Heat slows every ultramarathon runner, but it slows men more. In races run at 34°C, the average speed gap between men and women was less than half of what it was in cool weather. Researchers from Charles University in Prague and Seattle Pacific University published the finding on September 17 in Medicine & Science in Sports & Exercise.
Ultramarathons are races longer than the 42-kilometer marathon. Earlier work suggested that heat hurts women's marathon speed less than men's, but ultramarathons had barely been studied. Runners there move slower and so make less heat, which could have made the difference between the sexes disappear.
1.35 million finishes
The team analyzed 1,353,878 finish times from 8,582 races of 50 to 161 kilometers. Average race-day temperatures ranged from 0 to 34°C, relative humidity from 0 to 100 percent, and the strength of sunlight up to 700 watts per square meter. A statistical model then estimated how fast women and men ran in each kind of weather.
Closer in the heat
Both sexes ran slower when it was hot, but men's speed fell more steeply. The gap between them dropped from 0.73 km/h at 0 to 8°C to 0.34 km/h at 34°C.
Heat together with humid air had the strongest effect. At 34°C and 90 percent humidity, the predicted gap was just 0.21 km/h. Humidity on its own worked slightly the other way, widening the difference by 0.07 km/h across its whole range.
Sunshine slowed runners a little, and about equally for women and men.
What the numbers do not say
The study measures how much the gap changes, not why. It does not test what helps women hold their pace better in the heat.
It shows only that they do.
