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% V_O2peak at 19 degrees C followed by a 40 degrees C bath. EHA and CON involved a work-matched treadmill run at 65% V_O2peak at 33 degrees C or 19 degrees 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 degrees C, HWI -0.41 +/- 0.15 degrees 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 degrees C). No differences were identi-fied 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.