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Fatigue is suggested to be a general concept that is characterised by an acute impairment in performance and leads to the eventual inability to produce the necessary force, that is, volitional exhaustion.1 Homeostasis is maintained by the physiological regulatory processes occurring throughout the body,2 and during exercise, the development of fatigue and exhaustion is proposed to prevent homeostatic failure.3 Principle to the regulatory systems maintaining homeostasis is that of thermoregulation, and as such the combined stressors of exercise and heat stress are used as a model to investigate the regulatory processes that influence physical performance, the development of fatigue and the onset of exhaustion.4 5 Accordingly, theories have been developed, healthy (lively) debate has arisen and various journals have seen fit to devote special issues—for example, Sports Medicine (vol 37, issues 4–5), Journal of Applied Physiology (vol 104, issue 5) and timely reviews.4,–,7
Within the exercise and heat stress model, two exercise protocols have been used that both describe performance as a dependent variable of time: (1) fixed-intensity (ie, constant workload) exercise—exercise is undergone until volitional exhaustion, and (2) self-paced exercise—which evaluates the time to complete a given task (eg, a set amount of work) where the workload can be adjusted by the exerciser. Both exercise time-to-exhaustion and self-paced exercise performance have been found to be attenuated under heat stress.8,–,16 However, confounding these conclusions is that these exercise protocols evaluate a different combination of physiological processes. Fixed-intensity exercise provides a measure of exhaustion, whereas self-paced exercise provides an indication of the development of fatigue; and arising from this limitation, seemingly opposing theories have developed.
Thermoregulation is attained by both passive (ie, non-regulated) and active (ie, regulated) systems.17 During exercise in the heat, passive heat loss …