Intermittent post-exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle-distance runners

European Journal of Applied Physiology https://doi.org/10.1007/s00421-020-04541-z ORIGINAL ARTICLE Intermittent post‑exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle‑distance runners Nathalie V. Kirby1 · Samuel J. E. Lucas1 · Oliver J. Armstrong2 · Samuel R. Weaver1 · Rebekah A. I. Lucas1 Received: 6 April 2020 / Accepted: 24 October 2020 © The Author(s) 2020 Abstract Purpose This study investigated whether intermittent post-exercise sauna bathing across three-weeks endurance training improves exercise heat tolerance and exercise performance markers in temperate conditions, compared to endurance training alone. The subsidiary aim was to determine whether exercise-heat tolerance would further improve following 7-Weeks post-exercise sauna bathing. Methods Twenty middle-distance runners (13 female; mean ± SD, age 20 ± 2 years, V O2max 56.1 ± 8.7 ml kg−1 min−1) performed a running heat tolerance test (30-min, 9 km h−1/2% gradient, 40 °C/40%RH; HTT) and temperate (18 °C) exercise tests (maximal aerobic capacity [V O2max], speed at 4 mmol L−1 blood lactate concentration ( [La−]) before (Pre) and following three-weeks (3-Weeks) normal training (CON; n = 8) or normal training with 28 ± 2 min post-exercise sauna bathing (101–108 °C, 5–10%RH) 3 ± 1 times per week (SAUNA; n = 12). Changes from Pre to 3-Weeks were compared betweengroups using an analysis of co-variance. Six SAUNA participants continued the intervention for 7 weeks, completing an additional HTT (7-Weeks; data compared using a one-way repeated-measures analysis of variance). Results During the HTT, SAUNA reduced peak rectal temperature (Trec; − 0.2 °C), skin temperature (− 0.8 °C), and heart rate (− 11 beats min−1) more than CON at 3-Weeks compared to Pre (all p < 0.05). SAUNA also improved V O2max (+ 0.27 L−1 min−1; p = 0.02) and speed at 4 mmol L−1 [La−] (+ 0.6 km h−1; p = 0.01) more than CON at 3-Weeks compared to Pre. Only peak Trec (− 0.1 °C; p = 0.03 decreased further from 3-Weeks to 7-Weeks in SAUNA (other physiological variables p > 0.05). Conclusions Three-weeks post-exercise sauna bathing is an effective and pragmatic method of heat acclimation, and an effective ergogenic aid. Extending the intervention to seven weeks only marginally improved Trec. Keywords Post-exercise sauna · Thermoregulation · Exercise performance · Heat acclimation · Ergogenic aid Abbreviations 3-Weeks Tests following three-weeks intervention 7-Weeks Tests following seven-Weeks intervention ANOVA Analysis of variance Communicated by Narihiko Kondo. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00421-020-04541-z) contains supplementary material, which is available to authorized users. * Nathalie V. Kirby 1 University of Birmingham, Birmingham B15 2TT, UK 2 Performance Centre, University of Birmingham Sport, Birmingham, UK ANCOVA Analysis of co-variance CON Control group EPO Erythropoietin HR Heart rate HTT Running heat tolerance test Pre Pre-intervention tests RER Respiratory exchange ratio RPE Rating of perceived exertion SAUNA Sauna intervention group Trec Rectal temperature Tsk Mean weighted skin temperature TTE Time-to-exhaustion VEGF Vascular endothelial growth factor VO2max Maximal aerobic capacity 13 Vol.:(0123456789) European Journal of Applied Physiology Introduction Heat acclimation improves exercise performance in the heat (Sawka et al. 2011). The most common model studied in the literature is “medium-term” active heat acclimation (Tyler et al. 2016), where individuals exercise in a climatic/environmental chamber for 60–120 min for 7–14 consecutive days (Garrett et al. 2009). This model of heat acclimation carries considerable barriers related to financial and temporal costs, as well as accessibility to climatic/environmental chambers. Consequently, heat acclimation is not widely used by athletes (recently recorded prevalence of ~ 15%; Périard et al. 2015) despite its effectiveness in improving exercise performance in hot environments. With major athletic events in extreme heat fast approaching, such as the Tokyo 2020 Olympic Games (postponed; Gerrett et al. 2019) and the 2022 FIFA World Cup in Doha, it is imperative that athletes are prepared to compete in such challenging environments. Post-exercise sauna bathing presents a practical and accessible heat acclimation alternative to active heat acclimation that could be implemented without disruption to an athlete’s training programme. Repeated bouts of sauna bathing have been observed to elicit some heat acclimation adaptations [e.g., reduced resting core temperature with sauna independent of exercise (Leppäluoto et al. 1986) and plasma volume expansion using post-exercise sauna bathing (Scoon et al. 2007; Stanley et al. 2015)]. However, its efficacy to elicit hallmark heat acclimation adaptations during exercise heat stress (e.g., reduced exercising heart rate and body temperatures, etc.; Sawka et al. 2011) has not previously been investigated (Casadio et al. 2017). Despite the relatively sparse scientific evidence, post-exercise sauna bathing is recommended to athletes preparing for competition in the heat (Racinais et al. 2019). It more specifically recommends that ahtletes use post-exercise sauna bathing to prepare for competition in the heat than the reference currently used. Therefore, the first aim of this study was to assess exercise heat tolerance following repeated bouts of intermittent sauna bathing using a fixed-workload exercise heat stress test. The myriad of physiological adaptations attained through heat acclimation may improve exercise performance in cool or temperate conditions (Minson and Cotter 2016). Similar improvements may result from post-exercise sauna bathing, as a case study of a young female tennis player showed improvements in performance outcomes of timeto-exhaustion (TTE), maximal aerobic capacity (V O2max), and lactate threshold after 12 sauna sessions following exercise spread across 3 weeks (Novak et al. 2018). Similarly, Scoon and colleagues (2007) observed that running TTE increased in six male athletes after approximately 12 intermittent post-exercise sauna sessions. However, the effects of intermittent post-exercise sauna bathing on more objective 13 markers of temperate exercise performance (i.e., V O2max and blood lactate profile) have not been assessed in a larger cohort. Therefore, the second aim of this study was to assess whether integrating intermittent post-exercise sauna bathing (~ 3 sessions·week−1) into an endurance training programme further improves temperate exercise performance markers (V O2max and running speed at 4 mmol L−1 blood lactate concentration [La−]), as compared to an endurance training programme alone. Finally, long-term heat acclimation interventions (i.e., > 14 days) are expected to maximise exercise capacity in the heat (Tyler et al. 2016). These longer protocols m (...truncated)