"Forget the gym -- you just need special shoes!" With a pitch like that, no wonder toning shoes are generating sizzling sales. Is this craze a sustainable trend, or another fad that could leave investors with scrawny, underdeveloped returns?

Toning shoes supposedly work out the leg muscles while you go about your normal daily business. According to their promoters, the shoes not only tone wearers' legs, but also help reduce the "junk in your trunk." A Skechers (NYSE: SKX) ad says it all: "Get in shape without setting foot in the gym." I'm sold!

… Or not
A recent NPR report pointed out two new studies from the American Council on Exercise, which found "no significant difference between any of the toning shoes and the standard running shoe." Conversely (no pun intended), the American Podiatric Medical Association claims that certain brands of toning shoes, including Reebok's, are "beneficial to foot health and of significant value."

Toning shoes are currently the hottest segment in shoe fashion, with sales expected to surge to $1 billion this year. Companies like Skechers, Reebok, and MBT have been making a killing off this trend, selling the shoes for between $100 and $245 per pair.  

Nike (NYSE: NKE) has refused to play into the hot toning shoes market, though, expressing skepticism about the shoes' claims. It even launched a counter-ad campaign for its Nike Trainer One, claiming, "This shoe works if you do."

Fads can wreak havoc on stocks, making temporary eye-popping sales gains look tantalizingly sustainable to investors. Shoe stocks are particularly vulnerable to these wild swings. Look no further than the histories of Crocs (Nasdaq: CROX) and Heelys (Nasdaq: HLYS) to see the dangers of fading fads. To its credit, Skechers has a history of adeptly capitalizing off different shoe fads with its own offerings.

If toning shoes aren't a fad, and have staying power among consumers who feel their new kicks really are beneficial, Nike may have really put its foot in its mouth by missing the trend. The company behind the famous swoosh has lost share in women's footwear, thanks to its rebellion against the toning shoes craze.

Are toning shoes' robust sales sustainable, or juiced up by a fad? Are the companies making them nimble enough to evolve if the tide of consumer taste turns? What do you think of the toning-shoes craze? Let us know the comment box below.