Author Identifier

Hui Cheng CHOO

https://orcid.org/0000-0002-4631-6751

Date of Award

2019

Document Type

Thesis - ECU Access Only

Publisher

Edith Cowan University

Degree Name

Doctor of Philosophy

School

School of Medical and Health Sciences

First Supervisor

Dr Chris Abbiss

Second Supervisor

Dr Jeremiah Peiffer

Abstract

Background

Heat acclimation and precooling are strategies commonly used to mitigate heat stress during exercise in the heat. Physiological adaptations during heat acclimation include lower core temperature (Tcore), lower heart rate (HR), and increased sweat output during exercise at a given workload. Adjusting the training intensity to meet specific energetic demands during heat acclimation may be as important for performance improvements. However, the challenge of maintaining the quality of training during heat exposure without concomitant accumulative fatigue remains. Precooling presents a potential strategy to maintain the quality of training during heat exposure but may have some inhibitory effects on thermoregulatory adaptations. Of the various precooling techniques, cold water immersion (CWI) and ingestion of ice slushy (ICE) are different in terms of application (external versus internal) and their ensued effects on the sweat response, skin temperature (Tsk) and Tcore. Therefore, the primary purpose of this thesis was to examine the effects of CWI and ICE on sudo- and vasomotor responses and adaptations to heat acclimation.

Aims

Study 1 aimed to determine the effects of ICE and CWI on the psychophysiological responses and endurance exercise performance through meta-analysis. Subsequently, two studies were conducted to: 1) examine the effects of CWI and ICE on the changes in rectal temperature (Tre) and Tsk, perceived thermal sensation and sweat responses during constant-paced exercise in the heat; and 2) determine the effects of altered physical thermal state (Tre and Tsk) and perceived thermal sensation following ICE and CWI on thermoregulatory behaviour (i.e., total work output and mean power output [MPO]) during cycling in the heat. Finally, the purpose of study 4 was to investigate the influence of regular precooling by CWI as part of a heat acclimation regimen on thermoregulatory adaptations and changes in exercise performance.

Methods

For study 1, 22 studies were included in the meta-analysis based on the following criteria: 1) cooling was performed with ICE or CWI before the exercise; 2) exercise longer than 6 min was performed in ambient temperature ≥ 26°C; and 3) crossover study design with a noncooling passive control condition. Weighted average effect sizes in Hedges’ g and 95% confidence intervals (CIs) were calculated from the mean difference and pooled SD. Studies 2 and 3 were randomised crossover studies with three conditions (ICE, CWI and no cooling control [CON]). The participants in studies 2 and 3 were recreationally active males aged between 19-40 y. Each trial was preceded by 30 min of CWI (22.0 ± 0.2°C), ICE (- 0.3 ± 0.4°C) or CON. In study 2, 11 men cycled at 40 or 50% of peak aerobic power for 60 min (33.2 ± 0.3°C, 45.9 ± 0.5% relative humidity [RH]). In study 3, 11 men cycled for 60 min at perceived exertion (RPE) equivalent to 15 (i.e., “hard”) (33.9 ± 0.2°C and 42.5 ± 3.9% RH). Local sweat rate was measured by capacitance hygrometry. Using near-infrared spectroscopy and laser Doppler flowmetry, changes in muscle blood volume and oxygenation and skin perfusion were examined. In study 4, 20 male recreational triathletes and cyclists (27-50 y) completed 10 sessions of 60-min cycling at RPE 15 within 14 days in the heat (35.3 ± 0.3°C, 53.4 ± 1.9 % RH), preceded by no cooling (CON, n = 10) or 30 min of CWI at 21.9 ± 0.5°C (PRECOOL, n = 10). Only 19 participants (n = 9 and 10 for CON and PRECOOL, respectively) completed heat stress tests before and after heat acclimation, which involved 25 min of cycling at 60% V̇O2peak and a 20- km time trial in the heat (35.3 ± 0.2°C, 53.8 ± 0.7% RH).

Results

The meta-analysis (study 1) revealed that CWI improved exercise performance (Hedges’ g [95CI] +0.53 [0.28; 0.77]) and resulted in greater increase (ΔEX) in Tsk (+4.15 [3.1; 5.21]) during the exercise. Additionally, lower peak Tcore (-0.93 [-1.18; -0.67]), whole body sweat loss (-0.74 [-1.18; -0.3]), and thermal sensation (-0.5 [-0.8; -0.19]) were observed without concomitant changes in ΔEX-Tcore (+0.19 [-0.22; 0.6]), peak Tsk (-0.67 [-1.52; 0.18]), peak HR (-0.14 [-0.38; 0.11]), and RPE (-0.14 [-0.39; 0.12]). ICE had no clear effect on exercise performance (+0.2 [-0.07; 0.46]) but resulted in greater ΔEX-Tcore (+1.02 [0.59; 1.45]) and ΔEX-Tsk (+0.34 [0.02; 0.67]) without concomitant changes in peak Tcore (-0.1 [-0.48; 0.28]), peak Tsk (+0.1 [-0.22; 0.41]), peak HR (+0.08 [-0.19; 0.35]), whole body sweat loss (-0.12 [- 0.42; 0.18]), thermal sensation (-0.2 [-0.49; 0.1]) and RPE (-0.01 [-0.33; 0.31]). In studies 2 and 3, ICE decreased Tre by ~0.3°C during precooling, compared with CON and CWI (p < 0.05). CWI decreased Tsk by ~4°C (p < 0.05) during precooling, compared with CON but did not have any significant cooling effect on Tre. In both studies, ICE decreased Tre- Tsk gradient during the first 5 min of exercise when compared with CON (p < 0.05), and CWI increased Tre-Tsk gradient during the initial 15-20 min of exercise when compared with CON and ICE (p < 0.05). In both studies, CWI (p < 0.001) and ICE (p = 0.019) delayed sweat recruitment by 1-5 min, compared with CON but did not significantly affect the body temperature threshold for sweating. In studies 2 and 3, muscle blood volume was decreased during CWI and during the initial 10-20 min of exercise when compared with CON and ICE (p < 0.05); however, there was no significant condition effect on muscle oxygenation. In study 2, mean HR during the exercise was decreased by ~5 bpm in CWI when compared with CON only (p = 0.025). In study 3, thermal sensation was lower in CWI for up to 35-40 min during the exercise, compared with CON and ICE (p < 0.05). Additionally, thermal sensation was lower in ICE than in CON during the first 20 min of exercise (p < 0.05). Thigh skin perfusion was decreased during CWI (p = 0.012) and ICE (p = 0.044), compared with CON but was not different between conditions during the exercise (p > 0.05, study 3). MPO was greater in CWI when compared with CON only (p = 0.024, CON: 130 ± 20 W, ICE: 128 ± 25 W, CWI: 138 ± 18 W, study 3). In study 4, changes in MPO (p = 0.024) and HR (p = 0.028) during heat acclimation were lower in CON (MPO: 97.4 ± 8.1%, ΔHR: -7 ± 4 bpm) than in PRECOOL (MPO: 102.9 ± 6.6%, ΔHR: -1 ± 7 bpm). HR during cycling at 60% V̇O2peak was decreased from the first heat stress test in both groups (p < 0.001). Tre, Tsk, sweat responses, Tre threshold and sensitivity for sweating, thigh skin perfusion, thermal sensation and RPE during the heat stress tests were not affected by heat acclimation in both groups (all p > 0.05). MPO (p = 0.016, Cohen’s effect size [d] = 0.93) and finish time (p = 0.013, d = 0.97) for the 20-km time trials were improved from the first heat stress test in PRECOOL but were not significantly changed in CON (MPO: p = 0.052 and d = 0.76, finish time: p = 0.140 and d = 0.54).

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