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.

Change in Exercise Performance and Markers of Acute Kidney Injury Following Heat Acclimation with Permissive Dehydration

“…For example, heat acclimation significantly increases exercise performance when exercise is performed in a hot environment [ 1 , 2 ]; however, it is unclear whether heat acclimation improves exercise performance and VO 2max in a cool or temperate environment [ 4 , 5 ]. The process of heat acclimation involves various whole-body [ 6 , 7 ] and cellular changes [ 8 ] which are dependent on the frequency, duration, and level of thermal strain of the protocol [ 2 , 9 ]. It is reported that reductions in core and skin temperature typically occur by the fifth day [ 10 , 11 ], while increases in sweat rate usually develop by the tenth day of heat acclimation [ 11 , 12 ].…”

The Gastrointestinal Exertional Heat Stroke Paradigm: Pathophysiology, Assessment, Severity, Aetiology and Nutritional Countermeasures

“…Likewise, 10 days of fixed-intensity heat acclimation (one hour running at 50% VO 2max in 40 • C/25% RH), had no influence on post-exercise I-FABP concentration compared to day one [137]. In a recent study, neither seven nor thirteen days isothermic heat-acclimation (90 min to sustain T core~3 8.5 • C) blunted the rise in endotoxin concentration following 45 min low intensity (40% watt max ) cycling in the heat (40 • C/50% RH), despite large reductions in thermal strain [173]. In a non-heat acclimation study, 14 days of 20% increased training versus standard load, led to a reduction in resting endotoxin concentration (35%), but did not influence peak concentrations following a 70% VO 2max treadmill run (35 • C/40% RH) until a T core of 39.5 • C was attained [173].…”

“…In a recent study, neither seven nor thirteen days isothermic heat-acclimation (90 min to sustain T core~3 8.5 • C) blunted the rise in endotoxin concentration following 45 min low intensity (40% watt max ) cycling in the heat (40 • C/50% RH), despite large reductions in thermal strain [173]. In a non-heat acclimation study, 14 days of 20% increased training versus standard load, led to a reduction in resting endotoxin concentration (35%), but did not influence peak concentrations following a 70% VO 2max treadmill run (35 • C/40% RH) until a T core of 39.5 • C was attained [173]. The influence of aerobic fitness has been shown to both increase (I-FABP; [152]) and reduce (endotoxin; [165]) GI barrier integrity loss following exertional heat stress that evoked comparable thermal strain between groups.…”

The Effect of Medium-Term Sauna-Based Heat Acclimation (MPHA) on Thermophysiological and Plasma Volume Responses to Exercise Performed under Temperate Conditions in Elite Cross-Country Skiers

“…On the other hand, Garrett et al [ 10 ] showed that a daily 90-min isothermic HA protocol was an adequate stimulus for heat adaptation and improved cardiovascular stability in highly trained athletes. Moss et al [ 73 ] suggest that a 5-day 60-min isothermic HA regimen provides a sufficient thermal stimulus to elicit beneficial adaptations to reduce physiological and perceptual strain while exercising in heat in ultra-endurance runners. Furthermore, people with a high VO 2 max can acclimate faster than people with lower VO 2 max [ 74 ].…”