Short-term isothermic heat acclimation elicits beneficial adaptations but medium-term elicits a more complete adaptation

Abstract

Purpose To investigate the effects of 60 min daily, short-term (STHA) and medium-term (MTHA) isothermic heat acclimation (HA) on the physiological and perceptual responses to exercise heat stress. Methods Sixteen, ultra-endurance runners (female = 3) visited the laboratory on 13 occasions. A 45 min sub-maximal (40% W max ) cycling heat stress test (HST) was completed in the heat (40 °C, 50% relative humidity) on the first (HST PRE ), seventh (HST STHA ) and thirteenth (HST MTHA ) visit. Participants completed 5 consecutive days of a 60 min isothermic HA protocol (target T re 38.5 °C) between HST PRE and HST STHA and 5 more between HST STHA and HST MTHA . Heart rate (HR), rectal ( T re ), skin ( T sk ) and mean body temperature ( T body ), perceived exertion (RPE), thermal comfort (TC) and sensation (TS) were recorded every 5 min. During HSTs, cortisol was measured pre and post and expired air was collected at 15, 30 and 45 min. Results At rest, T re and T body were lower in HST STHA and HST MTHA compared to HST PRE, but resting HR was not different between trials. Mean exercising T re , T sk , T body , and HR were lower in both HST STHA and HST MTHA compared to HST PRE . There were no differences between HST STHA and HST MTHA . Perceptual measurements were lowered by HA and further reduced during HST MTHA . Conclusion A 60 min a day isothermic STHA was successful at reducing physiological and perceptual strain experienced when exercising in the heat; however, MTHA offered a more complete adaptation.

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