Heat Acclimation and Acclimatisation And Maximal Aerobic Performance in Thermoneutral and Hot Environments
by George Blackwell
It’s well established that specific heat acclimation (training in a hot but artificial environment, e.g. a heat lab or makeshift home setup) and heat acclimatisation (training in a hot but natural environment, e.g. a training camp in hot temperatures) can have huge impacts on maximal aerobic performance in high temperatures.
However, this meta-analysis reviewed the impacts that heat prep can have on both hot environments and when thermoneutral. Waldron et al. conducted four meta-analyses to evaluate the effects of heat acclimation or acclimatisation (HA) on maximal oxygen consumption and to research that factors that moderate these effects.
The conclusions of these analyses were that HA can enhance adaptions in BOTH thermoneutral (cool) and hot environments by at least a small and up to moderate-large amount, with the largest improvements occurring in the heat. Factors such as the level of ambient heat, the number of induction days and of post-testing days can partially moderate the change in hot VO2 Max adaptation. The positive effects of HA on VO2 Max were apparent with or without the inclusion of a control group, suggesting its capacity to enhance the effect of endurance training.
The specific physiological adaptions that can occur from this heat prep were outlined by Periard (2016), who observed an increase in total body water and plasma volume expansion (our volume of blood). These lead to a better sustainment of stroke volume (the amount of blood the heart pumps with each beat) and, therefore, mediate the increase in heart rate typically seen in hot conditions to accommodate for fluid loss. Alongside this, there are a number of further adaptations regarding skin blood flow and sweating responses to help improve heat loss.
How much an individual adapts to the heat depends on a number of factors in respect to the conditions of their heat prep (dry versus humid), as well as the intensity, duration and frequency of heat sessions. However, the research clearly shows the value of these heat sessions, depending on your training goals and race environment.
References:
Periard et al., (2016). Cardiovascular adaptations supporting human exercise-heat acclimation. Autonomic Neuroscience, 196, 52-62. http://www.sciencedirect.com/science/article/pii/S1566070216 300078
Waldron, M., Fowler, R., Heffernan, S. et al. Effects of Heat Acclimation and Acclimatisation on Maximal Aerobic Capacity Compared to Exercise Alone in Both Thermoneutral and Hot Environments: A Meta-Analysis and Meta-Regression. Sports Med 51, 1509–1525 (2021). https://doi.org/10.1007/s40279-021-01445-6