It’s a summer night in Oz Park and Michael Zernow, whom everyone here knows as “Frosti,” is undressed for action. Wearing nothing but black shorts, yellow sneakers, and a black skullcap, he stands on a two-inch-wide plank and prepares to run a precarious route on, over, and around the play lot equipment he’s using as an obstacle course.

Frosti, 21, communicates regularly with his parkour pals over online forums, but just about every Wednesday evening the locals gather in Oz Park, near Lincoln and Halsted, which has all kinds of uneven ledges, benches, and wooden railings that suit their purposes. No one trains harder than Frosti.

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Thousands of people are believed to practice parkour in the United States, and Frosti is one of the most widely recognized. At five-foot-eight with a sinewy build, multiethnic features, and short black hair that’s often pushed into a fauxhawk, he has a distinctive look. Among parkour enthusiasts he’s known for his daring acrobatics—he makes a move like a standing backflip look easy. He’s done stunts in K-Swiss commercials and performed on Madonna’s Confessions tour. Last year, at 20, he became the youngest person ever to appear on Survivor, when it was set in China, and now he’s working to develop a parkour clothing line.

When he was a sophomore, one of his aikido instructors told him about parkour, and Frosti tracked down a tape of Ripley’s Believe It or Not with footage of traceurs in action. “I watched it,” he says, “and later that day, me and my friend were doing it.”

Frosti’s youthful self-assurance rubbed some viewers the wrong way. During the month he was in China, Frosti received dozens of letters and e-mails, mostly from younger viewers telling him in so many words that he wasn’t all that. During the filming he lost 20 pounds, and despite his best efforts to make strategic alliances with his castmates, he was bounced from the show in week nine, about halfway through. But that’s not his main regret.

David Belle, a Frenchman, is considered by many to be the father of parkour. He came up with the name and started practicing his moves on video more than a decade ago. Over the last few years he’s helped popularize the sport by posting countless more videos of himself in action on the Internet, and his fans have responded with videos of their own performances, many set to raucous rock and rap music.

“The one thing I have against the videos,” says Toorock, “is that they don’t distinguish between the people like David Belle, who have been practicing it for years, and some jackass who’s just jumping off his roof for the first time.”