The Impact Of Short-Term Hot Water Immersion On Heat Acclimation And Thermotolerance

Abstract

BACKGROUND: Environmental heat stress increases physiological strain during exercise in non-acclimated individuals. Heat acclimation (HA) protocols are often used as countermeasures to preserve physiological function during exercise in the heat. Passive heat strategies could be a potential method of HA that reduces excess physical exertion prior to activity or relocation. PURPOSE: To determine the effect of hot water immersion (HWI) on heat acclimation and thermotolerance. METHODS: 6 males (Age: 23.8 ± 1.5; VO2max: 45.0 ± 7.5 mL/kg/min.) participated in a crossover, counterbalanced study with a four-week washout between conditions. Heat stress tests (HST) were performed PRE and POST acclimation sessions and consisted of 45 min of cycling at 50% of VO2max in 40 °C, 40% RH. Acclimation sessions were either three consecutive bouts of HWI or traditional heat-exercise training (TRAD). HWI sessions consisted of 40 min of submersion at 40 °C. TRAD sessions consisted of 40 min of cycling at 50% VO2max in 40 °C, 40% RH. Core body temperature (Tcore), heart rate (HR), rate of perceived exertion (RPE), and thermal sensation (TSS) were recorded during HSTs. Blood was drawn PRE and POST HST to determine change in plasma volume. Nude body mass was recorded before and after HSTs to calculate whole body sweat loss (WBSL). Tcore and HR were used to calculate physiological strain index (PSI). RESULTS: HWI decreased average HR (PRE: 158 ± 7; POST: 149 ± 7; p = < 0.05), peak HR (176 ± 7; 164 ± 5; p = < 0.05), and end exercise RPE (15.3 ± 1.8; 13.2 ± 1.9; p = < 0.05). HWI had no significant effect on resting TCore (37.2 ± 0.4; 36.8 ± 0.3; p = 0.66), end exercise TCore (38.7 ± 0.4; 38.4 ± 0.3; p = 0.20), PSI (7.8 ± 1.1; 7.0 ± 0.8; p = 0.56), or TSS (10.8 ± 1.0; 9.5 ± 1.2; p = 0.09). TRAD resulted in no significant changes in average HR (PRE: 153 ± 11; POST: 149 ± 11; p = 0.82), peak HR (170 ± 9; 165 ± 10; p = 0.51), end exercise RPE (15.1 ± 1.2; 13.3 ± 2.0; p = 0.87), PSI (9.3 ± 2.1; 8.6 ± 1.4; p = 0.73), or TSS (10.5 ± 0.8; 10.7 ± 1.5; p = 0.77). Plasma volume expansion (%) was observed in both groups (HWI: 6.6 ± 6.4; TRAD: 4.3 ± 2.0; p = 0.41). CONCLUSION: Three consecutive HWI sessions are effective in lowering HR during submaximal exercise in the heat. Compared to heat-exercise exposures, HWI is a method in which to more rapidly elicit a HA phenotype.