BELLINGHAM, Wash. – On a chilly night, seven members of the Whatcom Family YMCA’s new junior climbing team pick their way over two vertical slabs of “rock” like human spiders. Below, coach Kayla Erickson watches and corrects their form.

“Come on, Quinn, tidy feet. Don’t make them spastic. Make them tidy. You need a sequence before you go up next time,” Erickson says to Quinn Shultis, one of the team members, who range in age from 8 to 16.

Erickson turns her head and eyeballs Ruby Bender’s ascent, hands reaching for a grip here, feet braced on a grip there, as she works on completing her “Rambos.” The idea is to have Ruby climb up the 66-foot-high wall three times and back down three times, while team member Elise Chambers belays from below, keeping tight the rope that is attached to Ruby’s harness, the rope that is Ruby’s safety net if she falls.

“This builds endurance,” Erickson explains of “Rambos” during the 1 1/2-hour training session.

By the third cycle, Ruby will feel less like a human spider and more like a lump of lead as her muscles max out. Not that these youngsters can’t take it.

“They’re very strong and they’re very motivated,” Erickson, 18, says of her young team.

What this new team, which has existed only since September, is doing is called “climbing plastic.”

They’re climbing artificial rock, and they’re not alone as more children and adults take to rock walls inside three exercise facilities in Whatcom County and other indoor walls around the region, testing their muscles and flexing their brains.

“The nice thing about rock climbing is that it’s a good physical workout but you’re also mentally planning out your next move, so you’re using all of your resources to get up the wall or get over the wall,” says Aislinn Ronaghan, operations and climbing wall coordinator for Wade King Student Recreation Center at Western Washington University.

Going inside to hit the walls is more popular than going outside – 6.7 million vs. 5 million, according to the 2005 Outdoor Recreation Participation Study by the Outdoor Industry Foundation.

The Whatcom Family YMCA sees 2,500 new participants at its climbing walls each year, according to Robert Knowles, who is the organization’s adventure director.

The sport seems to be favored by youngsters. USA Climbing, the national governing body for the sport, has 6,000 members who compete and most are 19 and younger, according to Anne-Worley Moelter, executive director of USA Climbing.

Indoor rock climbing can be traced back to 1964 in the United Kingdom, where the first true wall was built by Don Robinson, a university lecturer teaching physical education and founder of DR Climbing Walls, according to the company’s Web site.

The idea of climbing inside spread from there, until the first climbing gym in the U.S. opened in Seattle in 1987, according to REI. Today, it remains popular during rainy, cold and dark Pacific Northwest winters.

“I remember the very first indoor climbing gyms being developed,” says Michael Medler, a 28-year climber and associate professor at WWU’s Huxley College of the Environment who recalls going to some of the first climbing gyms in Seattle and Portland about 20 years ago.

Most recently, he worke with colleagues and student researchers at WWU’s climbing walls to prepare them for a trip to Costa Rica. There, they are placing cameras in tall trees to see why the country’s revered macaws – “they’re to Costa Rica what bald eagles are to the United States” – are dying off.

They had to modify rope-climbing techniques for trees. “The nest we found, the lowest branch was 90 feet off the ground,” says Medler, a 45-year-old Bellingham resident who teaches geography.

And while they were learning at the center, they were themselves becoming a disappearing breed.

“Bouldering has become extremely popular. You go to the climbing gym at Western and nobody’s using ropes,” Medler says. Bouldering is a spin-off of rope climbing.

Climbers are not attached to ropes when bouldering. The routes are usually shorter, but tougher, and reach 10 to 12 feet off the ground. There are crash pads on the ground and spotters who try to guide a climber in the event of a fall. (Popular bouldering sites in the outdoors are the rocks at Clayton Beach and Larrabee State Park.)

“Bouldering is more challenging both mentally and in a physical sense. because you’re responsible for all of your weight,” says Ronaghan.

The YMCA and WWU center have climbing walls for those who want to do both types of climbing. as well as classes and private lessons for children and adults who want to learn. Leading Edge Gymnastics Academy North also has a small indoor climbing wall, but no classes.

“The great thing about having a wall like this in the YMCA is you get to dispel that intimidating feeling and sort of the myths of rock climbing for people who have never done it before,” says Adrienne Moore, climbing wall supervisor for the Whatcom Family YMCA.

She adds: “It’s a cool facility because it holds up to seasoned rock climbers who want to come in and train but it’s also an inviting place for young kids and families who have never done it before.”

The YMCA’s first climbing wall, which is 30 feet tall, opened in 1996. Its second wall, the one Ruby was climbing, is 66 feet tall. It opened in 2003 and is the tallest in the state, according to Knowles.

Both walls are 27 feet wide. The YMCA has considered adding a third wall for bouldering for at least five years.

“It’s one of those things that’s always in the mix, especially now because bouldering has become this popular spin-off of traditional roped climbing,” Moore says. “A lot of the younger kids are getting into it. It hasn’t been built yet, but we’re definitely trying to get it to go forward.”

The Wade King Recreation Center, which opened in 2003, has two climbing walls that are about 30 feet high as well as a 12-foot tall bouldering wall with overhangs.

Adults and children may take climbing classes through the center.

Ronaghan says she sees lots of youngsters climbing.

“Kids can go on the wall and do things I would cry about doing,” she says. She attributes indoor climbing’s popularity to its constant evolution.

“You never reach a limit. You always find a way to challenge yourself. It’s not like a video game where you beat the game and you beat the game. You keep going. You never stop,” she says.

The kids on Erickson’s climbing team don’t seem to be stopping one recent night as they train for rope season, which runs from March until nationals in July. The season for bouldering competitions runs September through February.

Their enthusiasm is strong, even if, at times, they can’t explain why.

Take Ricky Heilmann, a 12-year-old Fairhaven Middle School student, who’s wild for climbing.

“I love climbing. I just love climbing. I don’t know what I love about it.”

Then there’s Kayla Culmback, 9, who on Jan. 19 went to the bouldering regional championships at ClubSport in Tigard, Ore., with Erickson and Ruby, 13, both of whom also competed. (Erickson, herself a competitive climber, would go on to qualify for the national championships in Boulder, Colo., where she placed 20th among women ages 18-19.)

Kayla and Ruby are the original members of the team.

“I like challenging myself to do something. I like the feeling I get when you get to the top and (think), ‘I made it,”‘ the Happy Valley fourth-grader says.

Ruby, who goes to Mount Baker Junior High School, says she joined the team because she wanted to climb more and was encouraged to do so by her parents.

“My mom thinks it’s great. She wants me to do a lot of physical stuff,” says Ruby, who had been climbing for about two years before joining the team. And what’s the view like from up top?

“It’s scary at first. If you look down, you freak out,” she says.

Until you realize: Cool, I did this, Ruby adds.

Ruby’s mother, Swan Bender, is a fan of her daughter’s climbing.

“Ruby’s just a spider,” says Swan, adding that her daughter has learned confidence and problem-solving skills as well.

It’s akin to the motto for USA Climbing, the governing body for competitive climbing in the United States.

“Strength builds confidence,” Erickson says of the motto. “I really do believe that is a key factor when kids start climbing. It’s an important part of climbing for kids.”

She should know.

Erickson started climbing at the YMCA in Bellingham six years ago and would go on to climb competitively starting at age 14, most recently for Vertical World in Everett.

She started the Bellingham team because she wanted youngsters here to have what she had. “A team was a huge part of my life growing up,” Erickson says.

After practice has ended this night, after the children’s din has faded and other climbers take to the walls, Ruby sits with her mother as she talks about team-building.

“I like them working together, bolstering each other, supporting each other,” Swan Bender says. Now, Swan Bender says, she wants to take the rope certification classes through the YMCA and climb with her daughter.