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

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 degrees C dry bulb temperature and 81% relative humidity. Peak T(c) was 39.8 +/- 0.5 (38.5-40.7) degrees C with 24 runners achieving T(c) > 39.0 degrees C, 17 runners > or = 39.5 degrees C, and 10 runners > or = 40.0 degrees C. In 12 runners attaining peak T(c) > or = 39.8 degrees 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 T(c) at 21 km (R(2) = 0.42, P < 0.001) and was the main discriminating variable between hyperthermic (T(c) > or = 39.8 degrees C) and normothermic runners (T(c) < 39.8 degrees 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 degrees C, is common in trained individuals undertaking outdoor distance running in environmental heat, without evidence of fatigue or heat illness.