A comparison of heat acclimation by post-exercise hot water immersion and exercise in the heat

Abstract

Objectives: To compare heat acclimation adaptations after three and six days of either post-exercise hot water immersion (HWI) or exercise-heat-acclimation (EHA) in recreationally active individuals. Design: Randomised, mixed model, repeated measures. Methods: Post-exercise HWI involved a daily 40-min treadmill-run at 65% VȮ 2peak in temperate conditions (19°C, 45% RH) followed by HWI (≤40 min, 40°C water; n = 9). Daily EHA involved a ≤60-min treadmill-run in the heat (65% VȮ 2peak ; 33°C, 40% RH; n = 9), chosen to elicit a similar endogenous thermal stimulus to HWI. A thermoneutral exercise intervention (TNE, 19°C, 45% RH; n = 9), work-matched to EHA, was also included to determine thermoregulatory adaptations to daily exercise in temperate conditions. An exercise-heat-stresstest was performed before and after three and six intervention days and involved a 40-min treadmill-run and time-to-exhaustion (TTE) at 65% VȮ 2peak in the heat (33°C, 40% RH). Results: ANCOVA, using baseline values as the covariate, revealed no interaction effects but significant group effects demonstrated that compared to EHA, HWI elicited larger reductions in resting rectal temperature (T re ; p = 0.021), T re at sweating onset (p = 0.011), and end-exercise T re during exercise-heat-stress (−0.47°C; p = 0.042). Despite a similar endogenous thermal stimulus to HWI, EHA elicited a modest reduction in end-exercise T re (−0.26°C), which was not different from TNE (−0.25°C, p = 1.000). There were no main effects or interaction effects for end-exercise T sk , heart rate, physiological strain index, RPE, thermal sensation, plasma volume, or TTE (all p ≥ 0.154). Conclusions: Compared with conventional short-term exercise heat acclimation, short-term post-exercise hot water immersion elicited larger thermal adaptations.

A comparison of medium-term heat acclimation by post-exercise hot water immersion or exercise in the heat: adaptations, overreaching, and thyroid hormones

“…However, no study to date in humans has investigated the effect of heat acclimation on thyroid hormone concentrations or thyroid hormone influences on heat acclimation thermal adaptations. Specifically, it is unknown whether reductions in thyroid hormones are responsible for the pronounced reduction in resting core temperature observed after HWI heat acclimation (6,7,9). This research is presented in two parts.…”

“…post-exercise hot water immersion (HWI) also presents a more practical heat acclimation strategy than conventional exercise heat acclimation (EHA), as it eliminates the requirement for access to an environmental chamber and can be more easily incorporated into normal training and an athlete’s taper ( 8 ). Moreover, McIntyre et al ( 9 ) recently demonstrated that despite a similar endogenous thermal stimulus for adaptation, 6 days of HWI elicited larger thermal adaptations than EHA. Although the 6-day HWI intervention presents an effective, practical, and time-efficient short-term (< 7 days) heat acclimation strategy, previous literature suggests that medium- (7–14 days) ( 10 , 11 ) and long-term (>14-day) ( 10 ) interventions provide a more complete state of heat acclimation.…”

“…The reduction in thermal strain after HWI heat acclimation can be largely attributed to a reduction in resting core temperature ( 6 , 7 , 9 , 13 , 14 ). The underlying mechanism for this reduction in resting core temperature is currently unknown but may involve a reduction in metabolic heat production via reduced circulating thyroid hormone concentrations ( 15 ), a decrease in the thermoregulatory balance point ( 16 , 17 ), or hypothalamic neural network remodeling ( 18 , 19 ).…”

A Special Issue on Heat, Health, and Performance in Journal of Science and Medicine in Sport

“…Daily training sessions conducted in the heat, or while wearing thermal clothing in temperate conditions, or while wearing thermal clothing in temperate conditions combined with hot water immersion are all equally effective for improving exercise performance in the heat. McIntyre 3 A comparison of heat acclimation by post-exercise hot water immersion and exercise in the heat Compared to 6 consecutive days of exercise heat acclimation, post-exercise hot water immersion initiates larger heat acclimation adaptations in young, healthy adults. Pryor 9 Short term heat acclimation reduces heat strain during a first, but not second, consecutive work session in the heat Five days of heat acclimation reduced cardiovascular strain but not thermoregulatory strain during the second, same-day exercise-heat exposure.…”

“…Collectively, these data indicate that the type of cooling strategy used by an athlete in the heat should be selected based on whether the goal is to improve performance or attenuate physiological heat strain. Finally, McIntyre et al 3 report a study showing that hot water (40°C) bathing for up to 40 min after a 40-min treadmill run in temperate (19°C) conditions elicits larger physiological thermal adaptations, evidenced by a lower resting rectal temperature, lower sweating onset threshold temperature, and lower end-exercise core temperature, relative to a traditional short-term exercise heat acclimation protocol. Such findings are enormously useful for athletes or workers seeking to attain maximal heat adaptation responses without access to a climatic chamber or a hot ambient environment prior to exposure.…”