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

Abstract

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 (T_re) after HWI. HWI involved a treadmill run at 65% V̇o_2peak at 19°C followed by a 40°C bath. EHA and CON involved a work-matched treadmill run at 65% V̇o_2peak at 33°C or 19°C, respectively. Compared with CON, resting mean body temperature (T_b), resting and end-exercise T_re, T_re 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 T_b: CON -0.11 ± 0.15°C, HWI -0.41 ± 0.15°C). Moreover, resting T_b and T_re at sweating onset were lower after HWI than EHA (P ≤ 0.015, resting T_b: 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.

Keywords: core temperature; hot bath; thermoregulation; thyroxine; triiodothyronine.

Graphical abstract

Figure 1.

Schematic of the study design (part 1). CON; temperate exercise control; EHA, exercise heat acclimation; HWI, post-exercise hot water immersion.

Figure 2.

Flow diagram indicating the numbers of participants assessed for eligibility, commenced testing, and withdrew, were excluded, or completed the study protocol (part 1). CON, temperate exercise control; EHA, exercise heat acclimation; HWI, post-exercise hot water immersion.

Figure 3.

Influence of 3 (POST3), 6 (POST6), and 12 days (POST12) of a temperate exercise control (CON, n = 7 participants), exercise heat acclimation (EHA, n = 7 participants), or post-exercise hot water immersion (HWI, n = 7 participants) on resting mean body temperature (T_b, A), rectal core temperature (T_re, B), mean skin temperature (T_sk, C), and metabolic heat production (H, D) in temperate conditions (19°C, 45% relative humidity). Bars represent baseline-adjusted means; circles represent individual participant responses. Analyzed by two-way mixed model ANCOVA, with baseline (PRE) as the covariate and Bonferroni-adjusted pairwise comparisons; †HWI lower than CON, P < 0.05; ††HWI lower than CON, P < 0.01; ‡HWI lower than EHA, P < 0.05.

Figure 4.

Influence of 3 (POST3), 6 (POST6), and 12 days (POST12) of a temperate exercise control (CON, n = 7 participants), exercise heat acclimation (EHA, n = 7 participants), or post-exercise hot water immersion (HWI, n = 7 participants) on end-exercise mean body temperature (T_b, A), end-exercise rectal core temperature (T_re, B), T_re at sweating onset (C), whole-body sweat rate (D), end-exercise rating of perceived exertion (RPE, E), and end-exercise thermal sensation (TS, F) in the heat (33°C, 40% relative humidity). Bars represent baseline-adjusted means; circles represent individual participant responses. Analyzed by two-way mixed model ANCOVA, with baseline (PRE) as the covariate and Bonferroni-adjusted pairwise comparisons; †group difference to CON, P < 0.05; ††group difference to CON, P < 0.01; ‡HWI lower than EHA, P < 0.05.

Figure 5.

Influence of 3 (POST3), 6 (POST6), and 12 days (POST12) of a temperate exercise control (CON, n = 7 participants), exercise heat acclimation (EHA, n = 7 participants), or post-exercise hot water immersion (HWI, n = 7 participants) on plasma concentrations of free triiodothyronine (T3, A), free thyroxine (T4, B), total T3 (C), and total T4 (D). Bars represent baseline-adjusted means; circles represent individual participant responses. Analyzed by two-way mixed model ANCOVA, with baseline (PRE) as the covariate and Bonferroni-adjusted pairwise comparisons; †HWI lower than CON, P < 0.05; ‡HWI lower than EHA, P < 0.05.

Figure 6.

Influence of 6 days of a temperate exercise control (CON, n = 16 participants) or post-exercise hot water immersion (HWI, n = 32 participants) on resting rectal core temperature (T_re, A), resting metabolic heat production (H, B), and resting plasma concentrations of free triiodothyronine (T3, C), free thyroxine (T4, D), total T3 (E), and total T4 (F). Bars represent baseline-adjusted means; circles represent individual participant responses. Analyzed by one-way ANCOVA, with baseline (PRE) as the covariate; ††HWI lower than CON, P < 0.01.

Figure 7.

Relationships between the changes in resting core temperature (T_re) and plasma concentrations of thyroid hormones free triiodothyronine (T3, A), free thyroxine (T4, B), total T3 (C), and total T4 (D) after 6 days of post-exercise hot water immersion (n = 32 participants). Analyzed by Pearson's correlation.