Respiratory responses to intermittent and prolonged exercise in a hot-dry environment

Ventilatory gas exchange ratio (R), V?O₂, ventilation (V?_E), respiration rate (RR), rectal temperature (T_re), and heart rate (HR) were determined for four acclimatized subjects during intermittent and prolonged exercise on a treadmill at 24° and 45°C (dry) as follows: 1) 8 cycles (10 min. exercise and 5 min. rest), and 2) prolonged exercise lasting for 90 min. While during intermittent and prolonged exercise, V?O₂ and V?_E did not differ in the heat, RR, T_re, HR and the respiratory dead space were higher in the hot ambient environment. After steady-state attainment, exercise R was higher in the initial as compared to the last cycles with higher values in neutral as compared to the hot ambient condition. It was concluded that heat was more effective than time in lowering the R, probably with a greater dependence on fat oxidation in the latter exercise cycles which seemed to be more pronounced in the heat.     

Thermoregulatory responses during exercise and a hot water immersion and the affective responses to peripheral thermal stimuli

Tympanic (Tty), mean skin (¯Tsk) and mean body (¯Tb) temperatures and heart rate (HR) increased more in low Vo₂ max group (LG) than in high Vo₂ max group (HG) during exercise. The regression coefficient of body temperatures (Tty and ¯Tb) on HR and the increased rate of heat storage were larger in LG than in HG during exercise. The local sweat rate (per min/cm²) during a hot water bath exhibited a considerable large quantity in comparison with the amount during exercise. Internal and skin temperatures during a hot water bath increased more immediately than those during exercise. The levels of comfort sensation during the preovulatory phase in women and pre-exercise period in men were higher at 40C than at 20C as peripheral thermal stimulus. The levels during the postovulatory and post-exercise phases in the same subjects were higher with the cool stimuli than with the warm stimuli. Above results suggest that thermoregulatory responses during submaximal exercise are different according to physical fitness and that these responses are different from those during hot water immersion. In addition, these suggest that the scores of thermal sensation with warm and cool stimuli are different during the pre- and post-ovulatory phases and the pre- and post-exercise periods.     

Heat storage in horses during submaximal exercise before and after humid heat acclimation.

The effect of humid heat acclimation on thermoregulatory responses to humid and dry exercise-heat stress was studied in six exercise-trained Thoroughbred horses. Horses were heat acclimated by performing moderate-intensity exercise for 21 days in heat and humidity (HH) [34.2-35.7 degrees C; 84-86% relative humidity (RH); wet bulb globe temperature (WBGT) index approximately 32 degrees C]. Horses completed exercise tests at 50% of peak O(2) uptake until a pulmonary arterial temperature (T(pa)) of 41.5 degrees C was attained in cool dry (CD) (20-21.5 degrees C; 45-50% RH; WBGT approximately 16 degrees C), hot dry (HD 0) [32-34 degrees C room temperature (RT); 45-55% RH; WBGT approximately 25 degrees C], and HH conditions (HH 0), and during the second hour of HH on days 3, 7, 14, and 21, and in HD on the 18th day (HD 18) of heat acclimation. The ratios of required evaporative capacity to maximal evaporative capacity of the environment (E(req)/E(max)) for CD, HD, and HH were approximately 1.2, 1.6, and 2.5, respectively. Preexercise T(pa) and rectal temperature were approximately 0.5 degrees C lower (P < 0. 05) on days 7, 14, and 21 compared with day 0. With exercise in HH, there was no effect of heat acclimation on the rate of rise in T(pa) (and therefore exercise duration) nor the rate of heat storage. In contrast, exercise duration was longer, rate of rise in T(pa) was significantly slower, and rate of heat storage was decreased on HD 18 compared with HD 0. It was concluded that, during uncompensable heat stress in horses, heat acclimation provided modest heat strain advantages when E(req)/E(max) was approximately 1.6, but at higher E(req)/E(max) no advantages were observed.     

Physiological adaptations to thermal stress in tropical Asians

Young sedentary adult males of Malay, Indian, and Chinese origin who had established continuous residence in tropical Malaysia and presumed to be naturally acclimatized to heat, were studied to evaluate their physiological responses to a standard heat stress test. The Malay and Indian races have evolved in hot and humid geographical zones, whereas the Chinese originated from a temperate area. Subjects exercised at 50% VO2max alternating 18 minutes walking and 2 min rest during a 2-h exposure to an ambient of 34.9 degrees C dry bulb and 32.1 degrees C wet bulb. Heart rates, core and skin temperatures, sweat rates, and oxygen uptakes were measured during the heat exposure. The subjects of Malay origin exhibited the least circulatory stress of the three ethnic groups. The data obtained on these long-term residents of a hot-wet climate and who were considered acclimatized to this environment were compared to experimental data obtained by other investigators and other ethnic groups.     

Influence of heat stress and acclimation on maximal aerobic power

Thirteen male volunteers performed cycle ergometer maximal oxygen uptake (VO2max tests) in moderate (21 degrees C, 30% rh) and hot (49 degrees C, 20% rh) environments, before and after a 9-day heat acclimation program. This program resulted in significantly decreased (P less than 0.01) final heart rate (24 bt X min-1) and rectal temperature (0.4 degrees C) from the first to last day of acclimation. The VO2max was lower (P less than 0.01) in the hot environment relative to the moderate environment both before (8%) and after (7%) acclimation with no significant difference (P greater than 0.05) shown for maximal power output (PO max, watts) between environments either before or after acclimation. The VO2max was higher (P less than 0.01) by 4% after acclimation in both environments. Also, PO max was higher (P less than 0.05) after acclimation in both the moderate (4%) and hot (2%) environments. The reduction in VO2max in the hot compared to moderate environment was not related to the difference in core temperature at VO2max between moderate and hot trials, nor was it strongly related with aerobic fitness level. These findings indicate that heat stress, per se, reduced the VO2max. Further, the reduction in VO2max due to heat was not affect be state of heat acclimation, the degree of elevation in core temperature, or level of aerobic fitness.     

Comparison of techniques for the measurement of skin temperature during exercise in a hot,humid environment

Exercising or working in a hot, humid environment can results in the onset of heat-related illness when an individual''s temperature is not carefully monitored. The purpose of the present study was to compare three techniques (data loggers, thermal imaging, and wired electrodes) for the measurement of peripheral (bicep) and central (abdominal) skin temperature. Young men and women (N = 30) were recruited to complete the present study. The three skin temperature measurements were made at 0 and every 10-min during 40-min (60% VO₂max) of cycling in a hot (39±2°C), humid (45±5% RH) environment. Data was statistically analyzed using the Bland-Altman method and correlation analysis. For abdominal skin temperature, the Bland-Altman limits of agreement indicated that data loggers (1.5) were a better index of wired than was thermal imaging (3.5), For the bicep skin temperature the limits of agreement was similar between data loggers (1.9) and thermal (1.9), suggesting the both were suitable measurements. We also found that when skin temperature exceeded 35°C, we observed progressively better prediction between data loggers, thermal imaging, and wired skin sensors. This report describes the potential for the use of data loggers and thermal imaging to be used as alternative measures of skin temperature in exercising, human subjects.     

Thermoregulation during marathon running in cool, moderate, and hot environments.

A well-trained subject, age 38, ran continously for periods ranging from 60 to 165 min on a motor-driven treadmill at 255.7 m/min while confronted with an airflow equivalent to running speed in cool, moderate, and hot environments. After a period of intensive heat acclimatization, treadmill runs were repeated in the moderate and hot conditions. Measurements were also obtained outdoors in a competitive marathon race. Sweat rate (SR) and mean skin temperature (Ts) were linearly related to Tdb. Acclimatization did not alter VO2max or metabolic rate during the treadmill runs, but heart rat (HR),rectal temperature (Tre), and Ts were lower, SR was higher, and maximal run duration longer in the hot environment, postacclimatization. Maximum runs in the hot environment were terminated by a spiralling increase in Tre to hyperthermic levels, due largely to a marked reduction in cutaneous blood flow, probably reflecting cardiovascular overload from the combined muscular and thermoregulatory blood flow demands, coupled with the effects of progressive dehydration. Utilizing partitional calorimetry and the subject's metabolic heat production, two examples of limiting environmental conditions for his marathon running speed were given.     

The Canadian Home Fitness Test as a predictor of aerobic capacity

The purpose of this study was to determine if maximum oxygen consumption (Vo₂ max) could be predicted from independent variables measured during the administration of the Canadian Home Fitness Test. Fifty-nine subjects between the ages of 15 and 74 years underwent the fitness test and a progressive exercise treadmill test for the direct determination of volitional Vo₂ max. The results indicated that Vo₂ max could be adequately predicted by the following regression equation: Vo₂ max (ml/kg·min) = 42.5 + 16.6(Vo₂) - 0.12(W) - 0.12(H) - 0.24(A), where Vo₂ is the average oxygen cost of the last completed exercise stage (in l/min), W is the body weight (in kg), H is the postexercise heart rate (in beats/min) and A is the age (in years).     

Effect of primary hypohydration on physical work capacity

Physical work capacity (PWC₁₈₀) was assessed with different levels of hypohydration in 25 heat-acclimatized male volunteers in hot dry (45°C DB, 30% RH) and hot humid (39°C DB, 60% RH) conditions equated to a heat stress level of 34°C on the WBGT scale. Heat acclimatization was carried out by exposing the subjects for 8 consecutive days in a climatic chamber with moderate work for two 50 min work cycles and 10 min intervening rest pauses. Acclimatization resulted in significant decreases in heart rate (27 bpm), oral temperature (0.8°C), mean skin temperature (1.2°C) and a significant increase in sweating rate (120 g h^–1 m^–2). Day-to-day variations in body hypohydration levels during heat acclimatization were not significantly different, although water intake was found to increase significantly from day 3 onwards when the subjects were in ad lib water intake state. The heat acclimatized subjects were then hypohydrated to varying degrees, viz. 1%, 2% and 3% body weight deficit, with moderate work in heat in the climatic chamber and after successful recovery from the effects of thermal stress and exercise; their physical work capacity was assessed individually. Physical work capacity was found to decrease significantly with hypohydration as compared to controls. The decrease was of the order of 9%, 11% and 22% in the hot dry condition and 6%, 8% and 20% in the hot humid condition with hypohydration levels of 1%, 2% and 3% respectively. The decrease was more pronounced during 3% hypohydration level under both heat stress conditions. This decrease was in spite of significant increases in maximal ventilation. However, the PWC₁₈₀ under the two heat stress conditions, when compared, did not reveal any significant difference. It was concluded that the heat stress vehicle did not adversely affect the physical work capacity. On the other hand, the decreases in physical work capacity were found to be closely related to the primary hypohydration level in heat-acclimatized tropical subjects.Abbreviations WBGT wet bulb globe temperature - bam beats per minute - YSI Yellow Springs Instrument - EKG electrocardiogram     

Influence des conditions climatiques saisonnières sur quelques paramètres physiologiques dès boucs Créoles alimentés avec de l'ensilage de banane

Response of three groups of 12 male creole goats (weighing about 10 kg) to environmental variations was tested in Guadeloupe (French West Indies) respectively at three times in the year: end of humid season (October–November), dry season (February–March) and beginning of humid season (July–August). Voluntary free intake of banana silage (silage of mixed green banana, bagassa, wheat bran and urea complemented with molasse) was not significantly affected by climatic variations. Three physiological parameters: rectal temperature, respiratory frequency and cardiac frequency were measured. These parameters were correlated with heat production dependent factors such as metabolic body weight, body weight gain and voluntary free intake. Rectal temperature increased all through the day until sunset and then decreased during the night. Both minimal rectal temperature and daily increase of rectal temperature were correlated with ambient temperature. Cardiac frequency increased during feeding. Generally cardiac frequency seemed to be correlated with activity of animals and so with behavioural response to environmental variations. Respiratory frequency was the most sensitive index of goat response to climate. The daily increase of respiratory frequency was important at the end of the humid season but was not observed in dry season. This increase was dependent on ambient temperature increase but also on air humidity characteristics and air velocity. These points are discussed according to integration of those physiological parameters in thermoregulation.     

Some physiological and behavioural responses inBos indicus andBos taurus heifers acclimatized to the hot humid seasonal equatorial climate

Some physiological and behavioural responses were observed in German Brown, Holstein-Friesian and White Fulani heifers during four different quarters of the year in an attempt to assess their adaptability to the seasonal equatorial climate of Southern Nigeria. Rectal temperatures under shade conditions showed slight but significant (P<0.05) fluctuations. Holstein heifers experienced hyperthermia and panted in the sun, unlike the Brown and Fulani cattle. Breathing rate showed a highly significant (P<0.01) seasonal variation. The heifers were generally most comfortable during the wettest periods of the year. The Holstein and Fulani showed the highest and lowest responses respectively. The Fulani did not seek shade; the Holstein sought shade more frequently than the Brown heifers. The Brown are more adaptable than the Holstein to the hot humid environment in which the Fulani cattle are relatively comfortable.     

Effects of evaporative cooling on reproductive performance and milk production of dairy cows in hot wet conditions

Fourteen animals of second and third lactation of Thai Friesian crossbred cows (87.5% Friesian × 12.5% Bos indicus) located at Sakol Nakhon Research and Breeding Centre, Department of Livestock Development, Ministry of Agriculture and Cooperatives, were divided randomly into two groups of seven each to evaluate the effects of evaporative cooling on reproductive and physiological traits under hot, humid conditions. Results indicated that installation of evaporating cooling in the open shed gave a further improvement in ameliorating heat stress in dairy cows in hot-wet environments by utilising the low humidity conditions that naturally occur during the day. The cows housed in an evaporatively cooled environment had both a rectal temperature and respiration rate (39.09°C, 61.39 breaths/min, respectively) significantly lower than that of the non-cooled cows (41.21°C; 86.87 breaths/min). The former group also had higher milk yield and more efficient reproductive performance (pregnancy rate and reduced days open) than the latter group. It is suggested that the non-evaporatively cooled cows did not gain benefit from the naturally lower heat stress during night time.     

Experimental and numerical study of physiological responses in hot environments

This paper proposed a multi-node human thermal model to predict human thermal responses in hot environments. The model was extended based on the Tanabe's work by considering the effects of high temperature on heat production, blood flow rate, and heat exchange coefficients. Five healthy men dressed in shorts were exposed in thermal neutral (29 °C) and high temperature (45 °C) environments. The rectal temperatures and skin temperatures of seven human body segments were continuously measured during the experiment. Validation of this model was conducted with experimental data. The results showed that the current model could accurately predict the skin and core temperatures in terms of the tendency and absolute values. In the human body segments expect calf and trunk, the temperature differences between the experimental data and the predicted results in high temperature environment were smaller than those in the thermally neutral environment conditions. The extended model was proved to be capable of predicting accurately human physiological responses in hot environments.     

The relative influence of physical fitness, acclimatization state, anthropometric measures and gender on individual reactions to heat stress

An experiment was set up to quantify the relative influence of fitness, acclimatization, gender and anthropometric measures on physiological responses to heat stress. For this purpose, 12 male and 12 female subjects were exposed to a neutral [ambient temperature (Ta) 21 degrees C, relative humidity (r.h. 50%)], a warm, humid (Ta 34 degrees C, r.h. 80%) and a hot, dry (Ta 45 degrees C, r.h. 20%) climate at rest and at two exercise intensities [25%, and 45% maximal O2 intake (VO2max)], seated seminude in a net chair behind a cycle ergometer. Their physiological responses were recorded and the data submitted to a multiple regression analysis. It was shown that for the variance in heat storage, the percentage of body fat and the surface to mass ratio had relatively the largest influence of all the individual parameters, followed by VO2max and the sweat rate versus increase in core temperature (total r2 = 92%). For the skin temperature variation, the relative influence of individual parameters (sweat gain, VO2max) was small. For body core temperatures, individual parameters had a large influence. The largest effect was due to the percentage of fat and the surface to mass ratio, followed by the sweating setpoint and, finally, VO2max (total r2 = 54%-70%). For the variance in heart rate the VO2max was the most relevant parameter, followed by the setpoint of the sweat rate:rectal temperature relationship (total r2 = 88%). Blood pressure and skin blood flow predictions were also shown to improve by the addition of individual characteristics to the model.(ABSTRACT TRUNCATED AT 250 WORDS)     

Correlation between heat tolerance during exercise and maximum aerobic work capacity

Observation of the physiological responses during exercise in a hot environment and measurement of maximal work capacity were made on eight young male subjects, ages 20--22. Exercise was performed on a bicycle ergometer at a constant work load of 450 kg . m/min at a cycling rate of 50 rpm for 30 min in a climatic chamber at 30 degree C with 70% relative humidity. The maximum work capacity was measured by bicycle ergometer exercise. Heat tolerance during exercise was assessed by the magnitude of physiological strain expressed by the combination of relative rise in rectal temperature, relative water loss and relative salt loss. Heat load during exercise was calculated using metabolic rates at rest and during exercise, assuming heat loss through the respiratory tract to be 10 percent of metabolic rate. Fairly good correlations were found between the ratio of work done to maximum work capacity and rise in rectal temperature, ratio of body weight loss to body weight and heat tolerance during exercise. Close correlations were found among relative heat load during exercise and rise in rectal temperature, relative body weight loss and heat tolerance. Heat tolerance during exercise in a hot environment correlated well to capacity of heat dissipation and maximum work capacity.     

Elevated temperature is more effective than elevated [CO2] in exposing genotypic variation in Telopea speciosissima growth plasticity: implications for woody plant populations under climate change

Intraspecific variation in phenotypic plasticity is a critical determinant of plant species capacity to cope with climate change. A long‐standing hypothesis states that greater levels of environmental variability will select for genotypes with greater phenotypic plasticity. However, few studies have examined how genotypes of woody species originating from contrasting environments respond to multiple climate change factors. Here, we investigated the main and interactive effects of elevated [CO₂] (C_E) and elevated temperature (T_E) on growth and physiology of Coastal (warmer, less variable temperature environment) and Upland (cooler, more variable temperature environment) genotypes of an Australian woody species Telopea speciosissima. Both genotypes were positively responsive to C_E (35% and 29% increase in whole‐plant dry mass and leaf area, respectively), but only the Coastal genotype exhibited positive growth responses to T_E. We found that the Coastal genotype exhibited greater growth response to T_E (47% and 85% increase in whole‐plant dry mass and leaf area, respectively) when compared with the Upland genotype (no change in dry mass or leaf area). No intraspecific variation in physiological plasticity was detected under C_E or T_E, and the interactive effects of C_E and T_E on intraspecific variation in phenotypic plasticity were also largely absent. Overall, T_E was a more effective climate factor than C_E in exposing genotypic variation in our woody species. Our results contradict the paradigm that genotypes from more variable climates will exhibit greater phenotypic plasticity in future climate regimes.     

Effect of age on heat-activated sweat gland density and flow during exercise in dry heat

Physiological responses of eight postmenopausal older women (age 52-62 yr) and eight younger women (age 20-30 yr) were compared during moderate intensity exercise in a hot dry environment (48 degrees C dry bulb, 25 degrees C wet bulb). The age groups were matched on the basis of maximal O2 consumption (VO2max), body surface area, and body fatness. After heat acclimation the women walked at 40% VO2max for up to 2 h in the hot dry environment while heart rate (HR), rectal temperature (Tre), mean skin temperature (Tsk), whole-body sweating rate (Msw), and local sweating rates (msw; forearm, chest, and scapula) were measured. Additionally, the density of heat-activated sweat glands (HASG) was determined and average sweat gland flow (SGF) was calculated for the scapular area. Although no differences between age groups were found in HR response (when analyzed as percent of maximal HR) or Tsk, the older women had a significantly higher Tre throughout the heat-exercise session. The greater heat storage of the older women may be explained by their significantly lower Msw and msw. There were no differences between the younger and older women in the density of HASG after 30 min; therefore, the lower msw reflects a diminished output per HASG rather than a decrease in the number of sweat glands recruited. The diminished thermoregulatory ability of the older women, unrelated to differences in VO2max, appears to reflect either 1) a diminished response of the sweat glands to central and/or peripheral stimuli, or 2) an age-related structural alteration in the eccrine glands or surrounding skin cells.     

Comparison of heat dissipation response between Malaysian and Japanese males during exercise in humid heat stress

This study investigated the differences in heat dissipation response to intense heat stress during exercise in hot and humid environments between tropical and temperate indigenes with matched physical characteristics. Ten Japanese (JP) and ten Malaysian (MY) males participated in this study. Subjects performed exercise for 60 min at 55% peak oxygen uptake in 32°C air with 70% relative humidity, followed by 30 min recovery. The increase in rectal temperature (T _re) was smaller in MY during exercise compared to JP. The local sweat rate and total body mass loss were similar in both groups. Both skin blood flow and mean skin temperature was lower in MY compared to JP. A significantly greater increase in hand skin temperature was observed in MY during exercise, which is attributable to heat loss due to the greater surface area to mass ratio and large number of arteriovenous anastomoses. Also, the smaller increase in T _re in MY may be explained by the presence of a significantly greater core–skin temperature gradient in MY than JP. The thermal gradient is also a major factor in increasing the convective heat transfer from core to skin as well as skin blood flow. It is concluded that the greater core–skin temperature gradient observed in MY is responsible for the smaller increase in T _re.     

How does Spix's yellow-toothed cavy (Galea spixii Wagler, 1831) face the thermal challenges of the Brazilian tropical dry forest?

The aim of this work was to investigate the thermal biology of the Spix's yellow-toothed cavy (Galea spixii) from the hot and dry environment of the Brazilian Caatinga by infrared thermography and biophysical equations. We monitored the rectal temperature, as well as the non-evaporative (radiative and convective pathways) and evaporative heat exchanges of males and females. The mean rectal temperature of females and males was 37.58 ± 0.02 and 37.47 ± 0.02 °C, respectively. We identified thermal windows by infrared thermography. The surface temperatures and the long-wave radiation heat exchanges were higher in the periocular, preocular, pinnae and vibrissae regions, in that order. The surface temperature of the periocular and preocular regions correlated positively with rectal temperature. Convective heat exchange was insignificant for thermoregulation by G. spixii. Evaporative heat loss increased when the thermal environment inhibited the radiative pathway. Females showed higher evaporative thermolysis than males at times of greater thermal challenge, suggesting a lower tolerance to heat stress. Therefore, infrared thermography identified the thermal windows, which represented the first line of defense against overheating in G. spixii. The periocular and preocular surface temperatures could be used as predictors of the thermal state of G. spixii.     

Aerobic power as a factor in women's response to work in hot environments.

Twelve young women, athletes (n = 6) and nonathletes (n = 6), walked on a treadmill at loads equivalent to approximately 30% Vo2 max for two 50-min periods in three environments: 1) 28 degrees C, 45% rh, 2) 35 degrees C, 65% rh, and 3) 48 degrees C, 10% rh. There were no differences between groups in rectal temperature, heart rate, evaporative heat loss, or mean skin temperature at 28 or 35 degrees C or during the first work period in the 48 degrees C environment. However, a significantly lower cardiac output (Q) and stroke volume (SV) observed for nonathletes by the 46th min of work at 48 degrees C may explain why no nonathletes were able to complete a 2nd h of work while four of six athletes successfully finished the period. It appears that in conditions of severe heat stress (48 degrees C) athletes were able to maintain a cardiac output sufficient to meet the metabolic requirements and the large increase in peripheral blood flow for a longer period of time than nonathletes.