Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat

We examined whether daily hot water immersion (HWI) after exercise in temperate conditions induces heat acclimation and improves endurance performance in temperate and hot conditions. Seventeen non-heat-acclimatized males performed a 6-day intervention involving a daily treadmill run for 40min at 65% V.O-2max in temperate conditions (18 degrees C) followed immediately by either HWI (N=10; 40 degrees C) or thermoneutral (CON, N=7; 34 degrees C) immersion for 40min. Before and after the 6-day intervention, participants performed a treadmill run for 40min at 65% V.O-2max followed by a 5-km treadmill time trial (TT) in temperate (18 degrees C, 40% humidity) and hot (33 degrees C, 40% humidity) conditions. HWI induced heat acclimation demonstrated by lower resting rectal temperature (T-re, mean, -0.27 degrees C, P<0.01), and final T-re during submaximal exercise in 18 degrees C (-0.28 degrees C, P<0.01) and 33 degrees C (-0.36 degrees C, P<0.01). Skin temperature, T-re at sweating onset and RPE were lower during submaximal exercise in 18 degrees C and 33 degrees C after 6days in HWI (P<0.05). Physiological strain and thermal sensation were also lower during submaximal exercise in 33 degrees C after 6days in HWI (P<0.05). HWI improved TT performance in 33 degrees C (4.9%, P<0.01) but not in 18 degrees C. Thermoregulatory measures and performance did not change in CON. Hot water immersion after exercise on 6days presents a simple, practical, and effective heat acclimation strategy to improve endurance performance in the heat.