Rebuttal by Christopher T. Minson and James D. Cotter

In this CrossTalk debate, our opponents base much of their argument on two studies that utilized trained cyclists, in which no performance benefit was observed following heat acclimation (HA) (Nybo & Lundby, 2015). In both the Keiser (Keiser et al. 2015) and Karlsen studies (Karlsen et al. 2015), athletes in the experimental groups performed all training in hot conditions. This is a departure from the HA-training model used by Lorenzo (Lorenzo et al. 2010), in which subjects maintained their intense and distance training in cooler conditions, and added low-intensity exercise in the heat (or matched absolute workload in cool conditions for controls). A key aspect of Nybo and Lundby’s position is that performance gains following HA reported in earlier studies reflect a training effect, accrued through an increased training load or relative intensity during HA. The application of relative versus absolute exercise intensity to protocols and performance testing has challenged exercise physiologists for decades. We concur that this issue may explain some of the discordant findings in the literature, especially in uncontrolled studies performed at training camps (Buchheit et al. 2011; Racinais et al. 2014). Clearly, more studies are needed, as the thoughtful use of heat conditioning as part of team-sport training camps may result in training sessions performed at a higher percentage of maximal heart rate without additional muscular fatigue or damage, thereby maximizing fitness gains while minimizing fatigue and soreness. Whether this is considered a training effect or a component of HA may be a matter of semantics. That said, the effect of HA, independent of training, has been tested using post-exercise sauna bathing in a crossover-design study with trained male runners (Scoon et al. 2007). Performance runs, performed in duplicate, indicated a small but important benefit in temperate conditions. An increase in plasma volume (PV) has been shown to improve performance in some individuals when haemodilution is not a limiting factor (Coyle et al. 1990; Luetkemeier & Thomas, 1994), including some apparent responders in the data provided in our opponents’ CrossTalk position (Nybo & Lundby, 2015). However, we have not suggested PV expansion is a singular mechanism for performance enhancement, particularly in the absence of concomitant improvements in myocardial efficiency, vascularization, cardiac output and/or red-cell volume that might attend HA but not artificial PV expansion. Rather, we suggest that a constellation of small physiological adaptations following HA may underlie performance increases in some athletes (Corbett et al. 2014). Along these lines, potential effects of HA on psychological aspects of performance has received little attention but cannot be discounted. We acknowledge that the story is incomplete and controversial, but contend there is evidence that HA does improve performance in some individuals in cool conditions.