Physiological strain and countermeasures with firefighting.

Protective clothing is integral to the task of firefighting, but at the same time can increase physiological strain and impair work capacity. Encapsulation of the head and the high thermal resistance and/or low water vapor permeability of the clothing ensemble impede evaporative heat dissipation, thus elevating the rate of heat storage and creating a state of uncompensable heat stress (UHS). In addition, the additional weight from carrying a supplemental air supply and the greater respiratory work of breathing through a regulator can create a negative spiral of thermal hyperpnea from greater respiratory demands and metabolic heat production. The elevated respiratory demands also increase cardiac strain and potentially the risk for myocardial events. Tolerance time during UHS is determined by three factors: the core temperature at the beginning of the heat stress exposure, the core temperature that can be tolerated before exhaustion or collapse ensues, and the rate of increase in core temperature from the beginning to end of the heat stress exposure. Protective clothing is often employed in highly dynamic environments, making portability, longevity and integration with the task requirements and clothing critical design characteristics for countermeasures. To date, most countermeasures have been relatively indirect in nature, primarily with alterations in work scheduling along with physiological manipulations such as cooling manipulations during recovery periods. Advances are required in materials science to develop lighter and less restrictive protective equipment, concurrent with cooling strategies that target specific regions or which can be effectively implemented during exercise.

Influence of aerobic fitness and body fatness on tolerance to uncompensable heat stress.

This study examined the independent and combined importance of aerobic fitness and body fatness on physiological tolerance and exercise time during weight-bearing exercise while wearing a semipermeable protective ensemble. Twenty-four men and women were matched for aerobic fitness and body fatness in one of four groups (4 men and 2 women in each group). Aerobic fitness was expressed per kilogram of lean body mass (LBM) to eliminate the influence of body fatness on the expression of fitness. Subjects were defined as trained (T; regularly active with a peak aerobic power of 65 ml x kg LBM(-1) x min(-1)) or untrained (UT; sedentary with a peak aerobic power of 53 ml x kg LBM(-1) x min(-1)) with high (High; 20%) or low (Low; 11%) body fatness. Subjects exercised until exhaustion or until rectal temperature reached 39.5 degrees C or heart rate reached 95% of maximum. Exercise times were significantly greater in T(Low) (116 +/- 6.5 min) compared with their matched sedentary (UT(Low); 70 +/- 3.6 min) or fatness (T(High); 82 +/- 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness. However, similar effects were not evident between T(High) and UT(High) (74 +/- 4.1 min) or between the UT groups (UT(Low) and UT(High)). The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9 degrees C), whereas both body fatness and rate of heat storage affected the exercise time as independent factors.

Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress.

The purpose of the present study was to determine the separate and combined effects of aerobic fitness, short-term heat acclimation, and hypohydration on tolerance during light exercise while wearing nuclear, biological, and chemical protective clothing in the heat (40 degrees C, 30% relative humidity). Men who were moderately fit [(MF); <50 ml . kg-1 . min-1 maximal O2 consumption; n = 7] and highly fit [(HF); >55 ml . kg-1 . min-1 maximal O2 consumption; n = 8] were tested while they were euhydrated or hypohydrated by approximately 2.5% of body mass through exercise and fluid restriction the day preceding the trials. Tests were conducted before and after 2 wk of daily heat acclimation (1-h treadmill exercise at 40 degrees C, 30% relative humidity, while wearing the nuclear, biological, and chemical protective clothing). Heat acclimation increased sweat rate and decreased skin temperature and rectal temperature (Tre) in HF subjects but had no effect on tolerance time (TT). MF subjects increased sweat rate but did not alter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased Tre and heart rate and decreased the respiratory exchange ratio and the TT regardless of acclimation state. Overall, the rate of rise of skin temperature was less, while DeltaTre, the rate of rise of Tre, and the TT were greater in HF than in MF subjects. It was concluded that exercise-heat tolerance in this uncompensable heat-stress environment is not influenced by short-term heat acclimation but is significantly improved by long-term aerobic fitness.

Longitudinal study of ventilation threshold and maximal O2 uptake in athletic boys.

Ventilation threshold (VET) and peak O2 uptake (VO2max) were determined annually from ages 11 to 15 yr in 18 athletic boys. The treadmill protocol consisted of a constant-run speed with grade increments every second minute. Ventilation, VO2, and CO2 production were measured using online open-circuit spirometry. Coefficients of variation for determination of VO2max and VET were 3.4 and 5.6%, respectively. VO2max increased across age 11-15 yr, from 60.8 to 68.0 ml X kg-1 X min-1. VET at 11 yr was 34.4 and at 15 yr 41.9 ml X kg-1 X min-1, thus increasing from 56 to 62% of VO2max. Previous studies of children have shown a decline of VET relative to VO2max across age; however, in the present study the increase may have been due to the training of the boys in competitive athletics. However, the trained youth did not achieve the high relative threshold of trained adults. Across age, both VO2max and VET scaled to weight to the power 1 (in a log-log transformation). The increase in VO2max (l/min) showed greatest increments corresponding to gains in size (a growth curve), whereas increases of VET were consistent year to year. Thus VET was altered independently of VO2max. Factors other than size (and presumably muscle mass) such as the maturation of an enzymatic profile of fast glycolytic fibers might have an important influence on the threshold during youth.

Rate of formation of carboxyhemoglobin in exercising humans exposed to carbon monoxide.

The purpose of this study was to test the CFK equation for its prediction of the rate of formation of carboxyhemoglobin (HbCO) in exercising humans by use of measured values of the respiratory variables and to characterize the rate of appearance of HbCO with frequent blood sampling. Ten nonsmoking male subjects were exposed to carbon monoxide (CO) on two separate occasions distinguished by the level of activity. Steady-state exercise was conducted on a cycle ergometer at either a low (approximately 45 W) or moderate (approximately 90 W) power output. Each experiment began with an exposure of 3,000 ppm CO for 3 min during a rest period followed by three intermittent exposures ranging from 3,000 ppm CO for 1 min at low exercise to 667 ppm CO for 3 min at moderate exercise. Increases in HbCO were normalized against predicted values to account for individual differences in the variables that govern CO uptake. No difference in the normalized uptake of CO was found between the low- and moderate-exercise trials. However, the CFK equation underpredicted the increase in HbCO for the exposures at rest and the first exposure at exercise, whereas it overpredicted for the latter two exposures at exercise. The net increase in HbCO after all exposures (approximately 10% HbCO) deviated by less than 1% HbCO between the measured and predicted values. The rate of appearance of HbCO fits a sigmoidal shape with considerable overshoot at the end of exposure. This can be explained by delays in the delivery of CO to the blood sampling point (dorsal hand vein) and by a relatively small blood circulation time compared with other regions of the body. A simple circulation model is used to demonstrate the overshoot phenomenon.

Heat strain models applicable for protective clothing systems: comparison of core temperature response.

Core temperature (Tc) output comparisons were analyzed from thermal models applicable to persons wearing protective clothing. The two models evaluated were the United States (US) Army Research Institute of Environmental Medicine (USARIEM) heat strain experimental model and the United Kingdom (UK) Loughborough (LUT25) model. Data were derived from collaborative heat-acclimation studies conducted by three organizations and included an intermittent-work protocol (Canada) and a continuous-exercise/heat stress protocol (UK and US). Volunteers from the US and the UK were exposed to a standard exercise/heat stress protocol (ambient temperature 35 degrees C/50% relative humidity, wind speed 1 m/s, level treadmill speed 1.34 m/s). Canadian Forces volunteers did an intermittent-work protocol (15 min moderate work/15 min rest at ambient temperature of 40 degrees C/30% relative humidity, wind speed approximately 0.4 m/s). Each model reliably predicted Tc responses (within the margin of error determined by 1 root mean square deviation) during work in the heat with protective clothing. Models that are analytically similar to the classic Stolwijk-Hardy model serve as robust operational tools for prediction of physiological heat strain when modified to incorporate clothing heat-exchange factors.

Low doses of melatonin and diurnal effects on thermoregulation and tolerance to uncompensable heat stress.

This study examined whether the reported hypothermic effect of melatonin ingestion increased tolerance to exercise at 40 degrees C, for trials conducted either in the morning or afternoon, while subjects were wearing protective clothing. Nine men performed four randomly ordered trials; two each in the morning (0930) and afternoon (1330) after the double-blind ingestion of either two placebo capsules or two 1-mg capsules of melatonin. Despite significant elevations in plasma melatonin to over 1,000 ng/ml 1 h after the ingestion of the first 1-mg dose, rectal temperature (T(re)) was unchanged before or during the heat-stress exposure. Also, all other indexes of temperature regulation and the heart rate response during the uncompensable heat stress were unaffected by the ingestion of melatonin. Initial T(re) was increased during the afternoon (37.1 +/- 0.2 degrees C), compared with the morning (36.8 +/- 0.2 degrees C) exposures, and these differences remained throughout the uncompensable heat stress, such that final T(re) was also increased for the afternoon (39.2 +/- 0.2 degrees C) vs. the morning (39.0 +/- 0.3 degrees C) trials. Tolerance times and heat storage were not different among the exposures at approximately 110 min and 16 kJ/kg, respectively. It was concluded that this low dose of melatonin had no impact on tolerance to uncompensable heat stress and that trials conducted in the early afternoon were associated with an increased T(re) tolerated at exhaustion that offset the circadian influence on resting T(re) and thus maintained tolerance times similar to those of trials conducted in the morning.

Interactions of physical training and heat acclimation. The thermophysiology of exercising in a hot climate.

Physical training and heat acclimation are both commonly adopted tactics to improve performance and/or tolerance times when individuals must compete or work in the heat. Potential benefits include: (i) improved aerobic fitness and thus a greater cardiovascular reserve (probably seen mainly after training); (ii) a lower resting body temperature that allows greater heat storage (probably seen mainly after acclimation); (iii) a decreased energy cost of a given intensity of exercise (seen after acclimation and also as the learning component of training); (iv) an enhanced sweating response at a given percentage of maximal effort (probably developed by both treatments); (v) a slower increase in body temperature owing to (iii) and/or (iv) [seen after both treatments]; (vi) a reduced cardiovascular stress because of changes in the autonomic nervous system (probably realised mainly by training), expansion of blood volume (seen after both treatments) and/or a decreased peripheral pooling of blood (probably found after both treatments); and (vii) improved subjective tolerance reflecting a decrease in the relative intensity of a given activity (probably seen mainly after training), a reduction in the physiological strain (found after both treatments) and/or habituation to heat-exercise stress (probably developed by both treatments). Factors affecting improvements in physiological and psychological responses to a given set of conditions include: (i) the individual's initial fitness and acclimatisation to heat; (ii) age, gender, hydration, sleep deprivation, circadian rhythms and in women the menstrual cycle: (iii) use of ergogenic aids such as fluid ingestion, carbohydrate and/or electrolyte replacement and blood doping; (iv) event or test conditions such as the mode of exercise, the severity of environmental heat stress and the type of clothing worn; and (v) treatment conditions such as the intensity, duration and frequency of exercise and/or heat exposure, the length of any rest intervals and cumulative depletion of body water and minerals.

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

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

Metabolic and cardiorespiratory responses relative to the anaerobic threshold.

The present study has compared the metabolic and cardiorespiratory responses for two groups of male subjects during 20 min of exercise at the anaerobic threshold (AT), at AT + 1/3, and at AT + 2/3 of the difference (delta) between AT and VO2max. A log-log transformation of the lactate (LA)-power output relationship was used to define AT and divide subjects into a high (N = 7, AT = 51.9 +/- 1.5% VO2max) and low (N = 5; AT = 41.9 +/- 1.8% VO2max) AT group. No differences were observed between groups during exercise at AT for VE.VO2-(1), VE.VCO2(-1), pH, pCO2, blood LA, and plasma strong ions Na+, K+, and Cl-. Although blood LA values were significantly elevated for the low AT subjects (2.3 +/- 0.6 mmol.l-1) compared with the high AT group (1.0 +/- 0.1 mmol.l-1) during exercise at AT + 1/3 delta, no other differences between groups were noted. In contrast, marked differences were observed between groups during exercise at AT + 2/3 delta. The high AT group showed no change in VE (79.1 +/- 4.8 l.min-1), pH (7.367 +/- 0.01), pCO2 (37.3 +/- 1.2 mm Hg), and blood LA (2.9 +/- 0.3 mmol.l-1) during the final 10 min of the 20 min exercise test.(ABSTRACT TRUNCATED AT 250 WORDS)

Active recovery, endurance training, and the calculation of the individual anaerobic threshold.

The individual anaerobic threshold (IAT) is the highest metabolic rate at which blood lactate (LA) concentrations are maintained at a steady state during prolonged exercise. The purpose of this study was to compare the effects of active and passive recovery on the determination of the IAT before and after an endurance training program. Both before and after an 8-wk training program, nine subjects did two submaximal, incremental cycle exercise tests (30 W and 4 min per step) until LA was greater than or equal to 4 mmol.l-1. Blood was sampled repeatedly during exercise and for 12 min during the subsequent recovery period, which was passive for one test and active (approximately 35% VO2max) during the second test. An IAT metabolic rate and power output were calculated for the passive (IATp) and active (IATa) recovery protocols. On separate days, before and after training, five of the subjects exercised for 30 min at either the IATp or the IATa. Before training, IATa occurred at a higher (P less than 0.05) power output and absolute and relative VO2 compared to IATp. After training, VO2max and the power output and VO2 at IATa and IATp increased significantly; as a percent VO2max, IATp but not IATa increased. During the pretraining 30-min IAT rides, LA was higher during the IATa than the IATp test, but LA values did not change during the last 20 min of exercise. LA was similar for both 30-min IAT rides after training and did not change from 5 to 30 min of exercise. The LA steady-state concentrations ranged from 1.3 to 6.8 mmol.l-1.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative evaluation of the individual anaerobic threshold and the critical power.

The individual anaerobic threshold (IAT) is defined as the highest metabolic rate where blood lactate (La) concentrations are maintained at a steady-state during prolonged exercise. The asymptote of the hyperbolic relationship between power output and time to fatigue has been defined as the critical power (CP), which, in theory, represents the highest metabolic rate where a steady-state response can be achieved during prolonged exercise. Since IAT and CP may define the same power output, the purpose of this study was to compare the gas exchange, blood La, and acid-base responses during exercise at the metabolic rates defined as IAT and CP. Fourteen males performed a maximal incremental cycle exercise test that was followed by a light active recovery period to determine IAT. Subsequently, subjects exercised to fatigue at five power outputs (calculated to elicit from 90% to 110% VO2max) to determine CP. IAT occurred at a significantly lower power output and VO2 (235 +/- 44 W and 2.97 +/- 0.47 l.min-1, respectively) compared with CP (265 +/- 39 W and 3.35 +/- 0.41 l.min-1, respectively). During 30 min of exercise at IAT, blood La levels increased during the initial 10 min to 3.9 +/- 1.9 mmol.l-1 but did not change during the final 15 min. Blood pH decreased to 7.32 +/- 0.04 at 5 min and did not change thereafter, while PCO2 fell from 41.5 +/- 3.2 mm Hg at 5 min to 36.2 +/- 3.6 mm Hg at 30 min. Only one subject completed 30 min of exercise at CP.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma catecholamine and blood lactate responses to incremental arm and leg exercise.

PURPOSE AND METHODS: The present study was conducted to examine the pattern of plasma catecholamine and blood lactate responses to incremental arm and leg exercise. Seven untrained male subjects performed two incremental exercise tests on separate days in random order. One test consisted of 1-arm cranking (5W x 2 min(-1)), whereas the other exercise test was 2-leg cycling (20-25W x 2 min(-1)). Blood samples were obtained from the nonexercising arm during 1-arm cranking and from the same arm and vein during 2-leg cycling. Thresholds for blood lactate (T(La)), epinephrine (T(Epi)) and norepinephrine (T(NE)) were determined for each subject under both exercise conditions and defined as breakpoints when plotted as a function of power output. RESULTS: When the two modes of exercise were compared, T(La), T(Epi), and T(NE) were all significantly lower for 1-arm cranking than for 2-leg cycling (P < 0.01). During 1-arm cranking, T(La) (0.96 +/- 0.10 L x min(-1)), T(Epi) (1.02 +/- 0.07 L x min(-1)), and T(NE) (1.07 +/- 0.09 L x min(-1)) occurred simultaneously. During 2-leg cycling, T(La) (1.77 +/- 0.20 L x min(-1)), T(Epi) (1.74 +/- 0.17 L x min(-1)), and T(NE) (1.98 +/- 0.17 L x min(-1)) occurred at similar levels of VO2 and were not significantly different. The correlation observed between the VO2 measured at the T(La) and T(Epi) was 0.917 for arm and 0.929 for leg exercise (P < 0.001). The epinephrine concentration ([Epi]) obtained at the T(La) was not significantly different for arm (0.144 ng x mL(-1)) and leg (0.152 ng x mL(-1)) exercise. CONCLUSIONS: The breakpoint in plasma [Epi] shifted in an identical manner and occurred simultaneously with that of T(La) regardless of the mode of exercise (arm or leg). The Epi concentrations observed at the T(La) agree with those previously reported to produce a rise in blood lactate during Epi infusion at rest. These results support the hypothesis that a rise in plasma [Epi] may contribute to the breakpoint in blood lactate that occurs during incremental exercise.

A computational method for determination of the individual anaerobic threshold.

The individual anaerobic threshold (IAT) has received attention recently in the field of exercise physiology. The IAT is defined as the point during progressive exercise when lactate elimination from the blood is both maximal and equal to the diffusion from the working muscles. It has been theorized that an individual can maintain exercise for relatively long periods when working at the IAT. A common method for determination of the IAT is to perform a visual determination from plots of lactate concentration versus power output. This paper briefly describes the theoretical basis for determination of the IAT and presents an algorithm for its accurate calculation.

Heat strain while wearing the current Canadian or a new hot-weather French NBC protective clothing ensemble.

BACKGROUND: A new nuclear, biological and chemical (NBC) protective garment has been designed for use in hot environments. This NBC battle dress uniform (NBC-BDU) is intended to be worn over the skin or, at most, over underwear and an undershirt. It is unclear whether the documented reductions in heat strain associated with wearing this clothing configuration represent simply the removal of the combat clothing layer normally worn underneath a NBC protective overgarment or an improved heat transfer through the new NBC-BDU. HYPOTHESIS: It was hypothesized that the removal of the combat clothing layer would produce a significant reduction in heat strain. As a result, it was also hypothesized that there would be no difference in heat strain between this new NBC-BDU and the current Canadian protective overgarment when the remaining clothing was standardized. METHODS: There were 9 males who alternated 15 min of walking at 1.11 m.s-1 and 15 min of seated rest for a maximum of 4 h in a chamber set at 40 degrees C, 30% relative humidity and a wind speed less than 0.1 m.s-1 while wearing underwear, an undershirt and the current Canadian protective overgarment either with (C + C) or without (C-C) combat clothing underneath and the new NBC-BDU worn over underwear and an undershirt. RESULTS: All indices of heat strain which included tolerance time, sweat rates, sweat evaporation, Tre, Tsk, skin and garment vapor pressures, and heart rate indicated a significant improvement when the combat clothing was removed regardless of which NBC protective garment was worn. The new NBC-BDU was associated with a lower Tre after 2 h of exposure and lower skin and garment vapor pressures compared with the Canadian overgarment. Other indices of heat strain were not different between the NBC-BDU and C-C configurations. CONCLUSIONS: During light intermittent exercise when the rate of heat production is low, the removal of the combat clothing layer as part of the Canadian NBC protective ensemble is recommended to significantly reduce the heat strain.

Work performance at 40 degrees C with Canadian Forces biological and chemical protective clothing.

This study examined the effects of a hot environmental temperature (40 degrees C and 50% relative humidity) and metabolic rate on soldiers' tolerance time (TT) while wearing various levels of the Canadian Forces biological and chemical (BC) defence protective clothing. The subjects, 19 unacclimatized males, were assigned to exercise at either a light intermittent (LI) (N = 4), light continuous (LC) (N = 5), moderate continuous (MC) (N = 5) or heavy continuous (HC) (N = 5) metabolic rate. For groups LI and LC, exercise involved walking on a treadmill at 4.0 km.h-1 with a 0% grade and lifting 10 kg boxes. LI alternated between 15 min of exercise and 15 min of rest. Group MC walked at 4.8 km.h-1 with a 3% grade and lifted 15 kg boxes. Group HC walked at 4.8 km.h-1 with a 7.5% grade and lifted 20 kg. Subjects were tested wearing three levels of clothing protection: combat clothing (L); combat clothing and a semi-permeable BC overgarment (M); combat clothing and BC overgarment, gloves, boots and respirator (H). TT was the time until rectal temperature reached 39.3 degrees C, heart rate reached 95% maximum, dizziness or nausea precluded further exercise, or 5 h had elapsed. For group LI, TT was similar for L (137 +/- 15 min) and M (117 +/- 9 min) clothing. TT was significantly reduced for H (67 +/- 6 min) clothing. For group LC, TT was greater for L (91 +/- 11 min) compared with either M (68 +/- 7 min) or H (55 +/- 2 min) clothing.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of granisetron on thermoregulation during exercise in the heat.

BACKGROUND: A NATO project group has an interest in selecting an antiemetic agent that not only is effective in the prevention of emesis induced by chemical agents or radiation exposure but also has minimal, if any, side effects. Granisetron is the second candidate drug of a class of selective serotonin antagonists that has been shown to be an effective antiemetic agent for patients receiving radiation or chemotherapy treatment. The present study was designed to evaluate whether a single 2-mg oral dose of granisetron influenced temperature regulation during exercise in a hot and relatively dry environment. HYPOTHESIS: Based on our previous findings with the other candidate drug, ondanseton, we hypothesized that granisetron would not influence temperature regulation. METHODS: Nine unacclimatized males performed a drug and placebo trial in a double-blind manner. The sessions involved walking on a treadmill at 4.8 km.h-1 with a 2% elevation for a maximum of 3 h at 40 degrees C and 30% relative humidity while wearing combat clothing. RESULTS: Granisetron was associated with a small (0.2 degrees C) but significant elevation in mean skin temperature at the beginning and after 2 h of exercise. However, there was no difference between trials for the 1.6 degrees C increase in rectal temperature. Also, body heat gain (406 +/- 97 and 407 +/- 103 k) for the placebo and drug trial, respectively) and whole body sweat rates (0.72 +/- 0.10 and 0.73 +/- 0.10 kg.h-1 for the placebo and granisetron trial, respectively) were not different. Tolerance times also were not different for the placebo (157.4 +/- 16.7 min) and drug (159.4 +/- 20.4 min) sessions. CONCLUSIONS: For the environmental conditions used in this investigation, we would accept the null hypothesis that a single 2 mg oral dose of granisetron does not influence temperature regulation during exercise.

Comparison of short-term aerobic training and high aerobic power on tolerance to uncompensable heat stress.

This study investigated whether, in subjects of moderate aerobic fitness, short-term aerobic training could replicate the improved physiological responses to exercise-heat stress observed in individuals with a high level of aerobic fitness. Males of moderate (MF; <50 ml x kg(-1) min(-1) VO2peak, n = 8) and high (HF; >55 ml x kg(-1) x min(-1) VO2peak, n = 8) aerobic fitness walked at 3.5 km x h(-1) in the heat (40 degrees C, 30% relative humidity) wearing nuclear, biological, and chemical protective clothing. Tests were conducted once on HF subjects and on MF subjects before (MF-Pre) and after (MF-Post) a 2-week program 6 d x week(-1) of daily aerobic training (1 h treadmill exercise at 65% VO2peak for 12 d, 22 degrees C, 40% relative humidity). The training significantly increased VO2peak by 6.5%, while heart rate (fc) and rectal temperature (Tre) rise decreased during exercise in a thermoneutral environment. HF had lower body mass and body fat content than MF, and VO2peak remained lower in MF pre-or post-training. In the heat, MF-Post had a decreased skin temperature (Tsk) and an increased sweat rate compared with MF-Pre, but no changes were observed in fc, Tre, or tolerance time (TT). No significant differences during the first 60 min in Tre and fc were observed between the MF-Post and the HF subjects, though the HF subjects exhibited a lower Tsk. The endpoint Tre, deltaTre, and TT remained significantly higher in HF than in either the MF-Pre or MF-Post subjects. It was concluded that, in preparation for exercise in an uncompensable heat stress environment, short-term aerobic training offers little, if any, benefit and is not an adequate substitute for a high level of aerobic fitness resulting from habitual exercise and training.

Influence of temperature and metabolic rate on work performance with Canadian Forces NBC clothing.

This study examined the effects of environmental temperature and metabolic rate on soldiers' work tolerance time (WTT) while wearing various levels of nuclear, biological, and chemical (NBC) defence protective clothing. There were 23 unacclimatized males (23 +/- 3 years, 76 +/- 8 kg, 1.77 +/- 0.08 m) assigned to exercise at either a light (walking 1.11 m.s-1 0% grade, alternating with lifting 10 kg) or heavy metabolic rate (walking 1.33 m.s-1 7.5% grade, alternating with lifting 20 kg) in an environmental chamber at either 18 degrees C, 50% R.H. (cool) or 30 degrees C, 50% R.H. (warm). Subjects were tested wearing three levels of clothing protection: combat clothing (L); combats and a semi-permeable NBC overgarment (M); combats and NBC overgarment, gloves, boots and respiratory (H). WTT was the time until rectal temperature (Tre) reached 39.3 degrees C, heart rate reached 95% maximum, dizziness or nausea precluded further exercise, or 5 h had elapsed. During the light and cool trials (N = 5), wearing M or H did not impair WTT (277 +/- 47 min). For the light and warm experiments (N = 6), WTT was significantly impaired with H (82.7 +/- 10.6 min). With the heavy and cool condition (N = 6), WTT was reduced with M (240.5 +/- 73.8 min) and H (56.7 +/- 17.9 min). Finally, during the heavy and warm trials (N = 6), WTT was progressively impaired for L (172.5 +/- 52.8 min), M (65.8 +/- 18.2 min), and H (34.0 +/- 9.7 min) levels of protection.(ABSTRACT TRUNCATED AT 250 WORDS)