Mitigation of heat strain during exercise in hot-humid conditions: One size does not fit all

Hot-humid environmental conditions have a negative impact on exercise performance and increase the risk of exertional heat illness. To attenuate the adverse effects of heat stress, athletes can adopt heat mitigation strategies such as heat acclimation and cooling. In this thesis, we assessed individual responsiveness to exercise under heat stress and heat mitigation strategies. First, we focused on heat-induced performance decrements in able-bodied and para-athletes. In Chapter 2, we observed that, for both able-bodied and para-athletes, performance was substantially impaired in hot-humid compared to temperate conditions. We did, however, observe considerable inter-individual variability, suggesting that some athletes need to invest more in heat mitigation strategies (i.e., cooling and heat acclimation) than others. Second, we focused on heat acclimation in recreational athletes. In Chapter 3, we demonstrated that the magnitude of adaptations following humid heat acclimation varied across individuals. We found that individuals with a relatively large body mass showed large sweat rate adaptations, whereas individuals with a relatively small body mass showed large heart rate adaptations. In Chapter 4, we observed that humid heat acclimation did not negatively affect salivary immunoglobulin-A (i.e., an important biomarker of the mucosal immune system), self-reported respiratory illness symptoms, and self-reported wellness parameters. Third, we focused on heat mitigation strategies in para-athletes. Para-athletes with a spinal cord injury perform upper-body exercise, and therefore lower-body cooling may be convenient. However, in Chapter 5, we observed that for these athletes, lower-body cooling was less effective in lowering heat strain compared to upper-body cooling. In Chapter 6, we observed that many para-athletes at the Tokyo 2020 Paralympic Games used heat acclimation and cooling strategies, which may have contributed to the low incidence of exertional heat illness. Altogether, the observations in this thesis indicate that heat mitigation needs and responsiveness vary across athletes, emphasizing that a one-size-fits-all heat mitigation approach may be suboptimal. Practitioners are encouraged to measure (thermo)physiological responses of individual athletes in the lab and/or field to determine their personal heat mitigation needs and responsiveness. Future studies should aim to further elucidate what factors influence the individual responsiveness to exercise under heat stress, cooling, and heat acclimation.