I did a quick Google search and found scads of trendsetting celebs gushing about these shoes (Jennifer Lopez, the curvy girl's style icon, swears by FitFlops and even blogged it). Being the trend fiend that I am, I immediately went out and bought some of the toning sneakers last winter. Then this spring, because it's sandal season, I bought some of the newest models of toning flip-flops.

And guess what? They work.

I work out six days a week doing Pilates, Zumba, boot camp, body combat and running, so I was skeptical about them actually having any benefit. But after walking around for the past six months, just for errands, in my Reebok EasyTone sneakers and spending the past few weekends tooling about in my New Balance Rock & Tone sandals, I can definitely see a difference. I also own a pair of FitFlops (launched in May 2007, FitFlops are the original toning sandals, assures Katie Neiman of FitFlop), which work as well to me as the New Balance and Reebok.

After I began to wear the shoes regularly, I noticed that my behind was sporting a higher (if not smaller) profile, the back of my thighs had even less wiggle, and I could actually feel my stomach muscles working because the slight instability that the shoes create forced me to stand straighter and use my core to do so.

But was I imagining it? Did I just so believe in the power of shoes that I wanted them also to be able to give me flatter abs?

How they work

Kathleen Piercy, a physical therapist and owner of Piercy Pilates in Huntington Beach, Calif., says no, I'm not imagining things.

"These toning products create a micro-instability with every step, calling in core muscles to stabilize and stay on longer to correct the micro-imbalance," Piercy says. "The engineering is based on the [same principle as a] balance ball. When one does a sit-up on an even surface, so many muscles are used. Now do that same sit-up on a balance ball, and many other muscles are used, not only to sit and stabilize, but also to do the work."

The Microwobbleboard (that's what they call it) technology of the FitFlop and the engineering of the New Balance Rock & Tone and Reebok EasyTone sole (which looks like it balances on two halves of a rubber ball) force you to engage your glutes, thighs and calves while providing a comfortable, shock-absorbing padding for your foot and knee. Some of the shoes' engineering, such as that of the Reebok EasyTone, even increases the engagement of abdominal muscles as you walk and stand.

"All can benefit from the joint protection that the shock-absorbing midsole provides," says FitFlop's Neiman in an e-mail, because FitFlops absorb up to 22 percent more shock than regular footwear. (The percentages differ, but all such toning shoes absorb more shock.)

I could see how the design of the shoes' über-cushy sole, arch support and lightweight construction could make walking more fun and comfortable, but could it really burn up to 10 percent more calories, as some of the shoemakers claim? (I was skeptical because the shoes only work if you are moving, and if you are only moving because you are wearing them, then you are going to burn calories anyway because you are moving, right?)

Turns out, because the shoes cause that slight instability, they can not only increase muscle activation by up to 14 percent in your calves, 29 percent in your quads and 16-28 percent in your glutes and hamstrings (hello, toning!), but the increased engagement of more muscles increases the calorie burn, Piercy says.

"If you just do bicep curls brainlessly" you burn only so many calories and work only your arms, Piercy says. But "if you're engaging your abdominals, the more often more muscles kick in, the more calories you can burn," she says.