Background: Heat acclimatization (HAz) or acclimation (HA) is one of the most beneficial, heat illness prevention and performance strategies used during physical activity. However, the optimal method to maintain these methods are unknown. Purpose: To test the efficacy of a novel dual heat acc (DHA) induction protocol and to examine if there is a dose response relationship related to the frequency of intermittent heat training following HA on aerobically training athletes. Methods: Twenty-seven male endurance athletes (mean[M]±standard deviation[SD]; age, 34±12 years; height, 178.44±6.31 cm; body mass, 72.56±8.81 kg; VO2max 57.65±6.79 ml×kg-1×min-1) completed five tests (Un-acclimatized [Test#1], following HAz [Test#2], following HA [Test#3], the middle of heat training (HT) [Test#4] and the end of HT [Test#5]) following HA that involved sixty minutes of steady state exercise (59.12±1.74% vVO2maxTest#1) in an artificial environmental laboratory (M±SD; ambient temperature [Tamb], 35.42±1.06°C; relative humidity [%RH], 46.35±2.48%; Wet Bulb Globe Temperature [WBGT] 29.62±1.37°C; wind speed, 3.98±0.30 mph) on a motorized treadmill. The study, in its entirety, was approximately six months in length. Following Test#3, participants were randomly assigned to three groups: control group with no heat exposures (HTCON), once per week heat exposure group (HTMIN), and twice per week heat exposure group (HTMAX). Repeated measures ANOVA were utilized to determine differences in physiological variables between trials. Results: DHA resulted in significant mean differences in maximal HR (p<0.001), average HR (p<0.001), ending Trec (p<0.001), average Trec (p=0.001), delta Trec (p=0.026), sweat rate (p=0.033), and Tsk (p<0.001) between Test#1, Test#2, and Test#3. At Test#5, the highest trial HR was significantly higher in HTCON compared to HTMAX (M±SD, HTCON, 173.88±22.22 bpm; HTMAX, 151.00±16.52 bpm, p<0.05), but was not different than HTMIN (M±SD, 159.33 bpm). There were statistical differences between HTCON and HTMAX % change of rectal temperature from Test#3 (HTCON vs HTMAX, [95%CI] 0.46%, 2.7%; ES=1.37; p=0.009), but not between HTMIN (HTCON vs HTMIN, [95%CI] -0.26%, 2.8%; ES=0.85; p=0.098) at Test#5. Conclusions: HTMAX (twice weekly heat training) provides clear evidence for the ability to maintain and possibly improve physiological adaptations following DHA. HTMIN (once weekly heat training) may be sufficient for some individuals to maintain gains made from DHA.
