Optimising the acquisition and retention of heat acclimation

Heat acclimation (HA) often starts in a moderately hot environment to prevent thermal overload and stops immediately prior to athletic activities. The aims of this study were (1) to establish whether acclimation to a moderately hot climate is sufficient to provide full acclimation for extreme heat and (2) to investigate the physiological responses to heat stress during the HA decay period. 15 male subjects exercised for 9 consecutive days at 26° C Wet Bulb Globe Temperature (WBGT) and 3 days at 32° C WBGT on a cycle ergometer for up to 2?h per day and repeated the exercise 3, 7 and 18 days later in 26° C WBGT. Rectal temperature (T (re)) and heart rate (HR) were measured during 60?min of steady state exercise (～45% of maximum oxygen uptake). During days 1-9, end-exercise T (re) was reduced from 38.7±0.1 to a plateau of 38.2±0.1° C (p<0.05), HR was reduced from 156±10 to 131±11?bpm (p<0.05). No changes in HR and T (re) occurred during the 3 days in the very hot environment. However, T (re) during rest and exercise were significantly lower by 0.4-0.5° C after HA compared with day 9, suggesting that heat acclimation did not decay but resulted in further favourable adaptations.