An investigation into environmental variables influencing post-race exertional heat illness in thoroughbred racehorses in temperate eastern Australia.

Exertional heat illness (EHI) is a syndrome that occurs when metabolic heat production from muscular contraction exceeds the rate at which it can be dissipated. Core body temperature rises to critical levels, causing hyperthermia and central nervous system dysfunction. Best practice for the prevention of EHI centres around early detection, rapid response and aggressive cooling. Advance planning enables risk mitigation measures. The more that is known about EHI in horses, the better prepared those in the positions of responsibility can be to anticipate events in which the risk of EHI may be elevated. This prospective, observational study investigated environmental risk factors associated with the occurrence of EHI. From 2014 to 2018, the number of horses exhibiting the symptoms of post-race EHI was recorded at 73 convenience sampled race meetings. Of the 4809 starters, the signs of EHI were recorded for 457. Thermal environmental data were measured and included ambient temperature, radiant heat, vapor pressure (humidity) and wind speed (WS). Mixed linear regression models were computed to assess the associations between the occurrence and incidence of post-race EHI and the four thermal environmental variables. The analysis showed that vapor pressure and WS had the largest effects on the occurrence of post-race EHI. The major limitations were that the race meetings selected were convenience sampled according to attendance by the first author and individual horse data were not available. EHI is influenced by a complex interaction of independently acting environmental variables, but warm, windless, humid days are most likely to result in the cases of EHI.

Autonomic and behavioural thermoregulation in tennis.

OBJECTIVES: This report describes physiological and behavioural mechanisms behind the control of body temperature and thermal comfort during competitive singles tennis. METHODS: Thermoregulatory responses and workload were observed during "best of three sets" tennis matches among 25 players. In total, 94 matches were played in ambient temperatures ranging from 14.5 to 38.4 degrees C. The thermal environment was assessed by dry bulb, wet bulb and natural wet bulb temperatures, globe temperature and wind speed. Core body and skin temperatures were recorded each minute throughout the match, and body mass and fluid intake were measured before the match, after 30 minutes of play and at the completion of the match to determine sweat rate. Subjective ratings of thermal strain included thermal comfort, sweatiness and perceived exertion. Workload observations included match, game and point durations, and the proportion of match time spent in play (effective playing time). RESULTS: Change in rectal temperature was positively correlated with point duration (p<0.001) and effective playing time (p<0.05). Sweat rate showed positive associations with air (p<0.0001), rectal (p<0.03) and skin (p<0.0001) temperature. Thermal comfort was reduced with increasing rectal (p<0.03) and skin (p<0.0001) temperature. Point duration and effective playing time were reduced when conditions were rated increasingly difficult (p<0.002 and p<0.0002, respectively). CONCLUSION: Autonomic (increase in sweat rate) and behavioural (reduction in workload) thermoregulation are responsible for the control of body temperature and thermal comfort during tennis.

Thermoregulatory responses during competitive singles tennis.

OBJECTIVES: To provide examples of thermoregulatory responses during competitive singles tennis and comparisons with continuous, steady-state running. METHODS: Typical examples of body core (rectal) temperature, skin temperature and heart rate were selected to show the differing characteristics of tennis and running, and the corresponding thermal environments. Rectal and skin temperatures were logged each minute and heart rate logged every 15 seconds throughout the competitive "best of three sets" singles tennis matches and 60-minute continuous, steady-state running trials. Tennis matches were completed outdoors in widely varying thermal environments, and the running trials were completed in the laboratory under stable conditions. RESULTS: Rectal temperature in tennis was raised only slightly above resting levels, reaching a plateau relative to the exercise intensity. Rectal temperature during tennis was found to take longer to reach a plateau than continuous, steady-state exercise. Skin temperature during tennis varied widely depending on environmental air temperature, and was lower than that of runners at the same air temperature. Heart rate was very similar between opponents for both average and response characteristics during tennis. A wider range and higher peak values were found for tennis players compared with runners. CONCLUSIONS: This report provides a descriptive account of thermoregulatory response characteristics during singles tennis. Differences between outdoor tennis and continuous, steady-state running in the laboratory for each of these responses were found.

Plasma Hsp72 is higher in runners with more serious symptoms of exertional heat illness.

Exertional heat illness is a potentially fatal disorder that primarily affects fit young men. Plasma Hsp72 may be important in the aetiology of this disorder, acting as a danger signal to the organism and leading to an inflammatory response. The aim of this study was to determine whether patients with exertional heat illness following a 14 km run show a difference in their plasma Hsp72 concentration compared with control subjects who completed the event without incident. Patients (n = 22) and controls (n = 7) were all male. The patients were subdivided into two groups, one of which exhibited more serious symptoms indicating neurological impairment such as confusion (n = 13) (CNS) while the other group exhibited mild symptoms (MILD) (n = 9). The CNS group had a higher rectal temperature (T(rec)) compared with the control group (41.0 +/- 0.3 vs. 39.8 +/- 0.2 degrees C, P < 0.05, mean +/- SE). Immediately after the run plasma Hsp72 was higher in the CNS group compared to controls and patients with mild symptoms (37.9, 17.0, and 20.9 ng/ml, respectively, P < 0.005). There was a correlation between plasma Hsp72 and T(rec) measured immediately after the race (r = 0.597, P < 0.001, n = 29). However, core temperature was not the only factor leading to increased plasma Hsp72 immediately post race. Plasma Hsp72 was still higher in CNS patients compared with the control group (P < 0.05) when T(rec )was included as a covariate. In conclusion, plasma Hsp72 was elevated immediately after a 14 km run with higher levels in patients with more serious symptoms of heat illness.

Plasma glucose levels after prolonged strenuous exercise correlate inversely with glycemic response to food consumed before exercise.

It was hypothesized that slowly digested carbohydrates, that is, low glycemic index (GI) foods, eaten before prolonged strenuous exercise would increase the blood glucose concentration toward the end of exercise. Six trained cyclists pedaled on a cycle ergometer at 65-70% VO2max 60 min after ingestion of each of four test meals: a low-GI and a high-GI powdered food and a low-GI and a high-GI breakfast cereal, all providing 1 g of available carbohydrate per kilogram of body mass. Plasma glucose levels after more that 90 min of exercise were found to correlate inversely with the observed GI of the foods (p < .01). Free fatty acid levels during the last hour of exercise also correlated inversely with the GI (p < .05). The findings suggest that the slow digestion of carbohydrate in the prevent food favors higher concentrations of fuels in the blood toward the end of exercise.

Effects of acclimatization to cold baths on men's responses to whole-body cooling in air.

The purpose of this study was to investigate the thermoregulatory mechanisms underlying artificial acclimatization to cold and to compare them with those of naturally acclimatized men. Six white men were cooled, nude, in air at 10 degrees C for 2 h before and after they had been acclimatized by ten daily cold (15 degrees C) baths of 30-60 min followed by rapid rewarming in hot (38-42 degrees C) water, and again 4 months later after acclimatization had decayed. Six control subjects also underwent the same tests, providing an opportunity to discriminate between changes caused by the immersions and those caused by extraneous influences. Acclimatization significantly reduced heat production and heat loss (P < 0.05) but did not change heat debt. The reduced heat production was accompanied by reductions in shivering (P < 0.10) and in cold-induced muscle tenseness; no evidence of nonshivering thermogenesis or active brown fat was found. These findings are attributed to increased tissue insulation, mediated by an enhanced vascular response to cold that did not involve the cutaneous circulation and was probably located in skeletal muscle. Thermal sensation and discomfort did not change, although perceived strain tended to increase (P = 0.08). Acclimatization was accompanied by, but was unrelated to, slower cooling of the finger and toe. The main conclusions, and many specific findings, agree with those of two previous studies made by the same techniques in naturally acclimatized men wintering in Antarctica. Other significant findings included changes--in particular reduced thermoneutral rectal temperature and a delayed onset of shivering--that are commonly regarded as evidence of acclimatization but were in fact unrelated to it as they also occurred in the control group. They are attributed to extraneous influences, in particular the relaxation of heightened arousal ('first-time effects') found in the baseline tests.

Cardiovascular and metabolic responses to noradrenaline in men acclimatized to cold baths.

The purpose of this study was to see whether artificial acclimatization to cold would reduce the pressor response to noradrenaline (NA) as natural acclimatization has been shown to do, and whether it would induce nonshivering thermogenesis. Three white men were infused with NA at four dosage levels between 0.038 and 0.300 microgram.kg-1.min-1 (2-23 micrograms.min-1), before and after artificial acclimatization to cold and again 4 months later when acclimatization had decayed. Acclimatization was induced by ten daily cold (15 degrees C) baths of 30-60 min followed by rapid rewarming in hot (38-42 degrees C) water, and was confirmed by tests of the subjects' responses to whole-body cooling in air. Three control subjects also underwent the first and third tests. Acclimatization substantially reduced the pressor response to NA at 0.150 and 0.300 micrograms.kg-1.min-1, confirming earlier findings by the same technique in naturally acclimatized men, and its decay increased this response to beyond its initial levels (P < 0.05 for both changes). Acclimatization did not change the response to NA of heart rate, subjective impressions, skin temperature of finger and toe, pulmonary ventilation, or plasma free fatty acids and ketone bodies. At no time did NA increase oxygen consumption, or increase skin temperature or heat flow over reported sites of brown fat. These findings would seem to show that acclimatization to cold reduces sensitivity to the pressor effect of NA but does not induce nonshivering thermogenesis, and that the reduced sensitivity is replaced by a hypersensitivity to NA when acclimatization decays.

Effects of fitness, fatness, and age on men's responses to whole body cooling in air.

Simple and multiple regression analyses were used to assess the influence of 12 white men's fitness (aerobic capacity 44-58 ml O2.min-1.kg fat-free mass-1), fatness (mean skin-fold thickness 5-20 mm, body fat content 15-36%), and age (26-52 yr) on their thermal, metabolic, cardiovascular, and subjective responses to 2 h of whole body cooling, nude, in air at 10 degrees C. Fitter men had slower heart rates, and fatter men had higher blood pressures. Fitness had no effect (P greater than 0.39) on any measured response to cold. Fatness was associated (P less than 0.01) with reduced heat loss, heat production, and mean skin temperature; unchanged heat debt; and increased tissue insulation. Age had the opposite effects. When the confounding effects of fatness were held constant by multiple regression, older men responded to cold as though they were 1 mm of skinfold thickness leaner for each 3-4 yr of age. We conclude that aging, even between the relatively youthful ages of 26 and 52 yr, is accompanied by a progressive weakening of the vasoconstrictor response to cold.

Carbohydrate feeding before exercise: effect of glycemic index.

Low glycemic index (GI) foods may confer an advantage when eaten before prolonged strenuous exercise by providing a slow-release source of glucose to the blood without an accompanying insulin surge. To test this hypothesis, eight trained cyclists pedalled to exhaustion one hour after ingestion of equal carbohydrate portions of four test meals: lentils, a low GI food (LGI); potato, a high GI food (HGI), and glucose and water. Plasma glucose and insulin levels were lower after LGI than after HGI from 30 to 60 min after ingestion (p less than 0.05). Plasma free fatty acid (FFA) levels were highest after water (p less than 0.05) followed by LGI and then glucose and HGI. From 45 to 60 min after ingestion, plasma lactate was higher in the HGI trial than in the LGI trial (p less than 0.05) and remained higher throughout the period of exercise. The rank order from lowest to highest for total carbohydrate oxidation during exercise was water, lentils, glucose and potato. Endurance time was 20 min longer after LGI than after HGI (p less than 0.05). These findings suggest that a low GI pre-game meal may prolong endurance during strenuous exercise by inducing less post-prandial hyperglycemia and hyperinsulinemia, lower levels of plasma lactate before and during exercise, and by maintaining plasma glucose and FFA at higher levels during critical periods of exercise.

Fire fighters' exposure to carbon monoxide during Australian bushfires.

Fatal entrapments of Australian bushfire fighters have led to suggestions that carbon monoxide (CO) poisoning could have contributed to these accidents by impairing the fire fighters' judgement. Carboxyhemoglobin saturation (COHb%) levels were assessed from alveolar CO levels in 24 fire fighters working with handtools and in 12 accompanying scientific observers, before and after fire fighting (duration 37-187 min) on 15 experimental bushfires. Carboxyhemoglobin levels increased on average by 0.7% per hour in the fire fighters and by 0.3% per hour in the observers. Nonsmoking fire fighters had lower COHb% after fires than the smokers had before fires. Estimates of environmental CO concentrations (including cigarette smoke) during the fires averaged 31 parts per million (ppm) for the smokers, 17 ppm for the nonsmoking crew members, and 11 ppm for the observers, none of whom smoked. The highest estimates of environmental CO arising solely from bushfire smoke were 40 to 50 ppm. Smokers were exposed to as much CO from their cigarettes as from bushfire smoke. Carboxyhemoglobin levels at the end of 8-hr fire fighting shifts, predicted from these levels of environmental CO, averaged about 5% (maximum 11%) in smokers and about 3% (maximum 7%) in nonsmokers. Acute levels of COHb% of this degree are not considered to have significant effects on health or performance. These results indicate that bushfire fighters are generally unlikely to experience hazardous levels of CO exposure.

The physical characteristics of the members during the International Biomedical Expedition to the Antarctic.

Twelve male medical scientists formed the International Biomedical Expedition to the Antarctic (IBEA). Their physical characteristics and maximum oxygen uptakes (VO2max) were measured in association with three series of thermal tolerance tests in Sydney, twice before and once after going to the Antarctic. In the Antarctic they lived in tents and spent 15 days travelling by motor toboggan. Their body mass (BM) and skinfold thickness (SFT) were measured four times during the 69 days the expedition spent in the field. The characteristics of the group were (ranges): age 26-52 years, height 1680-1889 mm, BM 58.5-103.4 kg, fatness 16-34% BM and VO2max 33-49 ml X kg-1 X min-1. In the Antarctic 9 men lost between 0.7 and 5.5 kg (mean 2.7 kg) of BM with a decrease in SFT, whilst 2 men increased BM by 1.2 and 1.9 kg without change in SFT. One man retired early from the expedition. BM and SFT were regained and physical fitness lost during the return voyage to Australia. Consequently there was no difference in average SFT between the pre- and post-Antarctic laboratory tests, but BM was greater after the Antarctic implying gains in fat free mass. VO2max was lower in the final laboratory tests than in the tests before Antarctica.

Nutrition and sports performance.

During the past 20 years there have been great developments in the scientific understanding of the role of nutrition in health and physical performance. Epidemiological and physiological studies have provided evidence that certain forms of dietary behaviour may be linked with an increased risk of developing disorders such as high blood pressure, coronary artery disease and some cancers. This has resulted in dietary recommendations that are intended to reduce the incidence of these disorders in the community. The science of nutrition in relation to sports performance has progressed from empirical studies investigating the effects of dietary manipulations, such as restriction and supplementation, to the direct investigation of the physiological basis of the specific nutritional demands of hard physical exercise. This review is based on the premise that it is "what comes out' rather than "what goes in', which provides the clues to ideal nutrition for athletic performance. Various aspects of the physical demands of athletic exercise are viewed as stresses that induce specific biochemical, and hence nutritional, strains in the athlete. Training is the predominant demand in the athletic lifestyle. This is characterised by acute bouts of high power output. During one hour of hard training an athlete may expend 30% of his or her total 24-hour energy output. These high power outputs have important implications for energy substrate and water requirements. Carbohydrate, specifically muscle glycogen, is an obligatory fuel for the high power outputs demanded by athletic sports. Muscle glycogen is a limiting factor in hard exercise because it is held in limited amounts, utilised rapidly by intense exercise, and fatigue occurs when it is depleted to low levels in the active muscles. Liver glycogen may also be exhausted by hard exercise and low blood glucose contributes to fatigue. High sweat rates are demanded during severe exercise and large water deficits commensurate with energy expenditure are incurred during extended periods of hard training and competition. Salt, potassium, and magnesium are lost in nutritionally significant amounts in the sweat, but vitamins and trace elements are not. Adaptive mechanisms protect athletes against electrolyte depletion. Iron loss in sweat may contribute to the iron deficiency seen in some endurance runners. Protein is degraded and amino acids are oxidised during physical exercise. Protein is also retained during muscle building training. Recent investigations indicate that the minimal protein requirements of athletes may be substantially higher than those for sedentary persons.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term effects of physical training programme on risk factors for coronary heart disease in otherwise sedentary men.

Three hundred and seventy sedentary men aged 20-65 years enrolled in a physical training programme after a medical and fitness examination that included measurements of "classical" risk factors for coronary heart disease. Five years later re-examination showed (a) that on average the subjects had not changed significantly in weight, blood pressure, serum lipid concentrations, smoking habits, and physical working capacity; (b) that men who had remained active and therefore had a higher degree of fitness did not differ in risk factors from men who had returned to sedentary habits; and (c) that men who had improved substantially in fitness did not differ in risk factors from men whose fitness had not changed or had declined. Five years after the initial programme one-third of the men were continuing with regular vigorous exercise. These results do not support the view that classical risk factors for coronary heart disease improve with increased physical activity and fitness.

Effects of beta blockade and atropinisation on plasma catecholamine concentration during exercise.

The changes in plasma catecholamine concentration (deltaC) following beta-blockade (practolol, 15 mg) and atropinisation (Atropine, 1.8 mg) have been studied on 5 healthy male subjects during exercise on a motor driven treadmill. The results showed that for a given V-2 and % VO2 max, beta blockade was without effect on delta C (except in one athletic subject, but atropine produced a rise in deltaC. In relation to Q, both drugs produced an increase in deltaC, but for a given cardiac frequency (fH) deltaC was higher with beta blockade, and lower with atropinisation than found in control experiments. The intra- and inter-subject variability of delta C in relation of fH was resolved by considering the change in cardiac frequency calculated from baseline value obtained during walking at 6.44 km/h on the level, and expressed as a percentage of the maximal fH attainable for given individuals under the different drug and control conditions (%deltafH). It was concluded that during short term exercise, the rise of deltaC in relation to %deltafH reflects both the myocardial sensitivity to vagal and beta blockade, and the circulatory vasoconstrictor control of blood vessels which is required to sustain increases in systemic and muscle blood flow.

Temperature regulation during severe exercise with some observations on effects of skin wetting.

The thermoregulatory responses to severe exercise of 1-h duration have been studied in 11 healthy subjects, and the effects of skin wetting have been examined in 6 of them. Measurements in the laboratory included metabolic (M) and total (H) heat production, rectal (Tre) and mean skin (Tsk) temperatures, evaporative sweat loss (E), and peripheral tissue conductance (K). In addition Tre and changes in body weight were measured in 18 conpetitors following a 20-km road race run at approximately 90% of their maximal oxygen intake (Vo2 max). The results showed that Tre was curvilinearly related to %Vo2 max. In contrast Tsk was independent of M, H, E, and %Vo2 max, and reached similar and constant values at the end of exercise in all subjects. E was appropriate for a given H, but K was lower in the sedentary subjects than in the athletes. Skin wetting had no effect on Tre but reduced E by 10%. It is suggested that the pattern of Tre rise in excercise reflected the nature of the controlling system and was not due to a failure of the heat-dissipating mechanisms.

Energy expenditure and physiological performance of Sudanese cane cutters.

The thermal and exercise tolerances of 165 Sudanese cane cutters were measured in the laboratory and related to work performance and productivity in the cane fields. The results showed that the amount of cane cut per minute in the field was significantly correlated with changes in body weight (r = +0-53) during the third hour of work, aerobic energy expenditure (r = + 0-43), and cardiac frequency during work. These variables in turn were associated with predicted maximal power output (VO2 max) measured in the laboratory. The average energy expenditure during cane cutting was 1-66 +/- 0-33 1/min-1 (34-9 kJ/min-1) which represents approximately 60% of the workers predicted VO2 max. This rate of energy expenditure was sustained in the cane fields for at least three hours without significant pauses for rest. The sweat losses measured in 32 cane cutters during the two and three hours of work averaged 637 +/- 221 and 770 +/- 282 g/h-1 respectively, while the mean urine temperature immediately on cessation of effort was 37-74 +/- 0-46 degrees C. Despite the additional environmental heat load of the tropics, it would seem that cane cutters performing a self-paced task demanding heavy physical effort, are able to sustain work levels well in excess of those recommended for most European factory workers without obvious signs of fatigue or heat stress.