Abstract
Challenging environmental conditions including heat and humidity are associated with particular risks to the health of runners and triathletes during prolonged events. The heat production of a runner is the product of its energy cost of running (C r) by its velocity. Since C r varies greatly among humans, those individuals with high C r are more exposed to heat stress in warm and humid conditions. Although risk factor awareness is crucial to the prevention of heat stroke and potential fatalities associated therewith, how C r affects the highest sustainable velocity (V) at which maximal heat loss matches heat production has not been quantified to date. Here, we computed in virtual runners weighting 45-75kg, the influence of C r variability from 3.8 to 4.4J·m(-1)·kg(-1) on V. Heat loss by radiation, convection, and conduction was assessed from known equations including body dimensions, running velocity (3.4-6.2m·s(-1)), air temperature (T a, 10-35°C) and relative humidity (r h, 50, 70 and 90%). We demonstrated a marked and almost linear influence of C r on V in hot and humid conditions: +0.1J·kg(-1)·m(-1) in C r corresponded to -4% in V. For instance, in conditions25°C r h 70%, 65-kg runners with low C r could sustain a running speed of 5.7m·s(-1) as compared to only 4.3m·s(-1) in runners with high C r, which is huge. We conclude that prior knowledge of individual C r in athletes exposed to somewhat warm and humid environments during prolonged running is one obvious recommendation for minimizing heat illness risk.
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