Circulatory and thermal responses of men with different training status to prolonged physical work in dry and humid heat.

Eight physically trained and eight untrained, unacclimated men walked on a treadmill at 30% of their maximum oxygen consumption up to 3.5 h in a thermoneutral [20 degrees C/40% relative humidity (RH)], a warm humid (30 degrees C/80% RH), and a hot dry (40 degrees C/20% RH) environment while wearing industrial work clothing. Their oxygen consumption, rectal and skin temperatures, sweating, cardiac output, heart rate, stroke volume, and peripheral blood pressure were measured during the tests. Thirteen of the 32 heat stress tests were prematurely stopped due to high rectal temperature, high heart rate, subjective fatigue, or heat syncope. The physiological strain, as indicated by the rectal temperature and heart rate, was not significantly different between the warm humid and hot dry environments (wet bulb globe temperature approximately 28 degrees C). The rectal temperature and heart rate responses of the physically trained and untrained subjects did not differ in any of the environments. In the heat, the heart rate was significantly higher than in the thermoneutral environment, but because of the markedly reduced stroke volume the average cardiac output was not different between the three environments. The impaired work performance in the heat seemed mainly to be related to the circulatory instability accompanying the increased cutaneous circulation.

When people work for the first tim e in a hot enviro nment, their work performance is often reduced, their heart rate and internal body temperatures may increase greatly, and they are prone to heat disorders such as heat syncope and heat exhaustion (2,15,34). Depending on the severity of the environment and work load, man can adjust to a certain extent to heat stress by increased sweating and skin blood flow and thus atta in a physiological stead y state.
During physical exercise in the heat, the reduction of stro ke volume has been shown to be an important mechani sm limiting physical performance (29). In a hot environment at heavy work loads the lowered stroke volume seems to prevent any thermally induced rise in cardiac output (5,18,29,32), and increments in skin blood flow occur through redistribution of the blood flow from visceral organ s (27) and inactive muscles (14). Therefore, una cclimated subject s cannot reach a circulatory and ther mal steady sta te durin g heavy work in the heat, and the work period s tend to be short.
Although the metabolic demand s are usually relatively low ( < 30 070 of maximal oxygen consumption) in hot workplaces (16), the work period s last for several hours and work overalls are worn which can cause significan t extra heat strai n (11). Und er these cond ition s, the increase in cardiac output may provide part of the increased skin blood flow needed to maintain thermal balance, as suggested by Brouh a (3). Earlier obser vation s (4, 18,21 ,30,34) indicate that, durin g light exercise in the heat , card iac output may rise over control values. Ho wever, it is difficult to assess the significance of these findings in respect to work performance, because these studies employed either short -term exercise in an unu sual bod y position (supine, semiupr ight) or a limited numb er of subjects (usually three or four ). The purpose of the present stud y was to investigate the circulatory and the rmal responses of unacclimated men wearing ordinarily used industrial work clothing during 3.5 h of light treadmill work in a warm humid and a hot dry environment. Because the level of physical fitness has been shown to influence heat tolerance (1,8,9), the measur ements were made on two groups of men with a different physical tra ining status.

Subjects
The subjects were eight physically trained and eight untr ained healthy men, aged 28 -37 years (table 1). The subje cts were not acclimated to heat, and none of them worked permanently in a hot workplace. Before the tests, oral and written information about the pro cedures was given to the subjects, and their informed consents were obtained.

Exp erimental procedures
Before the experiments the subjects had a medical check-up including a clinical exercise test and routine examinations of blood and urine. Their maximum oxygen consumption (V0 2max) and anthropometric characteristics were then determined at a neutral room temperature. The experimental part of the stud y consisted of three prolonged treadmill tests performed randomly in a climatic chamber, once in a thermoneutral [20°C/40 0/0 relative humidity (RH») once in a warm humid (30°C /SO % R H), and once in a hot dry (40°CI20 % RH) environment (table 2). In addition , these three test s were preceded by a similar training walk in the thermoneutral climate in order to accustom the subjects to the experimental procedures. The two hot environments were chosen to give the same heat stress index value in terms of wet bulb globe temperature (WBGT), and they were within the range of thermal work conditions found in hot workplaces in Finland (22). All the tests took place in the morning, with an inter val of at least one week between the tests to avoid acclimation effects. During the tests the subjects wore short-sleeved and short-legged cotton underwear, cotton work overalls, sock s, and sneakers. The thermal insulation value of the cloth ing, measured from a thermal mannequin, was 0.109 K·m1·W -1 (0,7 clo).
During the 48 h preceding the actual tests the subjects were asked to maintain their normal diet, to avoid 38 any excessive physical exertion, and not to use any medication or alcoholic beverages . They reported to the laboratory at around 0645 after having a light breakfast without coffee or tea . They were also asked not to smoke in the morning before the tests . The thermistors and electrodes were attached , and the subjects had a 30-min rest period in a lying position in a thermone utral environment. The treadmill tests consisted of seven 30-min work periods at 30 % V0 2max interrupted by 5-min pauses for weighing. During the tests, flavored, lukewarm water was offered ad libitum, and drinking was encouraged. After exercising , the subjects rested for 30 min as they had done before the tests.
The tests were discontinued if the rectal temperature or heart rate increased constantly without achieving steady state . The fixed end points were 3S.6°C for rectal temperature and 160 beats· min -I for heart rate. Other termination criteria were subjective fat igue and syncopal symptoms.

Measurements
The V0 2max of the subjects was directly measured on a treadmill by a method modified from that of Oja et al (24). The expired air was directed through lowresistan ce tubing (modified Koegel Y-valve) to 150-1 neoprene bags . The ventilation volume was measured with a calibrated dry gas meter . Gas analyses were performed with an infrared carbon dioxide (C0 2 ) analyzer (Morgan SOld, PK Morgan Ltd, England) and a paramagnetic oxygen analyzer (Morgan 500d). Before each test the analyzers were calibrated with standard gas mixtures.
During the experiments in different environments the pulmonary ventilation and oxygen consumption were measured for every second work period by the Morgan Exercise Test System (PK Morgan, England), which, in addition to the aforementioned analyzers, includ es a microprocessor and ventilometer. During the tests the electrocardiogram was continuously monitored (OLLI 332, Kone , Finland).
Cardiac output during each 30-min exercise period was estimated two to four times by a simplified CO 2-rebreathing tech nique (6,18,23) utili zing a rapidly responding infrared CO 2 analyzer (Datex CD 101, Datex, Finland) and a portable chart recorder (YEW 3057-21, Yokogawa Electric Works, Japan). During the first training walk the subjects became accustomed to the rebreathing protocol. Table 3. Distribution of the tests discontinued in the warm humid and hot dry environments according to the criterion for discontinuation .

Metabolic and thermal responses
During the prolonged tests in the thermoneutral environment the physically untrained subjects had a lower absolute level of oxygen consumption in comparison to that of the trained men (table 4), but the relative work levels (070 V0 2 max) of the groups were equal, the mean and standard error (SE) of the mean being 30.7 and 1.0 %, respectively, for the untrained and 31.4 and 1.2 070, respectively, for the trained groups.

Results
The subjects had no difficulties in accomplishing the exercise task in the thermoneutral environment, as indicated by the stable heart rate and rectal temperature. In both of the hot environments, however , a physiological steady state was achieved only in nine of the 32 tests. A total of 13heat stress tests were prematurely discontinued (table 3). Three syncope episodes took place in the warm humid environment, and they all occurred in the trained group as the subject stood still beside the treadmill for a blood pressure measurement. (See the Subjects and Methods section.) Eight of the 13 premature cessations took place in the trained group.

Warm
Hot humid dry analysis of variance with repeated measures on ambient temperature (33). If a significant main effect was found between the environments, the Newman-Keuls procedure was performed to locate the significant differences between the means . In the case of significant interactions a modified F-test was used to test the group differences for each environmental condition. Because of the dropouts during the tests (see theResults section), the analysis of variance was performed over values obtained for the steadily changing variables (heart rate, stroke volume, digital systolic blood pressure, mean skin temperature, rectal temperature) during the fourth work period or over the mean values of the steady variables (oxygen consumption, thermal conductance, evaporation rate, cardiac output) and the derived variables (weight loss, sweat rate) over the entire test. The linear regression technique was used to correlate the thermoregulatory responses with changes in cardiac output. The differences were considered statistically significant when p< 0.05.

Criterion
The analysis of the obtained curve and the calculations were done with a digitizer (HP91I lA Graphics Tablet, Hewlett-Packard, United States) connected to a desk-top computer (HP85). A standard CO 2 dissociation curve for oxygenated blood was used to estimate CO 2 concentrations in the arterial and mixed venous blood (17,26). For all the accepted CO 2 -rebreathing curves, the correlation coefficients in the estimation of CO 2 pressure in the mixed venous blood were over 0.90. Heart rate at the beginning of the rebrcathing was used to estimate the stroke volume . The cardiac output determinations within one 3D-min work period were averaged for the analysis. For one subject working in the warm humid environment, the CO 2 rebreathing was not done for technical reasons .
During the last 5 min of each work period the digital systolic blood pressure was measured by straingauge plethysmography (Medimatic SP2, Medimatic, Denmark) from the left middle finger as described previously (7). For the measurement the forearm was placed on a padded support at heart level. The digital systolic blood pressure of the physically trained men was also measured while the men stood beside the treadmill. However, this procedure appeared to provoke syncopal symptoms (see the Results section), and it was not used with the untrained men.
Internal body temperature was measured during the tests with a rectal probe (YSI 40I, Yellow Springs Instruments, United States) inserted 10 em beyond the anal sphincter. Skin temperatures were measured with thermistors (YSI 427) attached at nine sites with surgical tape (Scanpor, Norgesplaster, Norway). The mean skin temperature was calculated as the areaweighted average of the nine skin temperatures (11). Body temperatures and heart rate were recorded every minute with a desk-top computer (HP85) via an analogue digital converter (HP2397 A).
The total body evaporation rate was determined after each 3D-min period from changes in clothed body weights, and the total body sweat rate was estimated by adding the weight gain in clothes to the total amount of evaporation, adjustment being made for water intake and urination. The weight gain in clothes was measured at the end of each test before the recovery period. In our tests, the amount of dripping sweat (from hands, legs, and face), which is difficult to quantify, was not accounted for, and it is thus included in the obtained evaporation rates. A differential balance with an accuracy of ± 10 g (Schember, Austria) was used.
Total skin blood flow was estimated from calculations of the whole-body thermal conductance (21).

Statistical methods
The differences in the circulatory and thermal parameters were tested between the three environmental conditions and between the two groups by the two-way Table 4. Met abolic and th ermal responses of t he physically tra ined (N = 8) and unt rained (N = 8) groups to prolonged light exerc ise in the thermoneutral , warm hum id, and hot dry environments and th e results from the analysis of variance. The means and standard errors of the means are presented for the fourth work period (mean rectal and skin temperature) or for all the work periods averaged over the entire test (thermal conductance, sweat rate , evaporation rate , weight loss , oxygen consumption) . • p <0.05, .. p-cn.nt for th e main effects of environment or t rain ing (NS =not statistically significant). In both groups, the mean oxygen consumption was slightly higher (p<O.OI) in the warm humid environment than in the thermoneutral and hot dry environments.
Before the subjects walked in the thermoneutral, warm humid, and hot dry environments, their rectal temperatures did not differ between the gro ups, the  (table 4). In the heat , the rectal temperature increased constantly in most of the subjects, and five tests were discontinued because the subject's rectal temperature reached~38.6°C (table 3). No significant group differences were found for the rectal temperatures, although they tended to be slightly higher in the trained group in the heat. At the end of the 30-min recovery period, the rectal temperatures were higher (p<0.01) after work in the warm humid and hot dry environments, but the differences between the groups were not significant.
The mean skin temperatures were higher (p < 0.01) in the warm humid and hot dry environments, and those of the physically trained subjects were about I.O°C higher than tho se of the untrained men (p<O.OI in the warm humid environment, p<0.05 in the hot dry environment). Consequently , the mean whole-body thermal conductance in the heat was 47 W·m -2 .oC-1 higher (p<O.OI) in the physically trained than in the untrained men (table 4).
The physically trained subjects had higher (p < 0.0 I) evaporation and sweat rates as compared to the untrained subjects (table 4 and figure I) . The sweat rates did not differ in the warm humid and hot dry envi-ronments, but the evaporation ra te was significantly lower in the warm humid environment, in which 49 070 of the sweat produced by the physically trained subjects evaporat ed as compared to 66 070 of the sweat o f the untrained subj ects (p< O.OI). The deficits in nud e body weight after work in the two hot environm ents were higher (p < 0.01) than the corresponding values in the thermoneutral environment.

Cardio vascular responses
Altogether 788 acceptable CO 2 -rebreathing curves (17 per test) were analyzed for cardiac output estima tion. The consecutive determinations of cardiac output varied little in the tests in the thermoneutral environm ent, the coefficient of var iation (standard deviation /mean X 100) averaging 7.2 ± 0.7 (Jlo. Due to the higher absolute work loads , the physically trained subjects exhibited a higher (p < 0.05) level of cardiac output than the untrained subjects (table 5 and figure 2), but there was no significant dif ference in cardiac output between the three environmental conditions. In the warm humid and hot dry environm ents, however, the ind ividual variat ion in the card iac output respon ses was higher than in the therm oneutral environment. Elevated , un chan ged or lowered cardiac output was found in different tests ( figure 3). In spite of the ind ividual variability, the time courses of the cardiac output responses were rather stead y th roughout each test. Th e change s in the cardiac output values in the warm humid and hot dry environments relative to the corresponding values under the thermoneutral conditions correlated significantly (r = 0.51, p < 0.0 I, N = 31) with the changes in thermal conductance but not with the changes in the evaporation (r = -0.11, NS) or sweat rate (r = -0.0 5, NS).
As shown in figure 2, the mean stroke volume de-d ined similarly in the warm humid and hot dr y environments (p <O.OI) as compared to mean stroke volume und er the thermoneutral conditions. Most of the reduction occurred during the first 2 h of exercise, after which the stroke volume reached a relatively stead y level of about 100 ml in the physically tr ained and 90 ml in the untrained subjects . In the th ermoneutral environment, the strok e volumes were constant, the mean level varying from 131 to 138 ml for the trained men and from 105 to 118 ml for the untrained men . In all the environments the stroke volumes of the physically trained subjects were significantly (p<O.OI) higher.
Because the mean level of cardia c output was not different in the three environments, the reductions in stroke volume in the heat were compensated by elevated hear t rate s (p <O .OI). As both groups exercised at similar relat ive wor k loads, the heart rates were nearly on the same level in the thermoneutral environment (figure 2). In the heat the heart rate response did not significantly dif fer between the groups .
At the end of the rest period before and after the tests the mean heart rate of the physically trained men was lower (p<O.OI) . Thirty minutes after the work in the warm humid and hot dry environm ents was completed, the mean heart rate was 15 beat s-min -I higher than 30 min after the test in the thermoneutral environm ent. Heart ra te recovery was similar in both groups.
The mean digital systolic blood pressure rema ined stable during the 3.5 h of work in the thermoneutral environment (table 5). In the warm humid and hot dr y environm ents it decrea sed during the first hour, whereafte r it remained steady. Figure 4 illustrates the circulatory respon ses of two subjects. One had an elevated cardi ac output in the warm humid and hot dry environments, his heart rate increased continuously, and his strok e volume was Table 5. Circulatory responses of the trained (N = 8) and unt rained (N = 8) subj ects to pro lon ged light exerc ise in the thermoneutr al, warm hum id , and hot dry enviro nments and the results from th e analysis of variance. The means and th e standard error of the means are present ed for th e fo urth wor k period (stroke volume, heart rate, digital systol ic blood pressure) or fo r all th e work period s averaged over the ent ire test (cardiac output).  .... markedly reduced. The other subject reached a steadystate level in both heat stress tests and had a slightly lowered cardiac output. The digital systolic blood pressure of both subjects remained steady throughout the experiments, although it varied markedly with time.

Discussion
During the prolonged treadmill work requiring 30 It/ o VOzmax in the warm humid and hot dry environ-ments, as compared to similar performance in the thermoneutral environment, our subjects exhibited the typical signs of unacclimated persons when initially exposed to work in a hot environment (2,5,15,34), ie, elevated heart rate and rectal temperature and reduced physical performance, as evidenced by the 41 070 rate of premature test interruptions. Only three subjects were able to achieve a circulatory and thermal steady state in both of the hot environments. It is therefore obvious that, in workplaces with thermal work con-ditions similar to our test environments (WBGT value of about 28°C as the heat stress index), it is necessary to reduce the heat stress, as indicated also by the reference values in the international heat stress standard (12). However, in spite of the equivalent WBGT values, the warm humid environment seemed to be somewhat more severe, as indicated by the slightly higher heart rates and higher incidence of heat syncope, than the hot dry environment.
With the use of the recently proposed (13) analytical heat stress index ("required sweat rate") the warm humid environment was also predicted to exert the highest thermal strain, especially in the physically trained group working at a higher metabolic rate. According to the model the duration-limited exposure times (danger level) predicted for humid heat were 1 h 8 min and 3 h 46 min for the physically trained and untrained men, respectively. The corresponding figures for the dry heat were 2 h 53 min and 6 h 18 min. The latter values are in contrast to our result of no significant difference in physiological strain between the two groups. The higher sweat rates observed for the trained men in our study, as compared to the maximum criterion value of 650 g·h-1 (250 W·m-2 ) used in the model, probably explains some part of this discrepancy. Actually, the predicted limit for exposure duration for an acclimated person in the hot dry environment would be 7 h 7 min. Therefore it is possible that the higher sweat rates coupled with aerobic physical fitness (I, 8,9) should be accounted for in the criteria used for unacclimated persons in calculations of allowable work times.

Circulatory adjustments
The present study employed a simple, noninvasive CO 2-rebreathing technique to estimate pulmonary blood flow, ie, cardiac output. Ohlsson et al (23) compared this technique with the direct Fick method at rest. In the determination of stroke volume there was no significant difference between the two methods (I' = 0.90), but cardiac output was, on the average, 22 0,10 higher with the CO 2 method because of increased heart rate. We observed that under neutral conditions the hyperventilation needed in the rebreathing procedure increased heart rate by 5-10 beats-min -1, and probably cardiac output also increased, but in the heat at a higher heart rate the increase was negligible. Thus, in our tests, possible increases in cardiac output in the heat, relative to that in the thermoneutral environment, may have been underestimated. In addition changes of about 0.81·min-1 or less (coefficient of variation 7.2 %) were probably undetectable. We feel, however, that using the averaged cardiac output values and accepting only CO 2 curves with a steady exponential rise (I' > 0.90) considerably reduced the inherent variation caused by the methodology.
In the present study, during prolonged treadmill work requiring 30 % V0 2 max , the heart rate was significantly higher in the warm humid and hot dry environments than under the thermoneutral conditions, but, because of the marked reduction in stroke volume, the average cardiac output was not different in the three different environments. The mechanism behind the reduced stroke volume in the heat seems to be related to the increased level of skin blood flow (30), which, together with the reduction in the tone of distensible cutaneous veins (35), increases the pressure and blood volume of the veins. Because 70 % of the blood volume is below heart level in humans and most of it is in the veins, the venous pooling of blood under heat stress reduces cardiac filling, central blood volume, and stroke volume (30). In an upright walking man the muscle pump empties the veins during each contraction, but the arterial inflow is so high that the average venous pressure and volume are increased (10). Dehydration also reduces venous return to the heart by lowering circulating blood volume (19). In spite of the unchanged mean cardiac output in the heat, there was substantial variation in the individual cardiac output responses (figure 3). A significant part of this variation was probably caused by the aforementioned methodological limitations and by day-to-day biological variation. In addition our subjects were voluntarily dehydrated to different degrees (from 0.1 to 2.9 %), and this difference may have augmented the variation, especially with respect to the lowered cardiac output values (19). On the other hand, the increases in cardiac output were significantly related to increases in skin blood flow, as estimated by the thermal conductance. Similarly, a significant correlation (0.74) between increases in cardiac output and skin temperature was found during light exercise in water-perfused suits, in which the skin temperature was maintained at different levels (30). Thus at lower work loads higher thermal stress, as indicated by skin temperature or skin blood flow, may be coupled with elevated cardiac output, despite reduced stroke volume. However, in our study, the cardiac output rose over the control value in only one of the nine tests with 44 steady-state responses (figure 4). In six tests it remained unchanged, and in two it decreased slightly. High cutaneous blood flows probably cannot be maintained for long periods of time because of the negative effects on central circulation.
During the prolonged walking tests in the warm humid and hot dry environments the average peripheral systolic blood pressure was significantly lower than in the thermoneutral environment, but after the second bout of exercise it remained stable. Three of the heat stress tests were terminated due to exhaustion of the subjects, whose systolic blood pressures, however, did not show any significant lowering. This result is in agreement with the finding of a relatively constant mean aortic blood pressure during exhaustive work in dry heat (28). Naturally, in the cases of heat syncope, a drastic fall in mean aortic blood pressure must have occurred. In the three exhausted subjects the exhaustion was characterized by a high heart rate (> 150 beats-min:"), a rectal temperature of ,=,38.4°C, and a steadily decreasing stroke volume. In two of the subjects cardiac output was elevated over the control values. The relatively low rectal temperatures imply that, during acute exercise-heat stress, circulatory instability rather than hyperthermia may be a factor limiting prolonged work performance.

Comparison of the responses of the trained and untrained men
In 1965 Piwonka et al (25) began the discussion on whether physical training can substitute heat acclimation. At present, the experimental evidence indicates that endurance-type physical training enhances sweating (8,9,20) and may increase heat tolerance by 50 % of that achieved by heat acclimation (I). Frequent rises in internal body temperature during training sessions seem to be the prerequisite for these changes (1,8).
A strict comparison of the trained and untrained groups is not justified in our study because the trained men were exposed to extra postural stress during the blood pressure measurement in a standing position. (See the Subjects and Methods section.) In the thermoneutral environment the trained subjects, because of their higher absolute work loads, had a higher mean cardiac output, a higher stroke volume and a higher oxygen consumption than the untrained men, but, due to the higher evaporation rates and thermal conductances of the trained men, the heart rates and rectal temperatures of the two groups were similar. However, in the light of earlier studies (1,8,9), it was somewhat surprising to note that, in the heat, the heart rate and rectal temperature responses did not differ between the two groups, and actually they were slightly higher in the trained men. The physically trained men had significantly higher sweat rates in the heat, but the relative amount of evaporation was significantly lower, especially in the warm humid environment (reduced sweating efficiency). Actually, in the warm humid environment , the predicted required skin wetness level (13) for the trained me n exceeded unity (1.13) . Therefore the evaporation rate required to maintain heat balance would be very difficult to achieve even b y a cclimated per sons. Because the study wa s performed in th e wintertim e a nd most o f th e ph ysicall y trai ned subje ct s exe rc ised ou tdoors (skii ng , jogging) , these su bje cts may have no t experienced the sa me elevati o ns in in tern al body temperature as in other st udies (I , 8,9).
Du ring long-distance skiing requiring 75 % V0 2 m ax, the mean skin temperature of one man varied between 21 .0 and 24 .0°C and the rectal temperatu re between 37.5 and 38.0°C (31) .
It is im po rt a nt to note, however, th at in our study the ad van tage of ph ysical training in respect to work performance was nearly maintained in the warm humid and hot dry environments, because the absolute amount of p hysical work done by the trained men was higher. In other words , in hot workplaces wit h fixed work ta sk s, the physiological strain will be lower in the more physically fit person .
In summary, a warm humid and a hot dry environment with nearly equal WBGT values produced marked circulatory and thermal strai n in un acclirnated m en performing prolonged, light ph ysical work whil e wearing work overalls. On the ba sis o f heart rat e and rectal temperature no stat istica lly sign ifican t difference in physiological st rain was fou nd between the wa rm humid a nd hot dry en vironmen ts or betwee n the ph ysicall y trained and untrained subjects. In th e he at , heart rate was significa ntly higher th an under the rmoneutral co nd itio ns , but because o f red uced st roke vo lu mes the mean cardiac output was not differ ent in the three environments. At the individual level, the increases in cardiac output were related to increases in skin blood flo w, but not to an attainment of a ph ysiological steady sta te during prolonged work . The pre sent res ults indicate that during a cute hea t exposure impaired work performance is mainly rela ted to circulatory instability caused by peripheral cir cu lation that is increased for heat di ssipation .

Practical conclusions
The marked extra circula to ry and therma l stra in observed in th e heat stress te st s indicates th at continuo us wo rk a t the WBGT value o f a bo ut 28°C sho uld be avo ided and rest pauses in cooler enviro nme nts sho uld be incorporated into the workshi ft. New wo r kers in hot wo rkplaces are at increased risk fo r heat syncope, and th ey sho uld be warned a bo u t statio na ry work po sitions. Physicall y trained person s seem to be as affected by acute heat exposur e as un tr ained perso ns if th e rela tive ae ro bic st rain is th e sa me . H ear t ra te monitoring may be a useful me thod fo r assessing wo rkers' ph ysiological st rain in a hot environ me nt becaus e, during prolonged wo r k, it wa s ass oci ated with reduced stro ke vo lume rather th an with increased ca rdiac output.