Continuous thermoregulatory responses to mass-participation distance running in heat

Purpose: To continuously measure core temperature (T_c) and heart rate (HR), and quantify fluid balance during a 21-km mass-participation road race in warm, humid environmental conditions. Methods: Eighteen heat-acclimatized male soldiers ingested a telemetric T_c sensor on the evening prior to the race and wore an ambulatory Tc data recorder and HR monitor during the race. Pre- to postrace changes in nude body mass quantified fluid balance. Results: Environmental wet bulb globe temperature averaged 26.5°C. All runners finished the race asymptomatic of heat illness in a mean ± SD (range) time of 118 ± 13 (105-146) min. corresponding to an average running speed of 10.8 ± 1.1 (8.6-12.0) km·h^-1. All runners recorded peak T_c > 39°C; 56% (N = 10) > 40°C; and 11% (N = 2) > 41°C. Peak T_c was 40.1 ± 0.7 (39.3-41.7) °C at 86 ± 36 (13-130) min, with T_c 39.9 ± 0.8 (38.3-41.7) °C at race finish. The magnitude of T_c response was unrelated (P > 0.05) to running time or fluid balance (e.g., fluid intake, % dehydration). Cumulative heat strain index was 2790 ± 1112 (1046-5144) units at race finish. Conclusion: Ingestible telemetric temperature sensors demonstrated utility for continuous measurement of T _c during mass-participation running. Successful application of this technology has highlighted the magnitude and duration of T_c elevation that runners will voluntarily achieve during mass-participation distance races in heat and high humidity without medical consequence. Copyright © 2006 by the American College of Sports Medicine.