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When veteran running journalist Scott Douglas took a reporting trip to the epicentre of international distance running, in the highlands of western Kenya, he noticed a curious pattern. Even though the temperatures were relatively comfortable, he was the only one running in shorts and a T-shirt.
"I was struck that all the runners did almost all their runs in full sweats," recalls Douglas, a contributing editor at Runner's World magazine and author of the forthcoming book Running Is My Therapy. "This was true even of the up-and-comers who aren't sponsored and have maybe two outfits, but are wearing them for two or three runs a day."
While it's tempting to dismiss this as a meaningless cultural quirk, a new study in the journal Medicine & Science in Sports & Exercise suggests that overdressing during exercise may have performance benefits – particularly if, like runners across Canada planning to run a spring marathon, you're training in cool conditions for a competition that might take place in hot weather. Enduring the discomfort of a few extra layers, it turns out, triggers physiological adaptations that help you better handle the heat.
The basic principles of heat adaptation have been understood for decades, dating back to pioneering experiments in the sweltering gold mines of South Africa in the 1930s. With the introduction of mandatory acclimatization periods of up to 14 days, miners developed higher sweat rates, greater blood volumes and lower core temperatures – and the heat-stroke deaths that had previously plagued the mines mostly disappeared.
The same principles apply for athletes, too. In the weeks leading up to hot-weather competitions, such as the 2020 Tokyo Olympics and the 2022 World Cup in Qatar, virtually all competitors will conduct hot-weather training camps to acclimatize, or use sophisticated heat chambers to simulate the conditions.
Not all of us have that option, of course. In recent years, researchers have suggested usinga sauna, or even just taking a hot bath, after runs in order to get your body used to heat. But overdressing is an even simpler option, and one that many runners have tried haphazardly over the years – including Brett Ely, a physiologist at the University of Oregon, who tried the approach before competing in the 2016 U.S. Olympic Marathon Trials in Los Angeles.
Ely and her colleagues decided to test the approach more rigorously in the laboratory, so they recruited 13 runners to complete two one-hour runs. In one run, they wore a singlet and shorts in a room set to 40 degrees Celsius. In the other run, the room was cooled to 15 degrees Celsius – but the runners donned five oppressive layers of clothing, including a fleece hat, mittens, a waterproof jacket and rain pants, with each layer tucked in and the sleeves and ankles cinched.
The result showed that overdressing didn't overheat the runners quite as much as hot conditions, but it did heat them up enough to trigger adaptations. Their sweat rate and heart rate spiked, and 10 of the 13 runners reached a core temperature above 38.5 degrees Celsius, which is thought to be a key threshold for heat adaptation.
There are, of course, some particular challenges in the context of Canadian winter. In serious cold, sweat will cool and recondense – or, worse, freeze – on the outer layers of clothing, meaning that you end up cold and wet instead of hot. "The best way to implement this approach while living in a very cold climate," Ely says, "would be to do some easy runs indoors on a treadmill or indoor track with a bit of extra clothing."
But if your target competition isn't until spring, you can probably wait until the outdoor weather is a little nicer, since you don't get any further acclimatization benefits after about two weeks. "I would suggest starting about two to three weeks out from competition and overdressing on easy runs so that there are 10 to 14 sessions before the race," Ely says.
For Douglas, the findings vindicate his own experiments with overdressing for muggy training runs in Maryland and Virginia in the 1980s – with one improvement. "It would probably be less miserable now that you'd be doing it in better gear than cotton shirts and nylon pants," he says, "both of which clung to me like it was pouring rain after a few miles."
Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.
Follow him on Twitter @sweatscience