How Elite Marathoners Handle Hot Conditions
In-race data collected during the world championships sheds new light on how top endurance athletes avoid overheating
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At the 2019 world track and field championships in the muggy heat of Qatar, a total of 108 marathoners and racewalkers swallowed wireless temperature pills a few hours before their races and posed for an infrared camera at the start and finish lines. The resulting data, some of which has just been published in the Journal of Applied Physiology, offers a peek inside the engines of the top athletes in the world in real-world competitive conditions—including some unexpected surprises.
The unsurprising part is that the athletes got very hot. The conditions for the events included in the study—marathon, 20K racewalk, and 50K racewalk—ranged between 85 and 91 degrees Fahrenheit, with relative humidity between 46 and 81 percent. And this was in the middle of the night! The races took place between 11:30 P.M. and 5:00 A.M. local time, which at least eliminated the additional heating effects of solar radiation, both from above and reflected from the road.
Still, it was much too hot for optimal performance. Only one of the athletes in the study matched their pre-event personal best, and on average they were 13 percent slower than their best. Core temperature, as measured by the temperature pills, drifted upward by an average of 2.7 degrees Fahrenheit over the course of the races. All the athletes finished with “feverish” core temperatures above 100 degrees, and 16 percent of them were above 104 degrees, which was long considered an approximate upper limit of tolerance.
Previous data, including some from the 2016 world cycling championships, which were also held in Qatar, has shown that many elite athletes are able to push above 104 degrees without negative consequences. In fact, in the new marathon and racewalk data, there was no correlation between core temperature and performance (relative to previous best).
The shape of the temperature curves also tells an interesting story. The athletes tended to experience a rapid rise in core temperature during the first part of the race, then a plateau during the middle of the race, then a further rapid rise at the end of the race coinciding with their finishing acceleration. That’s consistent with a revised view of how heat affects pacing. You don’t slow down because your body has already overheated. Instead, you carefully regulate your pace to avoid overheating—until the finish line is within reach, at which point you throw caution to the wind.
But the analysis also contained an unexpected twist. The research team, which brought together scientists from eight different countries and was led by Polly Aylwin and George Havenith of Loughborough University in Britain, also used infrared cameras to estimate the skin temperatures of the athletes as they ran or walked by during the race. And in this case, the trend was the opposite: average skin temperatures dropped by 2.7 degrees from start to finish.
To understand why this happened, we have—conveniently—another newly published study, this one in Medicine & Science in Sports & Exercise from a team at the University of Canberra led by Felicity Bright and Julien Périard. Bright and her colleagues put cyclists through a series of four time trials lasting approximately 45 minutes on stationary bikes in the lab. The four conditions were still air and three different wind speeds (10, 18.6, and 27.3 miles per hour), generated by a stack of three industrial fans blowing in their faces.
Here are the power outputs in the four trials, with the still air trial indicated with black triangles:
Contrary to the researchers’ expectations, the different wind speeds didn’t matter much. Performance with 10 mph winds (which is the low end of what you might see while mountain biking) was indistinguishable from 27.3 mph (which is towards the upper end of what good road cyclists see for prolonged periods). But still air was notably different: the cyclists started slowing down early in the time trial. That’s because they were hotter. Here’s the skin temperature in the four conditions:
Moving air matters, even at the relatively modest speed of 10 mph. By that point, Bright and her colleagues suggest, your sweating efficiency is probably close to 100 percent, meaning that it’s cooling you down by evaporating directly into the air rather than dripping into that disgusting pool you create when you cycle or run hard indoors.
In fact, the Doha track and field results suggest that moving air matters at even more modest speeds like those sustained by marathoners and racewalkers. There are a lot of factors that may have contributed to the observed drop in skin temperatures, including varying wind conditions, changes in air temperature as the night progressed, and cooling strategies like dumping water over your head. But the primary reason that results were so different from what lab studies have typically observed is that the athletes were moving through the air, generating their own wind.
That matters because a lot of our conventional wisdom about heat and hydration was generated in lab studies with no moving air, which creates a misleading impression of how given levels of heat and dehydration will affect performance in the real world. In recent years, studies have begun to use fans to make conditions more realistic. For example, a 2015 study found that even being dehydrated by 3 percent of body mass didn’t hurt cycling performance in a 20 mph wind.
None of this is to say that heat doesn’t affect performance. The Doha results offer a vivid illustration of how even the best athletes in the world have to slow down when temperatures rise. But with the heat of summer rapidly approaching, I’ll keep these results in mind to comfort myself during the inevitable sweatfests to come: it would be worse on the treadmill.
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