Post Exercise Hot Water Immersion Elicits Heat Acclimation Adaptations in Endurance Trained and Recreationally Active Individuals

Hot water immersion (HWI) after exercise on 6 consecutive days in temperate conditions has 27 been shown to provide heat acclimation adaptations in a recreationally active population. 28 Endurance athletes experience frequent, sustained elevations in body temperature during 29 training and competition; as a consequence, endurance athletes are considered to be partially 30 heat acclimatized. It is therefore important to understand the extent to which endurance 31 trained individuals may benefit from heat acclimation by post-exercise HWI. To this end, we 32 compared the responses of eight endurance trained and eight recreationally active males 33 (habitual weekly endurance exercise: 9 h vs. 3 h) to a 6-day intervention involving a daily 34 treadmill run for 40 min (65% V̇O2 max) in temperate conditions followed immediately by 35 HWI (≤ 40 min, 40°C). Before (PRE) and after the intervention (POST), hallmark heat 36 acclimation adaptations were assessed during a 40-min treadmill run at 65% V̇O2 max in the 37 heat (33°C, 40% RH). The 6 day, post-exercise HWI intervention induced heat acclimation 38 adaptations in both endurance trained and recreationally active individuals. Training status 39 did not significantly influence the magnitude of heat acclimation adaptations from PRE to 40 POST (interactions P > 0.05) for: the reduction in end-exercise rectal core temperature (Tre, 41 mean, endurance trained -0.36°C; recreationally active -0.47°C); the reduction in resting Tre 42 (endurance trained -0.17°C; recreationally active -0.23°C); the reduction in Tre at sweating 43 onset (endurance trained -0.22°C; recreationally active -0.23°C); and, the reduction in mean 44 skin temperature (endurance trained -0.67°C; recreationally active -0.75°C: PRE to POST P 45 < 0.01). Furthermore, training status did not significantly influence the observed reductions in 46 mean V̇O2, mean metabolic energy expenditure, end-exercise physiological strain index, 47 perceived exertion or thermal sensation (PRE to POST P < 0.05). Only end-exercise heart 48 rate was influenced by training status (P < 0.01, interaction); whereby, recreationally active 49 but not endurance trained individuals experienced a significant reduction in end-exercise 50 heart rate from PRE to POST (P < 0.01). In summary, these findings demonstrate that post-51 exercise hot water immersion presents a practical strategy to reduce thermal strain during 52 exercise-heat-stress in endurance trained and recreationally active individuals.