Heat Acclimation, Session Work and Thermal Strain: Three Comparisons of Endurance Adaptation

Abstract

Heat acclimation alters sweating, cardiovascular function and body-temperature regulation during exercise. These adaptations also change the thermal strain generated by subsequent sessions, so a constant exercise prescription need not deliver constant internal exposure. McDonald and colleagues' Bayesian analysis estimated an end-exercise heart-rate reduction of 17 beats/min (90% credible interval: 14–19), a core-temperature reduction of 0.43^oC (0.36–0.48) and a time-trial improvement of 3.1% (1.8–4.5). Individual experiments delimit these average responses: isothermic exercise reduced exercise requirements in a 24-man comparison, whereas hot-water immersion improved a 5-km trial in heat without a corresponding temperate-condition improvement. Three causal comparisons are defined: assignment to a complete acclimation programme, hot versus thermoneutral immersion after a common exercise prescription, and exercise versus passive heating under temperature-targeting rules. Mechanical work and the core-temperature time integral describe separate exposure dimensions; physiological adaptation and task performance remain separate outcomes. A temporal diagram identifies how adaptation and fatigue connect successive sessions and why adjustment for achieved work or temperature can alter an assignment-based comparison. Small samples, incomplete exposures and dependence between articles from the same cohort constrain inference.