The Impact of Air Temperature, Relative Humidity, and Solar Irradiance on Ultramarathon Running Speed in Women and Men.

INTRODUCTION/PURPOSE: The running speed of women was suggested to be less impacted by heat stress than that of men in marathons, but this has been scarcely studied in ultramarathons. Lower running speeds in ultramarathons may reduce heat stress and therefore attenuate weather-related sex differences compared with marathons. We examined how air temperature, relative humidity, and solar irradiance influence running speed and the relative difference in speed between sexes during 50-161-km ultramarathons. METHODS: We analyzed 1,353,878 finish times in 8,582 races held at daily-averaged air temperatures of 0-34 °C, relative humidity of 0-100%, and solar irradiance levels of 0-700 W/m 2. Using a linear mixed-effects model, we estimated running speed in women and men, and the difference in speed between sexes across weather conditions. RESULTS: Running speed declined in both sexes at high air temperatures, with the decline steeper in men, reducing the difference between sexes from 0.73 km/h at 0-8 °C to 0.34 km/h at 34 °C. The negative effect of high air temperature on the sex difference was most pronounced at high relative humidity, with the predicted difference declining to 0.21 km/h at 34 °C and 90% RH. Relative humidity had the opposite, but weaker, effect, increasing the sex difference by 0.07 km/h across its observed range. Solar irradiance had a small effect on running speed, and the effect did not differ significantly between men and women. CONCLUSIONS: In ultramarathons, women's running speed is less affected by high air temperature than men's running speed. This effect is most pronounced at high relative humidity.