Heat stress is one of many physical agents to which thousands of workers are under constant exposure. Oftentimes it is necessary to work above the WBGT-based heat stress exposure limits. It is therefore important to consider alternative measures that include an exposure time limit to manage the heat stress. Predicted Heat Strain (PHS) (ISO7933) is one of those alternatives. PHS uses both personal factors like height and weight and job factors of environment, metabolic rate and clothing. The purpose of this project is to determine whether the PHS is an adequate method to predict short term exposure limits. The project’s data were taken from a prior experimental study where twelve participants were exposed to five different heat stress levels while over three different clothing ensembles. A total of 15 combinations of clothing and environment were tried. The PHS process was adapted to an Excel function using Visual Basic for Applications (VBA) (called fPHSTre). fPHSTre predicted a rectal temperature (Tre) at the exposure limit using both personal and job factors and then using standard values for personal factors. Based on analysis of variance, the fPHSTre adequately accounted for clothing, specifically evaporative resistance, using either fixed or individual data for predicted Tre on the experimental trials. In general, the PHS model could be used to reliably assess time limiting safe exposures in occupational settings for workers in hot environments.

Performance during prolonged exercise capacity diminishes with increasing temperatures. The onset of fatigue under these conditions is not adequately explained by peripheral mechanisms. Recently, drugs which inhibit the reuptake of dopamine and noradrenaline in the brain have been found to improve exercise performance in warm conditions. The aim of this thesis was to further explore and characterise the role of these neurotransmitters during prolonged exercise in warm conditions by manipulating their reuptake or synthesis. The first series of experiments were designed to further investigate the effects of bupropion, a dopamine and noradrenaline reuptake inhibitor, which has been found to improve performance in warm conditions. To explore gender differences in response to acute bupropion administration, the effects of bupropion on prolonged exercise performance in warm conditions in women was investigated in Chapter 3. The results of this study suggest that during the follicular phase of the menstrual cycle, acute administration of bupropion improves exercise performance. To determine whether there are any dose-dependent effects of bupropion, the experiment in Chapter 4 was designed to test three different doses of bupropion. Exercise performance was only improved for the maximal dose, suggesting a threshold for the performance effects of bupropion. Catecholamine precursors do not appear to improve exercise performance as consistently as reuptake inhibitors. In agreement with previous studies, the dopamine precursor L-DOPA did not affect exercise performance in warm conditions in Chapter 5. In Chapter 6 the effect of the atypical antidepressant nutritional supplement S-adenosylmethionine was investigated for its role in the synthesis of dopamine and noradrenaline. S-adenosylmethionine appeared to negatively influence cognitive function, increased skin temperature and circulating prolactin concentrations, but no effects on exercise performance were observed.

Workers are exposed to stressful thermal work environments in multiple industries every day. Methods for assessing heat stress often struggle to balance productivity without compromising the health of the workers. The Thermal Work Limit (TWL) is a method that has been adopted in areas outside of the United States as a viable method for heat assessment that combines health with productivity. TWL recommends a maximum metabolic rate for a given set of environmental conditions, clothing ensemble and acclimatization state. The purpose of this paper was to evaluate the validity of the TWL against a set of heat stress data known to be at the maximum sustainable level. A range of conditions were combined through environmental (20%, 50% and 70% relative humidity), clothing (woven clothing, WC; particle barrier coveralls, PB; water barrier coveralls, WB; and vapor barrier coveralls, VB), and workload factors (metabolic rates at low, L; moderate, M; and high, H) at the transition from sustainable to unsustainable exposure to ensure that the TWL method is thoroughly explored. Data from previous heat stress studies were used to compare the difference in predicted TWL with a calculated value. An analysis of variance (ANOVA) demonstrated that there were significant effects of the TWL due to clothing, metabolic rate level and relative humidity level. TWL provided similar results for WC, PB and WB, but had systematically lower values for VB. This suggested a more protective recommendation with high evaporative resistance. As the metabolic rate increased, the recommended limiting TWL also went up out of proportion to the metabolic rate, which provided greater protection at increasing metabolic rates. Under drier conditions (20% relative humidity), the TWL was systematically lower than for 50% and 70% relative humidity. While there were significant differences due to the main effects, the TWL was designed to be used without defined limits on environmental conditions, metabolic rate or clothing. Therefore, all of the conditions represented a comprehensive test of the TWL. Overall, the TWL was less protective than the current methods used by ACGIH Threshold Limit Values (TLV) and National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limit (REL). At the threshold, the TWL had a 7% probability of being unsustainable compared to the threshold probability of 1% for the TLV and REL.

Heat stress is a common physical agent associated with many occupations. The most commonly used method of assessing heat stress exposure is an empirical method using the Wet Bulb Globe Temperature Index but his method is limited in its ability to parse out individual contributors to the heat stress. The International Organization for Standardization (ISO) published a rational model called Predicted Heat Strain (PHS) in 2004, and rational methods have the advantage of separating out the individual pathways for heat exchange. The objective of this research was a performance assessment of the current PHS model. This experimental design consisted of 15 trials (3 clothing ensembles and 5 heat stress levels) involving 12 men and women. The clothing ensembles were work clothes, NexGen® (microporous) coveralls, and Tychem® QC (vaporbarrier) coveralls. The heat stress levels were 1.0 , 2.0 , 3.5 , 5.5 and 9.0 °CWBGT above the average critical environment for each ensemble determined in prior studies. The metabolic rate was 190 W/m2. The two outcomes of each trial were an exposure time when core temperature reached 38 °C (ET38) and a Safe Exposure Time (SET) defined as the amount of time required to reach either a core temperature (Tre) = 38.5 ºC, a heart rate of 85% age-estimated maximum, or fatigue. ix Trial data for environment, metabolic rate and clothing were inputs to the (PHS) model to determine a predicted amount of time for the participants to reach a Tre = 38 ºC, which was the limiting condition in PHS for acute exposures. The first consideration was predictive validity for which PHS-Time was compared to ET38. The expectation would be that PHS-Time would predict the mean ET response. Results for predictive validity indicated a moderate agreement between ET38 and PHS-Time (r2 of 0.34 and Intraclass Correlation Coefficient at 0.33). When the method for accounting for clothing was changed to that recommended by ISO, the PHS predicted times moved systematically toward a shorter exposure time and modest agreement (r2 of 0.39 and Intraclass Correlation Coefficient at 0.31). Protective validity was the ability of the PHSTime to predict an exposure time that would be safe for most people. In this case, PHS-Time was compared to SET. The PHS was protective for 73% of the cases. When it was modified to account for clothing following the ISO method, the protective outcomes were 98%. In addition, the PHS model examined with respect to starting core temperature and fixed height and weight. Using the actual core temperature improved the outcomes somewhat, but changing from 36.8 to 37.0 would be sufficient. There is a strong tendency to over-predict PHS-Time for individuals with a low body surface area, usually short and lower than average weight.

Excessive exposure to heat stress can cause a host of heat-related illnesses. For laborers, job specific work demands and protective garments greatly increase the risk of succumbing to the effects of heat stress. Microclimate cooling has been used to control heat stress exposure where administrative or engineering controls are not adequate. This study tested the performance of four personal cooling vests for use with insulated protective clothing (gas extraction coveralls) in warm-humid (35 ° C, 50% relative humidity) and hot-dry (40°C, 30% relative humidity) conditions. On 10 separate occasions, 5 male volunteers walked on a treadmill to elicit a target metabolic rate of 300 watts, for 120 minutes, while wearing a (a) water cooled vest, (b) air cooled vest, (c) frozen polymer vest (FP) (d) liquid CO2 cooling (LCO2) vest, or (e) no cooling (NC). A three-way mixed effects ANOVA was used to assess the results and a Tukey's Honestly Significant Difference multiple comparison test was used to identify where significant differences occurred ( < 0.05). The air, water, and FP systems produced significantly lower heat storage rates compared to NC. To the extent that the gas extraction coverall is worn in an environment between 30°C and 45°C and the rate of work is moderate, the FP, air and water vest were shown to manage heat storage well, reducing storage rate by about 48%, 56% and 65% respectively.

Caffeine consistently enhances endurance performance in temperate environmental conditions, while far less research has examined its ergogenic and physiological effects during prolonged exercise in the heat. Despite the performance benefit of an acute caffeine doses being less pronounced in regular caffeine users versus those not habituated to the drug, few studies have examined the influence of a prolonged period of controlled caffeine intake on endurance performance. The endogenous trace amine octopamine is purported to possess stimulant-like properties and influence fat metabolism, although no study has examined these effects in humans. The aim of this thesis was to further characterise the performance and physiological effects of caffeine during prolonged exercise, while elucidating a potential ergogenic role for octopamine. The first two studies investigated the ergogenic and thermoregulatory effects of low to moderate caffeine doses during prolonged cycle exercise in the heat. Chapter 4 demonstrated that 3 mg kg-1 caffeine, administered either as a single or split-dose (2 x 1.5 mg kg-1) before exercise, improved endurance performance without influencing thermoregulation during prolonged exercise at a fixed work-rate. Dividing the caffeine bolus appeared to confer an additional performance benefit, suggesting repeated low dose may potentiate the efficiency of the same total caffeine dose under these conditions. Chapter 5 demonstrated that a 6 mg kg-1 caffeine dose improved endurance cycle performance without differentially influencing thermoregulation than placebo. The level of habituation to caffeine influences the ergogenic effect of an acute dose, yet previous studies have employed sub-chronic supplementation protocols. Chapter 6 investigated the effect of a twenty-eight day supplementation period on endurance cycle performance. Habituation to caffeine attenuated the ergogenic effect of an acute caffeine dose, without any change in circulating caffeine, substrate oxidation or hormonal concentrations. In chapter 7 the performance and metabolic effects of octopamine was investigated. Octopamine supplementation did not influence performance, hormonal concentrations or substrate oxidation, likely due to low serum concentrations of the drug.

This study was conducted to explore the feasibility of using sleeves, along with both physiological and perceptual measurements, to assess the thermal comfort and heat strain in chemical protective clothing wear tests. The effect of body sites, i.e. the arms and the chest, and the effect of a Tyvek® coverall on skin temperature and sweat amount were investigated as well. Ten male subjects were selected from college students. Each subject was assigned to wear a garment of either T-shirt and pants or a changeable left-sleeved Tyvek® coverall with or without an experimental sleeve on the left arm. Three experimental sleeves were of the same style, but made of three different fabrics: Tyvek®, Pro/Shield I®, and Pro/Shield II®. Each subject wore an assigned garment and pedaled on a cycle ergometer in an environmentally-controlled room. Skin temperature, sweat amount, and subjective comfort evaluations were recorded and later analyzed by repeated-measures ANOVA. Results showed there is a feasibility in using a sleeve with a T-shirt/pants standard garment for assessing thermal comfort in chemical protective garment wear tests. The T-shirt/pants standard garment yielded more consistent and reliable results than did the Tyvek®-coverall standard garment. Under the same conditions, there were no significant differences in skin temperature and sweat amount among the left and right upper arms and the chest. The Tyvek® sleeve in this study impeded the transfer of heat by the greatest amount, followed by the Pro/Shield II® sleeve and the Pro/Shield I® sleeve, respectively. / Master of Science

Heat stress is a recognized occupational hazard present in many work environments. Its effects increase with increasing environmental heat loads. There is good evidence that exertional heat illness is associated with ambient thermal conditions in outdoor environments. Further, there is reason to believe that risk of acute injury may also increase with the ambient environment. For these reasons, the assessment of heat stress, which can be done through the characterization of the wet bulb globe temperature (WBGT), is designed to limit exposures to those that could be sustained for an 8-h day. The ACGIH Threshold Limit Value (TLV) for heat stress was based on limited data from Lind in the 1960s. Because there are practical limitations of using thermal indices, measurement of physiological parameters, such as body temperature and heart rate are used with environmental indices or as their alternative. The illness and injury records from the Deepwater Horizon cleanup effort provided an opportunity to examine the effects of ambient thermal conditions on exertional heat illness and acute injury, and also the cumulative effect of the previous day’s environmental conditions. The ability of the current WBGT-based occupational exposure limits to discriminate unsustainable heat exposures, and the proposal of alternative occupational limits was performed on data from two progressive heat stress protocol trials performed at USF. The USF studies also provided the opportunity to explore physiological strain indicators (rectal temperature, heart rate, skin temperature and the Physiological Strain Index) to determine the threshold between unsustainable and sustainable heat exposures. Analysis were performed using Poisson models, conditional logistic regressions, logistic regressions, and receiver operator curves (ROC curves). It was found that the odds to present an acute event, either exertional heat illness or acute injuries increased significantly with rising environmental conditions above 20 °C (RR 1.40 and RR 1.06, respectively). There was evidence of the cumulative effect from the prior day’s temperature and increased risk of exertional heat illness (RRs from 1.0–10.4). Regarding the accuracy of the current TLV, the results of the present investigation showed that this occupational exposure limit is extremely sensitive to predict cases associated with unsustainable heat exposures, its area under the curve (AUC) was 0.85; however its specificity was very low (specificity=0.05), with a huge percentage of false positives (95%). The suggested alternative models improved the specificity of the occupational exposure limits (specificities from 0.36 to 0.50), maintaining large AUCs (between 0.84 and 0.89). Nevertheless, any decision in trading sensitivity for specificity must be taken with extreme caution because of the steeped increment risk of heat related illness associated with small increments in environmental heat found also in the present study. Physiologic heat strain indices were found as accurate predictors for unsustainable heat stress exposures (AUCs from 0.74 to 0.89), especially when measurements of heart rate and skin temperature are combined (AUC=0.89 with a specificity of 0.56 at a sensitivity=0.95). Their implementation in industrial settings seems to be practical to prevent unsustainable heat stress conditions.

Heat stress is influenced by environmental conditions, workload and clothing. A critical environment is the upper limit of compensable heat stress for a given metabolic rate and clothing ensemble. The physiological strains associated with heat stress are core and skin temperatures, heart rate and physiological strain index (PSI). Because heat dissipation mechanisms may differ between men and women, there may be gender differences in the critical environment and the associated physiological variables. Gender differences were explored between acclimated men (n = 20) and women (n = 9) at the upper limit of compensable heat stress. Participants walked on a motorized treadmill at a target metabolic rate of 160W/m2 while wearing five different clothing ensembles (cotton work clothes, cotton coveralls, and three coveralls of particle barrier, liquid barrier, and vapor barrier properties). The starting air temperature (Tdb) was 34°C and humidity was held constant at 50%. Once thermal equilibrium was achieved, Tdb was increased 1°C every five minutes until loss of thermal equilibrium or termination criteria were met. Upon initial analysis, several gender differences were found. A significant difference (p = 0.035) was found for WBGTcrit, where values were 32.5°C for men and 33.1°C for women. Women had higher average heart rates (hr = 125 and 112 bpm), average skin temperatures (Tsk =36.4 and 36.2°C), and psi values (4.5 and 3.8) than men. No significant difference was found between genders for core temperature (tre) (p = 0.147). The target metabolic rate of 160W/m2 was not achieved and there were significant differences (p <0.0001) between men (172 W/m2) and women (152 W/m2). The effects of metabolic rate on WBGTcrit was examined and it was discovered that the difference in WGBTcrit could be explained by the difference in metabolic rate. The same logic was applied to the physiological responses and confirmed a difference between genders for Tre, HR, and PSI. The differences for Tsk disappeared. These findings indicate that women experienced a greater cardiovascular strain at the critical conditon and also greater heat strain than men at the same heat load.

De multiples événements sportifs majeurs vont se dérouler prochainement dans des environnements chauds voire tropicaux, justifiant l’intérêt scientifique actuel pour les questions associées à l’effet de la chaleur sur la performance en endurance. Il est admis que l’effort est subjectivement perçu comme plus difficile, en parallèle de la dégradation de la performance, à mesure que la température ambiante s’élève au cours d’une épreuve prolongée. Contrecarrer les effets délétères de la chaleur passe notamment par une exposition répétée dans des conditions écologiques ou simulées d’exercice (i.e. stratégie d’acclimatation). Toutefois, le décalage actuel entre les recommandations d’application issues d’études scientifiques et les conditions réelles de préparation et d’organisation des athlètes de haut niveau est à l’origine d’un faible engouement actuel pour de telles méthodes. Par conséquent, ce travail de thèse a eu pour ambition de répondre à certaines inconnues relatives à l’application de contenus d’entraînement classiques en ambiance chaude, en particulier (i) leur conséquence sur la capacité de performance de l’athlète et (ii) les moyens d’optimiser le contrôle et la régulation de la charge d’entraînement. Dans ce contexte, nous avons proposé à des sujets entraînés de soutenir la production de seuils de RPE (i.e. exercice à RPE fixe) dans un cadre expérimental puis d’entraînement. Au-delà de la validité de ce modèle, nous soutenons que la régulation volontaire de l’intensité dépend, au-delà de la perception de l’effort, de l’état émotionnel et de la motivation de l’individu. Lors d’une première étude, nous avons recherché à comparer les performances de solutions techniques pour le suivi de la température centrale en conditions écologiques (i.e. température gastro-intestinale vs. température frontale par capteur à annulation de flux). Les résultats obtenus ont validé l’usage du capteur à annulation de flux comme alternative pendant l’exercice, en dépit de l’absence de corrélation avec les mesures gastro-intestinales. Ceci suggère d’éventuelles perspectives en matière de contrôle de la température corporelle pendant l’exercice. La seconde étude s’est intéressée aux possibles conséquences de la répétition de sessions d’entraînement exigeantes et des contraintes logistiques d’un stage d’acclimatation (i.e. accumulation de fatigue mentale) sur la perception de l’effort et la performance. En dépit de l’absence d’effets combinés de la tâche cognitive préexercice et de la chaleur ambiante, les résultats tendent à démontrer le rôle-clé de la température cutanée et de la sensation de chaleur sous-jacente dans la régulation de la puissance soutenue à RPE-15 (chaud vs. tempéré : -0,022 vs. -0,008 W.kg-1.min-1). Enfin, la troisième étude suggère un potentiel intérêt de l’application de hautes intensités autorégulées, associée à une diminution du volume d’entraînement (-23%), lors d’une période d’acclimatation de courte durée (i.e. 5 jours). Le moindre effet observé, en comparaison d’un protocole à intensité fixe, sur la performance au cours d’un exercice de contre-la-montre (i.e. expérimental vs. fixe : 1,4 vs. 2,8 %) soulève toutefois l’importance du rapport volume-intensité dans la construction d’un protocole d’acclimatation. De manière générale, l’ensemble des résultats de cette thèse offrent des perspectives pour une individualisation et une adaptation spécifique à l’activité sportive des protocoles d’acclimatation à la chaleur. / The increasing number of major sport events that will take place in hot and/or tropical environments justify the current scientific interest in the effects of heat strain on endurance performance. During a prolonged self-paced exercise, it is well known that the subjectively perceived effort is higher as the ambient temperature increases and the performance level decreases. A repeated exposure to the heat in ecological and/or simulated exercise conditions may counteract the subsequent deleterious effects. However, the discrepancy between guidelines from scientific research and training priorities of well-trained athletes causes a lack of interest in these methods. Therefore, we aimed through the current thesis work to improve the current knowledge about heat acclimation strategies, more precisely about i) its effects on athlete’s performance capacity and ii) the optimization of training load monitoring and building. To shed light on these issues, we proposed to our trained and/or well-trained subjects to cycle at a fixed RPE first in an experimental framework, and then during a training program. We submit that the self-regulation of fixed-RPE exercise work rate depends not only of perceived exertion but also on emotional and motivational parameters. The first study aimed to compare the performance of technical devices for core temperature monitoring in ecological conditions (i.e. gastrointestinal point vs. forehead point from a zero-heat-flux sensor). Results showed that zero-heat-flux measurements might be considered as relevant during exercise. In this way, some opportunities may be considered for the monitoring of body temperature during field-based exercise. The second study investigated the subsequent effects of repeated strenuous training sessions and logistical constraints during a heat camp (i.e. higher mental fatigue) on perceived exertion and endurance performance. Despite the lack of combined effects from the prior cognitive task and the ambient condition during exercise, skin temperature and underlying heat sensation impact the linear decrease of power output at RPE-15 (hot vs. neutral: -0,022 vs. -0,008 W.kg-1.min-1). Lastly, the third study suggested an interest for self-regulated high intensities, associated with a decrease of the total exposure duration (-23%), during a short-term heat acclimation protocol (i.e. 5 days). However, the slighter effect on the improvement of performance in comparison with a classic constant-power training program, (HIT vs. constant-power: 1,4 vs. 2,8 %) provides scope for the building of training load (i.e. volume vs. intensity) in this context. In summary, the overall results of this thesis work open some perspectives for individualizing or adapting heat acclimation strategies to sport-specific conditions.