A comparison of medium-term heat acclimation by post-exercise hot water immersion or exercise in the heat: adaptations, overreaching, and thyroid hormones
Robert D. McIntyre, Michael J. Zurawlew, Jessica A. Mee, Neil P. Walsh, Samuel J. Oliver
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.