Sweat Science

What does the evidence show?

Heat Slows You Down. Even When It's Not Real.

When it comes to the effects of temperature on performance, perception is reality

How you perform in heat is (mostly) in your head. (Curtis Kim/Stocksy)
Photo: Curtis Kim/Stocksy heat

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When it comes to the effects of temperature on performance, perception is reality

As a father, a husband, and a purveyor of training advice, one of the most valuable and hard-earned lessons I’ve learned over the years is that telling someone that a problem is all in his or her head is seldom helpful. So please don’t take this the wrong way—but when you slow down on a hot day, it’s not just a consequence of elevated core temperature or decreased blood volume. It’s at least partly in your head. It’s not me telling you this, it’s science.

The science in question is a new study in the journal Physiology and Behavior, a collaborative effort between Romain Meeusen’s research group at Vrije Universiteit Brussel in Belgium and Samuele Marcora’s group at the University of Kent, led by Jeroen Van Cutsem, a joint doctoral student in both labs. The goal of the study was to investigate whether the subjective experience of feeling hot is enough to hurt your performance, independent of whether your body is actually getting hot. That’s a tricky thing to test, but the results are telling.

Nobody disputes the fact that heat slows you down. But it’s unexpectedly difficult to pin down the precise reasons for this slowdown. It’s not just dehydration: if it’s 100 degrees out, no amount of drinking will enable to you to run a marathon as fast you would on a 50-degree day. There are numerous ways that heat makes you less efficient—oxygen-rich blood is diverted away from your muscles to instead ferry excess heat to the skin, for example—that tell part of the story, but not all of it.

In the 1990s, a promising line of research showed that if you put someone on a treadmill or an exercise bike in a hot room and ask them to continue for as long as possible, they’ll last until their core temperature hits a critical threshold of somewhere around 104 degrees Fahrenheit (40 degrees Celsius). This suggests that our brains are wired with some sort of self-protective circuit breaker that forces us to slow down or stop before we fry our circuits. But in the real world, researchers soon realized, virtually no one reaches that threshold while running marathons or doing other hot-weather exercise. No matter how motivated we are, we almost always slow down before the circuit breaker flips.

Instead, researchers studying endurance have begun to focus more on perception of effort, with the idea that your performance in the real world ultimately depends on how you feel. In a 2012 study, for example, the negative effects of cycling in 89-degree heat were partly erased when the thermometer in the room was rigged to read 79 degrees. And several recent studies have explored using things like menthol that create a perception of coolness to enhance performance in the heat. That may well be the biggest benefit of trendy sports tech like ice vests and Ashton Eaton’s cooling hood.

Still, it’s tricky to tease out the perception of heat from actual physiological responses to heat. The solution Van Cutsem and his colleagues came up with was to apply a small electric heat pad, tucked in the pocket of a specially designed shirt, to the upper back of their volunteers. The heat pad was turned up to about 104 degrees, just a few degrees above normal body temperature. Pilot testing showed that this was enough to make people feel hotter, but not enough to induce any measurable physiological changes: no increase in core temperature, no change in sweating, no shift in heart rate or any other cardiovascular parameters.

A dozen trained cyclists and triathletes completed a pair of time-to-exhaustion tests on an exercise bike at 70 percent of VO2max, with and without the heat pad. (As an aside, time-to-exhaustion tests are sometimes criticized because pedaling at a constant power output is different from racing at a freely chosen pace over a defined distance. But in this case, the advantage of a constant-power test was that it kept the level of heat being generated by each cyclist constant, rather than fluctuating depending on their pacing decisions.)

The results: subjects gave up 9 percent sooner when the heat pad was on, lasting 34:52 compared to 38:12 on average. They reported feeling hotter throughout the ride by about 1 point on a scale of -6 (very cold) to 6 (very hot). But none of the physiological measurements—blood lactate, core temperature, skin temperature, heart rate, stroke volume, cardiac output, oxygen uptake, ventilation—were different. The feeling of being a bit warmer was the only apparent difference.

Marcora, encouraged by the results, wants to take things a step further and try the heat-pad approach for heat acclimation. “My next study would be about using this cheap ‘fake heat’ method to induce psychological adaptations for people who compete in the heat but can’t afford training in exotic places or a climate chamber,” he says.

The study isn’t perfect, of course. Having a heat pad attached to your back could be distracting (for reasons that aren’t clear, the subjects didn’t wear the pad during the control ride). And it wasn’t blinded, so expectations may have played a role: in the crucible of an all-out test, we all look for excuses, and knowing that there’s an electric pad pumping heat into your body might serve as a tempting reason to stop a bit earlier than usual.

In a sense, though, the role of expectations is baked into the results. If you decide to alter your thermal sensations by swishing menthol in your mouth, or by wearing a cooling headband, then does it matter whether the cooling effect is acting on you consciously or unconsciously? Either way, if it makes you feel cooler, it may have the potential to help you push on in hot conditions. This does not, I hasten to point out, mean that you can run through Death Valley under the midday sun at the height of summer just by chanting to yourself “La la la, it’s all in my head, I’m not hot.” Thermal physiology, and its effects on human performance and safety, are real. But so is thermal psychology.

My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.

Filed To: Science / Racing / Weather