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

Michael J Zurawlew, Jessica A Mee, Neil P Walsh, Michael J Zurawlew, Jessica A Mee, Neil P Walsh

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 (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.

Keywords: acclimation; decay; heat; hot water; running; thermal strain; thermoregulation.

Figures

FIGURE 1
FIGURE 1
Influence of heat acclimation by post-exercise hot water immersion on resting rectal core temperature (Tre) (A–C) and end-exercise Tre(D–F) following 40-min treadmill running at 65% max in the heat (33°C, 40% RH). Bars show mean ± SD (A,D) or mean ± SD of the change from PRE, at POST, WK 1, and WK 2 (B,E). Lines represent individual responses (C,F). ∗P < 0.05, ∗∗P < 0.01 less than PRE.
FIGURE 2
FIGURE 2
Influence of heat acclimation by post-exercise hot water immersion on end-exercise heart rate (A), end-exercise rating of perceived exertion (RPE) (B), end-exercise mean skin temperature (Tsk) (C) and end-exercise thermal sensation (D) following 40-min treadmill running at 65% max in the heat (33°C, 40% RH). Bars show mean ± SD of the change from PRE, at POST, WK 1, and WK 2. ∗P < 0.05, ∗∗P < 0.01 less than PRE (post hoc effect).
FIGURE 3
FIGURE 3
Influence of heat acclimation by post-exercise hot water immersion on total hemoglobin (Hb) mass (A), blood volume (B) and plasma volume (C) at rest. Bars show mean ± SD at PRE and POST.
FIGURE 4
FIGURE 4
Influence of heat acclimation by post-exercise hot water immersion on mean local forearm sweating rate reported as a function of esophageal core temperature (Tes) (A) and as a function of the change in esophageal core temperature (Δ Tes) (B) during 12 min (1 min average) of submaximal treadmill running (65% max) in the heat (33°C, 40% RH) before (PRE) and after (POST) heat acclimation. Error bars removed for clarity.

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