Thermoregulation, pacing and fluid balance during mass participation distance running in a warm and humid environment

Abstract

Deep body temperature (T c), pacing strategy and fluid balance were investigated during a 21-km road race in a warm and humid environment. Thirty-one males (age 25.3 ± 3.2 years; maximal oxygen uptake 59.1 ± 4.2 ml kg-1 min-1) volunteered for this study. Continuous T c responses were obtained in 25 runners. Research stations at approximately 3-km intervals permitted accurate assessment of split times and fluid intake. Environmental conditions averaged 26.4°C dry bulb temperature and 81% relative humidity. Peak T c was 39.8 ± 0.5 (38.5-40.7) °C with 24 runners achieving T c > 39.0°C, 17 runners ≥39.5°C, and 10 runners ≥40.0°C. In 12 runners attaining peak T c ≥39.8°C, running speed did not differ significantly when T c was below or above this threshold (208 ± 15 cf. 205 ± 24 m min-1; P = 0.532). Running velocity was the main significant predictor variable of ΔT c at 21 km (R 2 = 0.42, P < 0.001) and was the main discriminating variable between hyperthermic (T c ≥39.8°C) and normothermic runners (T c < 39.8°C) up to 11.8 km. A reverse J-shaped pacing profile characterised by a marked reduction in running speed after 6.9 km and evidence of an end-spurt in 16 runners was observed. Variables relating to fluid balance were not associated with any T c parameters or pacing. We conclude that hyperthermia, defined by a deep body temperature greater than 39.5°C, is common in trained individuals undertaking outdoor distance running in environmental heat, without evidence of fatigue or heat illness. © Springer-Verlag 2010.