As a father, a husband, and a purveyor of training advice, one of the most valuable and hard-earned lessons I鈥檝e learned over the years is that telling someone that a problem is all in his or her head is seldom helpful. So please don鈥檛 take this the wrong way鈥攂ut when you slow down on a hot day, it鈥檚 not just a consequence of elevated core temperature or decreased blood volume. It鈥檚 at least partly in your head. It鈥檚 not just me telling you this鈥攊t鈥檚 science.
A published in the journal Physiology and Behavior investigated whether the subjective experience of feeling hot is enough to hurt your performance, independent of whether your body is actually getting hot. This is a tricky thing to test; the results of this study are telling.
Nobody disputes the fact that heat slows you down. But it鈥檚 unexpectedly difficult to pin down the precise reasons for this slowdown. It鈥檚 not just dehydration: if it鈥檚 100 degrees out, no amount of drinking will enable you to run a marathon as fast as you would on a 50-degree day. There are numerous ways that heat makes you less efficient鈥攐xygen-rich blood is diverted away from your muscles to instead ferry excess heat to the skin, for example鈥攖hat tells part of the story, but not all of it.
In the 1990s, a promising 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鈥檒l 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 the perception of effort, with the idea that your performance in the real world ultimately depends on how you feel. In , 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 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鈥檚 .
Still, it鈥檚 tricky to tease out the perception of heat from actual physiological responses to heat. The solution researchers from that 2019 study I mentioned earlier 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 nine 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鈥攂lood lactate, core temperature, skin temperature, heart rate, stroke volume, cardiac output, oxygen uptake, ventilation鈥攚ere different. The feeling of being a bit warmer was the only apparent difference.
Samuele Marcora, one of the researchers who conducted the study, feels encouraged by the results and wants to take things a step further and try the heat-pad approach for heat acclimation. 鈥淢y next study would be about using this cheap 鈥榝ake heat鈥 method to induce psychological adaptations for people who compete in the heat but can鈥檛 afford training in exotic places or a climate chamber,鈥 he says.
The study isn鈥檛 perfect, of course. Having a heat pad attached to your back could be distracting (for reasons that aren鈥檛 clear, the subjects didn鈥檛 wear the pad during the control ride). And it wasn鈥檛 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鈥檚 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. Suppose you decide to alter your thermal sensations by swishing menthol in your mouth or by wearing a cooling headband. 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鈥檚 all in my head, I鈥檓 not hot. Thermal physiology and its effects on human performance and safety are real. But so is thermal psychology.
My book, , with a foreword by Malcolm Gladwell, is now available. For more, join me on and , and sign up for the Sweat Science .
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