Purpose: Exercise performance is compromised in hot and humid conditions compared to temperate circumstances. However, it remains unclear which factors predispose individuals to either small or large performance decrements. This study aimed to identify factors associated with performance decrements in elite athletes during exercise under hot and humid ambient conditions. Methods: One hundred and six Dutch elite athletes performed a standardized incremental exercise test in a temperate condition (TEMP, ambient temperature 15.9 °C [1.2 °C], and relative humidity 55% [6%]) and hot—humid condition (HOT, ambient temperature 31.6 °C [1.0 °C], and relative humidity 74% [5%]). Performance loss was determined by the relative difference in time to exhaustion between both conditions. Multivariable linear regression analyses were performed to determine the association between performance loss and athlete characteristics, exercise responses, thermoregulatory responses, and perceptual parameters. Results: Time to exhaustion decreased from 60 (14) minutes in TEMP to 44 (10) minutes in HOT, corresponding to an average performance decrement of 26% (11%). Higher dehydration rates and more thermal discomfort in TEMP were associated with greater performance decrements. On the other hand, a higher BMI, female sex, greater exercise-induced increases in core temperature (TC) in HOT, and a higher peak heart rate (HR) and exercise-induced increases in HR in HOT versus TEMP were associated with less performance decrements (adjusted R2 = .58; P < .001). Conclusion: We identified that variables related to exercise responses (ie, HR, dehydration), athlete characteristics (ie, BMI, sex), thermoregulatory responses (ie, TC), and perception (ie, thermal discomfort) were associated with performance decrements during exercise in the heat. Our multivariable regression model explained 58% of the magnitude of performance decrements.
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