Short-term hot water immersion results in substantial thermal strain and partial heat acclimation; comparisons with heat-exercise exposures.

Abstract

To examine the effectiveness of hot water immersion (HWI) as a heat acclimation strategy in comparison to time and temperature matched, exercise-heat acclimation (EHA). 8 males performed heat stress tests (HST) (45min of cycling at 50% of VO2max in 40°C, 40% RH) before and after heat acclimation sessions. Acclimation sessions were either three consecutive bouts of HWI (40min of submersion at 40°C) or EHA (40min of cycling at 50% VO2max in 40°C, 40% RH). Average change in tympanic temperature (TTympanic) was significantly higher following HWI (2.1°C±0.4) compared to EHA (1.5°C±0.4) (P<0.05). Decreases in peak heart rate (HR) (HWI: -10 bpm±8; EHA: -6±7), average HR (-7 bpm±6; -3±4), and average core temperature (-0.4°C±0.3; -0.2±0.4) were evident following acclimation (P<0.05), but not different between interventions (P>0.05). Peak rate of perceived exertion (RPEPeak) decreased for HWI and EHA (P<0.05). Peak thermal sensation (TSPeak) decreased following HWI (P< 0.05) but was not different between interventions (P>0.05). Plasma volume increased in both intervention groups (HWI: 5.9%±5.1; EHA: 5.4%±3.7) but was not statistically different (P>0.05). HWI induced significantly greater thermal strain compared to EHA at equivalent temperatures during time-matched exposures. However, the greater degree of thermal strain did not result in between intervention differences for cardiovascular, thermoregulatory, or perceptual variables. Findings suggest three HWI sessions may be a potential means to lower HR, TCore, and perceptual strain during exercise in the heat.