FitFlops Supertone Suede in Cobalt Blue

FitFlops Supertone Suede in Cobalt Blue

Manufacturers say their shoes are safe if used properly.

"Just as the millions of pairs of high-heel shoes that are sold annually aren't suitable for everyone, Shape-ups may not be either," said Skechers' Armato. Shape-ups come with instructions and users are advised to limit wearing them to 25 to 45 minutes per day for the first two weeks, increasing usage time in 5- to 10-minute increments thereafter only if they're comfortable doing so. They should not be used for running.

Podiatrist Paul Langer of Twin Cities Orthopedics in Minneapolis said that although muscles likely do work harder to overcome the instability of the shoes, he is skeptical that it's to the extent that it will actually burn fat and boost muscle tone.

What they can do, Langer said, is redistribute the pressure on your foot, alleviating stress and strain on the joints, which is helpful for people suffering from arthritis in the foot. People should treat the shoes like fitness tools, Langer said. The most problems he sees are from overuse.

If you're in the market

Podiatrist Paul Langer offered these tips if you plan to buy toning shoes:

•Don't wear them if you have poor balance.

•Try on different kinds to assess which are most comfortable and suitable to your needs.

•Avoid cheap knock-offs, which tend to be flimsy on the sides.

•The shoes can be heavy, some weighing more than 16 ounces per shoe. Take that into account when buying, as heavier shoes can cause more muscle soreness.