Performance Changes Following Heat Acclimation and the Factors That Influence These Changes: Meta-Analysis and Meta-Regression

Abstract

Heat acclimation (HA) is the process of intentional and consistent exercise in the heat that results in positive physiological adaptations, which can improve exercise performance both in the heat and thermoneutral conditions. Previous research has indicated the many performance benefits of HA, however, a meta-analysis examining the magnitude of different types of performance improvement is absent. Additionally, there are several methodological discrepancies in the literature that could lead to increased variability in performance improvement following HA and no previous study has examined the impact of moderators on performance improvement following HA. Therefore, the aim of this study was two-fold; (1) to perform a meta-analysis to examine the magnitude of changes in performance following HA in maximal oxygen consumption (VO2max), time to exhaustion, time trial, mean power, and peak power tests; (2) to determine the impact of moderators on results of these performance tests. Thirty-five studies met the inclusion/exclusion criteria with 23 studies that assessed VO2max (n = 204), 24 studies that assessed time to exhaustion (n = 232), 10 studies that performed time trials (n = 101), 7 studies that assessed mean power (n = 67), and 10 papers that assessed peak power (n = 88). Data are reported as Hedge's g effect size (ES), and 95% confidence intervals (95% CI). Statistical significance was set to p < 0.05, a priori. The magnitude of change following HA was analyzed, with time to exhaustion demonstrating the largest performance enhancement (ES [95% CI], 0.86 [0.71, 1.01]), followed by time trial (0.49 [0.26, 0.71]), mean power (0.37 [0.05, 0.68]), VO2max (0.30 [0.07, 0.53]), and peak power (0.29 [0.09, 0.48]) (p < 0.05). When all of the covariates were analyzed as individual models, induction method, fitness level, heat index in time to exhaustion (coefficient [95% CI]; induction method, −0.69 [−1.01, −0.37], p < 0.001; fitness level, 0.04 [0.02, 0.06], p < 0.001; heat index, 0.04 [0.02, 0.07], p < 0.0001) and induction length in mean power (coefficient [95% CI]; induction length 0.15 [0.05, 0.25], p = 0.002) significantly impacted the magnitude of change. Sport scientists and researchers can use the findings from this meta-analysis to customize HA induction. For time to exhaustion improvements, HA implementation should focus on induction method and baseline fitness, while the training and recovery balance could lead to optimal time trial performance.

Heat acclimation during low‐intensity exercise increases and Hsp72, but not markers of mitochondrial biogenesis and oxidative phosphorylation, in skeletal tissue

“…The increase in following HA is not a universal observation. Our data agree with a recently published meta‐analysis with 23 studies which reported a small but significant effect of HA on (Hedge's g = 0.30) measured in a temperate environment (Benjamin et al., 2019). The inhomogeneous effect of HA in increasing may be due to the differences in protocols, including exercise mode (i.e.…”

“…C-I-C-IV, mitochondrial complexes I-IV; CaMK, Ca 2+ /calmodulin-dependent protein kinase; COX-4, cytochrome c oxidase subunit 4; Hsp72, heat shock protein 72; PGC-1α, peroxisome proliferator-activated receptor γ coactivator 1α; TFAM, mitochondrial transcription factor A following HA is not a universal observation. Our data agree with a recently published meta-analysis with 23 studies which reported a small but significant effect of HA onV O 2 max (Hedge's g = 0.30) measured in a temperate environment (Benjamin et al, 2019 or approximately 5%) in a temperate environment using trained cyclists . Recently, Waldron et al (2019) Figure 1).…”

“…It has been suggested that changes induced by HA may be considered as ergogenic under less stressful conditions where performance is not thermally limited (Corbett, Neal, Lunt, & Tipton, 2014). In this context, heat stress-induced increases in mitochondrial biogenesis may help to explain why previous studies report a 'transfer effect' in which a period of HA improved aerobic exercise performance andV O 2 max in a temperate environment (Benjamin, Sekiguchi, Fry, & Casa, 2019;Buchheit, Voss, Nybo, Mohr, & Racinais, 2011;. However, there are no published data on HA and markers of mitochondrial biogenesis and oxidative phosphorylation in skeletal muscle of human participants.…”

The Efficacy of Heat Acclimatization Pre-World Cup in Female Soccer Players

“…Previous findings in male soccer players (Buchheit et al, 2011(Buchheit et al, , 2013(Buchheit et al, , 2016Racinais et al, 2012) show that heat acclimatization can lead to an increase in PV and were corroborated in elite female soccer athletes in the current study, although the PV change failed to reach significance (Figure 4). Laboratory studies utilizing controlled hyperthermia acclimation protocols have reported PV increases of 4-15%, improving cardiovascular stability via maintenance of cardiac output and reductions in HRex, leading to performance enhancement (Chalmers et al, 2014;Periard et al, 2015Periard et al, , 2016Racinais et al, 2015;Casadio et al, 2017;Benjamin et al, 2019;Pryor et al, 2019). While laboratory HA protocols are more controlled, they are less practical and potentially less effective than field-based HA, which has been shown to induce better peripheral adaptations from sport-specific training and better maintenance of skills (Pryor et al, 2019).…”

“…Repeated training exposures in a hot environment have the potential to induce positive physiological adaptations that can attenuate the negative effects of heat stress by regulating cardiovascular strain while enhancing thermoregulation [e.g., increased plasma volumes (PV) and sweat rates] (Racinais et al, 2015 ). While isothermically controlled, lab-based heat acclimation protocols have demonstrated improvements in both team and endurance sport performance (Pethick et al, 2018 ; Benjamin et al, 2019 ), the resources required to execute an effective acclimation protocol can be expensive (core temperature monitoring), involve extensive equipment (heat chamber), and usually involve non-soccer-specific training (typically cycling) (Buchheit et al, 2011 ; Racinais et al, 2012 ; Chalmers et al, 2014 ). Therefore, the implementation of a field-based, sport-specific heat acclimatization protocol is more ecologically valid and practical for soccer players, while also minimizing time away from training often required for lab-based protocols.…”