INTRODUCTION

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HYPOHYDRATION INCREASES physiological strain during exercise in the heat. A loss of only 1% water of body weight compared with euhydration causes an increase in core temperature during exercise in normothermic and warm environments (3). Hertzman and Ferguson (12) were the first to describe hypohydration during heat stress as a "failure of the thermoregulatory system." The addition of hypohydration to the stress further reduces endurance and influences the thermoregulatory control systems, either through associated changes in blood volume (20) or through accompanying changes in plasma osmolality (11). The cardiovascular system is also affected by hypohydration during exercise in the heat. First, hypohydration results in an increase in heart rate (HR) to compensate for the fall in stroke volume. Second, hypohydration reduces cutaneous blood flow; thus the potential for dry heat exchange (by convection and radiation) between the body and the environment is lowered, impairing heat dissipation from the body (27). In 1979, Senay (28) suggested that the increased core temperature in hypohydrated individuals is necessarily the consequence of reduced heat transfer. In 1995, Sawka et al. (27) concluded that during exercise-induced heat stress, hypohydration compared with euhydration accelerates exhaustion from heat strain at a lower rectal temperature (T_re).

Hypohydration is usually associated with either a reduced or unchanged sweat rate (_sw) (27). When no change in _sw was reported during dehydration in a warm climate at a given metabolic rate, T_re was elevated, reflecting higher strain and delayed _sw threshold (26). Numerous investigators had attributed the higher core temperatures that accompany thermal hypohydration to either failure of the sweating response (3, 7) or to a redistribution of blood flow from the cutaneous regions. Some studies showed that different levels of hypohydration affected the sweating mechanism to different degrees (5, 25, 26). Montain et al. (17) found that the threshold temperature for sweating increased with hypohydration level, unlike sweating sensitivity, which decreased. In that study, the exercise intensity when combined with hypohydration increased sweating sensitivity but did not alter the sweating threshold temperature.

Heat strain indexes based on physiological parameters including _sw were suggested by a few researchers. McArdle et al. (15) developed the predicted 4-h sweat rate index (P4SR), which uses _sw as an indicator of heat strain and predicts _sw for 4 h of different combinations of metabolic rate and climatic condition. However, it was shown that sweat production by itself does not comprehensively represent heat strain (1, 9), and the P4SR was relevant only for fit-acclimatized men (14). Robinson et al. (23) suggested an index that relied on T_re, HR, _sw, and skin temperature. This index, based on an equal weighting of the four parameters with no relation to the metabolic rate, was developed on the basis of collected data involving heat-acclimatized subjects but was not validated for other conditions. Hall and Plote (8) suggested an index of physiological strain based on body heat storage which also used T_re, HR, and _sw. The complexity of calculating this index and the inability to assess the heat strain online were the main reasons that it has not been universally accepted.

In 1980, Lee (14) summarized his review of 75 years of searching for a universal heat stress index as follows: "any reader who was hoping for the evolution of a single heat index applicable to all aspects of human endeavor must by now be sadly disappointed." Although more inclusive and advanced indexes have been developed in the last 20 years, these indexes were unfortunately found to be complicated and difficult to apply (4, 6, 13, 22).

Recently, Moran et al. (19) introduced a physiological strain index (PSI) based on T_re and HR as representative of the combined strain reflected by the thermoregulatory and cardiovascular systems. This simple-to-use index scaled the strain to a range of 0-10 and can be used online or during data analysis. It was shown that the PSI can be applied at any time, including rest or recovery periods, whenever T_re and HR are measured (19). Furthermore, this index successfully rated and correctly discriminated between different clothing ensembles and climate conditions during heat stress.

The purpose of this study was to examine the ability of the PSI to assess and categorize heat strain at different combinations of hypohydration level and exercise intensity. In addition, we aimed to evaluate the interaction between PSI and _sw for these experimental conditions.

	MATERIAL AND METHODS

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The PSI was applied to two databases (16, 17). The first produced different levels of dehydration by having volunteers drink different volumes of fluid during prolonged exercise in the heat (16). The second database, taken from an independent study, examined the HR, core temperature, and sweating response to different combinations of hypohydration level and exercise intensity (17).

Protocol 1. Evaluation of PSI for different levels of dehydration during prolonged exercise was done using a database from Montain and Coyle (16) and was within the range of 53-175 beats/min for HR, 36.8-39.7°C for T_re, and 36.4-39.2°C for T_es. Eight endurance-trained male cyclists [age 23 ± 3 yr, body wt 72.2 ± 11.6 kg, and maximum O₂ consumption (O_{2 max}) 66.2 ± 7.6 ml · kg¹ · min¹] cycled at a power output eliciting 62-67% O_{2 max} for 120 min in a warm environment [33°C and 50% relative humidity (RH)]. Each subject completed four experimental exposures while ingesting different volumes of fluid during exercise: no fluid or a volume that replaced 20, 50, or 80% of the fluid lost in sweat [resulting in 4.2 ± 0.1, 3.4 ± 0.1, 2.3 ± 0.1, and 1.1 ± 0.1% body weight loss (BWL), respectively, after 120 min of cycling].

Protocol 2. Nine healthy young acclimated men participated in the study (17). The physical characteristics of the subjects were (means ± SE) age 24 ± 2 yr, height 176 ± 3 cm, body wt 81.7 ± 4.5 kg, and O_{2 max} 57 ± 2 ml · kg¹ · min¹. Subjects completed nine experimental exposures of 50 min of exercise in warm climate conditions (30°C and 50% RH). The exposures consisted of exercise on a treadmill at three intensities: 25, 45, and 65% of O_{2 max} when euhydrated or hypohydrated by 3 and 5% of the subjects baseline body weight. Hypohydration was achieved on the day before each trial using a standardized exercise-heat protocol. For the 5% BWL trials, subjects performed 2-3 h of exercise in the morning in addition to an afternoon exercise session. A number of experiments were terminated before the scheduled 50-min exposure time during the 65% O_{2 max} trials when a subject voluntarily withdrew, when a subject's esophageal temperature (T_es) reached 39.5°C, or when HR exceeded 90% of maximum HR for 3 consecutive minutes.

Calculations. The PSI was calculated using either T_es or T_re as suggested by Moran et al. (19) as follows where T_re0 and HR₀ are the initial T_re and HR, respectively, and T_ret and HR_t are simultaneous measurements taken at any time.

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Statistical analysis. Physiological responses at the different levels of hydration and the interaction of exercise intensity and hydration level on sweating were analyzed by two-way ANOVA for repeated measures. All statistical contrasts were accepted at the P < 0.05 level of significance. All experimental data are presented as means ± SE. The material and methods are presented in greater detail elsewhere (16, 17).

	RESULTS

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Database 1. Generally, T_re and T_es were elevated in proportion to the magnitude of the hypohydration levels, and the four trials were significantly different from each other (P < 0.05), with the exception of the 3.4 and 4.2% BWL exposures (Fig. 1). Similarly, HR increased progressively during exercise at the different levels of hypohydration. However, at 120 min of exercise, HR was not significantly different between the exposures of 1.1 and 2.3% BWL and the 3.4 and 4.2% BWL (Fig. 1).

View larger version (21K): [in this window] [in a new window]   Fig. 1.   Physiological strain index (PSI), calculated from rectal temperature (Tre) and heart rate (HR), applied to mean values obtained from 8 subjects exposed to heat stress [33°C, 50% relative humidity and 65% maximum O2 consumption (O2 max)] at 4 different levels of hypohydration [1.1, 2.3, 3.4, and 4.2% body weight loss (BWL)]. bpm, Beats/min.

View larger version (20K): [in this window] [in a new window]   Fig. 2.   PSI calculated from Tre (A) and esophageal temperature (Tes, B) on database of Montain and Coyle (16). Values obtained from 8 subjects exposed to 4 different levels of hypohydration (1.1, 2.3, 3.4, and 4.2% of BWL) during exercise.

Database 2. HR, T_re, and T_es dynamics during these experimental exposures are presented in Figs. 3 and 4. Generally, at the same exercise intensity HR, T_re and T_es values were higher with increasing levels of hypohydration. At the low exercise intensity (25% O_{2 max}), HR values were significantly less than for the other two intensities (45 and 65% O_{2 max}) across all hydration levels (P < 0.05). Similarly, HR values at 3 and 5% BWL for the moderate intensity were not significantly different from the euhydration values at the high exercise intensity (Fig. 3). Compared with simultaneous measurements of T_es, all T_re values were significantly higher (~0.1-0.4°C, P < 0.01). Analysis of the T_re and T_es dynamics during all the exposures revealed a pattern in which the low exercise intensity at 5% BWL overlapped with the high intensity during euhydration (Fig. 4).

View larger version (22K): [in this window] [in a new window]   Fig. 3.   HR dynamics (means ± SE) of all subjects who participated in 9 experimental exposures consisting of 3 exercise intensities (25, 45, and 65% O2 max) and 3 hydration levels (euhydration and hypohydration at 3 and 5% of body wt). * Drop due to subject attrition.

View larger version (23K): [in this window] [in a new window]   Fig. 4.   Tre (A) and Tes (B) dynamics (means ± SE) of all subjects who participated in 9 experimental exposures. * Drop due to subject attrition.

View larger version (22K): [in this window] [in a new window]   Fig. 5.   PSI (means ± SE) applied using Tre (A) and Tes (B) in evaluating 9 experimental exposures. * Drop due to subject attrition.

View larger version (15K): [in this window] [in a new window]   Fig. 6.   PSI and sweat rate (sw) (means ± SE) after 20 min of exercise across 3 hydration levels at 3 exercise intensities.

View larger version (15K): [in this window] [in a new window]   Fig. 7.   PSI and sw (means ± SE) after 20 min of exercise across 3 exercise intensities at 3 levels of hydration.

	DISCUSSION

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The PSI for the two different databases under investigation accurately described the heat strain of men dehydrated to four different levels during 120 min of cycling and the strain accompanying a matrix of three exercise intensities and three hypohydration levels. Our index succeeded in rating each one of these exposures on its universal scale of 0-10. The index, which is based on only two physiological parameters, HR and core temperature (T_re or T_es in this study), categorized every exposure in the proper and expected order, whereas HR, T_re, and T_es during the different exposures were limited in their individual ability to categorize each exercise intensity-hypohydration level combination separately (Figs. 1, 3, and 4).

During the last century, more then twenty heat strain indexes have been proposed (1, 14). However, none has been accepted as a universally valid index for rating heat stress. This is mainly attributable to the number and complexity of the interactions among the determining factors (1, 19). The ability to sustain exercise in the heat depends mainly on the effective heat transfer from the contracting muscles to the skin and from the skin to the environment. Dehydration compromises blood flow to the skin, resulting in greater thermal and cardiovascular strain. Thus, when hypohydration accompanies heat stress, it causes even more difficulties in evaluating the resultant physiological strain. The combination of many different levels of hypohydration and different exercise intensities provided by our two unique databases challenged the ability of the PSI to discriminate the relative strain of exercise in the heat.

It is well known that T_es values are generally lower than simultaneous T_re measurements (21, 24). T_es responds rapidly and quantitatively to changes in blood temperature with a time constant of ~1 min, whereas T_re responds more slowly with a time constant of ~12 min (e.g., during exercise) (27). To further the appreciation of the versatility of the PSI, we examined the physiological strain using both T_re (PSI_Tre) and T_es (PSI_Tes) measurements.

The simultaneous measurement of T_re and T_es in both database sets revealed higher T_re (P < 0.01) (Figs. 2 and 4). Therefore, it was expected that PSI_Tre would result in higher values than PSI_Tes. This was true for the first database, because PSI_Tre was significantly higher than PSI_Tes by ~0.5-1.0 unit (P < 0.01). However, in the second database, PSI_Tre was not significantly higher than PSI_Tes during exercise at 25 and 45% of O_{2 max}. PSI_Tre was highest during the higher exercise intensity (65% of O_{2 max}) (Fig. 4). These minor differences between PSI_Tre and PSI_Tes are attributed to the PSI construction, which normalized each physiologicalparameter (HR and T_re or T_es) to its initial value. Regardless, it can be concluded that PSI_Tes and the original PSI (PSI_Tre) are both able to provide meaningful values for estimating different levels of hypohydration during exercise heat stress, including severe conditions in which body heat balance is violated.

The two databases used supported earlier observations that hypohydration increased T_re and HR during exercise in the heat (25-27). Furthermore, as the severity of hypohydration increases during exercise in the heat, there is an associated increment in the elevation of T_re and HR. The incrementally increased T_re had been associated with a decreased _sw. Correspondingly, it was expected that T_re, T_es, and HR could be used for physiological strain assessment. T_re and T_es reflect the body heat storage and are elevated proportionally to exercise intensity during exercise. HR reflects the demands of the circulatory system. Unlike T_re, HR rapidly responds to changes in metabolic demands and environmental conditions (18). However, as depicted in Figs. 1, 3, and 4, T_re, T_es, and HR were limited in their ability to individually quantify and categorize the different experimental exposures. On the other hand, applying PSI to the same database containing T_re or T_es and HR measurements clearly evaluated the relative strain with a simple scale ranging from 0 to 10 (Fig. 5). In fact, the PSI described well the physiological strain at the different exercise intensities and hypohydration levels according to classic physiology: 1) exercise intensity correlated with the physiological stress and with _sw (Fig. 6) and 2) hypohydration level correlated with the physiological stress and inversely correlated with _sw (Fig. 7). The commonly used RPE scale was also correlated with hypohydration levels. However, although RPE correlated with PSI and discriminated among the different hydration levels, it was limited in significantly differentiating between two exposures (1.1 and 2.3% BWL), unlike the PSI.

The PSI, unlike other heat strain indexes, depicts the combined strain reflected by the cardiovascular and thermoregulatory systems. This enables the PSI to make comparisons between different studies. The first database analyzed in this study was collected for 120 min, whereas the second database was obtained for 50 min. However, a comparison of PSI between the two databases for similar exposures (65% O_{2 max} and ~3% BWL) after 50 min of exercise revealed the same moderate strain category values of 6.0 and 6.4 (for the first and the second databases, respectively). In a previous study, the PSI showed the ability to assess heat strain at different combinations of metabolic rate, climate condition, and clothing (19). In this study, we were able to extend its evaluation to different combinations of hypohydration levels and exercise intensities in the heat using either T_re or T_es and RPE.

In summary, the PSI successfully evaluated the heat stress in subjects who exercised in a warm environment at different exercise intensities combined with different levels of hypohydration. This index overcame the individual limits of the physiological parameters (T_re, T_es, and HR) in assessing heat stress for this study and continues to provide the potential to be accepted universally.