How to Heat-Proof Your Training

To prepare for the muggy Tokyo weather at this summer’s Olympics, athletes around the world are currently gearing up their heat adaptation strategies. Get your body used to hot-weather exercise, and you’ll benefit from a higher sweat rate, lower body temperature, lower heart rate during exercise, and various other helpful changes.

But getting the adaptations right isn’t straightforward. Traditional protocols, based on ones developed for the hellish conditions in South African gold mines in the 1920s and 1930s, call for at least an hour of sustained hot-weather exercise for 10 to 14 days—not a particularly pleasant or restful way to spend the final weeks before a big competition. Sports scientists have developed a variety of alternate protocols: shortening the length, using ingestible thermometers, adding hot tub or sauna sessions, and so on.

But there’s still plenty of debate over the best approach. Here are the highlights from three new studies on the nuances of facing the heat:

“Natural” Adaptation Isn’t Enough

The first study, published (and free to read) in the International Journal of Environmental Research and Public Health by a group led by Douglas Casa of the University of Connecticut’s Korey Stringer Institute, explores the differences between heat acclimatization and heat acclimation. The former is what happens naturally when you train outdoors in hot weather; the latter refers to a deliberate lab-based adaptation protocol—or, as the paper defines it, “the systematic process of repeated exposures to a thermally extreme environment that elicits positive physiological and perceptual adaptations in an artificial environment.”

The researchers asked 25 male runners and cyclists to do their normal training over the three months of a New England summer. Then, in September, they tacked on a five-day acclimation protocol that involved running in a heat chamber for about 80 minutes a day. (In comparison, the average run over the summer was 56 minutes long.) The heat chamber was set to just over 100 degrees Fahrenheit with 50 percent humidity; the goal of the session was to accumulate at least 60 minutes with a rectal temperature above 101.3 F.

The key question was whether this lab protocol would add any further adaptations, or whether the athletes would already be fully acclimatized from their summer training. The answer: the additional acclimation helped. In a 60-minute running test at 95 degrees Fahrenheit, the subjects dropped their average heart rate from 143 beats per minute in the spring to 138 after the summer of training, then to 134 after the five-day lab protocol. Similarly, sweat rate increased and rectal temperature decreased after the lab protocol.

The bottom line: if you’re preparing for a hot-weather competition and really want to dial in your preparation, you can’t assume that simply training through the summer will get you all the way there. Of course, the study took place in New England: you might have different results in Florida. And even if you don’t live in the hottest parts of the country, you might be able to max out your heat adaptation by deliberately timing your training for the hottest periods of the day for a week, and making sure you’re out there for more than an hour for each of those sessions.

The Benefits Don’t Fade

One of the most vexing problems for athletes is figuring out how to time their heat adaptation prior to a competition, without messing up their taper, their travel plans, and perhaps their final stint of altitude training. A study from Nicola Gerrett and her colleagues at Vrije Universiteit Amsterdam explored the benefits of reacclimation—that is, a short five-day hit of heat training to refresh the adaptations four weeks after an initial ten-day heat adaptation period.

The intended goal of the study, published (and also free to read) in Medicine & Science in Sports & Exercise, was to compare two five-day reacclimation options: hot exercise (cycling in 91-degree heat in order to maintain core temperature above 101.3 degrees for at least an hour) or a hot bath (40 minutes submerged to the neck in 104-degree water). But the most surprising finding was that many of the benefits of the initial ten days of cycling in the heat were still present after 28 days with no heat training.

The initial ten-day protocol lowered core temperature at rest and during exercise, lowered skin temperature, increased sweat rate, improved perceptions of thermal comfort, and increased time to exhaustion in a cycling test. Four weeks later, thermal comfort and core temperature, both at rest and during exercise, were still better than they’d been at the beginning of the experiment. Sweat rate had declined again, but both reacclimation protocols were equally effective at bringing it back up.

The researchers’ conclusion is a reassuring one: if you’ve done a good ten-day heat adaptation protocol, you probably don’t need to worry about doing another one for at least a month. This is reasonably consistent with a common rule of thumb suggesting that the benefits of heat adaptation last about twice as long as you spend acquiring them. And if you do want to top up your sweat rate, active (exercise) or passive (lounging in the hot tub) approaches both seem to work.

The Bath Is Best?

The third study takes a closer look at the merits of hot exercise versus hot baths. Researchers in Britain, including Jessica Mee of the University of Worcester and Neil Walsh of Liverpool John Moores University, tested six days of hot exercise versus six days of cool exercise plus a hot bath. The results, published in the Journal of Science and Medicine in Sport (and yet again free to read), suggest that baths may have an advantage, but with some additional wrinkles worth considering.

The crux of the new study is accessibility. Few athletes have access to heat chambers, and even fewer want to insert rectal thermometers (which are needed to get a reliable gauge of core temperature, unaffected by confounders like the mouth-cooling effect of a drink of cold water or the superficial skin heating of a warm bath) every time they train. The gold standard of exercise-based heat acclimation, as in the two studies above, involves monitoring your core temperature and adjusting your effort as needed to keep it above about 101 degrees Fahrenheit for the desired duration (usually an hour, which means running or cycling for a total of 80 to 90 minutes because it takes a while to get your temperature up). If you just go out and run at a steady pace, your temperature may end up being too low to spur adaptations or too high to be able to sustain the exercise for long enough.

The protocol in this study was simpler, reflecting how people actually tend to try heat acclimation. The subjects just ran for an hour at 65 percent of VO2 max (a fairly comfortable pace) in a heat chamber (91 degrees Fahrenheit, 40 percent humidity). The bath protocol involved running for only 40 minutes at an air temperature of 66 degrees Fahrenheit—just enough to get your core temperature above the threshold—after which they got into the 104-degree bath up to the neck to maintain their high core temperature for another 40 minutes.

The headline result is that the bath worked better: it produced larger changes in core temperature and sweating response. That’s a win, but not really all that surprising. Running for an hour at a comfortable pace, even in the heat, simply isn’t enough to keep your core temperature high enough for long enough. That’s why the other studies used 80- to 90-minute bouts. In fact, a control group in this study did 60 minutes of running in cooler temperatures (66 degrees), and produced similar results to the running-in-the-heat-for-an-hour group. If you’re just jogging for an hour, the heat simply isn’t stressful enough to maximize your adaptations.

Conversely, running for up to 90 minutes a day in the heat for six days in a row is probably too stressful for many athletes, particularly shortly before a race. The bath protocol, which keeps your core temperature up for close to an hour without forcing you to train for that long, seems like a nice compromise. But don’t make the mistake of thinking that 40 minutes up to the neck in 104-degree water is a pleasant soak in the jacuzzi. “The hot tub was nice for the first 15 to 20 minutes, but there was always a time when you would think ‘Alright, I’m over this, how much time do I have left,’” elite steepler Isaac Updike told me a few years ago. “Sadly, the answer was always longer than you hoped.”

Put all these findings together, and the overall conclusion is that the perfect heat adaptation protocol is in the eye of the beholder. Some people have 100-degree weather out of the front door; others have a hot tub in the backyard. Some train for 90 minutes or more daily as a matter of course; others would consider that unsustainable. Some athletes are peaking for a single big race; others need their heat adaptation to last for weeks or even months. Pick the best approach based on your own situation—but don’t assume that you’re ready to handle the heat just because it’s summer.

For more Sweat Science, join me on Twitter and Facebook, sign up for the email newsletter, and check out my book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.