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This Is How Your Body Adapts to Working Out in Hot Weather

I trained in a high-tech “sweat lab” with elite athletes to find out.
Frederick Bass/Getty Images

I board a mini-bus in Manhattan and am given an envelope containing hermetically sealed pill, a rubber bracelet, and a bulleted list of instructions. After skimming the instructions, I waste no time putting the bracelet on my wrist. Its purpose is to dissuade anyone from stuffing me into an MRI machine for the next two days or so. That's a fairly unlikely thing to happen, but the pill I’m trying to get down my gullet is chiefly made of metal. The concern is that the magnetic field in an MRI could move this foreign object around—and, if the events of a certain Black Mirror episode are to be believed—pull it out of my body via the shortest possible route.


The bullet-like pill is a thermometer designed to relay core temperature. I’m told that if the scientists I’m about to meet at the University of Connecticut’s Korey Stringer Institute had their druthers, they’d ask me and the two other journalists I’m road tripping there with to put thermometers in our asses instead.

“Apparently the temperature reading is more accurate that way, but we were like, ‘um…no’,” says Heather, one of two media chaperones from the PR firm representing a company called Mission.

Mission makes clothing and accessories that are intended to cool its wearers in the heat. The company counts Tennis great Serena Williams, NBA star Dwyane Wade, and Spanish soccer player David Villa among its celebrity endorsers. Today I've been invited to visit the Mission Heat Lab, a high-tech facility where the company tests what happens to athletic performance in a range of climatic conditions. Specifically, the Lab's aim is to improve human performance, stamina, and safety in the heat.

That last part is a big reason why they're housed in the Korey Stringer Institute (KSI—Stringer, you may recall, was a Minnesota Vikings offensive lineman who died in August 2001 from exertional heatstroke. His wife Kelci worked with Douglas Casa, an exertional heat stroke expert, to create the institute, which opened its doors at UCONN in April 2010.

Mission's Lab, which opened in the fall of 2017, has an outer room from where our party and a handful of Mission gear-wearing postgrad students watch a sinewy triathlete pedal on a stationary bike in sultry conditions—what's known as a "sweat wash down test," explains Robert Huggins, KSI's vice president of athlete performance and safety.


After an hour of cycling conditions ideal only for provoking hellacious swamp ass, the sweat on her body and soaked into her hair and clothing will be rinsed off with distilled water and a “sweat soup” will be made. From the the data gathered from this brimming pot of jock broth, the team can advise the athlete on how to optimally hydrate and take in electrolytes during her next event.

But what interests me most about the heat lab is how it can be used to help athletes acclimatize to hotter temperatures in which they are due to perform. Huggins uses an example of a football team from the Northeast traveling to compete in warmer climates. “Ordinarily the team that trains in warmer conditions will have a head start over those who don’t,” he says, adding that athletes who train in Florida, Louisiana, or Arizona will likely have a lower core temperature and heart rate than their northern counterparts while competing. “Additionally, their sweat rate and blood volume will be higher, and they’ll hold onto electrolytes better,” he says. “These are all advantages when exercising in the heat.”

You don’t need to be heading to a subtropical track meet to get the benefits. “A great deal of research has also shown that exercise in the heat also assists with cool weather events,” he adds. “In other words, if you get heat acclimatized and take part in a cool weather event, you will have a performance advantage that someone who doesn't exercise in the heat."


At that, I’m asked to give a urine sample, which is rated as a "one" when held up to a color chart that goes from virtually clear to something the color of Guinness. Excessively dark pee, the lab tech tells me, could be a sign of rhabdomyolosis, which entails pissing out rapidly damaged muscle tissue in the form of the muscle protein myoglobin. It can happen as a result of a crush injury, prolonged coma, or even over-exuberant CrossFitting.

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With that ghastly image in my head, I walk into a room that adjoins the heat chamber, strip naked, and get weighed—I’m found to be just shy of 141 pounds. Huggins tells me they're going to look at my core temperature and sweat rate. Anyone with a good scale can take the latter metric—you just weigh yourself, get on a bike or treadmill for 30 minutes, and weigh yourself again. Then, convert the differences to liters—a pound of water is about 453.5 ml—and double it. That's your sweat rate, expressed in liters per hour. “If you want to drink water when you’re on the treadmill," Huggins says, "just step on the scale with your bottle at the beginning and the end of the exercise."

On cue, I’m given a known quantity of water and a heart rate monitor, and I'm ushered into the heat chamber. At 93.3 degrees fahrenheit and 52.2 percent humidity, it’s certainly warm and muggy, but not infernal as I’d expected. (The heat and humidity can be dialed up to as high at 110 degrees fahrenheit and 95 percent, respectively.)


I’m shown to a treadmill, and soon I’m huffing along at a steady 6.0 mph. Every five minutes, my core temperature is taken and I’m asked how thirsty I am, how hard I’m working, and how hot I feel. On a monitor I see my baseline heart rate of 70bpm double, and by the end of the 30 minute exertion, top out at 188 bpm. After I and my remaining water are weighed, my sweat rate is determined to be 0.9 liters per hour, my urine got one gradation of color darker, and my core temp is 101.46 degrees fahrenheit.

“Your temperature was still going up by the time the experiment ended,” says Huggins, who suggested that this could mean that my sweat rate could in fact top 1 liter per hour.

Running in those hot and stuffy conditions was much more difficult than loping along the banks of the East River in early April, or—as is more often the case—powering along on the treadmill in my gym’s air conditioned environs. Over lunch, Huggins explains why, describing how oxygen and metabolites in the blood are needed in three places: the muscles, the skin, and the brain. “At the muscle, blood is needed to produce force and keep you moving. At the skin, blood is needed in capillaries to allow you to sweat and remove excess heat. And at the brain, blood keeps the whole system functioning,” he says, adding that when the environmental temperature gets hotter, the body needs to be able to get rid of the additional heat that is produced in the muscles in order for you to continue to exercise. Hence, sweating.


While other mammals pant or have circulatory systems that are closer to the skin to help them lose heat and stay cool, humans are generally adept at sweating to keep our core temperature in a "normal range" of 101-103 degrees Fahrenheit. If we don’t sweat or don’t sweat effectively, core temperature will rise to dangerous levels, and could lead to exertional heat stroke, which—as the name of the institute attests—can be fatal.

“When you exercise in the heat, there is a supply and demand issue between these three locations. The burden is placed primarily on the cardiovascular system or your heart, “ he says, explaining that as we sweat and lose fluids from your body, the amount of blood decreases. When there isn't enough blood—or supply to meet the demand—your heart rate and temperature go up. The higher they go, the harder it is to exercise, the more fatigued you feel, and the hotter you get. One of the reasons that the Mission Heat Lab had athletes shuttling in and out all day is because with as little as 5 to 7 days of training in the heat—while keeping their core temp between 101.5 to 103.0 for 60 minutes—an athlete can achieve 90 percent heat adaptation, enabling them to bring their best when competing in a hot climate.

Interestingly, the Lab worked with soccer star Cristiano Ronaldo (and the less swoon-inducing members of the Portuguese soccer team) ahead of the 2014 World Cup in Brazil. Portugal’s qualifying group games were set to be played in equatorial Manaus. For that reason, the KSI guys advised them to do their pre-tournament training in sweltering southern Florida. The team’s bosses, however, decided to train in temperate New York for promotional and marketing reasons. With one win, one loss, and one draw, Portugal never made it out of sultry Manaus. Of course, being heat adapted isn’t the only factor. Ghana, with a climate much more similar to Manaus than any other team in Group G, came dead last.

Huggins explains that being heat adapted means that the heart rate is lower at a given intensity and blood volume expands, allowing you to deliver more blood to the exercising muscles and to the skin for cooling via sweat. “Most importantly, your core temperature is lower both at rest and during exercise and your body becomes a more efficient sweater,” he says, adding that generally, the more fit you are, the more "efficient of a sweater" you are. In other words, it may not be that you sweat more all the time, but you sweat when you need to—and possibly sooner than the person running next to you.

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