Prediction and rectal
of heat tolerance from heart rate temperature in a temperate environment
E. SHVARTZ,
S. SHIBOLET,
Heller Sheba
of Medical Research, Center, Tel-Hashomer,
Institute Medical
A. MEROZ,
A. MAGAZANIK,
Tel-Aviv Israel
SHVARTZ, E., S. SHTBOLET, A. MEROZ, A. MAGAZANIK,AND Y. SHAPIRO. Prediction of heat tolerance from heart rate and rectal temperature in a temperate environment. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 43(4): 684-688, 1977. -To determine if heat tolerance could be predicted from responses to exercise in temperate conditions, 51 young men performed 15 min of bench stepping at an average work load of 80 W at 23°C. On the following day they attempted to perform 3 h of bench stepping at 40 W in heat (39.3”C dry bulb, 30.3”C wet bulb). Of these subjects, 4 were heat intolerant, judged by previous heat stroke episodes during field marches, 12 were heat acclimated, and 35 were unacclimated. The heat-intolerant subjects showed the highest heart rates (HR) and rectal temperatures (T,,) at 23°C and in heat, and the acclimated subjects showed the lowest corresponding values. HR and T,, in each environment were combined into a single score, from 10, indicating the poorest responses, to 100, indicating the best responses. These scores at 23OC when correlated with the corresponding scores in heat resulted in a linear correlation coefficient of r = 0.94 with a standard error of estimate of 8.6%. Scores of the heat-intolerant subjects were below 35, and those of the acclimated subjects were between 70 and 100. Thus heat tolerance can accurately be predicted for HR and T,, responses to exercise at room temperature.
heat intolerance heat
and acclimation;
prediction
of responses
to
TO WORK in heat results in high heart rates (HR) and rectal temperatures (T,,) in some unacclimatized men which could lead to heat stroke or heat syncope, while some unacclimatized men show good responses to heat on first exposure (7, 8, 12). Heat intolerance has been found to be related to age (9), low body weight (ll), and low Vo, max(12), but these three factors are absent in a significant number of heatintolerant men (12). Good responses in heat are related to high Vo, max (2, 12) and to physical training (2, 8, 10); but this relationship is only partial, and attempts to predict responses in heat from Vo, max (12) and submaximal HR recorded at room temperature (3, 5) have been only partially successful. In previous work (7) it was shown that HR and T,, responses recorded after 1 h of exercise at 35 W at room temperature were highly correlated with the same responses found after 4 h of exercise in heat; and heatacclimated subjects showed lower exercise HR and T,, at room temperature then unacclimated subjects. SimiEXPOSURE
University
Medical
AND
Y. SHAPIRO
School,
lar results were also recently found by the present authors in men exercising at 40 W (unpublished observations). It was therefore interesting to determine if heat tolerance could be predicted from a simple exercise test performed at room temperature, similar to the manner in which endurance fitness is predicted from submaximal HR. Like any other performance test, a test for the prediction of heat tolerance would have to be simple, valid, and practical. It was apparent that submaximal HR alone could not accurately predict heat tolerance because submaximal HR is a reflection of endurance fitness which is only partially related to heat tolerance (3, 5). Another parameter, like T,,, was needed for this purpose, but the known lag in its response during exercise would require a long and impractical test. After a considerable number of trials it was found that after 15 min of bench stepping at an average load of 80 W (energy expenditure of 640 W) at room temperature, T,, reached the same levels as those found after 1 h of exercise at 40 W (energy expenditure of 390 W). In many subjects this was not the equilibrium T,,, and the responses to heat of these subjects were poorer than those who did reach equilibrium. T,, however reached high enough levels (a mean of 38.03”C in 26 subjects in preliminary trials) to suggest that it could, in addition to heart rate, predict heat tolerance. The proposed test, therefore, consisted of 15 min of bench stepping at a rate of 24 steps min+ on a bench 30 cm high, which constituted an average work load of 80 W. METHODS
Subjects. The subjects were 51 healthy young men. Their physical characteristics are shown in Table 1. All subjects were given a physical examination before the experiment began. Thirty-five subjects were unacclimated to heat, but six of them had trained in endurance sports for a few years, and ten had engaged in heavy agriculture work periodically. These subjects were tested during late summer so some of them were partially or fully heat acclimatized, as the results later showed. Twelve subjects were acclimated to heat (described in Procedures). Four subjects, who were classified as “heat-intolerant,” had suffered from heat stroke during military marches in the summer seasons of 1971-73. The criteria for heat stroke were T, above 41.OO”Cand collapse. These subjects had been hospitalized for l-3 wk and the occurrence of heat stroke had
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PREDICTION
OF
HEAT
1. Physical
TABLE
Group
Data
are means
characteristics
Age, yr
Ht, cm
20.2 22.5 22.1 22.0 23.6 21.0
175.6 k4.5 176.7 24.8 175.0 54.2
Unacclimated (N = 35) Acclimated (N = 12) Heat intolerant (N = 4)
685
TOLERANCE
of subjects Wt,
kg
68.7 25.2 69.1 k9.5 68.2 + 10.6
VO
2
Surface Area, m2
ml * kg-’ . min-’
1.85 +o.ia 1.86 20.13 1.83 20.11
48.2 28.5 54.1 k5.3 41.2 k4.9
max9
+ SD.
min because of high T,, or HR, and exhaustion. All 12 acclimated subjects and 29 of the unacclimated subjects completed the 3-h heat exposures. Six unacclimated subjects were removed from the climatic chamber after 2-2.5 h for the same reasons as the heat-intolerant subjects. The acclimated subjects showed the lowest HR and T,, at 23°C and in heat (Table 2). Their responses before acclimation were very similar to those shown by the entire group of 35 unacclimated subjects (mean HR and T,, of 148 beats minl and 38.8”C, respectively, in heat, and 133 beats min+ and 38.0°C, respectively, at 23°C). Table 2 also shows that the acclimated subjects had the highest sweat rates in heat and the lowest at 23”C, with the lowest oxygen consumption values shown in both environments. There were no significant differences in voluntary dehydration among the groups, although water intake in the acclimated subjects was higher than in the other groups. Water intake of the unacclimated, acclimated, and heat-intolerant subjects during the heat exposures were 510, 767, and 545 ml h-l, respectively. The value for the heat-acclimated subjects differed significantly (P < 0.05) from those of the other groups. Water intake correlated poorly with final T,, in heat (r = -0.20). The heat-intolerant subjects showed the lowest Oo, max (Table l), and heat acclimation resulted in a 10% increase in 00, max from a mean of 48.7 to 54.1 ml kg-l 0min+ (P < 0.05). Table 2 shows that the correlation coefficients between HR, T,,, sweat rate, and oxygen consumption responses at 23°C and the corresponding responses in heat ranges from 0.59 to 0.68. Prediction of heat tolerance. As shown in Table 2, responses in temperate and hot environments are related, but the efficiency of prediction of a particular parameter in heat from the corresponding parameter at 23°C would be less than 30%. When one parameter at 23°C was correlated with another parameter in heat, such as HR versus T,,, similar correlation coefficients were obtained, ranging from 0.52 to 0.68. It was, therefore, clear that to predict heat tolerance it was necessary to consider more than one parameter at 23°C. Obviously, HR and T,, were considered for this purpose because these parameters were more valid represental
l
been confirmed by blood analyses. None of these cases occurred during water deprivation. Procedures. The 35 unacclimated subjects were tested during late summer (September, October) and the acclimated and heat-intolerant subjects were tested in the winter (December, January). The subjects reported to the laboratory at 9 A.M., rested for 1 h, and performed the test (bench stepping at a rate of 24 steps minl on a bench 30 cm high) for 15 min at room temperature of 23°C dry bulb, 16°C wet bulb, and wind speed less than 0.2 m s-l. The subjects wore shorts and tennis shoes only. HR, T,,, and oxygen consumption were recorded before the end of exercise, and weight loss was determined for the entire 15min period. After 3 h of rest, VO 2 max was determined on a treadmill. The subject ran at a constant speed of PO km* h-l and the grade was elevated every 2 min to cause exhaustion within 7-9 min. Air samples were collected during the last minute of exercise. On the following day, the subjects attempted to perform 3 h of bench stepping at a rate of 12 steps min-l (average work load of 40 W) in heat (39.3”C db, 30.3”C wb, and wind speed less than 0.2 me s-l). HR and T,, were recorded every 30 min, oxygen consumption was determined after 1 h, and weight loss was determined each hour. Exposure was discontinued when T,, exceeded 39.6”C, or when the subject was exhausted because of a HR above 180 beats min. The subjects wore shorts and tennis shoes only and drank water freely. Acclimation in 12 subjects was performed by an additional 7 days of exposure to the conditions described above, and the 15min test at 23°C was administered again on the day following acclimation. Measurements. HR was recorded with the aid of stethoscopes by experienced observers. T, was recorded with thermistors inserted 10 cm beyond the anal sphincter. Oxygen consumption was determined by the collection of the expired air into Douglas bags, and the analysis of gas was performed with a paramagnetic oxygen analyzer (Beckman E,) and an infrared Co, analyzer (Beckman LB,). Sweat rate was determined by weighing the nude subject before and after exposure on a scale sensitive to t10 g and correcting for water intake, urine output, and respiratory and metabolic weight losses (6). l
l
l
l
TABLE
2. Responses to exercise at 23°C and in heat Subj
Unacclimated (N = 35)
HR, beats * min-’
T l-2, “C
Sweat Rate, ml. h-l
Oxygen Cons, 1. min-’
23°C
38.0* kO.26 38.7” 20.51 37.7* kO.20 38.2* 20.33 38.3* ko.10 39.5” kO.33 0.62
444 579 545 2136 368t 291 632t &lo9 457 +70 575 227 0.59
1.916 20.225 1.198 kO.260 1.795 kO.201 1.044” to.090 1.856 20.127 1.114 20.038 0.68
Heat Acclimated (N = 12)
23°C Heat
Heat intolerant (N = 4)
23°C Heat
RESULTS
Responses at 23°C and in heat. Table 2 shows that the heat-intolerant subjects had the highest HR and T,, during the 15-min test at 23°C and in heat. None of these subjects could tolerate 3 h of heat exposure and were removed from the climatic chamber after 60-100
Data are means + SD. HR and T , are at 23”C, and during 3 h of exercise at 40 the 3-h exposures. Correlation coefficients < 0.01 compared with each of the other 2 groups. All correlation coefficients were operations and Pearson product correlation
134.2” 217.7 146.5* t17.9 120.5” 210.5 122.1” k9.4 158.0* k4.0 167.1” +ll.l 0.68
final responses after 15 min of exercise at 80 W W in heat. Sweat rates in heat are averages for * P are between responses at 23°C and in heat. groups. t P < 0.05 compared with the other 2 significant (P < 0.01). Analysis of variance coefficients were used.
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686
SHVARTZ
tives of heat tolerance than oxygen consumption and sweat rate. Thus, it was decided to consider the possibility of predicting heat tolerance, in terms of HR and T,, responses during 3 h of work in heat, from the same parameters recorded after 15 min of exercise at a load of 80 W (energy expenditure of 640 W) at 23°C. HR and T,, recorded in each environment were combined into a single composite score which represented responses to either the temperate or hot environment. Equal weighting was given to HR and T,,. This was done after other alternatives were considered. One alternative was to give more weighting to T,, than to HR, because T,, has been considered to be a more valid indicator of heat tolerance than HR. However, it has been our experience in the present study as well as in many previous studies that about half of a population of young men, working at different work loads in a wide range of hot environments, are exhausted because of high HR. The correlation coefficient between HR and T,, in warm environments is about 0.7 (4), and a similar relationship was found in the present study. Furthermore, subjects who display high HR in heat have a greater chance to develop heat syncope than subjects who show high T,, (8) . HR and T,, in each environment were given arbitrary scores from 10 to 100 as shown in Table 3. A score of 10 indicated very high HR or T,, (poor responses) and a score of 100 indicated very low values- and best responses. The composite scores for each environment were obtained by adding the HR and T, scores and dividing the sum by 2. The same value can be obtained by finding the appropriate score which corresponds to a particular HR and T,, response. For example, if an individual shows HR and T,, values of 125 beats min-l and 37.9”C, respectively, at 23”C, this corresponds to a HR score of 70 and a T,, score of 60 (Table 3), and therefore a composite score at 23°C of 65. The composite scores at 23°C were correlated with the composite scores in heat (Fig. 1). It is noted that all
ET
AL.
four heat-intolerant subjects received low scores of lo30 at 23°C and in heat. Five of the unacclimated subjects also received very low scores. These were the subjects who could not complete the 3-h exposures, showing similar responses to those of the heat-intolerant subjects. Ten of the heat-acclimated subjects received high scores of 75-100. Two acclimated subjects received scores of 65 and 70 because they acclimated only partially (HR during last heat exposure of 124 and 126 beats min+, and T,, 38.7 and 38.9”C, respectively). Figure 1 also shows that 10 unacclimated subjects received high scores of 75-95. These were 4 of the 6 trained subjects and 6 of the 10 subjects who had been engaged in outdoor work and tested during late summer. l
100
0v
. . .. 28 A0
20 . ,I(
,
0
20
I
40 COMPOSITE
1
60 SCOR,-
AT r..
FS
1
80
100
7’?‘r. v -v
+ r)c)oyc FIG. 1. Correlation coerricienl; - - - rP -’ L ‘-oer;ween * composir;e -1 scoreb .” n51~ ~3 b and in heat. 0 = Unac climated (35 + 12 subjects before acclimation); l = acclimated* subiects. r = 0.94: y = 1.07 x . and kL = heat-tolerant ‘d - 3.33; SD, = i2.38; SD, = 25.67; SE,,, = 8v.59.
TABLE 3. Scores and composite scores for heart rate and rectal temperature responses to exercise at 23°C and in heat 23°C:
Score: * rate,
37.7
37.8
38.1
38.2
38.3
38.4+
38.2 I 38.3
38.4 I 38.5
38.6 I 38.7
38.8 I 38.9
39.0 I 39.1
39.2 I 39.3
39.4
39.6+
-37.9
38.0 I 38.1
39.5
100
90
80
70
60
50
40
30
20
10
70 65 60 55 50 45 40 35 30 25
65 60 55 50 45 40 35 30 25 20
60 55 50 45 40 35 30 25 20 15
55 50 45 40 35 30 25 20 15 10
I
beats. min-l Score*
23°C
Heat
-105 106-113 114-121 122-129 130-137 138-145 146-151 152-159 160-167 168 +
-110 111-118 119-126 127-134 135-142 143-150 151-158 159-166 167-173 174 +
*Scores temperature
“C
37.6
Heat:
Heart
Rectal Temperature, 37.9 38.0
-37.5
for heart score)/2
100 90 80 70 60 50 40 30 20 10
Composite
100 95 90 85 80 75 70 65 60 55
95 90 85 80 75 70 65 60 55 50
90 85 80 75 70 65 60 55 50 45
rate or rectal temperature responses to exercise equal the appropriate composite score.
85 80 75 70 65 60 55 50 45 40
80 75 70 65 60 55 50 45 40 35
at 23°C or in heat.
scores
75 70 65 60 55 50 45 40 35 30 In each
environment,
(heart
rate
score
+ rectal
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PREDICTION
OF
HEAT
687
TOLERANCE
DISCUSSION
The results of the present study show that heat tolerance can accurately be predicted in men from a simple 15min exercise test performed in temperate conditions. Until more information is available with respect to sex and age, this test would apply to men aged 17-35 yr only. The test consists of bench stepping on a bench 30 cm high, at a rate of 24 steps min. This arrangement, in which the same bench is used for everybody, is the simplest and probably the most practical and realistic one. Having each individual work at a given percentage of his vo, max would greatly complicate this test, and having each individual work at a fixed work load would require different benches and it would not constitute a realistic test in most cases. We are usually interested in the performance of men standing, walking, or running in heat, sometimes uphill. In most cases, therefore, the heavier man performs more work than the lightweight one and the present test simulates these conditions. To predict heat tolerance in men, the following procedure should be followed. 1) Make sure that the subject has an appropriate rest period before starting the test and that he has not engaged in exercise for several hours before taking the test. 2) The subject performs bench stepping on a bench 30 cm high at a rate of 24 steps min-l for 15 min, wearing shorts only or also tennis shoes. Testing should be performed in ambient conditions of 21-25°C db, 15-19°C wb, and wind speed not more than 0.4 ms-l. 3) HR (preferably with stethoscopes) is recorded during the last minute of exercise, and T,, is recorded at the end of exercise or immediately after the cessation of exercise if a clinical thermometer is used. On rare occasions when a subject cannot complete the 15-min test, his HR at the time of exhaustion should be recorded, and if it is above 170 beats min+ this HR and the corresponding T,, should be considered. It is very unlikely that exhaustion could occur before 15 min because of high T,,, even in men suffering from malfunction of the thermoregulatory system. If such an unlikely event occurs, T,, at the time of exhaustion should be considered, if it is above 39.0°C, and the corresponding HR. 4) The subject’s heat tolerance score is found by adding the HR score and the T,, score, shown in Table 3, and dividing the sum by 2; or simply by finding the composite score which corresponds to given HR and T,,, also shown in Table 3. The heat tolerance score is the composite at 23°C. The prediction of heat tolerance from HR and T,, responses recorded in temperate conditions is made possible because responses in temperate and hot environments are related (Table 2). An individual in poor HR physical condition who shows a high submaximal in -a temperate environment, also tends to show high HR during exercise in heat. An individual who shows a relatively high heat production value and a relatively low sweat rate value in a temperate environment would l
l
also tend to show the corresponding responses in heat, which would affect his T,, response. There is an exception with respect to sweat-rate responses. Heat-acclimated men show high sweat rates in heat but low sweat rates in a temperate environment (Table 2). The correlation coefficient for sweat rate responses between the two environments, excluding the responses of the heat-acclimated subjects, was 0.72 instead of 0.59 shown in Table 2 for all subjects. The low sweat-rate responses of heat-acclimated men in temperate conditions, however, does not result in high T,, because these men also show low resting T,, values, low exercise heat production, and efficient cardiovascular systems which result, in low exercise T,,, as shown in Table 2. In short, as Fig. 1 shows, heat tolerance can accurately be predicted from HR and T,, responses to exercise in a temperate environment. Physical fitness or submaximal HR alone could not accurately predict heat tolerance. As shown in Table 2, the correlation coefficient between HR at 23°C and in heat was only 0.68, and similar relationships were found when HR at 23°C was correlated with T,, in heat or with the composite scores in heat. Although Table 1 subjects is shows that 00, max of the heat-intolerant lower than that of the other groups (41.2 ml kg-l min?, this value is not very low by any standards, and the correlation coefficient between vo, max and final T,, in heat of all subjects was only -0.57. It is clear then that although physically fit men tend to show better responses to heat than unfit men and heat-intolerant men tend to be unfit, it is not possible to accurately predict heat tolerance from physical fitness alone. The various correlation coefficients found in the present study suggest that the efficiency of prediction of heat tolerance on the basis of submaximal HR recorded in a temperate environment is 30%. Figure 1 shows that the corresponding predictive efficiency for a combined score of HR and T,, is 66%. The validity of the present test is very similar to that described by Astrand and Rodahl (1) for the prediction from submaximal HR. In the prediction of Of vQ? max heat tolerance, however, we are usually interested in qualitative classification of responses. For this purpose, the present test provides very useful information. Individuals who score below 35 can be expected to become heat intolerant when exercising in heat, and individuals who score above 75 can be expected to show typical responses of acclimatized men. More work is needed for the possible improvement of this test with respect to duration, load, and mode of exercise, and the effect of age, sex, and different hot environments. It is also possible that oral, instead of rectal, temperature could be used, which would greatly simplify the administration of this test. l
l
The authors express their appreciation to Y. Shoenfeld, A. Lev, G. Ben-Baruch, H. Birnfeld, A. Mechtinger, and S. Strasburg for very helpful assistance, and to E. Kamon for a critical review of the manuscript. This research was supported by a grant from the United StatesIsrael Binational Science Foundation (BSF), Jerusalem, Israel. Received
for publication
10 March
1977.
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688
SHVARTZ
ET
AL.
REFERENCES 1. ASTRAND, P.-O., AND K. RODAHL. Textbook of Work Physiology. New York: McGraw, 1970, p 617-620. 2. GISOLFI, C. V. Work-heat tolerance derived from interval training. J. Appl. Physiol. 35: 349 - 354, 1973. 3. HAUSMANN, A., D. BALEYEW, AND J. PATIGAY. Selection criteria for rescue workers operating in hot atmospheres. Rev. Inst. Hyg. Mines 21: 36-48, 1966. 4. KAMON, E., AND H. S. BELDING. Heart rate and rectal temperature relationships during work in hot humid environments. J. Appl. Physiol. 31: 472-477, 1971. 5. LAVENNE, L., AND D. BELAYEW. Exercise tolerance test at room temperature for the purpose of selecting rescue teams for training in a hot climate. Rev. Inst. Hyg. Mines 21: 48-58, 1966. 6. MITCHELL, J. W., E. R. NADEL, AND J. A. J. STOLWIJK. Respiratory weight losses during exercise. J. Appl. Physiol. 32: 474476, 1972. 7. SHVARTZ, E. Physical performance of heat-adapted individuals.
8. 9. 10.
11.
12
In: Environmental Biology, edited by Bhatia, Chhina, and Singh. New Delhi: Interprint, 1976, chapt. 20. SHVARTZ, E., N. B. STRYDOM, AND H. KOTZE. Orthostatism and heat acclimation. J. Appl. Physiol. 39: 590-595, 1975. STRYDOM, N. B. Age as a causal factor in heat stroke. J. S. African Inst. Mining Met. 72: 112-114, 1971. STRYDOM, N. B., AND C. G. WILLIAMS. Effect of physical conditioning on state of heat acclimatization of Bantu laborers. J. AppZ. Physiol. 27: 262-265, 1969. STRYDOM, N. B., C. H. WYNDHAM, AND A. J. S. BENADE. The responses of men weighing less than 50 kg to the standard climatic room acclimatization procedure. J. S. African Inst. Mining Met. 72: 101-104, 1971. WYNDHAM, C. H., N. B. STRYDOM, C. G. WILLIAMS, AND A. HEYNS. An examination of certain individual factors affecting the heat tolerance of mine workers. J. S. African Inst. Mining Met. 68: 79-91, 1967.
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of heat tolerance from heart rate temperature in a temperate environment
E. SHVARTZ,
S. SHIBOLET,
Heller Sheba
of Medical Research, Center, Tel-Hashomer,
Institute Medical
A. MEROZ,
A. MAGAZANIK,
Tel-Aviv Israel
SHVARTZ, E., S. SHTBOLET, A. MEROZ, A. MAGAZANIK,AND Y. SHAPIRO. Prediction of heat tolerance from heart rate and rectal temperature in a temperate environment. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 43(4): 684-688, 1977. -To determine if heat tolerance could be predicted from responses to exercise in temperate conditions, 51 young men performed 15 min of bench stepping at an average work load of 80 W at 23°C. On the following day they attempted to perform 3 h of bench stepping at 40 W in heat (39.3”C dry bulb, 30.3”C wet bulb). Of these subjects, 4 were heat intolerant, judged by previous heat stroke episodes during field marches, 12 were heat acclimated, and 35 were unacclimated. The heat-intolerant subjects showed the highest heart rates (HR) and rectal temperatures (T,,) at 23°C and in heat, and the acclimated subjects showed the lowest corresponding values. HR and T,, in each environment were combined into a single score, from 10, indicating the poorest responses, to 100, indicating the best responses. These scores at 23OC when correlated with the corresponding scores in heat resulted in a linear correlation coefficient of r = 0.94 with a standard error of estimate of 8.6%. Scores of the heat-intolerant subjects were below 35, and those of the acclimated subjects were between 70 and 100. Thus heat tolerance can accurately be predicted for HR and T,, responses to exercise at room temperature.
heat intolerance heat
and acclimation;
prediction
of responses
to
TO WORK in heat results in high heart rates (HR) and rectal temperatures (T,,) in some unacclimatized men which could lead to heat stroke or heat syncope, while some unacclimatized men show good responses to heat on first exposure (7, 8, 12). Heat intolerance has been found to be related to age (9), low body weight (ll), and low Vo, max(12), but these three factors are absent in a significant number of heatintolerant men (12). Good responses in heat are related to high Vo, max (2, 12) and to physical training (2, 8, 10); but this relationship is only partial, and attempts to predict responses in heat from Vo, max (12) and submaximal HR recorded at room temperature (3, 5) have been only partially successful. In previous work (7) it was shown that HR and T,, responses recorded after 1 h of exercise at 35 W at room temperature were highly correlated with the same responses found after 4 h of exercise in heat; and heatacclimated subjects showed lower exercise HR and T,, at room temperature then unacclimated subjects. SimiEXPOSURE
University
Medical
AND
Y. SHAPIRO
School,
lar results were also recently found by the present authors in men exercising at 40 W (unpublished observations). It was therefore interesting to determine if heat tolerance could be predicted from a simple exercise test performed at room temperature, similar to the manner in which endurance fitness is predicted from submaximal HR. Like any other performance test, a test for the prediction of heat tolerance would have to be simple, valid, and practical. It was apparent that submaximal HR alone could not accurately predict heat tolerance because submaximal HR is a reflection of endurance fitness which is only partially related to heat tolerance (3, 5). Another parameter, like T,,, was needed for this purpose, but the known lag in its response during exercise would require a long and impractical test. After a considerable number of trials it was found that after 15 min of bench stepping at an average load of 80 W (energy expenditure of 640 W) at room temperature, T,, reached the same levels as those found after 1 h of exercise at 40 W (energy expenditure of 390 W). In many subjects this was not the equilibrium T,,, and the responses to heat of these subjects were poorer than those who did reach equilibrium. T,, however reached high enough levels (a mean of 38.03”C in 26 subjects in preliminary trials) to suggest that it could, in addition to heart rate, predict heat tolerance. The proposed test, therefore, consisted of 15 min of bench stepping at a rate of 24 steps min+ on a bench 30 cm high, which constituted an average work load of 80 W. METHODS
Subjects. The subjects were 51 healthy young men. Their physical characteristics are shown in Table 1. All subjects were given a physical examination before the experiment began. Thirty-five subjects were unacclimated to heat, but six of them had trained in endurance sports for a few years, and ten had engaged in heavy agriculture work periodically. These subjects were tested during late summer so some of them were partially or fully heat acclimatized, as the results later showed. Twelve subjects were acclimated to heat (described in Procedures). Four subjects, who were classified as “heat-intolerant,” had suffered from heat stroke during military marches in the summer seasons of 1971-73. The criteria for heat stroke were T, above 41.OO”Cand collapse. These subjects had been hospitalized for l-3 wk and the occurrence of heat stroke had
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
PREDICTION
OF
HEAT
1. Physical
TABLE
Group
Data
are means
characteristics
Age, yr
Ht, cm
20.2 22.5 22.1 22.0 23.6 21.0
175.6 k4.5 176.7 24.8 175.0 54.2
Unacclimated (N = 35) Acclimated (N = 12) Heat intolerant (N = 4)
685
TOLERANCE
of subjects Wt,
kg
68.7 25.2 69.1 k9.5 68.2 + 10.6
VO
2
Surface Area, m2
ml * kg-’ . min-’
1.85 +o.ia 1.86 20.13 1.83 20.11
48.2 28.5 54.1 k5.3 41.2 k4.9
max9
+ SD.
min because of high T,, or HR, and exhaustion. All 12 acclimated subjects and 29 of the unacclimated subjects completed the 3-h heat exposures. Six unacclimated subjects were removed from the climatic chamber after 2-2.5 h for the same reasons as the heat-intolerant subjects. The acclimated subjects showed the lowest HR and T,, at 23°C and in heat (Table 2). Their responses before acclimation were very similar to those shown by the entire group of 35 unacclimated subjects (mean HR and T,, of 148 beats minl and 38.8”C, respectively, in heat, and 133 beats min+ and 38.0°C, respectively, at 23°C). Table 2 also shows that the acclimated subjects had the highest sweat rates in heat and the lowest at 23”C, with the lowest oxygen consumption values shown in both environments. There were no significant differences in voluntary dehydration among the groups, although water intake in the acclimated subjects was higher than in the other groups. Water intake of the unacclimated, acclimated, and heat-intolerant subjects during the heat exposures were 510, 767, and 545 ml h-l, respectively. The value for the heat-acclimated subjects differed significantly (P < 0.05) from those of the other groups. Water intake correlated poorly with final T,, in heat (r = -0.20). The heat-intolerant subjects showed the lowest Oo, max (Table l), and heat acclimation resulted in a 10% increase in 00, max from a mean of 48.7 to 54.1 ml kg-l 0min+ (P < 0.05). Table 2 shows that the correlation coefficients between HR, T,,, sweat rate, and oxygen consumption responses at 23°C and the corresponding responses in heat ranges from 0.59 to 0.68. Prediction of heat tolerance. As shown in Table 2, responses in temperate and hot environments are related, but the efficiency of prediction of a particular parameter in heat from the corresponding parameter at 23°C would be less than 30%. When one parameter at 23°C was correlated with another parameter in heat, such as HR versus T,,, similar correlation coefficients were obtained, ranging from 0.52 to 0.68. It was, therefore, clear that to predict heat tolerance it was necessary to consider more than one parameter at 23°C. Obviously, HR and T,, were considered for this purpose because these parameters were more valid represental
l
been confirmed by blood analyses. None of these cases occurred during water deprivation. Procedures. The 35 unacclimated subjects were tested during late summer (September, October) and the acclimated and heat-intolerant subjects were tested in the winter (December, January). The subjects reported to the laboratory at 9 A.M., rested for 1 h, and performed the test (bench stepping at a rate of 24 steps minl on a bench 30 cm high) for 15 min at room temperature of 23°C dry bulb, 16°C wet bulb, and wind speed less than 0.2 m s-l. The subjects wore shorts and tennis shoes only. HR, T,,, and oxygen consumption were recorded before the end of exercise, and weight loss was determined for the entire 15min period. After 3 h of rest, VO 2 max was determined on a treadmill. The subject ran at a constant speed of PO km* h-l and the grade was elevated every 2 min to cause exhaustion within 7-9 min. Air samples were collected during the last minute of exercise. On the following day, the subjects attempted to perform 3 h of bench stepping at a rate of 12 steps min-l (average work load of 40 W) in heat (39.3”C db, 30.3”C wb, and wind speed less than 0.2 me s-l). HR and T,, were recorded every 30 min, oxygen consumption was determined after 1 h, and weight loss was determined each hour. Exposure was discontinued when T,, exceeded 39.6”C, or when the subject was exhausted because of a HR above 180 beats min. The subjects wore shorts and tennis shoes only and drank water freely. Acclimation in 12 subjects was performed by an additional 7 days of exposure to the conditions described above, and the 15min test at 23°C was administered again on the day following acclimation. Measurements. HR was recorded with the aid of stethoscopes by experienced observers. T, was recorded with thermistors inserted 10 cm beyond the anal sphincter. Oxygen consumption was determined by the collection of the expired air into Douglas bags, and the analysis of gas was performed with a paramagnetic oxygen analyzer (Beckman E,) and an infrared Co, analyzer (Beckman LB,). Sweat rate was determined by weighing the nude subject before and after exposure on a scale sensitive to t10 g and correcting for water intake, urine output, and respiratory and metabolic weight losses (6). l
l
l
l
TABLE
2. Responses to exercise at 23°C and in heat Subj
Unacclimated (N = 35)
HR, beats * min-’
T l-2, “C
Sweat Rate, ml. h-l
Oxygen Cons, 1. min-’
23°C
38.0* kO.26 38.7” 20.51 37.7* kO.20 38.2* 20.33 38.3* ko.10 39.5” kO.33 0.62
444 579 545 2136 368t 291 632t &lo9 457 +70 575 227 0.59
1.916 20.225 1.198 kO.260 1.795 kO.201 1.044” to.090 1.856 20.127 1.114 20.038 0.68
Heat Acclimated (N = 12)
23°C Heat
Heat intolerant (N = 4)
23°C Heat
RESULTS
Responses at 23°C and in heat. Table 2 shows that the heat-intolerant subjects had the highest HR and T,, during the 15-min test at 23°C and in heat. None of these subjects could tolerate 3 h of heat exposure and were removed from the climatic chamber after 60-100
Data are means + SD. HR and T , are at 23”C, and during 3 h of exercise at 40 the 3-h exposures. Correlation coefficients < 0.01 compared with each of the other 2 groups. All correlation coefficients were operations and Pearson product correlation
134.2” 217.7 146.5* t17.9 120.5” 210.5 122.1” k9.4 158.0* k4.0 167.1” +ll.l 0.68
final responses after 15 min of exercise at 80 W W in heat. Sweat rates in heat are averages for * P are between responses at 23°C and in heat. groups. t P < 0.05 compared with the other 2 significant (P < 0.01). Analysis of variance coefficients were used.
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686
SHVARTZ
tives of heat tolerance than oxygen consumption and sweat rate. Thus, it was decided to consider the possibility of predicting heat tolerance, in terms of HR and T,, responses during 3 h of work in heat, from the same parameters recorded after 15 min of exercise at a load of 80 W (energy expenditure of 640 W) at 23°C. HR and T,, recorded in each environment were combined into a single composite score which represented responses to either the temperate or hot environment. Equal weighting was given to HR and T,,. This was done after other alternatives were considered. One alternative was to give more weighting to T,, than to HR, because T,, has been considered to be a more valid indicator of heat tolerance than HR. However, it has been our experience in the present study as well as in many previous studies that about half of a population of young men, working at different work loads in a wide range of hot environments, are exhausted because of high HR. The correlation coefficient between HR and T,, in warm environments is about 0.7 (4), and a similar relationship was found in the present study. Furthermore, subjects who display high HR in heat have a greater chance to develop heat syncope than subjects who show high T,, (8) . HR and T,, in each environment were given arbitrary scores from 10 to 100 as shown in Table 3. A score of 10 indicated very high HR or T,, (poor responses) and a score of 100 indicated very low values- and best responses. The composite scores for each environment were obtained by adding the HR and T, scores and dividing the sum by 2. The same value can be obtained by finding the appropriate score which corresponds to a particular HR and T,, response. For example, if an individual shows HR and T,, values of 125 beats min-l and 37.9”C, respectively, at 23”C, this corresponds to a HR score of 70 and a T,, score of 60 (Table 3), and therefore a composite score at 23°C of 65. The composite scores at 23°C were correlated with the composite scores in heat (Fig. 1). It is noted that all
ET
AL.
four heat-intolerant subjects received low scores of lo30 at 23°C and in heat. Five of the unacclimated subjects also received very low scores. These were the subjects who could not complete the 3-h exposures, showing similar responses to those of the heat-intolerant subjects. Ten of the heat-acclimated subjects received high scores of 75-100. Two acclimated subjects received scores of 65 and 70 because they acclimated only partially (HR during last heat exposure of 124 and 126 beats min+, and T,, 38.7 and 38.9”C, respectively). Figure 1 also shows that 10 unacclimated subjects received high scores of 75-95. These were 4 of the 6 trained subjects and 6 of the 10 subjects who had been engaged in outdoor work and tested during late summer. l
100
0v
. . .. 28 A0
20 . ,I(
,
0
20
I
40 COMPOSITE
1
60 SCOR,-
AT r..
FS
1
80
100
7’?‘r. v -v
+ r)c)oyc FIG. 1. Correlation coerricienl; - - - rP -’ L ‘-oer;ween * composir;e -1 scoreb .” n51~ ~3 b and in heat. 0 = Unac climated (35 + 12 subjects before acclimation); l = acclimated* subiects. r = 0.94: y = 1.07 x . and kL = heat-tolerant ‘d - 3.33; SD, = i2.38; SD, = 25.67; SE,,, = 8v.59.
TABLE 3. Scores and composite scores for heart rate and rectal temperature responses to exercise at 23°C and in heat 23°C:
Score: * rate,
37.7
37.8
38.1
38.2
38.3
38.4+
38.2 I 38.3
38.4 I 38.5
38.6 I 38.7
38.8 I 38.9
39.0 I 39.1
39.2 I 39.3
39.4
39.6+
-37.9
38.0 I 38.1
39.5
100
90
80
70
60
50
40
30
20
10
70 65 60 55 50 45 40 35 30 25
65 60 55 50 45 40 35 30 25 20
60 55 50 45 40 35 30 25 20 15
55 50 45 40 35 30 25 20 15 10
I
beats. min-l Score*
23°C
Heat
-105 106-113 114-121 122-129 130-137 138-145 146-151 152-159 160-167 168 +
-110 111-118 119-126 127-134 135-142 143-150 151-158 159-166 167-173 174 +
*Scores temperature
“C
37.6
Heat:
Heart
Rectal Temperature, 37.9 38.0
-37.5
for heart score)/2
100 90 80 70 60 50 40 30 20 10
Composite
100 95 90 85 80 75 70 65 60 55
95 90 85 80 75 70 65 60 55 50
90 85 80 75 70 65 60 55 50 45
rate or rectal temperature responses to exercise equal the appropriate composite score.
85 80 75 70 65 60 55 50 45 40
80 75 70 65 60 55 50 45 40 35
at 23°C or in heat.
scores
75 70 65 60 55 50 45 40 35 30 In each
environment,
(heart
rate
score
+ rectal
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PREDICTION
OF
HEAT
687
TOLERANCE
DISCUSSION
The results of the present study show that heat tolerance can accurately be predicted in men from a simple 15min exercise test performed in temperate conditions. Until more information is available with respect to sex and age, this test would apply to men aged 17-35 yr only. The test consists of bench stepping on a bench 30 cm high, at a rate of 24 steps min. This arrangement, in which the same bench is used for everybody, is the simplest and probably the most practical and realistic one. Having each individual work at a given percentage of his vo, max would greatly complicate this test, and having each individual work at a fixed work load would require different benches and it would not constitute a realistic test in most cases. We are usually interested in the performance of men standing, walking, or running in heat, sometimes uphill. In most cases, therefore, the heavier man performs more work than the lightweight one and the present test simulates these conditions. To predict heat tolerance in men, the following procedure should be followed. 1) Make sure that the subject has an appropriate rest period before starting the test and that he has not engaged in exercise for several hours before taking the test. 2) The subject performs bench stepping on a bench 30 cm high at a rate of 24 steps min-l for 15 min, wearing shorts only or also tennis shoes. Testing should be performed in ambient conditions of 21-25°C db, 15-19°C wb, and wind speed not more than 0.4 ms-l. 3) HR (preferably with stethoscopes) is recorded during the last minute of exercise, and T,, is recorded at the end of exercise or immediately after the cessation of exercise if a clinical thermometer is used. On rare occasions when a subject cannot complete the 15-min test, his HR at the time of exhaustion should be recorded, and if it is above 170 beats min+ this HR and the corresponding T,, should be considered. It is very unlikely that exhaustion could occur before 15 min because of high T,,, even in men suffering from malfunction of the thermoregulatory system. If such an unlikely event occurs, T,, at the time of exhaustion should be considered, if it is above 39.0°C, and the corresponding HR. 4) The subject’s heat tolerance score is found by adding the HR score and the T,, score, shown in Table 3, and dividing the sum by 2; or simply by finding the composite score which corresponds to given HR and T,,, also shown in Table 3. The heat tolerance score is the composite at 23°C. The prediction of heat tolerance from HR and T,, responses recorded in temperate conditions is made possible because responses in temperate and hot environments are related (Table 2). An individual in poor HR physical condition who shows a high submaximal in -a temperate environment, also tends to show high HR during exercise in heat. An individual who shows a relatively high heat production value and a relatively low sweat rate value in a temperate environment would l
l
also tend to show the corresponding responses in heat, which would affect his T,, response. There is an exception with respect to sweat-rate responses. Heat-acclimated men show high sweat rates in heat but low sweat rates in a temperate environment (Table 2). The correlation coefficient for sweat rate responses between the two environments, excluding the responses of the heat-acclimated subjects, was 0.72 instead of 0.59 shown in Table 2 for all subjects. The low sweat-rate responses of heat-acclimated men in temperate conditions, however, does not result in high T,, because these men also show low resting T,, values, low exercise heat production, and efficient cardiovascular systems which result, in low exercise T,,, as shown in Table 2. In short, as Fig. 1 shows, heat tolerance can accurately be predicted from HR and T,, responses to exercise in a temperate environment. Physical fitness or submaximal HR alone could not accurately predict heat tolerance. As shown in Table 2, the correlation coefficient between HR at 23°C and in heat was only 0.68, and similar relationships were found when HR at 23°C was correlated with T,, in heat or with the composite scores in heat. Although Table 1 subjects is shows that 00, max of the heat-intolerant lower than that of the other groups (41.2 ml kg-l min?, this value is not very low by any standards, and the correlation coefficient between vo, max and final T,, in heat of all subjects was only -0.57. It is clear then that although physically fit men tend to show better responses to heat than unfit men and heat-intolerant men tend to be unfit, it is not possible to accurately predict heat tolerance from physical fitness alone. The various correlation coefficients found in the present study suggest that the efficiency of prediction of heat tolerance on the basis of submaximal HR recorded in a temperate environment is 30%. Figure 1 shows that the corresponding predictive efficiency for a combined score of HR and T,, is 66%. The validity of the present test is very similar to that described by Astrand and Rodahl (1) for the prediction from submaximal HR. In the prediction of Of vQ? max heat tolerance, however, we are usually interested in qualitative classification of responses. For this purpose, the present test provides very useful information. Individuals who score below 35 can be expected to become heat intolerant when exercising in heat, and individuals who score above 75 can be expected to show typical responses of acclimatized men. More work is needed for the possible improvement of this test with respect to duration, load, and mode of exercise, and the effect of age, sex, and different hot environments. It is also possible that oral, instead of rectal, temperature could be used, which would greatly simplify the administration of this test. l
l
The authors express their appreciation to Y. Shoenfeld, A. Lev, G. Ben-Baruch, H. Birnfeld, A. Mechtinger, and S. Strasburg for very helpful assistance, and to E. Kamon for a critical review of the manuscript. This research was supported by a grant from the United StatesIsrael Binational Science Foundation (BSF), Jerusalem, Israel. Received
for publication
10 March
1977.
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688
SHVARTZ
ET
AL.
REFERENCES 1. ASTRAND, P.-O., AND K. RODAHL. Textbook of Work Physiology. New York: McGraw, 1970, p 617-620. 2. GISOLFI, C. V. Work-heat tolerance derived from interval training. J. Appl. Physiol. 35: 349 - 354, 1973. 3. HAUSMANN, A., D. BALEYEW, AND J. PATIGAY. Selection criteria for rescue workers operating in hot atmospheres. Rev. Inst. Hyg. Mines 21: 36-48, 1966. 4. KAMON, E., AND H. S. BELDING. Heart rate and rectal temperature relationships during work in hot humid environments. J. Appl. Physiol. 31: 472-477, 1971. 5. LAVENNE, L., AND D. BELAYEW. Exercise tolerance test at room temperature for the purpose of selecting rescue teams for training in a hot climate. Rev. Inst. Hyg. Mines 21: 48-58, 1966. 6. MITCHELL, J. W., E. R. NADEL, AND J. A. J. STOLWIJK. Respiratory weight losses during exercise. J. Appl. Physiol. 32: 474476, 1972. 7. SHVARTZ, E. Physical performance of heat-adapted individuals.
8. 9. 10.
11.
12
In: Environmental Biology, edited by Bhatia, Chhina, and Singh. New Delhi: Interprint, 1976, chapt. 20. SHVARTZ, E., N. B. STRYDOM, AND H. KOTZE. Orthostatism and heat acclimation. J. Appl. Physiol. 39: 590-595, 1975. STRYDOM, N. B. Age as a causal factor in heat stroke. J. S. African Inst. Mining Met. 72: 112-114, 1971. STRYDOM, N. B., AND C. G. WILLIAMS. Effect of physical conditioning on state of heat acclimatization of Bantu laborers. J. AppZ. Physiol. 27: 262-265, 1969. STRYDOM, N. B., C. H. WYNDHAM, AND A. J. S. BENADE. The responses of men weighing less than 50 kg to the standard climatic room acclimatization procedure. J. S. African Inst. Mining Met. 72: 101-104, 1971. WYNDHAM, C. H., N. B. STRYDOM, C. G. WILLIAMS, AND A. HEYNS. An examination of certain individual factors affecting the heat tolerance of mine workers. J. S. African Inst. Mining Met. 68: 79-91, 1967.
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