As I wait my turn in line at Gleason's Gym outside Minneapolis, I get tips from the guy ahead of me about how to get more power out of my Kong vault—a move where you sprint toward a four-foot-tall gymnastics apparatus, extend your arms, push off with your palms, and swing both legs over as you sail forward. He is probably 12 and I could be his dad, but at this moment we're just two guys trying to clear a wall.

It might seem weird to work out with guys who aren't old enough to drive, but this parkour class is for ages 11 and up, and the middle-school demo is all over it. They are at an age when jumping over things is considered an acceptable way to blow off steam.

Lucky them. If adults threw more fun into their time at the gym, we would go there more often, do smarter exercises, and be better-looking. We'd also feel younger.

"One of the sayings of parkour is that you don't stop playing because you grow up̬you grow up because you stop playing," says Chad Zwadlo, my teacher at Gleason's. The trend extends beyond parkour, however: Look in any corner of the fitness world and you'll discover a return to play. Not play as in beer-league softball. We're talking about training with ropes, rings, ladders, and crossbeams; classes on crab-crawling, hopping, vaulting, hurdling, balancing, tumbling; guys throwing rocks, pulling logs, and muscling over just about any tall obstacle.

Play is more organic than gym exercises because it involves more chances to control your descent and to use joints the way they are called upon in nature—to link the firing of the hip with the knee and then the ankle. This protects you from injuries and makes you more athletic, not just muscular.

"Our bodies are designed for success in a world that no longer exists," says Frank Forencich, author of Play as if Your Life Depends on It. What defines play, he says, is its sense of the unknown and the hint of risk that goes with it. Using play as exercise can mean the difference between becoming adapted, as you do in the gym, and becoming adaptable, as you do by tackling constantly varying challenges. It helps you mentally as well. "Deprive young animals of play and they become dysfunctional, aggressive, and don't know how to relate to one another," he says. "Play turns out to have benefits other than making your workout more effective."

Sure, serenity is nice, but the fitness benefits of play alone are drawing a global following. MovNat, the creation of French exercise and cultural critic Erwan Le Corre, promotes the idea that it's time to train like members of a species rather than a health club. Gym training gives you "slow and dumb muscles," Le Corre says. "Natural training means something the human animal would perform in nature, for their survival." Thanks to a slew of Web videos starring a barefoot Le Corre wordlessly running, swimming, scurrying, carrying animals, and leaping in the wilderness, MovNat has begun selling out multiday camps and training seminars around the world.

Researchers in Denmark recently studied how 12 weeks of soccer, speed running, and jogging variously affected balance and control. While jogging is the essence of health-club-style "cardio" training, soccer is a great example of vigorous free play—it requires sudden starts and stops, movement in three planes of motion, and changes in direction. The study found that play not only makes you more athletic but also increases your overall fitness. Likewise, research has shown that doing strength training while on your feet activates many more muscles than doing it while on a bench.

Natural movement is pretty much the opposite of the typical workout routine of trudging into the gym and doing the same thing, by yourself, day after day. "What I'm seeing over the last 10 years is a big trend towards being liberated from the tyranny of the treadmill and the realization that we have to get outside if we want to get healthy," Forencich says. "If I was a gym owner with a big investment in machines, I would be worried. A lot of that stuff is going to go unused in the years to come."

• • •

Yes, You Can Do Parkour

Parkour, which employs X Games-style gymnastics on the street, is held up as the paragon of modern natural movement, but backflipping off walls and roofs may seem daunting to the uninitiated. Let's get a few misconceptions out of the way: It isn't only for 20-year-old extreme athletes, and it isn't competitive. "My body moves differently than your body, so why would we try to compete?" asks Chad Zwadlo, an instructor in Minneapolis. "It's your body against the environment, not someone else." Getting started isn't as hard as you might think, either: You begin by learning how to land and roll, then you practice a half-dozen ways to vault over waist-high obstacles, and from there it's on to running along walls and bar swings—stuff any thirtysomething in decent shape can quickly master. Starting small and indoors is vital, because much of your success in parkour comes from building confidence. There aren't many parkour schools per se, but organized groups and classes meet in nearly every urban center. And gymnastics centers and CrossFit gyms are adding classes. For a list of places that teach parkour and helpful animations of key movements for self-instruction, check out

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