Heat adaptation in humans: the significance of controlled and regulated variables for experimental design and interpreta
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INVITED REVIEW
Heat adaptation in humans: the significance of controlled and regulated variables for experimental design and interpretation Nigel A. S. Taylor1 · Sean R. Notley2 · Michael I. Lindinger3 Received: 13 June 2020 / Accepted: 3 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Herein, the principles of homoeostasis are re-visited, but with an emphasis upon repeated homoeostatic disturbances that give rise to physiological adaptation. The central focus is human heat adaptation, and how, for experimental purposes, one might standardise successive adaptation stimuli, and then evaluate and compare the resulting adaptations. To provide sufficient background for that discussion, the principles of physiological control and regulation have been reviewed. The case is presented that, since it is the regulated variables that drive both the effector organs and the processes of physiological adaptation, then it is those variables (e.g., body temperature) that should be used to set and standardise the adaptation stimuli. Alternatively, some have proposed that the same outcome can be achieved through standardising a controlled variable (e.g., heart rate), and so the merits of that proposition are evaluated. Indeed, it can be an effective approach, although some experimental pitfalls are described to highlight its limitations with regard to between-group (e.g., able-bodied versus spinalinjured participants) and between-treatment comparisons (e.g., hot-water versus hot-air adaptation stimuli). The concept of setting the adaptation stimulus relative to an anaerobic or lactate threshold is also critically evaluated. Finally, an appraisal is offered concerning the merits of three different strategies for using deep-body and mean body temperature changes for evaluating thermoeffector adaptations. Keywords Adaptation · Deep-body temperature · Heat adaptation · Heart rate · Mean body temperature · Temperature regulation Abbreviation ATP Adenosine triphosphate
Introduction Homo sapiens acquired or inherited the behavioural, physiological and morphological characteristics necessary to avoid, tolerate and adapt to stress (Darwin 1859; Selye Communicated by Narihiko Kondo. * Nigel A. S. Taylor [email protected] 1
Research Institute of Human Ecology, College of Human Ecology, Seoul National University, Seoul, Republic of Korea
2
Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Canada
3
The Nutraceutical Alliance, Avenida Doctor Gregorio Maranon 68, 03185 Torrevieja, Alicante, Spain
1973). Indeed, routine stress exposure is essential for good health and the survival of all species (Selye 1973). Not surprisingly, the contemporary (and apparent) obsession with stress avoidance is causally linked with many disease states and epidemics of lifestyle disorders (Chakravarthy 2008; Booth et al. 2017). From a physiological perspective, adaptation is driven by repeated disturbances, wherein regulated variables (e.g., mean bo
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