LOS ANGELES -- Swept by a craze for barefoot running, ultramarathoner Ryan Carter ditched his sneakers for footwear that mimics the experience of striding unshod.

When he first tried it two years ago, he ran a third of a mile on grass. Within three weeks of switching over, he was clocking six miles on the road.

During a training run with a friend along a picturesque bike path near downtown Minneapolis, Carter suddenly stopped, unable to take another step. His right foot seared in pain.

"It was as though someone had taken a hammer and hit me with it," he recalled.

Carter hobbled home and rested his foot. Days later, when the throbbing became unbearable, he went to the doctor. The diagnosis: a stress fracture.

As more avid runners and casual athletes experiment with barefoot running, doctors say they are treating injuries ranging from pulled calf muscles to Achilles tendinitis to metatarsal stress fractures, mainly in people who ramped up too fast. In serious cases, they are laid up for several months.

Many converts were inspired by Christopher McDougall's 2009 best-seller "Born To Run," widely credited with sparking the barefoot running trend in the Western world. The book focuses on an Indian tribe in Mexico whose members run long distances without pain in little more than sandals.

While the ranks of people running barefoot or in "barefoot running shoes" have grown, they still represent the minority of runners. Some devotees swear they are less prone to injuries after kicking off their athletic shoes, though there's no evidence that barefoot runners suffer fewer problems.

Barefoot running uses different muscles. Shod runners tend to have a longer stride and land on the heel, compared with barefoot runners, who are likely to have a shorter stride and land on the midfoot or forefoot. Injuries can occur when people transition too fast and put too much pressure on calf and foot muscles, or don't shorten their stride and end up landing on the heel with no padding.

Pre-human ancestors walked and ran in bare feet for millions of years, often on rough surfaces, yet researchers surprisingly know very little about the science of barefoot running. The modern running shoe with its cushioned heel and stiff sole was not invented until the 1970s. And in parts of Africa and other places today, running barefoot is still a lifestyle.

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