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Canada's Megan Imrie leaves shooting range during the women's 4x6 km relay biathlon final at the Vancouver 2010 Winter Olympics in Whistler, British Columbia, February 23, 2010.ISSEI KATO/Reuters

The first in a four-part series on physiological extremes encountered by Olympic athletes.

If swimming were a Winter Olympic sport, Lewis Pugh would be a heavy favourite. The 44-year-old South African adventurer has completed long-distance swims at preposterously cold temperatures in every ocean, clad only in a Speedo, goggles and swim cap. Perhaps the most extreme: a one-kilometre swim in icy, 1.7 C water at the North Pole in 2007, which he completed in 18 minutes and 50 seconds.

None of the events in Sochi, Russia, will take place underwater, but all the athletes have to overcome some combination of bitter cold, cutting wind, snow, ice and sleet in training and sometimes in competition. Simply surviving the conditions isn't enough: Numb hands or a chilled core can be the difference between victory and defeat. Pugh's feats, which have been monitored and analyzed by thermal physiologists, illustrate just how remarkable the body's capacity to endure – and excel – in extreme cold is.

In general, your body strives to keep its core temperature around 37 C, in order to keep your brain and heart running smoothly. A drop of just 2 degrees signals the onset of hypothermia, which begins with confusion and loss of co-ordination and can progress to organ failure and eventually death – though that's not a universal rule. Pugh's rectal temperature dropped to 33.6 C after one of his Antarctic swims, and even that's a far cry from the record of 13.7 C in a Swedish skier who was trapped under a frozen stream for 80 minutes and somehow still made a full recovery.

Miracles aside, keeping your core temperature in a safe range depends as much on how much heat you generate as it does on how much you lose to the cold air or water around you. "The human body is a furnace," Norwegian explorer Roald Amundsen, the first person to reach the South Pole, observed a century ago. This is true in a literal sense: For every calorie of energy your muscles burn, only a quarter is translated in motion, while three-quarters is emitted as heat.

That ability to generate your own heat means there's no absolute limit on the temperature humans can withstand. How much heat you can generate is directly proportional to your VO2 max, a measure of aerobic capacity that is particularly high in athletes. Olympic cross-country skiers can turn their furnaces up higher and keep them burning for longer than the rest of us – which is why they're able to endure multi-hour training runs clad only in a thin layer of fluorescent spandex.

On the other side of the equation, the amount of heat you lose to the environment depends mostly on what you're wearing. "Where we live, under normal circumstances it's very rare for people to reach the limits of their cold tolerance if they're appropriately dressed," says Dr. Ira Jacobs, dean of the University of Toronto's faculty of kinesiology and physical education and former chief scientist with the Department of National Defence.

But the balance between heat generation and loss can change rapidly if your activity level changes or if your clothes lose their insulative properties, for example by getting wet, Jacobs cautions. That's what triggers "hiker's hypothermia," a phenomenon first studied after three hikers died in the relatively temperate climes of England in 1964. The hikers were fine as long as they kept moving at a brisk pace, but their core temperatures plummeted as soon as they stopped to rest. It took less than 90 minutes for the sweaty hikers to collapse, even though the temperature was above freezing. In the 1990s, Jacobs adds, four U.S. Army Rangers died from hypothermia during training exercises in Florida, of all places. Once the furnace goes out, even moderate cold can kill.

The other key challenge athletes face is keeping their hands and feet working properly. As skin temperature drops, your peripheral blood vessels constrict to reduce the amount of heat lost from circulating warm blood near the body's surface.

"The body will basically sacrifice the periphery to maintain the core," explains Dr. Ollie Jay, a thermal physiologist at the University of Sydney. "The term for is it 'physiological amputation.'"

This effect is especially challenging in sports such as biathlon, which alternates between furnace-on-max skiing and completely motionless shooting. "Trying to load an inch-long bullet into the rifle's small breech requires enormous dexterity at the best of times," says Chris Lindsay, Biathlon Canada's high performance director, so finger numbness is simply not an option.

As a result, biathletes have a near-obsession with avoiding dampness, which sucks heat from their fingers. They'll warm up with one set of gloves, put on a fresh set of pre-heated gloves for the race, and if it's particularly cold add some overmitts that they'll discard at the first shooting. The goal: Stay warm but don't, under any circumstances, sweat.

Just as importantly, "I always keep my core as hot as possible," says Megan Imrie, a biathlete from Falcon Lake, Man., who will be competing in her second Olympics in Sochi. That means carefully adding and subtracting layers as needed, carrying reusable hot packs and wearing a "huge" down jacket until seconds before the start. "I've found that if my core feels cold even for a second, my hands are almost guaranteed to be cold," she says.

That's exactly what Canadian military researchers found in a series of studies conducted in the 1990s. A shirt outfitted with electric heaters at strategic points on the torso enabled subjects to sit in a chamber at minus 25 C for three hours with no gloves on, and maintain better finger temperature and dexterity than with gloves but no torso-heating. The heaters kept the core warm enough to suppress the "physiological amputation" reflex.

Similarly, a U.S. military study in 2011 showed that wearing a face-covering balaclava conserved enough heat to produce significantly warmer fingers in a cold chamber at minus-15 degrees C.

For winter athletes, soldiers and, well, Canadians in general, that's the best approach to handling cold. Stay dry, protect the core and cover exposed skin to prevent frostbite.

Or you can try Pugh's method.

To their amazement, researchers monitoring Pugh found that his core temperature spontaneously jumps by as much as 1.4 C just before a polar swim, apparently a Pavlovian response to the repeated shocks of his progressively colder ice-water training swims.

This effect has never been observed in any other human and, according to Pugh, involves unspeakable pain and possibly lasting nerve damage. Your choice.

Alex Hutchinson blogs about exercise research at