Continuous Thermoregulatory Responses to a Mass-Participation 89-km Ultramarathon Road Race

To continuously measure body core temperature (Tc) throughout a mass-participation ultramarathon in subelite recreational runners to quantify Tc magnitude and the influence of aerobic fitness and body fat.

Twenty-three participants (19 men and 4 women; age 45 [9] y; body mass 72.0 [9.3] kg; body fat 26% [6%]; peak oxygen uptake 50 [6] mL·kg-1·min-1) had gastrointestinal temperature measured during an 89-km ultramarathon. Prerace-to-postrace changes in body mass, plasma sodium, and fluid and food recall quantified body water balance.

In maximal environmental conditions of 26.3 °C and 53% humidity, 21 of the 23 participants finished in 10:28 (01:10) h:min while replacing 49% (27%) of sweat losses, maintaining plasma sodium (140 [3] mmol·L-1), and dehydrating by 4.1% (1.3%). Mean maximum Tc was 39.0 (0.5) (range 38.2-40.1 °C) with 90% of race duration ≤39.0 °C. Mean maximum ΔTc was 1.9 (0.9) (0.9-2.7 °C) with 95% of race duration ≤2.0 °C. Over 0 to 45 km, associations between ΔTc and peak oxygen uptake (positive) and body fat (negative) were observed. Over 58 to 89 km, associations between Tc and peak oxygen uptake (negative) and body fat (positive) were observed.

Modest Tc responses were observed in recreational ultramarathon runners. Runners with higher levels of aerobic fitness and lower levels of body fat demonstrated the greatest changes in Tc during the first half of the race. Conversely, runners with lower levels of aerobic fitness and higher levels of body fat demonstrated the greatest absolute Tc in the final third of the race.