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. 2020 Jan;120(1):243-254.
doi: 10.1007/s00421-019-04269-5. Epub 2019 Nov 25.

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

Jodie N Moss  1 , Freya M Bayne  1   2 , Federico Castelli  1 , Mitchell R Naughton  1   3 , Thomas C Reeve  1 , Steven J Trangmar  1 , Richard W A Mackenzie  1 , Christopher J Tyler  4
Affiliations

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

Jodie N Moss et al. Eur J Appl Physiol. 2020 Jan.

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% Wmax) cycling heat stress test (HST) was completed in the heat (40 °C, 50% relative humidity) on the first (HSTPRE), seventh (HSTSTHA) and thirteenth (HSTMTHA) visit. Participants completed 5 consecutive days of a 60 min isothermic HA protocol (target Tre 38.5 °C) between HSTPRE and HSTSTHA and 5 more between HSTSTHA and HSTMTHA. Heart rate (HR), rectal (Tre), skin (Tsk) and mean body temperature (Tbody), 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, Tre and Tbody were lower in HSTSTHA and HSTMTHA compared to HSTPRE, but resting HR was not different between trials. Mean exercising Tre, Tsk, Tbody, and HR were lower in both HSTSTHA and HSTMTHA compared to HSTPRE. There were no differences between HSTSTHA and HSTMTHA. Perceptual measurements were lowered by HA and further reduced during HSTMTHA.

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.

Keywords: Acclimatisation; Endurance performance; Heat strain; Taper; Thermoregulation.

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Conflict of interest statement

The authors declare that they have no conflicts of interest.

Figures

Fig. 1
Fig. 1
Experimental Schematic. Salivary cortisol (C) and venous blood (B) samples were collected pre- and post-HST on the first (HSTPRE), seventh (HSTSTHA) and thirteenth (HSTMTHA) visit. Between HSTPRE and HSTSTHA and again between HSTSTHA and HSTMTHA, participants completed 5 consecutive days of 60 min isothermic heat acclimation during which time rectal temperature (Tre) was elevated to, and maintained at ~ 38.5 °C
Fig. 2
Fig. 2
Rectal temperature (Tre) on each time point during HSTPRE, HSTSTHA, and HSTMTHA. There were main effects of trial (P < 0.001) and time (P < 0.001) for Tre, *Significant (P < 0.05) difference between HSTPRE and both HSTSTHA and HSTMTHA. Data mean ± SD
Fig. 3
Fig. 3
Heart rate during HSTPRE, HSTSTHA, and HSTMTHA. There were main effects of trial (P < 0.001) and time (P < 0.001) for HR. †Significant (P < 0.05) difference between HSTPRE and HSTSTHA. *Significant (P < 0.05) difference between HSTPRE and HSTMTHA. #Significant (P < 0.05) difference between HSTMTHA and HSTSTHA. Data mean ± SD
Fig. 4
Fig. 4
a Thermal sensation and b thermal comfort were recorded before exercise and every 5 min intervals during HSTPRE, HSTSTHA, and HSTMTHA. †Significant (P < 0.05) difference between HSTPRE and HSTSTHA. *Significant (P < 0.05) difference between HSTPRE and HSTMTHA. #Significant (P < 0.05) difference between HSTMTHA and HSTSTHA. Data mean ± SD
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