Continuous thermoregulatory responses to a mass-participation 89-km ultramarathon road race

This is the final version. Available on open access from Human Kinetics via the DOI in this recordAbstract Purpose: To continuously measure body core temperature (Tc) throughout a mass-participation ultramarathon in sub-elite recreational runners to quantify Tc magnitude and the influence of aerobic fitness and body fat. Methods: Twenty-three participants (19 males, 4 females; age 45 ± 9 y; body mass 72.0 ± 9.3 kg; body fat 26 ± 6%; VO2peak 50 ± 6 ml∙kg-1 ·min-1 ) had gastrointestinal temperature measured during an 89 km ultramarathon. Pre-to-post-race changes in body mass, plasma sodium, and fluid and food recall quantified body water balance. Results: In maximal environmental conditions of 26.3°C and 53% humidity, 21 of the 23 participants finished in 10:28 ± 01:10 hh:mm whilst 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 ∆Tc was 1.9 ± 0.9 (0.9-2.7°C) with 95% of race duration ≤2.0°C. Over 0-45 km, associations between ΔTc and VO2peak (positive) and body fat (negative) were observed. Over 58-89 km, associations between Tc and VO2peak (negative) and body fat (positive) were observed. Conclusions: 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