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

Objective: 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). Methods: 8 males performed heat stress tests (HST) (45 min of cycling at 50% of VO2max in 40 degrees C, 40% RH) before and after heat acclimation sessions. Acclimation sessions were either three consecutive bouts of HWI (40 min of submersion at 40 degrees C) or EHA (40 min of cycling at 50% VO2max in 40 degrees C, 40% RH). Results: Average change in tympanic temperature (T-Tympanic) was significantly higher following HWI (2.1 degrees C +/- 0.4) compared to EHA (1.5 degrees 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 degrees 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). Conclusion: 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.