Stephen S. Cheung

The thermophysiology of uncompensable heat stress: Physiological manipulations and individual characteristics

In many athletic and occupational settings, the wearing of protective clothing in warm or hot environments creates conditions of uncompensable heat stress where the body is unable to maintain a thermal steady state. Therefore, special precautions must be taken to minimise the threat of thermal injury. Assuming that manipulations known to reduce thermoregulatory strain during compensable heat stress would be equally effective in an uncompensable heat stress environment is not valid. In this review, we discuss the impact of hydration status, aerobic fitness, endurance training, heat acclimation, gender, menstrual cycle, oral contraceptive use, body composition and circadian rhythm on heat tolerance while wearing protective clothing in hot environments. The most effective countermeasure is ensuring that the individual is adequately hydrated both before and throughout the exercise or work session. In contrast, neither short term aerobic training or heat acclimation significantly improve exercise-heat tolerance during uncompensable heat stress. While short term aerobic training is relatively ineffective, long term improvements in physical fitness appear to provide some degree of protection. Individuals with higher proportions of body fat have a lower heat tolerance because of a reduced capacity to store heat. Women not using oral contraceptives are at a thermoregulatory disadvantage during the luteal phase of the menstrual cycle. The use of oral contraceptives eliminates any differences in heat tolerance throughout the menstrual cycle but tolerance is reduced during the quasi-follicular phase compared with non-users. Diurnal variations in resting core temperature do not appear to influence tolerance to uncompensable heat stress.

Multiple triggers for hyperthermic fatigue and exhaustion.

CHEUNG, S. S., and G. G. SLEIVERT. Multiple triggers for hyperthermic fatigue and exhaustion. Exerc. Sport Sci. Rev., Vol. 32, No. 3, pp. 100–106, 2004. It has been proposed that a critical body temperature exists at which muscle activation is impaired through a direct effect of high brain temperature decreasing the central drive to exercise, but other factors may also inhibit performance in the heat. An integrative physiological model is presented to stimulate research into mechanisms of hyperthermic fatigue and exhaustion.

Guidelines to Classify Subject Groups in Sport-Science Research

AIM To review current cycling-related sport-science literature to formulate guidelines to classify female subject groups and to compare this classification system for female subject groups with the classification system for male subject groups. METHODS A database of 82 papers that described female subject groups containing information on preexperimental maximal cycle-protocol designs, terminology, biometrical and physiological parameters, and cycling experience was analyzed. Subject groups were divided into performance levels (PLs), according to the nomenclature. Body mass, body-mass index, maximal oxygen consumption (VO2max), peak power output (PPO), and training status were compared between PLs and between female and male PLs. RESULTS Five female PLs were defined, representing untrained, active, trained, well-trained, and professional female subjects. VO2max and PPO significantly increased with PL, except for PL3 and PL4 (P < .01). For each PL, significant differences were observed in absolute and relative VO2max and PPO between male and female subject groups. Relative VO2max is the most cited parameter for female subject groups and is proposed as the principal parameter to classify the groups. CONCLUSION This systematic review shows the large variety in the description of female subject groups in the existing literature. The authors propose a standardized preexperimental testing protocol and guidelines to classify female subject groups into 5 PLs based on relative VO2max, relative PPO, training status, absolute VO2max, and absolute PPO.

Influence of hydration status and fluid replacement on heat tolerance while wearing NBC protective clothing.

Abstract The purpose of the present study was to investigate the influence of hypohydration and fluid replacement on tolerance to an uncompensable heat stress. Eight healthy young males completed a matrix of six trials in an environmental chamber, set at 40°C and 30% relative humidity, while wearing nuclear, biological, and chemical protective clothing. Subjects performed either light (3.5 km · h−1, 0% grade, no wind) or heavy (4.8 km · h−1, 4% grade, no wind) treadmill exercise combined with three hydration states [euhydration with fluid replacement (EU/F), euhydration without fluid replacement (EU/NF), and hypohydration with fluid replacement (H/F)]. Hypohydration of 2.2% body mass was achieved by exercise and fluid restriction on the day preceding the trials. No differences in the endpoint mean skin temperature (T¯sk), sweat rate, or rectal temperature (Tre) were observed among the hydration conditions for either work rate. During light exercise, the change in Tre (ΔTre) was significantly higher with H/F than EU/F after 40 min, and heart rate was greater after 25 min. The heart rate was greater during EU/NF than during EU/F after 60 min. Tolerance times were significantly greater for EU/F than for either EU/NF or H/F. With heavy exercise, no differences in ΔTre were observed across hydration conditions. Compared to EU/F, heart rates were higher after 10 and 30 min for H/F and EU/NF, respectively. Tolerance times were significantly less during H/F than with either of the EU conditions. Stroke volume was significantly decreased in H/F trials compared to EU/F trials for both light and heavy work rates, but no differences in cardiac output were observed. It was concluded that even minor levels of hypohydration significantly impaired exercise tolerance in a severely uncompensable heat stress environment at both light and heavy exercise intensities.

Design and Control Optimization of Microclimate Liquid Cooling Systems Underneath Protective Clothing

The use of protective clothing, whether in space suits, hazardous waste disposal, or sporting equipment, generally increases the risk of heat stress and hyperthermia by impairing the capacity for evaporative heat exchange from the body to the environment. To date the most efficient method of microclimate cooling underneath protective clothing has been via conductive heat exchange from circulating cooling fluid next to the skin. In order to make the use of liquid microclimate cooling systems (LQMCSs) as portable and practical as possible, the physiological and biomedical engineering design goals should be towards maximizing the efficiency of cooling to maintain thermal comfort/neutrality with the least cooling possible to minimize coolant and power requirements. Meeting these conditions is an extremely complex task that requires designing for a plethora of different factors. The optimal fitting of the LQMCSs, along with placement and design of tubing and control of cooling, appear to be key avenues towards maximizing efficiency of heat exchange. We review the history and major design constraints of LQMCSs, the basic principles of human thermoregulation underneath protective clothing, and explore potential areas of research into tubing/fabric technology, coolant distribution, and control optimization that may enhance the efficiency of LQMCSs.

The effect of cooling prior to and during exercise on exercise performance and capacity in the heat: a meta-analysis

Exercise is impaired in hot, compared with moderate, conditions. The development of hyperthermia is strongly linked to the impairment and as a result various strategies have been investigated to combat this condition. This meta-analysis focused on the most popular strategy: cooling. Precooling has received the most attention but recently cooling applied during the bout of exercise has been investigated and both were reviewed. We conducted a literature search and retrieved 28 articles which investigated the effect of cooling administered either prior to (n=23) or during (n=5) an exercise test in hot (wet bulb globe temperature >26°C) conditions. Mean and weighted effect size (Cohens d) were calculated. Overall, precooling has a moderate (d=0.73) effect on subsequent performance but the magnitude of the effect is dependent on the nature of the test. Sprint performance is impaired (d=−0.26) but intermittent performance and prolonged exercise are both improved following cooling (d=0.47 and d=1.91, respectively). Cooling during exercise has a positive effect on performance and capacity (d=0.76). Improvements were observed in studies with and without cooling-induced physiological alterations, and the literature supports the suggestion of a dose–response relationship among cooling, thermal strain and improvements in performance and capacity. In summary, precooling can improve subsequent intermittent and prolonged exercise performance and capacity in a hot environment but sprint performance is impaired. Cooling during exercise also has a positive effect on exercise performance and capacity in a hot environment.

Effect of body temperature on cold induced vasodilation

Cold-induced vasodilation (CIVD) is an acute increase in peripheral blood flow observed during cold exposures. It is hypothesized to protect against cold injuries, yet despite continuous research it remains an unexplained phenomenon. Contrary to the traditionally held view, we propose that CIVD is a thermoregulatory reflex mechanism contributing to heat loss. Ten adults (4 females; 23.8 ± 2.0 years) randomly underwent three 130-min exposures to −20°C incorporating a 10-min moderate exercise period at the 65th min, while wearing a liquid conditioning garment (LCG) and military arctic clothing. In the pre-warming condition, rectal temperature was increased by 0.5°C via the LCG before the cold exposure. In the warming condition, participants regulated the LCG throughout the cold exposure to subjective comfort. In the control condition, the LCG was worn but was not operated either before or during the cold exposure. Results demonstrated that the majority of CIVD occurred during the warming condition when the thermometrically-estimated mean body temperature (Tb) was at its highest. A thermoregulatory pattern was identified whereby CIVD occurred soon after Tb increased past a threshold (~36.65°C in warming and pre-warming; ~36.4°C in control). When CIVD occurred, Tb was reduced and CIVD ceased when Tb fell below the threshold. These findings were independent of extremity temperature since CIVD episodes occurred at a large range of finger temperatures (7.2–33.5°C). These observations were statistically confirmed by auto-regressive integrated moving average analysis (t = 9.602, P < 0.001). We conclude that CIVD is triggered by increased Tb supporting the hypothesis that CIVD is a thermoregulatory mechanism contributing to heat loss.

Hyperthermia and voluntary exhaustion: integrating models and future challenges

Over the past decade, research interest has risen on the direct effects of temperature on exercise capacity and tolerance, particular in the heat. Two major paradigms have been proposed for how hyperthermia may contribute to voluntary fatigue during exercise in the heat. One suggests that voluntary exhaustion occurs upon the approach or attainment of a critical internal temperature through impairment in a variety of physiological systems. An alternate perspective proposes that thermal inputs modulate the regulation of self-paced workload to minimize heat storage. This review seeks to summarize recent research leading to the development of these two models for hyperthermia and fatigue and explore possible bridges between them. Key areas for future research and development into voluntary exhaustion in the heat include (i) the development of valid and non-invasive means to measure brain temperature, (ii) understanding variability in perception and physiological responses to heat stress across individuals, (iii) extrapolating laboratory studies to field settings, (iv) understanding the failure in behavioural and physiological thermoregulation that leads to exertional heat illness, and (v) the integration of physiological and psychological parameters limiting voluntary exercise in the heat.

Effects of different types of acute and chronic (training) exercise on glycaemic control in type 1 diabetes mellitus: a meta-analysis

OBJECTIVE Exercise has been accepted and generally recommended for the management of type 1 diabetes mellitus (T1D) and for improving the overall quality of life in affected individuals. This meta-analysis was conducted to determine the overall effects of exercise (acute bouts of exercise and chronic exercise [or training]) on acute and chronic glycaemic control in patients with T1D, the effects of different types of exercise on glycaemic control and which conditions are required to obtain these positive effects. METHODS PubMed, ISI Web of Knowledge and SPORTDiscus™ were consulted to identify studies on T1D and exercise. Cohens d statistics were used for calculating mean effect sizes (ES) as follows: small d = 0.3, medium d = 0.5 and large d = 0.8. Ninety-five percent confidence intervals (95% CIs) were used to establish the significance of our findings. RESULTS From a total of 937 studies, 33 that met the inclusion criteria were selected. Nine studies were used to calculate the ES of a single bout of aerobic exercise; 13 studies to calculate the ES of aerobic training; 2 studies to calculate the ES of strength training; 4 studies to calculate the ES of combined (aerobic and strength) training and 6 studies to calculate the ES of high-intensity exercise (HIE) and training. ES for exercise on acute glycaemic control were large, while they were small for chronic glycaemic control. Aerobic exercise, resistance exercise, mixed exercise (aerobic combined with resistance training) and HIE acutely decreased blood glucose levels. To prevent late-onset hypoglycaemic episodes, the use of single bouts of sprints into an aerobic exercise can be recommended. This meta-analysis also showed that a regular exercise training programme has a significant effect on acute and chronic glycaemic control, although not all exercise forms showed significant results. Specifically, aerobic training is a favourable tool for decreasing chronic glycaemic control, while resistance training, mixed and HIE did not significantly improve chronic glycaemic control. Although, this meta-analysis showed there was a tendency for improvement in glycaemic control due to resistance training or resistance training combined with endurance training, there were not enough studies and/or subjects to confirm this statistically. CONCLUSIONS Based on this meta-analysis, we can conclude that the addition of brief bouts of high-intensity, sprint-type exercise to aerobic exercise can minimize the risk of sustaining a hypoglycaemic episode. We can also conclude that only regular aerobic training will improve the glycated haemoglobin level of a patient with T1D.

Human conscious response to thermal input is adjusted to changes in mean body temperature

Objective and Design: To detect the dependable criteria of behavioural thermoregulation through modelling temperature fluctuations of individuals allowed to freely manipulate inlet water temperature of a liquid conditioning garment (LCG) during 130 min of passive exposure to −20°C interspersed with a 10 min period of moderate exercise at the 65th minute using a double-blind experiment. Participants: Eleven volunteers (5 women; 23.40 (SD 2.09) years; BMI: 23.24 (SD 2.19)) who lacked previous experience with LCG and cold exposure experiments. Results: Despite variations in core and skin temperatures, thermal comfort, thermal sensation, and mean body temperature did not fluctuate significantly over time. Participants were able to find a desired level of LCG inlet temperature within 25 minutes which was maintained at similar levels until the 65th minute of the cold exposure. During exercise, LCG inlet water temperature decreased significantly. Regression models demonstrated that mean skin temperature and change in mean body temperature were significantly associated with thermal comfort and thermal sensation. Subsequent models revealed that, although all temperature variables were associated with LCG inlet water temperature, the coefficient of determination mainly depended on mean skin temperature and change in mean body temperature. The involvement of skin temperature was anticipated as the liquid conditioning garment was in contact with the skin. Conclusions: Humans generate conscious thermoregulatory responses in resting and exercise conditions during exposures to cold environments that are aimed towards maintaining a threshold mean body temperature, rather than temperature changes in individual body regions.