This thesis examined why NCAA Division 1 American football “linemen” experience greater heat strain than “backs” during summer training camps. In study #1, exercise at a heat production of 350 W/m2 in a hot environment (Tdb:32.4±1.0ºC; Twb:26.3±0.6ºC) resulted in greater local sweating on the upper body (head, arm, shoulder and chest; all <0.05) and a greater core temperature (P=0.033) in linemen despite a ~25% lower heat production per unit mass (L:6.0±0.5 W/kg; B:8.2±0.8 W/kg). In study #2, greater convective and evaporative heat transfer coefficients (P<0.05) were found in backs during live summer training camp drills, but these did not lead to a greater dry heat transfer or evaporative capacity. However, the maximum metabolic rate per unit mass was lower in linemen due to differences surface area-to-mass ratio. In conclusion, the greater heat strain previously reported in linemen likely arises, in part, from differences in sweating efficiency and body morphology.

Three main factors that influence heat stress are clothing, work demands and environmental conditions. Gender may also influence the amount of heat stress an individual can tolerate. The purpose of this study was to examine the role of gender in heat stress limits (critical WBGT) and heat strain (heart rate and core temperature). The null hypothesis was that there was no gender difference among critical WBGT, heart rate and core temperature. Fifteen subjects (11 men and 4 women) wore five different clothing ensembles (cotton work clothes, cotton coveralls, particle barrier Tyvek, water-barrier/vapor permeable NexGen LS417, and vapor barrier Tychem QC made by Dupont) at three levels of metabolic rate (115, 175 and 250 W m-2). A treadmill was used to set the metabolic workload. A climatic chamber was used to control the environmental conditions. The participants continued to walk on the treadmill until their core temperature (Tre) reached a steady state. Then the air temperature and humidity were slowly increased. The point at which the core temperature increased steadily was defined as the inflection point. Environmental data as well as core temperature and heart rate were recorded at five minute intervals. The critical conditions were noted at five minutes before the inflection point. Metabolic rate, critical WBGT, core temperature and heart rate were analyzed by 3-way ANOVAs (participants nested by ensemble by metabolic rate) with all two way and three way interactions. Significant differences were observed between genders for metabolic rate and heart rate, but not for core temperature and critical WBGT across metabolic level and ensembles. While there were differences between genders in metabolic rate they did not affect the overall conclusions. The heart rate was significantly higher (12 bpm) for women than for men.

This study investigated the effects of progressive dehydration by 1.5-2.0% body mass (BM) (NF) on core temperature (Tc), heart rate (HR), on-ice performance, and mental fatigue during a 70-min scrimmage, compared to maintaining BM with a carbohydrate-electrolyte solution (CES). Compared to CES, Tc was significantly higher throughout the scrimmage in NF. Players in NF had reduced mean skating speed and time at high effort between 30-50 min of the scrimmage. Players in NF committed more puck turnovers and completed a lower percentage of passes in the last 20 min of play. Post-scrimmage shuttle skating time was higher in NF. Hockey fatigue questionnaire total score and Profile of Mood States fatigue score was higher in NF. The results indicate that mild dehydration compared to maintaining BM with a CES resulted in increased Tc, decreased skating and puck handling performance, and increased mental fatigue during an ice hockey scrimmage. / Gatorade Sports Science Institute, Natural Sciences and Engineering Research Council of Canada

This thesis examined the effect of differences in body surface area-to-lean body mass ratio (AD/LBM) on core temperature cooling rates during cold water immersion (2°C, CWI) and temperate water immersion (26°C, TWI) following exercise-induced hyperthermia (end-exercise rectal temperature of 40°C). Individuals with a High AD/LBM (315 cm2/kg) had a ~1.7-fold greater overall rectal cooling rate relative to those with Low AD/LBM (275 cm2/kg) during both CWI and TWI. Further, overall rectal cooling rates during CWI were ~2.7-fold greater than during TWI for both the High and Low AD/LBM groups. Study findings show that AD/LBM must be considered when determining the duration of the immersion period. However, CWI provides the most effective cooling treatment for EHS patients irrespective of physical differences between individuals.

This thesis examined why NCAA Division 1 American football “linemen” experience greater heat strain than “backs” during summer training camps. In study #1, exercise at a heat production of 350 W/m2 in a hot environment (Tdb:32.4±1.0ºC; Twb:26.3±0.6ºC) resulted in greater local sweating on the upper body (head, arm, shoulder and chest; all <0.05) and a greater core temperature (P=0.033) in linemen despite a ~25% lower heat production per unit mass (L:6.0±0.5 W/kg; B:8.2±0.8 W/kg). In study #2, greater convective and evaporative heat transfer coefficients (P<0.05) were found in backs during live summer training camp drills, but these did not lead to a greater dry heat transfer or evaporative capacity. However, the maximum metabolic rate per unit mass was lower in linemen due to differences surface area-to-mass ratio. In conclusion, the greater heat strain previously reported in linemen likely arises, in part, from differences in sweating efficiency and body morphology.

Four studies were performed to evaluate the independent influence of core temperature and heat acclimation on sweating responses when exercise is fixed for a given evaporative heat balance requirement (Ereq) during compensable and uncompensable heat stress. By using circadian rhythm to modulate absolute core temperature, study 1 investigated whether absolute core temperature altered the steady-state sweat rate during compensable heat stress at a fixed Ereq. Study 2 compared the influence of partial and complete heat acclimation on core temperature and sweating responses between a compensable and uncompensable heat stress condition. Study 3 quantified how maximum skin wettedness is altered with partial or complete heat acclimation. Study 4 determined whether aerobic fitness (i.e. maximum rate of oxygen consumption; VO2max) per se independently alters the sweating and core temperature responses to uncompensable heat stress or if the frequent bouts of exercise-induced heat stress that accompany aerobic training are required to augment thermoregulatory capacity. Study 1 demonstrated that when absolute core temperature is different between AM and PM by ~0.2°C, steady-state sweat rates were the same for a fixed Ereq. Only when a different level of Ereq was attained, were differences in steady-state sweating observed. Moreover, steady-state sweat rates were similar despite differences in skin and core temperature when exercise intensity was matched to elicit a fixed Ereq in two different ambient temperatures (23°C and 33°C). In study 2, neither partial nor complete heat acclimation altered the core temperature response to compensable heat stress despite a marginally greater sweat rate compared to an unacclimated state. However, the sudomotor adaptations associated with heat acclimation were evident during uncompensable heat stress and mitigated the rise in core temperature during 60 minutes of exercise compared to an unacclimated state. Study 3 determined that the biophysical parameter that defines the upper limit for evaporative heat loss, that is the maximum skin wettedness achievable, increased following partial (0.84±0.08) and complete heat acclimation (0.95±0.05) compared to unacclimated (0.72±0.06) which directly explains the reduced change in core temperature reported in study 2 during uncompensable heat stress. Lastly, study 4 demonstrated that VO2max per se does not alter the sudomotor responses to uncompensable heat stress. Rather, it is the repetitive exercise-induced heat stress experienced during aerobic training that induces a partial heat acclimation thereby mitigating the rise in core temperature during uncompensable heat stress. Taken together, when exercise is prescribed in a compensable environment, the steady-state sweat rate observed will be primarily determined by Ereq independent of absolute core temperature, while heat acclimation will slightly increase the sweat rate despite providing no additional reduction in the change in core temperature. However, progressive heat acclimation increases the upper limit of compensability via a greater maximum skin wettedness thereby mitigating the rise in core temperature during uncompensable heat stress.

Introduction: Assessment of heat strain is an alternative approach to assessing heat stress exposures. Two common measures of heat strain are body core temperature (TC) and heart rate (HR). In this study TC was assessed by rectal temperature (Tre). Physiological Strain Index (PSI) was developed to combine both Tre and HR into one metric. Data collected from progressive heat stress trials were used to (1) demonstrate that PSI can distinguish between Sustainable and Unsustainable heat stress; (2) suggest values for PSI that demonstrate a sustainable level of heat stress; and (3) determine if clothing or metabolic rate were effect modifiers. Methods: Two previous progressive heat stress studies included 494 trials with 988 pairs of Sustainable and Unsustainable exposures over a range of relative humidity (rh), metabolic rates (M) and clothing using 29 participants. To assess the discrimination ability of PSI, conditional logistic regression and logistic regression were used. The accuracy of PSI was assessed using Receiver Operating Characteristic curves (ROC). Results: The present study found that primary (Tre, HR, and Tsk) and derived (PSI and ΔTre-sk) HSMs can accurately predict Unsustainable heat stress exposures based on AUCs that ranged from 0.73 to 0.86. Skin temperature had the highest AUC (0.86) with PSI in the mid-range (0.79). The values of the HSMs associated with a predicted probability of 0.25 were considered as screening values (PSI < 2.6, ΔTre-sk > 1.9 °C, Tre < 37.5, HR < 109, and Tsk < 35.8). The value of using any one of these individual indicators is that they act as a screening tool to decide if an exposure assessment is needed. Metabolic rate was found to be a confounder for all the HSMs except for RTsk. It was not statistically significant for HSMs derived models (PSI and ΔTre-sk). And its effect modification was not significant in any model. Conclusions: Based on the ROC curve, PSI can accurately predict Unsustainable heat stress exposures (AUC 0.79). HR alone has a similar capacity to distinguish Unsustainable exposures (AUC 0.78) under relatively constant exposure (metabolic rate and environment) for an hour or so. Screening limits with high sensitivity, however, have low thresholds. This limits the utility of these heat strain metrics. To the extent that the observed strain is low, there is good evidence that the exposure is Sustainable.

This thesis examined why NCAA Division 1 American football “linemen” experience greater heat strain than “backs” during summer training camps. In study #1, exercise at a heat production of 350 W/m2 in a hot environment (Tdb:32.4±1.0ºC; Twb:26.3±0.6ºC) resulted in greater local sweating on the upper body (head, arm, shoulder and chest; all <0.05) and a greater core temperature (P=0.033) in linemen despite a ~25% lower heat production per unit mass (L:6.0±0.5 W/kg; B:8.2±0.8 W/kg). In study #2, greater convective and evaporative heat transfer coefficients (P<0.05) were found in backs during live summer training camp drills, but these did not lead to a greater dry heat transfer or evaporative capacity. However, the maximum metabolic rate per unit mass was lower in linemen due to differences surface area-to-mass ratio. In conclusion, the greater heat strain previously reported in linemen likely arises, in part, from differences in sweating efficiency and body morphology.

This thesis examined the effect of differences in body surface area-to-lean body mass ratio (AD/LBM) on core temperature cooling rates during cold water immersion (2°C, CWI) and temperate water immersion (26°C, TWI) following exercise-induced hyperthermia (end-exercise rectal temperature of 40°C). Individuals with a High AD/LBM (315 cm2/kg) had a ~1.7-fold greater overall rectal cooling rate relative to those with Low AD/LBM (275 cm2/kg) during both CWI and TWI. Further, overall rectal cooling rates during CWI were ~2.7-fold greater than during TWI for both the High and Low AD/LBM groups. Study findings show that AD/LBM must be considered when determining the duration of the immersion period. However, CWI provides the most effective cooling treatment for EHS patients irrespective of physical differences between individuals.

The aim of this study was to evaluate temperature data collected from Thoroughbred foals between birth and shortly after weaning. It provides a valuable survey with epidemiological conclusions providing insight into the temperature trends and pyretic occurrences of Thoroughbred foals during this age period. Temperature data were collected using telemetry from temperature sensitive microchips implanted into newborn foals. The system of inputting and storing temperature data was completely electronic and this study evaluated this system. It was found that this system was stable and allowed the evaluation of large amounts of frequently acquired data with little human intervention. The data obtained resulted in the valuable evaluation of age associated body temperature trends within the foals as well as providing an indication of the extent and epidemiology of pyrexia within the study cohort. The system of evaluating temperatures based both on the individual day value as well as on each individual foals prior series of temperatures shows that the use of these two criteria can be utilised simultaneously. The study provides basic information which future researchers using similar systems can use to objectively set criteria for pyrexia. An outbreak of equine encephalosis also occurred during the study period and this provided much needed prospective epidemiological information for such an outbreak, something which has not previously been documented. Copyright / Dissertation (MSc (Veterinary Science))--University of Pretoria, 2009. / Production Animal Studies / unrestricted