Heat Acclimation by Postexercise Hot-Water Immersion: Reduction of Thermal Strain During Morning and Afternoon Exercise-Heat Stress After Morning Hot-Water Immersion

Purpose: Recommendations state that to acquire the greatest benefit from heat-acclimation, the clock time of heat-acclimation sessions should match that of expected exercise-heat stress. It remains unknown if adaptations by postexercise hot-water immersion (HWI) demonstrate time-of-day-dependent adaptations. Thus, the authors examined whether adaptations following postexercise HWI completed in the morning were present during morning and afternoon exercise-heat stress. Methods: Ten males completed an exercise-heat stress test commencing in the morning (9:45 AM) and afternoon (2:45 PM; 40 min; 65% of maximal oxygen uptake treadmill run) before and after heat-acclimation. The 6-d heat-acclimation intervention involved a daily 40-min treadmill run (65% of maximal oxygen uptake) in temperate conditions followed by <= 40-min HWI (40 degrees C; 6:30-11:00 AM). Results: Adaptations by 6-d postexercise HWI in the morning were similar in the morning and afternoon. Reductions in resting rectal temperature (T-re) (AM -0.34 degrees C [0.24 degrees C], PM -0.27 degrees C [0.23 degrees C]; P= .002), T-re at sweating onset (AM -0.34 degrees C [0.24 degrees C], PM -0.31 degrees C [0.25 degrees C]; P= .001), and end-exercise T-re (AM -0.47 degrees C [0.33 degrees C], PM -0.43 degrees C [0.29 degrees C]; P= .001), heart rate (AM -14 [7] beats.min(-1), PM -13 [6] beats.min(-l); P < .01), rating of perceived exertion (P = .01), and thermal sensation (P= .005) were not different in the morning compared with the afternoon. Conclusion: Morning heat acclimation by postexercise HWI induced adaptations at rest and during exercise-heat stress in the morning and midafternoon.