Swept by the barefoot running craze, ultramarathoner Ryan Carter ditched his sneakers for footwear that mimics the experience of striding unshod. Within three weeks of switching over, he was clocking six miles on the road. During a training run with a friend, 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. The diagnosis: a stress fracture.
As more runners experiment with barefoot running, doctors say they are treating injuries ranging from pulled calf muscles to Achilles tendinitis to metatarsal stress fractures. "Most just jumped in a little too enthusiastically," says a podiatrist, who now treats between three and four barefoot runners a week. Experts say people can successfully lose the laces—the key is to break in slowly. But there's one group foot experts say should avoid barefoot running: People with decreased sensation in their feet, a problem common among diabetics, because they won't be able to know when they get injured. (Read more barefoot stories.)