The Complex Physiology of Ski Mountaineering

In July, the International Olympic Committee announced that ski mountaineering will make its Olympic debut at the 2026 Winter Games in Milano-Cortina, Italy. The transition from niche backcountry passion to a source of five Olympic medals will bring new attention to the sport—including from sports scientists. To kick things off, a team of researchers led by Lorenzo Bortolan of Italy’s University of Verona (along with colleagues from Sweden and Slovenia) just published a primer on what’s currently known about skimo, as the sport is known to the cognoscenti, in the journal Frontiers in Physiology. Here are some highlights:

The Basics

Skimo involves climbing mountains, mostly on skis fitted with non-slip skins. There are also some technical sections, for example along ridges and couloirs, where you climb on foot with your skis strapped to your backpack. And the payoff is skiing back down the mountain with your skins removed.

The Olympic program includes a men’s and women’s sprint, which typically involves a single ascent of about 250 vertical feet, then skiing back down, with a total duration of 3 to 3.5 minutes. There’s also a men’s and women’s individual event which lasts 1.5 to 2 hours and involves at least three ascents and descents, with a total vertical gain of a mile or more and skis-off technical sections totaling up to ten percent of the race. Finally, there’s a mixed-gender relay lasting about 15 minutes.

The Physiology

It takes a lot longer to skin up a mountain than it does to ski down it. That means it’s primarily an endurance sport. If you’re a few percent better than your rivals at climbing, you’ll gain minutes on them; if you’re a few percent better at descending, you’ll only gain seconds. Sure enough, in an Austrian study published earlier this year, the best predictor of race performance in a group of elite and sub-elite skimo racers was how they did in a ski-specific VO2 max test. Top pros reportedly train 16 to 24 hours a week during base periods, about half on skis and the rest biking, running, and roller skiing. In a given year, they’ll rack up 150 to 190 miles of vertical.

Of course, downhill proficiency also matters. You have to be good enough to make it down some pretty daunting slopes without bailing or snowplowing the whole way. This is not trivial: another study found that heart rate stays around 85 percent of max even during the descents—in part, the authors write, due to “the psycho-emotional and physical stress associated with choosing the optimal trajectory on the challenging course with varying snow conditions.”

Altitude is another factor to consider. While the rules for cross-country skiing events at the Olympics limit the elevation to a little under 6,000 feet above sea level, skimo doesn’t have an upper limit. Events often take place at 10,000 feet and occasionally as high as 13,000 feet. For every 3,000 feet of elevation, VO2 max declines by about six percent, and some people (in particular highly trained athletes) are affected more than others—so your ability to handle thin mountain air could be a limiting factor.

Finally, weight matters because you have to lug it uphill, so studies have found an inverse correlation between body fat and race time. That also affects gear choices, which we’ll get to below.

The Biomechanics

Skinning up a mountain looks a lot like the classic cross-country skiing technique, the main differences being that your skins don’t really glide uphill, and if you lose your grip you’ll tumble hundreds of feet to your death. (Okay, not quite, although a rule that goes into effect in 2022 stipulates that bindings must have a safety system that automatically stops the ski if it comes off—an innovation I would have appreciated in my own very limited experience!)

Because of the lack of glide imposed by skins, ski mountaineers take shorter, quicker strides than cross-country skiers. And when the terrain gets steeper, they shorten the strides even further and slow them down. In comparison, cross-country skiers tend to keep their stride frequency roughly constant and control their speed by altering only stride length. Upper-body strength and technique for poling uphill is also crucial.

On the downhills, ski mountaineers don’t get into as deep a tuck as alpine skiers, both because the terrain is a lot more uneven and unpredictable and, let’s be honest, because their legs are still shot from the uphill.

The Gear

Weight matters: an extra kilogram (2.2 pounds) at the ankles of a ski mountaineer burns two to three percent more energy. Safety also matters, which is why the International Ski Mountaineering Federation has a very long and detailed list of specifications for required gear to prevent athletes from competing to wear the lightest and flimsiest equipment. In particular, there are minimum weights for ski boots (1,000 grams for men, 900 for women) and skis and bindings (1,500 grams for men, 1,400 for women).

You also have to pack an approved snow shovel, snow probe, survival blanket, whistle, crampons, and avalanche detector, and have a minimum number of clothing layers. Bortolan’s paper has a nice infographic giving the details.

So what remains to be discovered between now and 2026? Bortolan and his colleagues point to environmental conditions as a current blindspot. How do high altitudes, cold temperatures, and varying snow conditions affect the demands of a race and dictate the best way to prepare for it? Wearable tech that’s specifically optimized to collect relevant data on pacing and effort during skimo runs will provide a more detailed picture of what athletes are experiencing. Better equipment that boosts performance and simplifies the transitions between skinning, hiking, and downhill skiing will undoubtedly emerge.

Still, as Olympic glory and its attendant perks beckon, it’s maybe worth finishing with this video of a skimo day that the maestro, Kilian Jornet, took from his home in Norway a few years ago. Every sport yearns on some level for more attention and bigger crowds—but for many ski mountaineers, the obsessive pursuit of faster, higher, stronger is just a means to another, less quantifiable end.

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