From hockey to gymnastics, fencing to rowing, kids in the Twin Cities have had their choice of gyms, clubs and teams to join if they want to try out a new sport.

Now you can add being a ninja warrior.

In the past couple of years, at least six ninja warrior gyms have sprung up in the Twin Cities, inspired by the obstacle-course competition made popular on NBC’s television show “American Ninja Warrior.”

And more are coming. Across the state — and across the country — new gyms are offering these “playgrounds on steroids” courses. Gymnastics schools, rock-climbing gyms and trampoline parks are also opting to join the ninja tsunami by adding obstacle programs. In turn, they’re attracting thousands of kids, some who have given up more traditional sports for the strength, agility and mental challenge of an obstacle course.

“The course is like a massive problem that you need to learn to solve,” said Tristan Anderson, 10, of Maple Grove.

The idea of ninja gyms originated with adults who built homemade obstacles in their garages and backyards to train for a spot on the television show. But when local gym owners turned their hobby into a business, they found that up to 75 percent of their customers are kids who are flocking to the gyms to climb, swing and leap over obstacles in classes, day camps, parties and competitions.

On a recent Friday when there was no school, nearly 1,000 kids came to the four Conquer Ninja gym locations in the Twin Cities, said Sheila Gullickson, sales manager for the gym chain.

“The main customers are kids between the ages of 5 and 14,” said Kevin Hogan, an accountant turned ninja warrior competitor who owns Ninjas United gyms in St. Cloud and Buffalo.

Gym owners and instructors, who often include people who have competed on “American Ninja Warrior,” say that kids are doing more than just having fun imitating the stunts seen on the show: Some hope to be athletes in an emerging sport.

“Our intention is to be the pioneer, to turn this into a legitimate sport,” said Sarah Schoback, a stay-at-home mom from St. Louis Park who won a spot on “American Ninja Warrior” and started her own gym, Obstacle Academy in Edina, in 2016.

“A new sport is started every decade and we truly believe this is it,” said Megan Noel, marketing director for NinjaZone, a youth obstacle course program marketed to gymnastics clubs worldwide.

Over about two years, the NinjaZone program has spread to more than 250 locations, including 10 in the Twin Cities, enrolling more than 80,000 kid ninjas nationally.

“It might be trendy right now, but it’s not a fad,” Noel said.

“There’s been a lot of talk about making this into an Olympic sport,” Hogan said.

Wiry and relatable

“American Ninja Warrior,” which was based on a 20-year-old Japanese show called “Sasuke,” is going into its 10th season. It draws about 6.8 million viewers overall and has a top-10 ranking for prime-time summer shows among kids ages 2-17, according to NBC. About 70,000 adults vie each year to get one of about 750 spots to compete on the show.

Some parents of ninja warriors say the television show competitors, who come from all walks of life, may be more relatable to kids than a typical pro athlete.

“It’s the little wiry guys who do well,” said Minneapolis ninja mom Amy Wozniak. “They’re just normal guys.”

Her son, Max, 10, is in an Obstacle Academy youth team program that costs $180 a month. He trains two nights a week on the gym’s versions of obstacles like the warped wall, swinging cannonballs and cliffhanger ledges.

Overcoming the obstacles involves the swinging, climbing and jumping they might need to climb a tree or a jungle gym, but with more padding.

“It’s like a playground on steroids,” Gullickson said.

Wozniak said her son loves the show and overcoming obstacles himself so much that he offered to drop hockey so he could work out and compete at the ninja gym. “He even said he’d put in his own money to do it,” she said. In addition to attending the team practices, he begs to go every weekend.

“What kind of kid doesn’t want to swing on stuff and conquer the world?” Wozniak said.

Kid ninjas say they also like the freestyle creativity that competitors can use to beat the obstacles.

“It’s definitely given me more confidence. I’m stronger than when I started,” said Addie Muckenhirn, a 10-year-old from Edina who is on an Obstacle Academy team.

You have to be 21 to get on “American Ninja Warrior.” But even if the show is off the air by the time Addie is old enough, she can hope that sport governing bodies like the National Ninja League or Ninja Sports International will still be around to sponsor competitions.

Too new to certify

Hogan guesses that there are about 400 ninja gyms in the country. He hopes to open another gym in the Twin Cities next year.

Jake Marshman, owner of the four Conquer Ninja gyms in the Twin Cities, expects to open a fifth one in Mankato by the end of the year. He hopes to franchise up to 20 other locations in the Midwest next year, and possibly start a business selling obstacle equipment to other gyms. Another entry into the field, Five Star Ninja Warriors, opened in Roseville in October.

Unlike other sports, ninja gyms are too new to have a national certification program for instructors.

“It’s in need of something like that,” Hogan said. “It has inherent risks.”

Local gyms say their instructors have certification in other programs like personal fitness training or indoor climbing.

“I’m a mom. Safety is the top priority,” said Schoback, the Obstacle Academy owner.

Hogan said the main injuries he’s heard of have been to the Achilles tendons of men who try to run up a warped wall without warming up first.

“We need to watch it and be aware of it,” said Heather Bergeson, a pediatrician and sports medicine physician with the TRIA orthopedic practice in Bloomington.

But so far she hasn’t seen any ninja gym injuries in her office. Bergeson said her own kids have tried ninja obstacles at birthday parties.

“I see way more kids injured coming off the playground,” she said.