Many notable trail runners will compete in the 2015 Gore-Tex U.S. Nationals Ski Mountaineering Championships, March 13-15.

In the summer months, Colorado’s Stevie Kremer is one of the world’s best trail runners, having won numerous domestic and international races over the past several years. But during the winter, she really doesn’t run at all. The same goes for Catalan phenom Kilian Jornet.

It’s not that they’re not training. They’re just giving themselves a mental and physical break from the rigors of running while building aerobic fitness and strength with different sports, primarily randonee skiing and ski mountaineering (skimo) races.

“I fully take off the winter from running,” says the 31-year-old Kremer, the 2013 SkyRunner World Series champion and winner of the 2013 Pikes Peak Marathon. “I feel like skimo is less pounding on your knees—it really does give your legs a break.”

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Anton Krupicka, Chris Vargo, Joe Grant, Dakota Jones, Rob Krar, Alicia Shay, Mike Foote and Nico Barraza are among the many other top American trail runners who have spent considerable time on skis this winter. Runners in colder climates have been cross-country skiing (classical or skate skiing) as a form of cross-training for years, but ski mountaineering is entirely different. Randonee skiing (or alpine touring) involves traveling up mountains on skis—without using a chair lift—either at a ski resort that allows it or in wild, backcountry locations.

Skiing uphill is made possible by attaching synthetic “skins” to the bottoms of skis (uphill skis are usually much lighter than standard alpine boards and have special bindings to allow for both uphill and downhill travel) and using a kick-and-glide technique to move forward. If terrain becomes too steep, skiers continue hiking while carrying their skis over their shoulders or in a pack. Once uphillers reach the top of the climb, they remove the skins, point skis downhill and enjoy the thrill of hard-earned turns back to the bottom or to the base of the next climb.

“It’s a great workout because you wind up getting a good amount of vertical in just a few laps and it’s a very low-impact way of getting it,” says Krupicka, who has skied 25 days near Allenspark, Colo., in 2015, often with Grant.

On the international level, ski mountaineering, long a common mode of winter travel in Europe, has a robust racing scene. Follow Jornet or Emelie Forsberg (champion of the 2014 SkyRunner World Series) on Instagram and you’ll see photo feeds filled with winter wonderland images from their home in Chamonix, France. The two spend the winter heading up big hills on little skis during the week and racing to podium finishes on the weekends. Plus, they emerge from the dark days with a strong fitness base for running rugged mountain races during the summer.

Skimo racing is starting to take off in the U.S., and American trail runners are trading running shoes for ski boots for year-round fitness and mountain training.

Kremer, a schoolteacher from Crested Butte, Colo., has been channeling her winning ways in trail running to uphill skiing for years, taking third in the 2012 U.S. Ski Mountaineering Championships. On Feb. 28, Kremer and her teammate Lindsay Plant, finished first at the Audi Power of Four Ski Mountaineering Race in Aspen, Colo. The course took skiers up Aspen’s four ski mountains for 11,600 feet of vertical climbing over 26 miles.

A lot of notable trail runners will be competing in the 2015 Gore-Tex U.S. Nationals Ski Mountaineering Championships from March 13-15 in Crested Butte, Colo., and the March 27-28 Gore-Tex Grand Traverse, a 40-mile through-the-night race from Crested Butte to Aspen with 7,800 feet of vertical gain.

Kremer plans to compete in several more races in March—including the national championships—citing the fact that running and skimo “complement each other so well.”

David Ruttum, a physician from Vail, Colo., couldn’t agree more. The four-time Ultra-Trail du Mont-Blanc finisher grew up skiing and began ski mountaineering a few years ago once the equipment became more user friendly.

“From mid-November to mid-May I ski, then, for the rest of the year, I trail run,” says Ruttum, 36. “It’s important to use different movement systems in your body and change your mental focus. Skiing works large muscle groups and improves muscular endurance, and in running I focus on speed.”

Ruttum praises ski mountaineering for developing a strong baseline of fitness.

“After hiking and skiing, your body is fatigued, but still feels really good,” Ruttum says. “You get a challenging workout without the physical pounding.”

Skiing and running are both lifelong pursuits for Paul Hamilton, a Durango-based ultrarunner who grew up in Colorado. But this is his first season doing skimo, and he’s a big fan.

“Skimo is low impact, aerobic work,” the 28-year-old says. “It’s great for strength training and it’s super fun. When it comes time to run in the spring, I’ll be that much more excited to get back into it after taking a mental and physical break.”

Hamilton also enjoys the adrenaline rush.

“Skimo racing is super rowdy,” he says. “It’s basically survival skiing downhill on toothpick skis—I love it!”

And much like the trail racing scene, a key component to ski mountaineering is spending time in beautiful places with a group of friends.

“It’s really cool to see the guys I compete against in the trail scene out here too,” Hamilton says.

That camaraderie is what attracted Ben Kadlec, 31, a software engineer from Boulder, to the sport.

“This is like mountain running, but on skis—it’s the same sort of racing,” Kadlec says. “You see many of the same people throughout summer running and winter skiing.”

Kadlec also touts the low impact effort as a great complementary workout for running. “It’s a full body hurt, every muscle hurts, but your joints and knees don’t,” Kadlec says. “It’s a great pain! I’m trying to convince more of my running friends to try it.”

According to Ruttum, the complementary nature of the sports happens because athletes tend to use hip flexors, glutes and their upper body more in ski mountaineering, and hamstrings and quadriceps in trail running. He says the one challenge can be foot strength. Skiing doesn’t always require using as much intrinsic foot strength, meaning feet may be tired and sore for the first week or two of running.

Some regular racers were surprised to see two new faces as the starting line in Aspen last weekend. Rob Krar and Mike Foote competed as a team in what was their first skimo race. Both are skiers, but the lightweight racing gear was a new challenge. Krar, 38, had 11 days to test his skimo rig and Foote got his about a month before the race. Even with the new gear Foote, 31, who lives in Missoula, Mont., said he managed to have his best ski run of the year so far.

“It’s a new kind of hurt, but I like it,” says Krar, who regularly hikes and skis in the mountains around Flagstaff, Ariz.

“You are working hard the entire time, even the descents,” Foote says. “It’s a fun way to travel through the mountains.”

The two finished fourth overall.

“I had no idea that was Mike Foote and Rob Krar ahead of me,” says an enthusiastic Ruttum, who finished 5 minutes behind them for fifth place.