The benefits of being passive

As an integrative physiologist I get excited when I come across research with compelling results built upon strong experimental design and technical approaches. As a performance physiologist who works with collegiate and elite athletes I get equally excited when I come across a research paper with novel strategies I can use to improve my athletes’ performance. Thus I have trouble containing my excitement when I see a study that bridges my integrative and performance physiologist sides, as is the case with the study by Jenkins et al. in the current issue of the Journal of Physiology (Jenkins et al., 2025). The classic 10–14 day heat acclimation protocol was designed for workers in mines and other occupational and military settings, but it has proven beneficial for athletes competing in hot environments. In this context the mechanisms of adaptation are reasonably well understood, albeit not completely. However the performance physiologist in me knows this is not the ideal protocol for most athletes and is far from ideal for elite endurance athletes with extremely high training loads. Heat stress, particularly during exercise, is a significant physiological stressor. If not considered and appropriately managed, heat acclimation can result in maladaptation to the training process, similar to concerns raised with traditional altitude training. It may still be debated whether the classic heat acclimation protocol can be used to improve performance in more temperate or even cool environments. What has become abundantly clear to me is that there are wrong ways to implement heat acclimation for high-level athletes. These include protocols that do not account for training load and fatigue management, especially with repetitive days of hard training in the heat. In his recent commentary Dr. Lars Nybo recounts his participation as a subject in his own study investigating whether heat acclimatization could improve performance in a cool environment (Nybo, 2025). This was a very challenging study that few research teams in the world could pull off. However their impressive study did not result in improvements in performance markers, even 2 weeks after the training camp, when the participants might have recovered from the training load (Karlsen et al., 2015). Based on his narrative of participation in the study, load and stress management may have been issues that undermined any beneficial performance adaptations. The use of passive heating protocols, such as hot water immersion or sauna use, has shown promise as a way to confer heat adaptation with less risk of negatively impacting training quality (Kissling et al., 2022). Over the past number of years, a few studies have employed longer-term heat acclimation (of at least 4–5 weeks) and have shown to be an effective method to increase blood volume and haemoglobin mass (Hgmass; an important factor for oxygen delivery to skeletal muscle and a major determinant of performance) and key laboratory-based markers of performance, even when the testing environment was not hot (Rønnestad et al., 2021). With long-term heat acclimation protocols in elite athletes consideration of the balance among training load, physiological stress and recovery strategies becomes exceedingly important. To date all studies on long-term heat acclimation in athletes have used exercise-heat acclimation protocols. Well-designed integrative studies in athletes employing heat-acclimation strategies that preserve training, recovery and adaptation quality are needed. In their study Jenkins et al. achieved this and provided novel insights into potential mechanisms of enhanced performance following heat acclimation. The authors utilized a within-subject, balanced, crossover design, with 10 well-trained runners undergoing 5 weeks of heat acclimation. They observed an increased Hgmass and an increase in left-ventricular end diastolic volume, resulting in an increased VO2max of 2.7 ml O2 kg−1 min−1. The fact that an extended heat acclimation protocol could result in increased Hgmass, and the performance was already established in a few studies. This study is important for broadening the support of this fact. But what is particularly exciting about this paper is that the heat acclimation protocol was passive, performed with hot-water immersion after the training session. Heating is a physiological stress, in addition to the training demands. With passive heating, management of the overall training load is disentangled from heat acclimation, and disruption to key workouts is minimized. Jenkins and colleagues also measured the cardiac adaptations to long-term heat acclimation (using 4D echocardiology), a novel and critically important observation. This excited the integrative physiologist side of me, which wants to understand the mechanisms that underpin performance, and I hope it will stimulate more research on this aspect of heat acclimation. We do not know whether these lab-based improvements in performance will translate into real-world, race-situation performance gains. We know we need more performance studies that include appropriate numbers of female athletes. That said a study, such as this, satisfies both the integrative and performance physiologist sides of me. Well-designed, rigorous studies demonstrating performance gains, grounded in physiological mechanisms, are needed to convince athletes, coaches and performance physiologists that a heat-adapted phenotype is essential for achieving top performance across all environments. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. No competing interests declared. C.M.: conception or design of the work; drafting the work or revising it critically for important intellectual content; final approval of the version to be published; agreement to be accountable for all aspects of the work. Wu Tsai Human Performance Alliance and the Joe and Clara Tsai Foundation.