The world, at least the only part of it that mattered at the moment, moved beneath Sayar Kuchenski’s bare feet, swaying and bouncing like an earthquake.

But he was casual about it, no worries, just going with the flow as he walked a slender length of nylon webbing suspended several feet off the ground.

Sometimes his steps would cause the webbing to dip nearly a foot. It would bounce back up and Kuchenski with it.

Sometimes Kuchenski’s steps caused the webbing to sway out from under him. He would angle his outstretched arms for balance or stretch out a leg like a counterweight.

Nothing else mattered except the inch-wide webbing he walked on. If he thought of anything besides his steps and the dubstep in his earphones, Kuchenski would fall.

And he just kept walking.

It’s called slacklining and the object is simple: Tie a length of nylon webbing between two trees and walk on it as far as you can without falling. If 1-inch webbing feels too narrow, you can buy pieces that are 2 inches wide.

"You don’t really require any skill other than being able to balance," said Kuchenski, a 26-year-old Navy reservist and University of Phoenix student of information technology. "You kind of learn how to slackline as you go. You get progressively better."

That won’t happen overnight, though.

"When I first started out, I couldn’t stand on the line for five seconds," he said. "I would stand up and immediately fall off. It took 500 attempts over six months before I could even walk 40 feet or so. After that it got easier."

The height of the line depends on how far it’s stretched. On this day at Kakaako Waterfront Park, Kuchenski stretched his line 230 feet between a pair of palm trees, a span that puts either end of the line about head-high to the 5-foot-8-inch, 150-pound athlete.

But whenever Kuchenski walks the line, especially when he nears the center of the span, it dips to about two feet off the ground.

"There is some risk of getting hurt," he said. "People break limbs on a regular basis. But relatively speaking, it’s safe as long as you use the equipment properly and you watch your limits and you don’t try to do something that you aren’t capable of doing."

THE ROOTS of slacklining are literally in the trees of Yosemite National Park in California. Rock climbers living in the park in the mid-1970s would tie their nylon climbing straps between trees and walk on them, according to Jaime Pletcher, a marketing manager at Gibbon Slacklines, which sells slackline equipment in 3,000 stores nationwide.

The practice improved their core strength and balancing skills, she said.

Slacklining remained primarily a climber’s pastime until about 2007, when companies such as Gibbon, which is based in Germany, began selling inexpensive kits with lengths of webbing as wide as a seat belt.

It has grown ever since. There are now professional competitors who do tricks, such as spins, flips and bounces, and outreach programs that teach slacklining in school PE classes, Pletcher said.

Amateur enthusiasts, many of them not climbers, now set up lines in their yards or at parks, he said. Some estimates put their ranks at between 5,000 to 10,000 people in the United States.

In Hawaii there might be as many as 200 slackliners, said Mike Richardson, owner of Climb Aloha, a Kaimuki store that offers rock-climbing gear and classes. With all the palm trees and beach settings, Hawaii is the perfect place to slackline. Enthusiasts often practice at Kapiolani Park, Queen’s Beach and Magic Island, he said.

"It’s growing by leaps and bounds in the last few years," Richardson said. "It is not as dangerous or as scary as climbing, but certainly takes a lot of concentration and mental willpower to control your thoughts and stay focused on the task at hand."

KUCHENSKI’S real passion is rock climbing. Originally from North Dakota, he moved to Hawaii five years ago while still in the Navy.

To get his climbing fix, he would head to a spot on Oahu’s North Shore and spend vacations traveling to climbing havens in California, Colo­rado, Kentucky and Wyoming.

But his local climbing spot, which is on state land, was closed a year ago after a 12-year-old girl was injured by falling rocks.

So Kuchenski turned to slacklining.

"There is no limit to how challenging you can make it: You just make the line longer," he said. "You don’t need to be in great shape for slacklining, but the heavier you are, the more challenging it will be."

For Kuchenski the biggest challenge was patience. At first he thought it was just too hard. Then it became a mental game.

"You have to be extremely focused because if your center of gravity shifts even the slightest bit, you will fall off," he said. "And the wind moves the line around, and you have to correct your balance."

Watching Kuchenski get up on the webbing is just as amazing as watching him walk its length.

He’ll put both hands on the webbing, hop up so he can straddle it, then sit there for a few moments, staring down its length while he finds an imaginary centerline.

It looks a bit painful, and you half expect him to spin like a propeller.

Then he’ll raise one leg — up so high his thigh brushes his chest — and place a foot on the webbing. Slowly … steadily … he’ll lift himself. His balance, shifting in breezy conditions, comes first on one leg.

And then he just starts walking.