Maintaining a desired core temperature during exercise in the heat is preferable for preventing fatigue as well as avoiding hyperthermia and possible heat illness. During exercise in the heat, autonomic thermoregulatory responses have a limited capacity to regulate body temperature, whilst behavioural thermoregulation can be more effective and is virtually limitless in regulating body temperature [1]. The findings from previous studies provide robust evidence that behavioural thermoregulation can reduce thermoregulatory strain in men but to a lesser extent, under more stressful environments [2], [3], [4]. Schlader et al. [3] demonstrated that when given the opportunity to behave, thermoregulatory strain was lower in self-paced than in fixed-intensity trials, due to the voluntary reduction of exercise intensity. Collectively, these results highlight the merits of behavioural thermoregulation on thermal homeostasis, and demonstrate that the voluntary reduction of exercise intensity in the heat is, in fact, part of our behaviour. Furthermore, this voluntary reduction of exercise intensity is essential to prevent premature fatigue and to allow greater end spurt performance at the end of the time trial [5].

Although a growing body of evidence has suggested that self-paced is a proven means to reduce core temperature, this issue still remains incomplete, as the comparison between self-selected pace and fixed-intensity does not have a common starting point. For example, Schlader et al. [3] eloquently demonstrated that self-paced reduced thermoregulatory strain in an uncompensable heat stress environment compared to fixed-intensity, yet, the exercise intensity in the fixed-intensity trial was significantly greater than the self-paced trial. This corresponded to higher metabolic heat production, and, therefore resulted in a higher core temperature response. Furthermore, it was conducted in a dry heat environment, something that may not translate to a humid heat setting. Therefore, to provide a definitive assessment of whether self-paced exercise is a means to reducing thermoregulatory strain, it is necessary to match the average workloads (hence, metabolic heat production) between self-paced and fixed-intensity trials and to compare this in differing environments matched for vapour pressure; as even if average workloads are matched, periodic fluctuations in power output, thereby metabolic heat production, could transiently affect thermoregulation.

Therefore, this study aims to investigate different exercise modalities, at the same average workload, in both warm (self-paced vs. fixed-intensity) and cool heat environments (self-paced vs. fixed-intensity) matched for vapour pressure in a compensable heat stress environment. We hypothesized that self-paced exercise does not modulate thermoregulatory strain when both exercise duration and intensity are matched.