The sport that isn't quite tightrope walking has spread from rock-climbing culture to the urban mainstream, and you see people doing it in parks all over Seattle.

Especially on nice weekends, you’re likely to see them at local parks such as Green Lake, Gas Works or Golden Gardens — barefoot people with flapping arms who seem to be tightrope walking a few feet off the ground between a couple of trees.

Except it’s not a tightrope. The line is slightly slack, hence the name: slackline, a sport that originated with rock climbers and has spread to the urban mainstream.

The looseness of the line is what makes it so challenging, says Adam Burtle, 29, a lanky documentary filmmaker who discovered slacklining when he worked in Los Angeles.

“I saw it at Muscle Beach and the first time I tried it, I fell in love with it,” he said. Returning home to Seattle to study medicine, Burtle noticed a few people slacklining in parks “but they were always doing it alone. I wanted it to be a community.”

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Here and growing

Thanks to him and other devoted followers of the sport, that community is growing. Out at Golden Gardens recently, Burtle and friends strung up a cobweb of 1-inch climbing webbing used in slacklining, affixed to trees (padded by towels and carpet) with a system of knots, carabiners and other hardware.

“Basically, the longer and looser the line, the harder it is because the line moves so much,” Burtle explained. “Beginners start on shorter, tighter lines.”

Although it looks hard, the sport is accessible to almost everyone. “You already know how to balance well, so most people can learn how to slackline in about an hour.”

Once you can balance on both feet consistently for about 10 seconds, it’s time to master the basics: “You learn to stand on just one foot,” said Burtle. “Then you learn to take a step and walk. Then to turn.”

Getting it right provides a great workout for the whole body. “It’s all in the movement of your hips, and the core. You have to retrain them. Your arms are used for fine tuning.”

Once you’ve got the fundamentals, try some tricks. As executed by Jenna McLennan, 24, and Austin Josephs, 22, of Orcas Island, tricks include leaping, jumping, surfing and even a kind of slackline dancing.

“It can be harder to walk in tandem,” McLennan noted, with Josephs adding: “If the other person loses their balance, you have to counter.”

But it was beautiful to watch these two lithe, young people glide across 1-inch webbing as though they were on a vast dance floor. Then she demonstrated a “drop knee,” which looked like some kind of pretzelly yoga thing, and apparently there is an entire subculture of the sport devoted to slackline Zen.

“Slacklining is a great meditation,” McLennan said.

Josephs attempted a back flip. He almost nailed it, too, but then the line snapped up and tossed him into the sand.

The couple also takes part in a hair-raising version of the sport called highline, walking a slackline way, way off the ground, as McLennan did last year — 3,000 feet in the air at Yosemite. “She was the second woman to do that,” Josephs said.

“It was amazing,” McLennan remembered. Of course, she wore a harness and leash attached to the line, a good thing since she fell. “The hardest part was getting back on but highlining is my passion!”

Attracts an audience

At Golden Gardens, most of the lines were a reasonable distance from the sand, and full of people wildly waving their arms. The spectacle attracted a circle of bystanders, many of them children.

“It’s a kid magnet,” Burtle said. “They love doing this and learn fast. The best ones are probably 12-year-old girls. They weigh almost nothing, they’re not very tall and they’re fearless.”

If parents seem interested in getting their kids into slacklining, Burtle hands them a flyer about how to get started for about $35-$50 to buy the basic equipment.

He’s a crusader for bringing the sport to children. “So many of them are drinking sugar, sitting on a couch. One of my karmic duties is to teach kids slackline.”

In fact, this July and August he’ll be taking slackline equipment to Ghana, where he’s going to film a Red Cross group providing measles vaccinations to children. “I think the kids will like it, where they struggle for even a soccer ball.”

Meanwhile, he works to spread the word in the Northwest, converting people every time he walks the line in some park.

“You know, I never used to get why people sit around drinking beer and watching a game on TV, but now I get it. It’s not just about the beer or the game, it’s about community. This is the same thing. It’s half-slackline and half-community.”

Connie McDougall is a freelancer writer who lives in Seattle.