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Drayton John secures a slackline in Crockett Park in Oakley, Calif., on Wednesday, Feb. 12, 2014. (Kristopher Skinner/Bay Area News Group)

OAKLEY -- Oblivious to the chill and overcast skies, the young men were passing a few hours in the nearly deserted park doing what they often do when there are trees handy.

Drayton John bounced on a nylon strap stretched at chest height between two trunks, lurching in slow motion from a sitting position to lying on his chest and then reversing directions.

"It was a lot harder than I thought," the 22-year-old Oakley resident laughed as he recalled his early experiences with the still-obscure sport known as slacklining.

John often can be found chilling with a handful of friends at Oakley's Crockett Park, where they wrap flat, stretchy strips of nylon and polyester around a couple of trees and spend hours practicing tricks about 5 feet off the ground as the lines bobble and shake.

John's interest in the pastime was sparked when he saw a video of a daredevil traverse a line stretched between a rock pillar and the main face of a mountain 2,890 feet above the floor of Yosemite Valley.

"It seemed interesting, seeing (slackliners) do all kind of tricks. I'd like to be able to get into something like that," said John, who also rock climbs.

And so a year ago, he grabbed a length of 2-inch nylon webbing and began practicing with a couple of friends.

At first, John only could stay on the line a few seconds because his leg muscles were unaccustomed to the bouncing, but it wasn't long before he was able to walk the full length.

These days, he spends most of his time on a 65-foot strip of webbing known as a trick line, but the sport also involves the use of so-called longlines -- those more than 100 feet -- and highlines, the latter often used to traverse deep chasms or span the gap between two tall buildings.

John works on butt and chest bounces, as well as fancifully named moves such as the "mojo tap spin" and "nasty chest."

He once overextended his elbow practicing a new trick but has avoided serious injury; the worst injury he has had was a broken collarbone, "but that was more of a freak accident than anything," he says.

The nature of the physical challenges that slacklining presents -- a balancing act that demands intense concentration -- is one of the appeals for 21-year-old Richard Duncan.

"It's like a meditation-type thing," the Oakley resident said. "I don't think about work. You're so focused that you don't think about any problems you have. A lot of people drink -- I just do this."

In addition to slacklining at Crockett Park at least once a week, John goes to Oakland's Lake Merritt, where many in the slacklining community congregate to demonstrate feats of agility and introduce the sport to onlookers.

Other haunts of his include spots along the Delta shoreline, where he'll spring up and down above the water on lines stretched between two pylons.

John has also dabbled in highlining across the Consumnes River Gorge, near Sacramento, where he's met an international crowd of fellow enthusiasts that has included professional slackliners.

They aren't welcome everywhere, however; John says he's been kicked out of Brentwood's City Park and notes that the East Bay Regional Park District prohibits the activity because those who don't take precautions can damage the trees with their gear.

On the other hand, he was playing around on a line at a local park once when a police car pulled up, responding to a report by someone who had seen John from afar and mistaken his wobbling for public drunkenness.

With the confusion cleared up, John invited the officer to give it a try -- and he accepted, briefly hopping on the line before leaving.

John predicts he'll be slacklining for a very long time to come -- "for the rest of my life, I guess," he said, noting that he knows a 65-year-old man who does it.

"You can't get tired of it because you're always getting better."

East County residents interested in learning more about slacklining can contact John and his friends at https://www.facebook.com/groups/Oakleyslackers/.

Contact Rowena Coetsee at 925-779-7141. Follow her at Twitter.com/RowenaCoetsee.