Mixed-Method Heat Acclimation Induces Heat Adaptations in International Triathletes Without Training Modification

Purpose: Heat acclimation (HA) kinetics often necessitates that the intervention is conducted in the weeks immediately preceding athletic competitions, potentially interfering with a training taper. Therefore, we investigated the efficacy of a mixed-method HA protocol, superimposed over planned external training loads, during the 3-weeks prior to the 2022 U23 World Triathlon Championships. Methods: Six international triathletes completed 8 pre-competition HA sessions (5 active: running/cycling, 3 passive: hot water immersion [HWI]), across 2-weeks. Outdoor high-intensity training sessions were followed by 30–60 min HWI, whilst low-intensity cycling/running sessions were completed in a hot, humid environmental chamber. To assess heat adaptations, participants completed three 25 min heat stress tests (HST) involving iso-speed treadmill running (session 1 = HST1, session 5 = HST2, and session 8 = HST3). Physiological, haematological and wellbeing monitoring were conducted throughout HA. Results: Reduced heart rate (~ − 6 beats/min) was observed within HST3 (P= 0.01, η_p²= 0.64), versus HST1 and HST2. No changes in core temperature were observed across HSTs (P= 0.055,η_p²= 0.44). Sweat sodium concentration was lower by HST2 at the arm (− 23 ± 16 mmol/L, P= 0.02) and back (− 27 ± 17 mmol/L, P= 0.01). White blood cell count reduced from baseline to the end of HA (P= 0.02, η_p²= 0.27), but no changes were found in any other haematological markers (all P> 0.05). Perceptual wellbeing measures did not change across HA (all P> 0.05). Conclusion: By HST3, seven prior mixed-method HA sessions improved markers of heat adaptation (exercising HR and sweat concentration) within international triathletes. Mixed-method HA may be implemented without modifying training load, with no apparent detrimental effects on athlete health or training stress markers.