Backstory: Why walk when you can fly?

Rui Cho, a chatty teen, is preparing to pounce. He cuts a lonely figure atop a six-foot-high wall, his knees bent and fists clenched, rocking gently back and forth as he contemplates the five-foot gap between this wall and the one he's about to - I hope - land on. There's nothing to cushion his fall, if it should come to that: just unforgiving pavement that will not treat this wiry kid kindly.

Are you mad, I ask?

"I know," he says, through a mischievous smile, clearly anticipating objections forming in my imagination. "This is more than a sport. It's a way of life," he offers. He's wearing nothing but a T-shirt, jeans, and sneakers. A helmet or kneepads would only "ruin the moment," making the jump "too safe," he says.

There's nothing safe here; he's about to take a leap I can only imagine taking if I was, say, fleeing a raging fire. But Rui's not fleeing anything; he's going to fling himself across this space for sport (sorry, as a way of life). He launches, clawing the air with a peculiar grace, legs reaching toward the target wall. I fight the temptation to close my eyes.

* * *

It's a crisp, clear Saturday morning in a public square on the south bank of the Thames. Normally buzzing with traffic, the square is empty and silent, save for Rui and his friends and the dull thud of sneakers on stone as the teens leap from wall to wall and run precariously along low building-ledges.

The square has ramps, paths, a monument, interesting nooks, crevices; the kind of area once overrun by baggy-trousered skateboarders, who transformed city-spaces like these into stunt parks when workers and bosses went home for the weekend. Today it's the property of a new kind of urban outfit: "free runners." The craze has swept Europe, its rising numbers of young practitioners ricocheting off walls, somersaulting off phone booths, and jumping the gaps between buildings.

Why walk through a city when you can "fly" over its railings and rooftops?

* * *

"See? I overcame danger!" beams Rui after landing safely, and standing perfectly still, on the opposite wall. I resist pointing out that he created this "danger" by climbing up the wall in the first place.

Free running, or "le parkour," its original French name, is all about overcoming obstacles. The sport (that's what I'm calling it) originated in the Paris suburbs in the '90s, when founders David Belle and Sebastien Foucan took the games they'd played as bored teens - ninja-style prancing on rooftops or imitating Jackie Chan - and christened them a sport. It's a ragbag of gymnastics, rock-climbing, and stuntman daring, with a little bit of philosophy thrown in for good measure. (Mr. Foucan has described parkour as "an evolution of the mind.")

The aim is to "flow" through the city, without stopping or pausing and never going backward. And if you encounter an obstacle, such as a wall, railing, or concrete bollard? You do what Rui did: overcome it. It's a strange sport to behold. There are no teams or scorecards; it's not a competition to see who can "win." Most strikingly, there's no audience to watch and cheer, except those bystanders who, by chance, spy the free runners as they traverse the city.

What, exactly, is the point?

"It is about you, the individual," says Richard Sheehan, a tall, energetic 15-year-old who has just executed a "360-degree cat," a leap five feet in the air - feet first - over a railing while turning a full circle. "It's about testing your own limits," he says.

Shay Hunter, the lone female among the 12 free runners here today, describes the sport as a way of working through angst. "If you have some trouble in your life, you can come to the city and push yourself, prove yourself. It helps," says the 22-year-old filmmaker, as she practices an "underbar" leap through a railing and pulls up on the other side.

Free running to thrash out your "issues" on cold urban slabs is part of the rise in individual "extreme sports," such as base-jumping (parachuting off buildings), mountain boarding, and freestyle biking. It coincides with a decline in traditional, rules-based team sports on fields and courts. Earlier this year, researchers at Brighton University in England found these new sports "ousting football, rugby, and cricket in popularity" in Britain.

The shift from team sports to individual sports suggests that young people feel increasingly isolated, and are "casting around" for new ways to push, prove, or simply entertain themselves, says Josie Appleton, a cultural commentator in London who has researched extreme sports. "Traditional sports provided a way for kids to develop through the challenge of competition or by working as a team. But in lifestyle sports, young people seem to ... focus on individual performance rather than pushing the limits of human achievement."

* * *

There's definitely an edgy, anti-rules air to the parkour culture, which creates danger to overcome it. This has earned free runners a place on the official watch list of health and safety organizations. Police and private security guards around the city are on orders to stop free runners.

Roger Vincent of the Royal Society for the Prevention of Accidents cautions that inexperienced, perhaps unfit, kids can get hurt trying the new trend. Indeed, in August, a 14-year-old died when he fell 20 feet while trying to jump the eight-foot gap from one roof to another at his school in Oxfordshire.

But while the safety police might fret over parkour, the phenomenon has spread as it's been glorified in the media. All the free runners I interviewed trace their initial step, skip, and jump into the sport to the 2003 film "Jump London." The movie showcased Foucan running up the walls and traversing the gaps of London's most famous landmarks - the Tate Modern, the Royal National Theatre, and, most breathtakingly, the sprawling rooftop of the Royal Albert Hall.

As the sport has captured the imaginations of increasing numbers of youth, advertisers have seized on it, too. A mesmerizing BBC ad showed a man rushing home from work in death-defying leaps across roofs to be in time for his favorite radio show; and professional free runners have signed with Adidas under the slogan: "Don't run. Fly free. Forever."

In the gasps inspired by parkour is a message of "ambition and individuality" - at least that's the message one firm in London is hoping to get across in a TV commercial. As it tapes a spot in central London, Chase Armitage, a 20-year-old "semi-pro" free runner, dashes toward a concrete ramp, leaps, and flips backward, landing where he started. It makes my bones creak. Later he tells me about the website he runs (www.3run.co.uk) - where he and his team show photos and videos of their "street stunts" - and how he hopes to make a "real living from parkour."

* * *

Back in the square by the Thames, the young amateurs tell me that taking risks reflects their "way of life." Richard shows me a scar on his shin, proud of the proof of his ability, or at least derring-do, in a sport that lacks medals or trophies.

"They sometimes throw us out of this square and other places, but we just go somewhere else," he says of the safety-conscious security brigade. "They can send us on, but they can't stop us."