The influence of heat acclimation protocol characteristics on the adaptive response

Abstract

Heat acclimation (HA) is a widely adopted strategy to mitigate the increased physiological strain and performance decrements associated with exercise under heat stress. However, several protocol characteristics employed during HA can influence the adaptive response. Therefore, this doctoral thesis aimed to develop a comprehensive understanding of how HA protocol characteristics influence thermoregulatory, cardiovascular, and haematological adaptations, and performance under heat stress. Additionally, the cross-acclimation effects between hot-dry (Dry-HA) and warm-humid (Humid-HA) environments were examined.
Chapter Three employed a novel meta-analytic approach to identify the impact of HA protocol characteristics, including ambient temperature, humidity, exposure duration, and number of exposures. The chapter also highlights how increasing ambient temperature by 5°C, elevating humidity (i.e. partial pressure of water vapour in air) by 1 kPa, extending exposure duration by 15 min, and adding one exposure day influences HA adaptations. Chapters Four and Five utilised a crossover design to directly compare Dry-HA and Humid-HA during constant work rate exercise and self-paced time trial performance in each respective environment. The findings demonstrated that Dry-HA and Humid-HA elicited a similar cross-adaptive benefit constant work rate exercise in hot-dry and warm-humid conditions, alongside similar haematological adaptations (Chapter Four). Further, these cross-adaptive benefits extended to self-paced performance, with an environment specific benefit noted following Dry-HA during the hot-dry time trial, which may be attributed to the greater work rate sustained during controlled heart rate exercise with Dry-HA (Chapter Five).
Collectively, this thesis highlights how HA protocols characteristics can be manipulated to optimise the adaptive response to improve performance and health in the heat. The findings also demonstrate the cross-adaptive benefits between Dry-HA and Humid-HA, with Dry-HA conferring an additional performance benefit in hot-dry conditions. An online predictor tool was also developed with the review in Chapter Three to guide athletes and practitioners in optimising HA protocols based on specific parameters.

Date of Award	2025
Original language	English
Supervisor	Julien PERIARD (Supervisor) & Brad CLARK (Supervisor)