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Defence and Civil Institute of Environmental Medicine, Human Protection and Performance Section, North York M3M 3B9; Graduate Department of Community Health, University of Toronto, Toronto, Ontario, Canada M5S 1A8
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ABSTRACT |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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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°C, 30% relative
humidity). Men who were moderately fit [(MF); <50
ml · kg
1 · min1
maximal O2 consumption;
n = 7] and highly fit
[(HF); >55
ml · kg1 · min1
maximal O2 consumption;
n = 8] were tested while they
were euhydrated or hypohydrated by ~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°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
Tre, 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.
heat exhaustion; temperature regulation; hypohydration
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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IN SITUATIONS of compensable heat stress, where the
evaporative heat-loss capacity
(Emax) of the environment
exceeds the evaporative heat loss required
(Ereq) to maintain a thermal
steady state, repeated heat exposures over 4 days to 2 wk have been
shown to produce cardiovascular and thermoregulatory adaptations which result in decreased physiological strain and increased tolerance during
exercise in the heat (13, 14). Classic adaptations after heat
acclimation include an increase in sweating response and a decrease in
heart rate (HR), core temperature and skin temperature (Tsk), and perceived exertion during exercise in the heat
(35). Two factors known to modify the dynamics of heat acclimation are the aerobic fitness and hydration status of the individual. The efficacy of a heat-acclimation program may be dependent on the fitness
status of the individual. Fit individuals adapt more rapidly to heat
exposure, with an inverse relationship reported between maximal aerobic
power
(
O2 max)
and the number of days required to reach a heat-acclimated state (24).
However, individuals of low- to moderate-aerobic fitness, without any
prior adaptations to heat from long-term training, may experience a
greater potential benefit from a period of heat acclimation than fit
individuals experience, as larger decreases in HR and rectal
temperature (Tre) postacclimation were observed in subjects
with low as opposed to high
O2 max (6, 30). Any
thermoregulatory benefits derived from heat acclimation during exercise
in the heat are overwhelmed by the elevated stress imposed by
hypohydration (HY). Whereas heat acclimation produced a decrease in
core temperature when subjects were euhydrated (EU), HY of 5% body
weight, regardless of acclimation status, resulted in a significantly
higher final Tre compared with EU condition (6, 27).
Some of the adaptations to long-term training or habitual exercise,
including an increase in evaporative heat-loss capacity along with a
decrease in resting core temperature, are common to those adaptations
observed with heat acclimation, such that improvements in aerobic
fitness, typically characterized by
O2 max, have been
associated with an improved tolerance to exercise in the heat (see Ref.
3). Cadarette et al. (6) reported that, during compensable heat stress,
a decreasing cardiovascular and thermoregulatory strain occurred with
increasing fitness before acclimation, although only cardiovascular
benefits achieved with fitness were retained
postacclimation. In a cross-sectional design (34), a lower
HR and aural temperature and Tsk, along with increased tolerance time (TT), were reported during compensable heat stress in
very fit subjects compared with subjects of average fitness.
The wearing of protective clothing in the heat can result in a
situation of uncompensable heat stress, where evaporative heat loss is
limited and is less than that required to maintain thermal equilibrium
(15). In these situations, increased aerobic fitness or heat
acclimation may be of limited effectiveness in decreasing physiological
strain or prolonging tolerance. Because of the limited water vapor
permeability through the clothing, it is possible that the increased
sweat production in trained or heat-acclimated subjects may increase
physiological strain by promoting a faster rate of dehydration rather
than increasing evaporative heat loss (22). After 8 wk of aerobic
training or 6 days of heat acclimation, little or no improvements in
physiological responses or TT have been observed in subjects wearing
nuclear, biological, and chemical (NBC) protective clothing (1). In
addition, no differences in aural temperature or Tsk and
only minor improvements in TT were observed in very fit subjects
compared with subjects of average fitness during exercise in the heat
while wearing NBC clothing (34). However, the metabolic rate used to
evaluate the changes in heat tolerance in these two studies may have
produced heat exhaustion before a significant amount of heat transfer
could occur through the clothing layers. Thus, the physiological
benefit of an increased sweat rate (SR) with aerobic fitness or heat
acclimation may have been negated, and the influence of
O2 max and fitness status on tolerance to uncompensable heat stress remains unclear. In
contrast, 12-day heat-acclimation programs produced a significant improvement in exercise-heat tolerance during light exercise in the
heat while wearing NBC clothing (2, 20). In these studies, subjects
underwent progressive dehydration with no fluid replacement. Recently,
we reported that fluid replacement during uncompensable heat stress
resulted in a significantly lower HR and an increased TT (10). Thus, it
remains unknown whether benefits from heat acclimation would be present
where fluid replacement is provided.
The purpose of the present study was to determine the separate and combined effects of heat acclimation, aerobic fitness, and hydration status on exercise-heat tolerance during uncompensable heat stress. Subjects of both moderate fitness (MF) and high aerobic fitness (HF) exercised in a hot environment while wearing the NBC clothing both while EU and while HY by ~2.5% of body mass. In particular, we investigated the following questions. 1) Do subjects with a high level of aerobic fitness derived from long-term training and habitual exercise have an improved tolerance during light exercise compared with individuals of MF and sedentary lifestyle? 2) When fluid replacement is provided, will heat acclimation produce similar benefits as observed previously without fluid replacement? 3) Is the magnitude of improvement with heat acclimation greater in subjects of MF? 4) Does mild HY impair exercise-heat tolerance regardless of fitness or heat-acclimation status?
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Subjects
Fifteen healthy men between the ages of 18 and 40 yr, who were recruited from the university population or the military community, participated in the study. Subjects underwent a medical examination and were informed of all details of the experimental procedures and the associated risks and discomforts before they provided their consent. Subjects were grouped into two general categories, either MF or HF, on the basis of both an interview concerning their exercise habits and the results of a treadmill test of maximal aerobic power. MF subjects were either inactive at the time of the study or engaged in physical activity only on an irregular basis; they had aO2 max between 40 and
50 ml · kg1 · min1.
All MF subjects agreed to abstain from regular aerobic activities for
the duration of the experiment. HF subjects were defined as those
engaged in a regular program of physical activity and having a
O2 max in excess of
55 ml · kg1 · min1.
Experimental Design
The experimental protocol and instrumentation used in the present study were approved by the Ethics Review Committees of the University of Toronto and the Defence and Civil Institute of Environmental Medicine (DCIEM). Testing was conducted at the DCIEM from late September to March to limit initial heat acclimation through casual exposure to high ambient temperatures. On five separate occasions, each subject performed a heat-stress test (HST), which consisted of walking on a motorized treadmill in a hot environment [40°C, 30% relative humidity (RH), wind speed <0.1 m/s] while wearing the Canadian Forces NBC protective clothing ensemble. On the afternoons before the sessions were conducted, subjects exercised in the heat until they became dehydrated by 2.5% of their body mass. For all subjects, the first session was used as a familiarization trial, and the results were discarded. A minimum of 1 wk separated experimental trials to avoid the effects of partial heat acclimation (5).Responses to the HST were evaluated during exercise on the treadmill at 3.5 km/h and 0% grade while the subjects' level of hydration were manipulated. After the dehydration protocol, subjects were either rehydrated to baseline body mass overnight (EU) or they maintained the 2.5% decrease in body mass overnight (HY). During all HST, subjects underwent a fluid-replacement program consisting of 200 ml of water every 15 min, with the water temperature maintained near 37°C. The order in which the different conditions were presented was randomized to minimize order effects or the effects of partial heat acclimation. To counterbalance order effects, the order of the hydration trials for each subject was reversed after the heat-acclimation period.
After an HST in each of the conditions (EU and HY), all subjects
underwent a 2-wk heat-acclimation program. For 5 days/wk, subjects
walked on a motorized treadmill at 4.8 km/h and a grade between 3 and
7% for 1 h while wearing combat clothing and the NBC overgarment in
the same hot environmental conditions (40°C, 30% RH) as were
employed for the HST. The intensity was chosen such that the increase
in Tre during the 1 h was 
1.5°C on the first day of
acclimation. A similar heat acclimation program of 12 days resulted in
an increase in sweat rate (SR) and a decrease in HR and core
temperature during subsequent exercise in a hot environment (20).
On the completion of the 2-wk heat-acclimation period, all subjects again performed an HST in the EU and HY conditions. Between the first and second postacclimation HST, all subjects performed two to three heat-acclimation sessions to maintain their heat-acclimated status.
NBC Protective Clothing
The Canadian Forces NBC protective clothing ensemble worn in all trials consisted of shorts, T-shirt, socks, combat shirt and trousers, running shoes, semipermeable NBC overgarment, gas mask and cannister, and impermeable rubber gloves and overboots. Total mass of the ensemble was ~6.0 kg. To allow some sweat evaporation, there is limited mass penetration of charcoal-filtered air through the fabric. Thermal resistance and the Woodcock vapor-permeability coefficient of the ensemble, as determined on a heated and wetted manikin at a wind speed of 1.12 m/s, were 0.291 m2 · °C · W1
and 0.33, respectively (16).
Determination of
O2 max
O2 max was determined
on a motorized treadmill by using open-circuit spirometry before and
after the series of experiments in the climatic chamber. After subjects
ran for 3 min at a self-selected pace, the treadmill grade was
increased 1% each minute to 10%. Thereafter, increases in treadmill
speed and grade of 0.22 m/s (0.8 km/h) or 2%, respectively, alternated
each minute until the subject could no longer continue. Subjects were
given verbal encouragement throughout the test.
O2 max was defined as
the highest 30-s O2 consumption
(O2) observed
during the incremental test. HR was monitored throughout the
incremental test from a telemetry unit (Polar Vantage XL). The value
recorded at the end of the exercise test was considered to be the
individual's maximal HR. Body fatness was estimated from skinfold
measurements by using a gender-specific regression equation developed
from hydrostatic measurements of body density (12).
Dehydration Protocol
An identical dehydration and overnight protocol was employed in a previous investigation in this laboratory and was found to be effective in manipulating the hydration status of the subjects (10). In the afternoons before the exercise sessions, subjects reported to the laboratory at ~1330 h for the dehydration protocol. This allowed ~15 h for body fluid compartments to stabilize between the dehydration protocol and the HST. Dehydration sessions took place in the same environmental chamber (40°C, 30% RH) that was used for the HST. Weights, measured when subjects were both nude and dressed (shorts, socks, shoes), were recorded before entry into the chamber. Subjects walked on a motorized treadmill at an exercise intensity (4.5-6.0 km/h, 3-7% grade) that induced weight loss at a rate of 0.8-1.5 kg/h. Tre and body mass were monitored throughout the dehydration, and subjects were removed from the chamber when they lost 2.5% of their baseline body mass.Nutrition was controlled for all trials by providing subjects with a set meal plan consisting of PowerBar meal replacement bars. For subjects undergoing EU trials, sufficient Gatorade was provided immediately after the dehydration session to replace the amount of weight loss. Subjects in the EU trial were also instructed to drink 600 ml/h of Gatorade or juices that evening and at least 600 ml in the morning before they reported to the laboratory. Subjects undergoing the HY trials were given a total ration of 800 ml of Gatorade, based on expected basal weight losses over a 15-h period.
Dressing and Weighing Procedure
Preparation of subjects, insertion of the rectal thermistor, and placement of skin thermistors have been detailed previously (19). Before the dressing procedure, subjects remained in an upright posture for 10 min; a 5-ml blood sample was obtained within 90 s of the subjects' lying down. Plasma osmolality was calculated from plasma concentrations of glucose, sodium, and blood urea nitrogen (Nova Ultra Stat, Nova Biomedical). Before subjects entered the chamber, their weight, both nude and dressed, was recorded. After subjects entered the chamber, their skin and rectal thermistor-monitoring cables were connected to a computerized data-acquisition system. Then the subjects began to exercise. Mean values over 1-min periods for Tre, and a 12-point weighted mean Tsk (
sk) (32) were calculated, recorded, and printed by the data-acquisition system. HR was recorded every 5 min from the Polar Vantage XL unit. After each trial was completed, subjects' weight while dressed was recorded within 1 min
after their exit from the chamber. Nude weight was recorded within 5 min after subjects undressed and were toweled dry.
Differences in nude and dressed body masses before and after each trial
were corrected for respiratory and metabolic weight losses (see
Gas-Exchange Analyses). The amount
of sweat produced was calculated as pretrial nude body mass 
posttrial nude body mass (corrected) + water given. Evaporative sweat
loss from the clothing was calculated as pretrial dressed mass posttrial dressed body mass (corrected) + water given.
TT
TT for all trials was defined as the time until Tre reached 39.3°C, HR remained at or >95% of maximal HR for 3 min, dizziness or nausea precluded further exercise, either the subject or the experimenter terminated the experiment, or 4 h had elapsed.Gas-Exchange Analyses
During each trial, open-circuit spirometry was used to determine expired minute ventilation,O2, and carbon dioxide
production from a 2-min average obtained every 15 min. To collect
expired air, an adaptor was attached to the respirator. Respiratory
water loss was calculated by using the measured
O2 and the equation of
Mitchell et al. (21). Metabolic body mass loss was calculated from the
O2 and the respiratory
exchange ratio (RER) by using the equation described by Snellen (31).
Statistics
Data are presented as means ± SD. A three-factor (period × hydration × time) repeated-measures ANOVA was used to compare Tre,sk, and
HR of the HF and MF subjects undergoing the short-term heat
acclimation. A two-factor (period × hydration) repeated-measures ANOVA was used to compare the responses of TT, body mass changes, SR,
plasma osmolality, metabolic rate, and RER. When a significant F-ratio (corrected for the
repeated-measures factor) was obtained, a Newman-Keuls post hoc
analysis was performed to isolate differences among treatment means.
After separate data analyses within either the HF or the MF group, a
comparison was performed across the two groups to detect differences in
exercise-heat tolerance as a result of long-term fitness. For all
statistical analyses, the 0.05 level of significance was used.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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The physical characteristics of the subjects are shown in Table
1. The HF and MF subjects were similar in
age and height. As expected, the HF subjects were distinguished from
the MF subjects by their much higher
O2 max and a
lower body fat content, respectively. Furthermore, the HF subjects were
generally smaller than the MF group, with a lower body mass and a
smaller surface area. The effects of the heat-acclimation regimes on
physiological responses during the first and the final day of exposure
are summarized in Tables 1 and 2.
Acclimation did not induce an increase in aerobic capacity, with no
changes in O2 max
following acclimation in either the HF or MF group (Table 1). MF
subjects experienced a significantly lower HR along with a higher SR
during the final day of acclimation (Table 2). Adaptation also occurred
in HF individuals, with an increased SR along with a decreased
Tre and HR in response to the heat-acclimation exercise on
day 10.
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Tolerance to the HST was determined by time, physiological end points imposed for ethical considerations, or subject exhaustion. The reasons for trial termination are presented in Table 3. None of the trials approached the 4-h time limit. For MF subjects, the large majority of the experimental trials were terminated because of exhaustion, as determined by the subject or the experimenter. Only four trials, all by the same subject, were terminated because of reaching the ethical limit for HR. In contrast, HF subjects generally terminated trials because of reaching the ethical limit for core temperature (39.3°C). Voluntary exhaustion often coincided with reaching the Tre limit; however, in many cases, the subjects felt that they were not completely exhausted and could have continued further. Every attempt was made to thoroughly familiarize all subjects with the protocol and to eliminate the influence of learning or order effects. Some subjects in both groups were very familiar with the NBC clothing and the HST because they had been subjects in previous heat experiments in this laboratory. The initial familiarization session served to accustom all subjects to the protocol and to any associated discomforts before the actual test. All subjects were highly motivated, and little doubt existed about the presence of exhaustion and imminent collapse at the end of the HST.
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The hydration schedule after the dehydration protocol was successful either in reinstating EU or in maintaining HY overnight (Table 4). In EU trials, body mass returned to baseline levels overnight, and mass was similar for all EU trials. The HY protocol resulted in a relative decrease in mass of 1.9 to 2.8% in the HF and MF groups, respectively, compared with the pre-EU trial. HY also increased serum osmolality in both groups.
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Table 5 summarizes some of the physiological responses to the HST. In both groups, one major adaptation to the acclimation program was an increased SR during the HST. However, because of the difficulty in water vapor transfer through the NBC ensemble, no significant changes in evaporation rate were observed. Heat acclimation did not significantly prolong TT in either fitness group. While heat acclimation had no effect on exercise-heat tolerance, HY resulted in a significant degree of impairment regardless of acclimation status. While end-point Tre was unaffected by hydration status within either group, the initial Tre for MF was significantly higher before HY trials. Overall, in both the HF and MF groups, HY resulted in a significantly shorter TT, regardless of acclimation status. HY resulted in a significantly lower RER during HY in both the MF and HF groups. This decrease in RER was not because of a difference in glucose availability before exercise; there were with no differences in serum glucose levels between EU and HY.
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The sk, Tre, and HR
responses to the HST are presented in Figs.
1-3, respectively. In the MF group, despite the cardiovascular and sweating
adaptations observed over the course of the acclimation program, no
differences in sk,
Tre, and HR response were evident during the HSTs after the
2-wk acclimation period. Interestingly, the acclimation period had a
greater impact on the thermoregulatory responses to the HST in subjects
who were already HF aerobically. In HF subjects, the acclimation
program was successful in decreasing the thermoregulatory strain during
the postacclimation HSTs, with a main effect found for a lower
sk and Tre.
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HY significantly impaired thermal and cardiovascular responses to the
HST in both fitness groups. In both MF and HF, Tre was significantly higher overall during HY than EU trials, with the primary
factor in MF being an elevated initial Tre (Fig. 2). In the
HF group, HY trials also resulted in an elevated rate of rise of
Tre. HR response to the HST was influenced by hydration
status in both fitness groups (Fig. 3), with a significant elevation in
HR in both the HF and MF groups during HY. In MF subjects, HY also
elicited a higher rate of rise of HR. In MF subjects, sk was significantly elevated
during HY. In HF subjects, HY did not affect overall
sk, but the initial increase in
sk over the first 25 min was
significantly slower during post-EU compared with all other conditions.
When the results in the HF and MF groups during the HST are compared,
the HF group had a greater change in Tre
(Tre), a result of both a significantly lower initial
Tre and a higher final Tre. The greater
Tre was a major contributing factor to an overall increased TT in HF subjects. The overall rate of rise in
sk was significantly lower in HF
than in MF subjects. No significant between-group differences were
observed in cardiovascular response, SR, and evaporation rate during
the HST.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Heat acclimation, hydration status, and aerobic fitness are factors that have been demonstrated to influence exercise-heat tolerance during compensable heat stress, where the Emax exceeds the Ereq (see Refs. 3, 26, 33 for reviews). The purpose of the present study was to extend the investigation of these factors into an environment of uncompensable heat stress, where the Ereq exceeds or matches the Emax. Even very light exercise in the heat while wearing clothing with limited permeability to water vapor will result in uncompensable heat stress (15), and the light exercise employed in the present study was sufficient to produce a heat-stress index (HSI) of ~2.5 (HSI = Ereq/Emax). Under these conditions, when fluid replacement is present, heat acclimation had no influence on exercise-heat tolerance in both MF and HF individuals. In contrast, HY before exercise significantly impaired tolerance regardless of acclimation status or aerobic fitness. Long-term aerobic fitness resulted in a significant improvement in exercise-heat tolerance, regardless of hydration or acclimation status.
Significant physiological adaptations occurred in both the HF and MF groups over the 10 days of the heat-acclimation protocol, notably an increased SR which was also evident during the full encapsulation conditions of the HST. The physiological mechanisms underlying this increased sweating response could not be determined with the present methodology. However, during the HST, the increased sweat production did not result in an elevation in evaporative heat loss or a slowing of the rate of Tre increase in either group because of the limited water vapor permeability of the NBC clothing. In uncompensable heat stress, therefore, the higher SR from heat acclimation is a negative adaptation which, instead of enhancing evaporative heat loss and attenuating thermal strain, increases the rate of dehydration and physiological strain (22).
In addition to the evaporative impairment caused by the protective clothing, several explanations may account for our finding of a lack of significant improvement in exercise-heat tolerance after heat acclimation. One explanation is that the subjects may not have been fully acclimated by our program. According to the regression equation for the attainment of heat acclimation presented by Pandolf et al. (24), the MF and HF subjects in the present study should have reached a plateau in response by 7 and 4.5 days, respectively. However, hot-wet acclimation may produce a slower plateau rate. In addition, full heat acclimation in a hot-humid environment required >1 h exposure/day (14). Counteracting these arguments are the reports of significant adaptations while wearing clothing with limited water vapor permeability after 1 h of heat acclimation for 4-6 (29) or 12 (20) days. In the present study, comparison of responses to the heat-acclimation exercise on days 9 and 10 revealed a plateau in SR and in the HR and Tre after 60 min in both HF and MF. In addition, because an increased SR is one of the physiological adaptations with the slowest time course (17), the elevated SR in both groups suggests the attainment of near-maximal adaptations in other physiological systems. We are therefore confident that the subjects achieved a nearly complete state of heat acclimation.
Any benefits accruing from heat acclimation in an uncompensable heat-stress environment may have been masked by the presence of fluid replacement. In the present study, the finding of no heat-acclimation effects on exercise-heat tolerance in both fitness groups contrasts with two previous studies in our laboratory that demonstrated a decrease in physiological strain and an increase in TT during exercise in the heat with NBC clothing after heat acclimation with (20) or without (2) NBC clothing. The disparity in the effects of acclimation may be caused by the fact that, whereas subjects in the previous studies were not provided with any fluid replacement during the HST, water was provided to subjects in the present study at regular intervals. Fluid replacement has been demonstrated to reduce physiological strain during exercise in the heat (8) and specifically to decrease cardiovascular strain and to increase TT while exercising in the heat with NBC clothing (10). Thus, the fluid replacement may have extended exercise-heat tolerance during the preacclimation HST to near the maximum possible given the uncompensable heat-stress environment, thereby limiting the amount of improvement that could be observed with subsequent heat acclimation. If this is the case, it would reinforce the importance of fluid replacement in an uncompensable heat-stress environment regardless of fitness or acclimation status.
The deleterious influence of HY on exercise performance in thermoneutral and hot environments has been reviewed in detail elsewhere, and it is well known that HY is associated with an increase in HR, Tre, and ratings of perceived exertion (7, 26). HY also altered the metabolic response to the HST, with a significantly lower RER during the HST in both the MF and HF groups. This decrease in RER has been reported previously (11, 28) and was not because of a decrease in glucose availability, as no differences were observed in serum glucose levels between EU and HY (unpublished observations). In a EU state, heat acclimation significantly decreased final Tre in environments with a HSI of ~1.0 (27). However, in the same study, HY significantly increased thermal strain, regardless of acclimation status, with similar final Tre both before and after heat acclimation. The present study extends these findings to a more severe uncompensable heat-stress environment. Compared with EU, HY resulted in a decreased TT, regardless of fitness or acclimation status, with an elevated resting Tre in MF and an increased rate of rise in Tre during the HST in HF subjects. Therefore, not only fluid replacement during exercise but also hydration status before exercise appears to take precedence over acclimation status in determining physiological strain during uncompensable heat stress.
Exercise-heat tolerance was improved by fitness, regardless of
hydration or heat-acclimation status, with an average combined TT of
110 and 88 min in HF and MF, respectively. These observations support
the general consensus that an association exists between the level of
cardiorespiratory fitness and improvements in physiological responses
to exercise in a hot environment (3). In subjects who exercised at a
higher intensity in the heat while wearing NBC clothing, trained
subjects also had a slight increase of TT compared with untrained
subjects (34). The present study used both the
O2 max and the level of
regular physical activity to define the division of subjects into two
distinct fitness groups. The inclusion of activity level as a selection
criterion was prompted by the observation that
O2 max by itself was
only moderately correlated with heat tolerance and that the amount of
regular physical activity may be a better indicator of the presence of training-induced adaptations to heat exposure (4, 18, 25).
One major difference in the response to the HST between the two fitness
groups was the significantly higher degree of hyperthermia and
Tre tolerated by the HF group. HF subjects had both a
significantly lower initial and a higher end-point Tre than
the MF subjects, resulting in a Tre over the course of
the HST of 2.3°C in the HF subjects compared with 1.6°C in MF
subjects. Extrapolating an increase of the Tre in the MF
group by 0.7°C, and given their linear rate of Tre
increase of ~1.0°C/45 min, the MF TT would increase by 31.5 min.
In contrast, if the Tre of the HF group decreasedby
0.7°C, the linear rate of Tre increase of
~1.0°C/40 min would decrease their TT by 28 min.
With an actual observed difference in overall TT of ~22 min between
the MF and HF groups, it is evident that the difference in TT between
the two fitness groups could be largely accounted for by the difference
in Tre.
The difference in tolerance to hyperthermia is also evident from the
disparity in the reasons for the termination of the HST (Table 3). The
large majority of the HSTs with HF subjects were terminated because of
the individuals' reaching the ethically imposed limit of 39.3°C
Tre, whereas nonfit subjects generally reached voluntary
exhaustion at a Tre well below 39.3°C. All subjects underwent an initial familiarization trial involving the complete experimental protocol, and no effects of order or training were observed on TT or endpoint Tre. HF individuals may be
capable of tolerating a higher level of subjective discomfort or
physiological strain because of their regular program of physical
activity. Alternatively, a given combination of Tre and
sk may produce a greater degree
of subjective discomfort in nonfit subjects. While thermal convergence
of core temperature and Tsk may not be a reliable
determinant of tolerance in an uncompensable heat-stress environment
(23), a particular Tsk-core temperature difference or the
increased rate of sk increase
could result in greater discomfort in MF subjects. HF subjects may also
be better able to tolerate high levels of skin wettedness and the
effects of hidromeiosis (9).
In summary, this study leads to the following observations regarding exercise-heat tolerance in an uncompensable heat-stress environment. 1) High aerobic fitness from long-term training and habitual exercise is of significant benefit. 2) When fluid replacement is provided, heat acclimation does not provide significant benefit regardless of fitness status. Fluid replacement may, therefore, be an effective substitute for a heat-acclimation program. 3) The magnitude of improvements in physiological strain with heat acclimation are greater in those subjects with high aerobic fitness, but the improvements are still insufficient to improve exercise-heat tolerance. 4) Mild HY of 2-3% of body mass results in significant impairment, regardless of fitness or heat acclimation status.
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ACKNOWLEDGEMENTS |
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The authors wish to express their gratitude to the subjects for their participation in this investigation. Thanks are extended to R. Limmer, J. Pope, and I. Smith for their technical assistance throughout the study.
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FOOTNOTES |
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PowerFoods Inc. provided the PowerBar meal-replacement bars.
S. Cheung was supported by a Department of National Defence research contract.
Address for reprint requests: T. M. McLellan, DCIEM, Human Protection and Performance Section, PO Box 2000, North York, Ontario, Canada M3M 3B9.
Received 4 August 1997; accepted in final form 19 December 1997.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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1.
Aoyagi, Y.,
T. M. McLellan,
and
R. J. Shephard.
Effects of training and acclimation on heat tolerance in exercising men wearing protective clothing.
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1994.
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Aoyagi, Y.,
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Effects of 6 versus 12 days of heat acclimation on heat tolerance in lightly exercising men wearing protective clothing.
Eur. J. Appl. Physiol.
71:
187-196,
1995.
3.
Armstrong, L. E.,
and
K. B. Pandolf.
Physical training, cardiorespiratory physical fitness and exercise-heat tolerance.
In: Human Performance Physiology and Environmental Medicine in Terrestrial Extremes, edited by K. B. Pandolf,
M. N. Sawka,
and R. R. Gonzalez. Indianapolis, IN: Benchmark, 1988, p. 199-226.
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Avellini, B. A.,
Y. Shapiro,
S. M. Fortney,
C. B. Wenger,
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