A comparison of medium-term heat acclimation by post-exercise hot water immersion or exercise in the heat: adaptations, overreaching, and thyroid hormones.

This research compared thermal and perceptual adaptations, endurance capacity, and overreaching markers in men after 3, 6, and 12 days of post-exercise hot water immersion (HWI) or exercise heat acclimation (EHA) with a temperate exercise control (CON), and examined thyroid hormones as a mechanism for the reduction in resting and exercising core temperature (Tre) after HWI. HWI involved a treadmill run at 65% V̇o2peak at 19°C followed by a 40°C bath. EHA and CON involved a work-matched treadmill run at 65% V̇o2peak at 33°C or 19°C, respectively. Compared with CON, resting mean body temperature (Tb), resting and end-exercise Tre, Tre at sweating onset, thermal sensation, and perceived exertion were lower and whole-body sweat rate (WBSR) was higher after 12 days of HWI (all P ≤ 0.049, resting Tb: CON -0.11 ± 0.15°C, HWI -0.41 ± 0.15°C). Moreover, resting Tb and Tre at sweating onset were lower after HWI than EHA (P ≤ 0.015, resting Tb: EHA -0.14 ± 0.14°C). No differences were identified between EHA and CON (P ≥ 0.157) except WBSR that was greater after EHA (P = 0.013). No differences were observed between interventions for endurance capacity or overreaching markers (mood, sleep, Stroop, P ≥ 0.190). Thermal adaptations observed after HWI were not related to changes in thyroid hormone concentrations (P ≥ 0.086). In conclusion, 12 days of post-exercise hot water immersion conferred more complete heat acclimation than exercise heat acclimation without increasing overreaching risk, and changes in thyroid hormones are not related to thermal adaptations after post-exercise hot water immersion.

A comparison of heat acclimation by post-exercise hot water immersion and exercise in the heat.

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% V̇O2peak 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% V̇O2peak; 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-stress-test was performed before and after three and six intervention days and involved a 40-min treadmill-run and time-to-exhaustion (TTE) at 65% V̇O2peak 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 (Tre; p = 0.021), Tre at sweating onset (p = 0.011), and end-exercise Tre 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 Tre (-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 Tsk, 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.

Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations That Are Retained for at Least Two Weeks.

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.

Heat Acclimation by Postexercise Hot-Water Immersion: Reduction of Thermal Strain During Morning and Afternoon Exercise-Heat Stress After Morning Hot-Water Immersion.

PURPOSE: Recommendations state that to acquire the greatest benefit from heat-acclimation, the clock time of heat-acclimation sessions should match that of expected exercise-heat stress. It remains unknown if adaptations by postexercise hot-water immersion (HWI) demonstrate time-of-day-dependent adaptations. Thus, the authors examined whether adaptations following postexercise HWI completed in the morning were present during morning and afternoon exercise-heat stress. METHODS: Ten males completed an exercise-heat stress test commencing in the morning (9:45 AM) and afternoon (2:45 PM; 40 min; 65% of maximal oxygen uptake treadmill run) before and after heat-acclimation. The 6-d heat-acclimation intervention involved a daily 40-min treadmill run (65% of maximal oxygen uptake) in temperate conditions followed by ≤40-min HWI (40°C; 6:30-11:00 AM). RESULTS: Adaptations by 6-d postexercise HWI in the morning were similar in the morning and afternoon. Reductions in resting rectal temperature (Tre) (AM -0.34°C [0.24°C], PM -0.27°C [0.23°C]; P = .002), Tre at sweating onset (AM -0.34°C [0.24°C], PM -0.31°C [0.25°C]; P = .001), and end-exercise Tre (AM -0.47°C [0.33°C], PM -0.43°C [0.29°C]; P = .001), heart rate (AM -14 [7] beats·min-1, PM -13 [6] beats·min-1; P < .01), rating of perceived exertion (P = .01), and thermal sensation (P = .005) were not different in the morning compared with the afternoon. CONCLUSION: Morning heat acclimation by postexercise HWI induced adaptations at rest and during exercise-heat stress in the morning and midafternoon.

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 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 (T re, mean, endurance trained -0.36°C; recreationally active -0.47°C); the reduction in resting T re (endurance trained -0.17°C; recreationally active -0.23°C); the reduction in T re 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.