Subjective thermal strain impairs endurance performance in a temperate environment

Purpose: The aim of this study was to test the hypothesis that subjective thermal strain can reduce endurance performance independently from the general physiological strain normally associated with impaired endurance performance in the heat. Methods: In 20 °C and 44% relative humidity, 12 endurance-trained athletes (1♀ 11♂; mean ± SD; age: 27 ± 6 y; VO 2max : 61 ± 6 ml/kg/min) performed a time to exhaustion (TTE) test in two different experimental conditions: with an electric heat pad applied to the subjects' upper back (HP) and control (CON: without heat pad). In both conditions, subjects cycled to volitional exhaustion at 70% of their VO 2max . Cardiorespiratory, metabolic, thermoregulatory and perceptual responses were measured throughout the TTE test and compared at 0%, 50% and 100% isotime and at exhaustion. Results: TTE was reduced by 9% in HP (2092 ± 305 s) compared to CON (2292 ± 344 s; p =.023). The main effect of condition on thermal discomfort at isotime (p =.002), the effect of condition on thermal sensation at 0% isotime (p =.004) and the condition by isotime interaction on rating of perceived exertion (p =.036) indicated higher subjective thermal strain in HP compared to CON. None of the measured cardiorespiratory, metabolic and thermoregulatory variables differed significantly between conditions. Conclusion: Our novel experimental manipulation (HP) was able to induce significant subjective thermal strain and reduce endurance performance in a temperate environment without inducing the general physiological strain normally associated with impaired endurance performance in the heat. These results suggest that subjective thermal strain is an important and independent mediator of the heat-induced impairment in endurance performance.