If you received a pair of Vibram FiveFingers or other barefoot-style running shoes as a gift this holiday season, read the directions and follow them to a T.

Then cross your fingers and hope they work with your biomechanics.

Running in Vibram FiveFingers seemed like a good idea to Caroline Cue, 46, of St. Charles. Several of her running and triathlon buddies had successfully transitioned to the quirky glove-like shoes with individual toe compartments and thin, flexible soles.

So Cue bought a pair and worked her way up to running five miles in them over about a year, just as the instructions recommend. But when she ran farther than that, she ended up with severe bursitis in her Achilles tendon. She was forced to use crutches for three weeks and stop running for five months.

"It fully changes your running stride," said Cue. "It ended up being '$100 in shoes, three months of PT (physical therapy),' and boy was that a bad idea."

Barefoot-style running shoes have grown in popularity in recent years. So has running in no shoes, though to a lesser degree.

Sales of Vibram FiveFingers grew from about $450,000 in 2006 to an estimated $50 million last year, while barefoot-style running shoes in general were a $1.7 billion industry last year. Other brands include Soft Star Run Amocs, Vivobarefoot and Merrell Gloves for road- and trail-running.

The barefoot trend stems from fan websites like birthdayshoes.com and books such as Christopher McDougall's best-selling "Born to Run," about a tribe of reclusive Tarahumara Indians in Mexico who run ultra-marathons either barefoot or in thin-soled sandals.

Several local retailers carry Vibram FiveFingers.

Sales of the shoes have been especially brisk this year at Little Shark Athletic Co. in University City and seem to be appealing to a more mainstream crowd, said sales manager Natalie Carroll.

"You see them a lot more at the YMCA or other fitness centers, and you see a lot more people in them in general," she said. "I have three pair, and they're the most comfortable shoes I have. Once you start wearing them, you understand why people love them."

But a debate about the benefits and safety of barefoot running has developed.

Proponents believe that traditional running shoes, which often have arch supports, stiff soles and thick, cushy heels, are over-engineered; that they make the feet weak, promote a hard heel-strike and don't prevent injuries. Running barefoot or in shoes like the Vibrams, they say, forces runners to land on their forefeet with a softer impact and strengthens the feet and lower legs.

Skeptics want to see proof. They believe that messing with a runner's natural stride leads to injuries. They even have their own website, runningbarefootisbad.com.

Daniel Lieberman, professor of human evolutionary biology at Harvard University, studies the biomechanics of endurance running by comparing barefoot runners with those who wear traditional running shoes. He has found that barefoot runners land more gingerly than shod runners who land heavily on their heels.

But even he admits that little data are available to support his hypothesis that running barefoot reduces the risk for injury.

Right now, there's just anecdotal evidence.

Chris Mavromatis, 39, of Crestwood, for instance, ran the St. Louis Rock 'n' Roll Half-Marathon, sans shoes, in October.

"I was close to giving up running — as much as I love it — because of shin splints and Achilles tendinitis," he said. "I haven't been injured since I started this."

Jennie Schmitz bought her first pair of Vibram FiveFingers in 2007 and runs in them all the time on all types of surfaces.

"I know they're not for everyone," said Schmitz, 33, of Creve Coeur. "But for me, they increased my speed. And I've noticed that I haven't had any knee or back injuries."

RUNNING ON HARD SURFACES

It's reasonable to think that if you engage the intrinsic small muscles of the feet to work, which barefoot running does, it might afford biomechanic benefits, says Dr. Anthony Margherita, medical director at West County Spine & Sports Medicine. But he's treated numerous injuries from barefoot running and isn't ready to recommend it.

"The problem is that here in the U.S., people run on hard surfaces," he said. "Most people run on asphalt or concrete, and each time you hit the ground with the foot, five times your body weight is driven through the body. If there's no energy absorption of a shoe at the interface, all that energy is driven into the body and must be absorbed somewhere."

He's also not so sure that everyone will automatically run on their forefeet if they switch to barefoot-style shoes. People have natural running patterns that they should stick with and enhance, he says. "I've seen people change their style and slow dramatically from an 8-minute mile to an 11-minute mile."

Kent Bohling, owner of the Running Center in Rock Hill, refuses to sell barefoot-style running shoes.

He doesn't have a problem with people wearing them as casual wear, because they're no different than sandals. But he's seen runners come into his store with calve strains, Achilles tendinitis and metatarsal stress fractures from wearing them.

"The believers of this barefoot cult, if you want to call it that, will say those people just didn't work up to it properly," Bohling said. "Well I ask the question: Why would you need to work up to it at all? There's 40 years of research that's gone into running shoe design, and it's to help you perform better and not worse."

He and other experts like to point out that there's not a single professional runner today who competes barefoot.

"Their ability to do what they do is enhanced by footwear. It's no different than computers, cell phones and great technology. You want to go back to where we were? It just doesn't work," he says.

Margherita treats a lot of patients who wear rigid motion-control running shoes because their feet roll in or out. He usually prescribes a custom orthotic to wear in neutral running shoes.

"Are there shoes that are over-engineered? Yes," he says. "But that's a relatively small number that are trying to be everything for everyone."

Dr. John O. Krause, an orthopedic surgeon who specializes in disorders of the foot, ankle and lower extremity with the Orthopedic Center of St. Louis, has also treated several patients with injuries from running in barefoot shoes.

He readily concedes that some runners probably have great success with the shoes and their feet probably are stronger. But Krause, who runs himself, is not sure why that is beneficial.

"It's like, 'Boy you can really flex your toes now,' " he says. "If you're falling from a tree, and you can catch yourself by the toes, that might be good. But you're an accountant, so who cares?"

Cue just recently resumed running — in Asics running shoes. She knows which side of the debate she's on.

"I would fall into the 'don't do anything that changes your natural stride too much' (side). Any time you do that, your body doesn't run the way it wants to run," she said. "I'm never going to be a Kenyan."

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EDITOR'S NOTE: An earlier version of this story had an incorrect link to a website about barefoot running. This version has been corrected.