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, $$V$$ 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 [$$V$$ 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 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 (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 $$V$$ 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.

Background The purpose of this study was to investigate the effect of sex on the efficacy of intermittent post-exercise sauna bathing to induce heat acclimation and improve markers of temperate exercise performance in trained athletes. Methods Twenty-six trained runners (16 female; mean ± SD, age 19 ± 1 years, V̇O2max F: 52.6 ± 6.9 mL⋅kg−1⋅min−1, M: 64.6 ± 2.4 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 [V̇O2max] and lactate profile) pre and post 3 weeks of normal exercise training plus 29 ± 1 min post-exercise sauna bathing (101–108 °C) 3 ± 1 times per week. Results Females and males exhibited similar reductions (interactions p > 0.05) in peak rectal temperature (− 0.3 °C; p < 0.001), skin temperature (− 0.9 °C; p < 0.001) and heart rate (− 9 beats·min−1; p = 0.001) during the HTT at post- vs pre-intervention. Only females exhibited an increase in active sweat glands on the forearm (measured via modified iodine technique; F: + 57%, p < 0.001; M: + 1%, p = 0.47). Conversely, only males increased forearm blood flow (measured via venous occlusion plethysmography; F: + 31%, p = 0.61; M: + 123%; p < 0.001). Females and males showed similar (interactions p > 0.05) improvements in V̇O2max (+ 5%; p = 0.02) and running speed at 4 mmol·L−1 blood lactate concentration (+ 0.4 km·h−1; p = 0.001). Conclusions Three weeks of post-exercise sauna bathing effectively induces heat acclimation in females and males, though possibly amid different thermoeffector adaptations. Post-exercise sauna bathing is also an effective ergogenic aid for both sexes.