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

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, [Formula: see text] O(2max) 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 [[Formula: see text] O(2max)], 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 between-groups 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 (T(rec); − 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 [Formula: see text] O(2max) (+ 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 T(rec) (− 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 T(rec). ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00421-020-04541-z) contains supplementary material, which is available to authorized users.