Those infernal Heelys.

The trend started innocuously enough: A few kids popped up in malls, zig-zagging around shoppers quick and nimble as fleas, in pint-size tennis shoes with rollers hidden in the heels.

They seemed almost cute at first — budding figure skaters landing triple axels in store aisles.

Heelys had all the marks of a passing fad — poor-man roller skates that don't go particularly fast or far — but kids found that the shoes gave them secret powers. They could walk around normally, then quietly shift their weight to the back heel and transform themselves from ordinary kids to superheroes quicker than their parents could say "Stop that!"

Clearly, the shoes have been on a roll. Sales topped $40 million in 2005 and $188 million in 2006, the same year Heelys Inc. went public. In addition, Heelys logged nearly $50 million in sales the first quarter of this year, putting it on track to top last year's sales.

In fact, since the introduction of Heelys in late 2000, sales have been nothing short of phenomenal, creating an even more alarming development: Of the millions spent on Heelys last year in more than 70 countries, an estimated 15 percent of the shoes sold were for adults. Big, brittle-boned adults. Recent research suggests that kids are at an increased risk for injury in Heelys. One can only imagine what happens when an adult straps on a pair.

At the Sport Chalet in Long Beach, Calif., sales rep Eli Ortega said it wasn't unusual for adults — often mothers wanting to accompany their kids — to buy the shoes. He rummaged around and emerged with a pair of pink-trimmed Heelys roughly the size of the Queen Mary.

"If you're going to buy Heelys, you'll need protective gear," he added.

Hmmm. Sunglasses?

"Wrist guards. It's not that you might fall," he said. "You will fall."

There's precious little data on adults using the shoes — they're manufactured in children's size 13 through men's size 12 — but research on children indicates adults should proceed with caution. Investigators at Children's University Hospital in Dublin, Ireland, for example, tracked Heely injuries at the hospital during a 10-week period and attributed 67 injuries to Heelys or Street Gliders, a similar product.

Most of the injuries — fractures to the upper limbs and hands and dislocated elbows — occurred while wearers were learning how to use the shoes.

This learning curve poses a problem for adults, who tend to be slower in picking up new skills. Whereas the average preteen can master the shoes in a few days, adults take about a week, says Heelys president Mike Staffaroni.

But it's not all bad news for Heelys. The American Academy of Orthopaedic Surgeons — you might think they would discourage Heelys — has not taken a position on the shoes.

On the one hand, the academy sees the shoes as promoting exercise, says AAOS spokesman Dr. Leon Benson, chief of hand surgery at Evanston Northwestern Healthcare. On the other, it acknowledges safety issues and recommends protective gear — helmet, wrist guards, knee pads and elbow pads. When those are worn, Benson says, he believes the shoes are reasonably safe.

"Probably the biggest problem with Heelys is that often they are purchased in shoe stores, which aren't in the business of selling protective gear," he says. That increases the likelihood that the purchaser will not get the proper safety equipment when they most need it — the first few times they try to use the shoes.

My own experience was pretty dismal. After lacing up the shoes, I confidently stood, only to have my feet go out from under me, catapulting me backward into the chair. Under admittedly poor test conditions, lurching around a "Dilbert"-like office holding onto waist-high cubicle partitions for support, I told myself, "Surely this will get better." It didn't, and a few bruises later, I hung up my spurs.