When he’s driving into a city, Jason Larson’s eyes are drawn to the skyline. But the Golden Valley resident has a different perspective than your typical gawker: He’s conquered those buildings.

“It’s kind of cool to go into a city and see the tallest building and say to yourself, ‘I climbed that,’ ” he said.

Larson has run up the stairwells of the Western Hemisphere’s tallest building, Chicago’s 110-story Willis Tower (still commonly called by its original name, the Sears Tower). In town, he’s run up the 30-floor Accenture Tower in downtown Minneapolis. Ten times. In 60 minutes. And he sprinted up 50 flights in the IDS Tower in under 8 minutes.

And every time, the stairwell gets more crowded.

Although stair-climbing races have been around since 1977, when 15 competitors ran up 86 floors in the Empire State Building, the activity has exploded in the past few years. The U.S. Stair Climbing Assocation, part of the newly formed Towerrunning World Association, reports that there were more than 200 races in 2012 that drew a combined 105,000 participants.

It’s part of the movement toward extreme fitness challenges, said JR Haines, a personal trainer at the Life Time Fitness Club in St. Louis Park.

“People are basically tired of the same old workouts and want to be challenged in new and interesting ways,” he said. “This is the reason that Tough Mudders and Warrior Dashes have become really popular. Running on the treadmill just isn’t doing it for people anymore.”

The climbing interest in climbing stairs is evident locally. The American Lung Association’s Fight for Air Climb drew more than 800 participants to the Accenture Tower, while 1,500 people tackled the Climb for the Cure for cystic fibrosis in the IDS Tower.

The racers certainly aren’t doing it for the scenery; there isn’t any. Stairwells have little, if any, ventilation, so fresh air’s not a big draw, either. (Because of the ventilation issue, nearly all stair-climbing events are held between November and April, when it’s cooler.) And because most of the events are staged as fundraisers, there aren’t any prizes.

In fact, in order to win the championships at the recent local events, Larson had to make donations to the sponsoring charities as part of his entry fee, but he didn’t mind.

“I just do it for fun,” he said. “And it raises money for good causes.”

But the races are increasingly attracting participants who are there primarily to compete. In fact, stair-climbing reached something of a dubious milestone last year when the winner of a charity event in Los Angeles was disqualified after security cameras caught him riding an elevator part of the way.

Up, up and away

Speed stair-climbing is not an endeavor for people who don’t like pushing themselves.

The Empire State Building climb still uses a massed start, but that’s a rarity because sponsors don’t want racers pushing and shoving for position on the stairs. Most races use computer chips to time staggered starts, with racers leaving the starting line at 5- or 10-second intervals.

As a result, racers have no way of knowing how they are doing compared with their competitors, who might have started several minutes before them or may still be in line behind them. The only strategy under such circumstances is for the racers to run as hard as they can for the entire climb.

There are no cheering crowds to bolster the spirits. Every few flights there’s a water station, usually a table tucked into the open door of that floor. But beyond that, the stairwell is eerily quiet as each climber focuses on the next flight of stairs.

Even avid distance runners report that forcing themselves to keep going is challenging.

“That hurt really bad,” Mary Jane Lund of Maple Grove admitted as she reached the top of the Accenture Tower. “By the 10th floor, it was getting painful. I’m a runner, so I have pretty good endurance, but I can really feel it in my quads.”

The stress comes from having to lift up your body weight with each step instead of carrying it along horizontally the way you do when running on flat ground, said Adam Rozmenoski, a personal trainer at Life Time’s Eden Prairie club.

“It’s a high-intensity workout,” he said. “That’s one reason that it’s gotten trendy: You get more bang for the buck. You burn more calories with 10 minutes of stair climbing than with 10 minutes of running because you’re carrying your body weight vertically.”

It can take a toll in a hurry, especially on racers who start too fast. “The first two levels were great,” said Josh Karry of Woodbury, “but then I realized that I’d better pace myself.”

Charles Rhea of Plymouth used a more scientific approach based on the heart monitor he wore on his wrist. “I do a lot of marathons, duathalons and triathlons,” he said. “I’d never done this before, but I just kept my heart rate between 155 and 160, and I feel fine. But you might want to check with me in a day or so.”

Stair racing can be tough on the knees, calves and lower back, Haines said.

“It is important to have strong glutes, quads and core,” he said. “Good flexibility is also important to maximize full range of motion.”

The best training is done in real stairwells, but for insurance reasons, building managers frown on that. Stair-climbing exercise machines are a good alternative, Haines said. Other techniques include running up hills or on a treadmill set at a steep incline, in addition to exercises that strengthen glutes and core muscles.

Cross-training is vital, Rozmenoski said.

“Stair climbing breaks down muscle tissue, and you have to allow your body to recover” between sessions, he said. “Cross-training allows you to stress the body in other ways.”

One element of stair climbing should appeal to even the most dedicated couch potato: When you reach the top of the building, you get to take the elevator back down.