We examined whether daily hot water immersion (HWI) after exercise in temperate conditions induces heat acclimation and improves endurance performance in temperate and hot conditions. Seventeen non-heatacclimatized males performed a 6-day intervention involving a daily treadmill run for 40 min at 65% V O 2max in temperate conditions (18°C) followed immediately by either HWI (N = 10; 40°C) or thermoneutral (CON, N = 7; 34°C) immersion for 40 min. Before and after the 6-day intervention, participants performed a treadmill run for 40 min at 65% V O 2max followed by a 5-km treadmill time trial (TT) in temperate (18°C, 40% humidity) and hot (33°C, 40% humidity) conditions. HWI induced heat acclimation demonstrated by lower resting rectal temperature (T re , mean, À0.27°C, P < 0.01), and final T re during submaximal exercise in 18°C (À0.28°C, P < 0.01) and 33°C (À0.36°C, P < 0.01). Skin temperature, T re at sweating onset and RPE were lower during submaximal exercise in 18°C and 33°C after 6 days in HWI (P < 0.05). Physiological strain and thermal sensation were also lower during submaximal exercise in 33°C after 6 days in HWI (P < 0.05). HWI improved TT performance in 33°C (4.9%, P < 0.01) but not in 18°C. Thermoregulatory measures and performance did not change in CON. Hot water immersion after exercise on 6 days presents a simple, practical, and effective heat acclimation strategy to improve endurance performance in the heat.

Morning heat acclimation by postexercise HWI induced adaptations at rest and during exercise-heat stress in the morning and midafternoon.

Hot water immersion (HWI) after exercise on 6 consecutive days in temperate conditions has been shown to provide heat acclimation adaptations in a recreationally active population. Endurance athletes experience frequent, sustained elevations in body temperature during training and competition; as a consequence, endurance athletes are considered to be partially heat acclimatized. It is therefore important to understand the extent to which endurance trained individuals may benefit from heat acclimation by post-exercise HWI. To this end, we compared the responses of eight endurance trained and eight recreationally active males (habitual weekly endurance exercise: 9 h vs. 3 h) to a 6-day intervention involving a daily treadmill run for 40 min (65% O2max) in temperate conditions followed immediately by HWI (≤40 min, 40°C). Before (PRE) and after the intervention (POST), hallmark heat acclimation adaptations were assessed during a 40-min treadmill run at 65% O2max in the heat (33°C, 40% RH). The 6 day, post-exercise HWI intervention induced heat acclimation adaptations in both endurance trained and recreationally active individuals. Training status did not significantly influence the magnitude of heat acclimation adaptations from PRE to POST (interactions P > 0.05) for: the reduction in end-exercise rectal core temperature (Tre, mean, endurance trained -0.36°C; recreationally active -0.47°C); the reduction in resting Tre (endurance trained -0.17°C; recreationally active -0.23°C); the reduction in Tre at sweating onset (endurance trained -0.22°C; recreationally active -0.23°C); and, the reduction in mean skin temperature (endurance trained -0.67°C; recreationally active -0.75°C: PRE to POST P < 0.01). Furthermore, training status did not significantly influence the observed reductions in mean O2, mean metabolic energy expenditure, end-exercise physiological strain index, perceived exertion or thermal sensation (PRE to POST P < 0.05). Only end-exercise heart rate was influenced by training status (P < 0.01, interaction); whereby, recreationally active but not endurance trained individuals experienced a significant reduction in end-exercise heart rate from PRE to POST (P < 0.01). In summary, these findings demonstrate that post-exercise HWI presents a practical strategy to reduce thermal strain during exercise-heat-stress in endurance trained and recreationally active individuals.

Heat acclimation by post-exercise hot water immersion (HWI) on six consecutive days reduces thermal strain and improves exercise performance during heat stress. However, the retention of adaptations by this method remains unknown. Typically, adaptations to short-term, exercise-heat-acclimation (<7 heat exposures) decay rapidly and are lost within 2 weeks. Short-term protocols should therefore be completed within 2 weeks of relocating to the heat; potentially compromising pre-competition/deployment training. To establish whether adaptations from post-exercise HWI are retained for up to 2 weeks, participants completed a 40-min treadmill run at 65% max in the heat (33°C, 40% RH) before (PRE) and 24 h after (POST) the HWI intervention ( n = 13) and then at 1 week (WK 1) and 2 weeks (WK 2) after the HWI intervention ( n = 9). Heat acclimation involved a 40-min treadmill run (65% max ) on six consecutive days in temperate conditions (20°C), followed by ≤40 min HWI (40°C). Post-exercise HWI induced heat acclimation adaptations that were retained for at least 2 weeks, evidenced by reductions from PRE to WK 2 in: resting rectal core temperature ( T re , −0.36 ± 0.25°C), T re at sweating onset (−0.26 ± 0.24°C), and end-exercise T re (−0.36 ± 0.37°C). Furthermore, mean skin temperature ( T sk ) (−0.77 ± 0.70°C), heart rate (−14 ± 10 beats⋅min –1 ), rating of perceived exertion (−1 ± 2), and thermal sensation (−1 ± 1) were reduced from PRE to WK 2 ( P < 0.05). However, PRE to POST changes in total hemoglobin mass, blood volume, plasma volume, the drive for sweating onset, sweating sensitivity and whole body sweating rate did not reach significance ( P > 0.05). As such, the reduction in thermal strain during exercise-heat stress appears likely due to the reduction in resting T re evident at POST, WK 1, and WK 2. In summary, 6 days of post-exercise HWI is an effective, practical and accessible heat acclimation strategy that induces adaptations, which are retained for at least 2 weeks. Therefore, post-exercise HWI can be completed during an athlete’s pre-taper phase and does not suffer from the same practical limitations as short-term, exercise-heat-acclimation.

Objectives: To compare heat acclimation adaptations after three and six days of either post-exercise hot water immersion (HWI) or exercise-heat-acclimation (EHA) in recreationally active individuals. Design: Randomised, mixed model, repeated measures. Methods: Post-exercise HWI involved a daily 40-min treadmill-run at 65% VȮ 2peak in temperate conditions (19°C, 45% RH) followed by HWI (≤40 min, 40°C water; n = 9). Daily EHA involved a ≤60-min treadmill-run in the heat (65% VȮ 2peak ; 33°C, 40% RH; n = 9), chosen to elicit a similar endogenous thermal stimulus to HWI. A thermoneutral exercise intervention (TNE, 19°C, 45% RH; n = 9), work-matched to EHA, was also included to determine thermoregulatory adaptations to daily exercise in temperate conditions. An exercise-heat-stresstest was performed before and after three and six intervention days and involved a 40-min treadmill-run and time-to-exhaustion (TTE) at 65% VȮ 2peak in the heat (33°C, 40% RH). Results: ANCOVA, using baseline values as the covariate, revealed no interaction effects but significant group effects demonstrated that compared to EHA, HWI elicited larger reductions in resting rectal temperature (T re ; p = 0.021), T re at sweating onset (p = 0.011), and end-exercise T re during exercise-heat-stress (−0.47°C; p = 0.042). Despite a similar endogenous thermal stimulus to HWI, EHA elicited a modest reduction in end-exercise T re (−0.26°C), which was not different from TNE (−0.25°C, p = 1.000). There were no main effects or interaction effects for end-exercise T sk , heart rate, physiological strain index, RPE, thermal sensation, plasma volume, or TTE (all p ≥ 0.154). Conclusions: Compared with conventional short-term exercise heat acclimation, short-term post-exercise hot water immersion elicited larger thermal adaptations.