Abstract
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 (Tre, −0.36 ± 0.25°C), Tre at sweating onset (−0.26 ± 0.24°C), and end-exercise Tre (−0.36 ± 0.37°C). Furthermore, mean skin temperature (Tsk) (−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 Tre 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.
Highlights
Before relocating to a hot climate, athletes, military personnel and occupational workers are advised to complete 5–14 days of heat acclimation to reduce thermal strain during exercise-heat stress, improve performance, and reduce susceptibility to exertional heat illness (Cluver, 1932; Periard et al, 2015)
All 13 participants completed a 40-min submaximal treadmill run followed by hot water immersion (HWI) (≤40 min) on six consecutive days
On day 1, 10 of the 13 participants removed themselves from the HWI due to discomfort; whereas, on day 6, all participants completed the 40-min HWI protocol (Table 1)
Summary
Before relocating to a hot climate, athletes, military personnel and occupational workers are advised to complete 5–14 days of heat acclimation to reduce thermal strain during exercise-heat stress, improve performance, and reduce susceptibility to exertional heat illness (Cluver, 1932; Periard et al, 2015). Recommendations for heat acclimation suggest that daily exposures should elevate body temperatures (≥38.5◦C) and initiate perfuse sweating through exercise-heat stress (Taylor, 2014; Periard et al, 2015; Casadio et al, 2017; Saunders et al, 2019). One explanation for poor athlete engagement with heat acclimatization is that protocols rely on sufficiently stressful natural environmental conditions in the days preceding competition to induce adaptation, which cannot be controlled or guaranteed (Gerrett et al, 2019). Recommendations suggest that heat acclimation protocols should be completed in the days preceding relocation to a hot climate; likely compromising competition taper or pre-deployment training (Garrett et al, 2009; Saunders et al, 2019). Completing an accessible and time efficient alternative method of heat acclimation prior to or during pre-competition taper, such as post-exercise hot water immersion (HWI) (Zurawlew et al, 2016) or sauna bathing (Scoon et al, 2007), may minimize disturbances to training and taper (Saunders et al, 2019)
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