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Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
Scheduling rest for consecutive light and heavy work loads under hot ambient conditions a
a
JANET TORMA KRAJEWSKI , ELIEZER KAMON & BARBARA AVELLINI
a
a
The Noll Laboratory for Human Performance Research, Pennsylvania State University , University Park, Pa, 16802, U.S.A Published online: 27 Mar 2007.
To cite this article: JANET TORMA KRAJEWSKI , ELIEZER KAMON & BARBARA AVELLINI (1979) Scheduling rest for consecutive light and heavy work loads under hot ambient conditions, Ergonomics, 22:8, 975-987, DOI: 10.1080/00140137908924671 To link to this article: http://dx.doi.org/10.1080/00140137908924671
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
ERGONOMICS, 1979, VOL. 22, No.8, 975-987
Scheduling rest for consecutive light and heavy work loads under hot ambient conditions By JANET TORMA KRAJEWSKI, ELIEZER KAMON and BARBARA AVELLINI The Noll Laboratory for Human Performance Research, Pennsylvania State University,
Downloaded by [The University of Manchester Library] at 13:41 08 October 2014
University Park, Pa. 16802, U.S.A. Three male and three female heat-acclimated subjects participated in a series of five testing sessions aimed at validating a resting period which was assigned to follow work under
Cr..
warm-humid 36°C, Tw • 31°C) and hot-dry (T". 50°C, Tw • 25°C) ambient conditions. Each working period consisted of 25 min of walking at 30% VO,mu followed by five minutes of carrying a load uphill at 75%
V02max'
The working period was based on the expected HR
as it could be derived from: (I) the work-specific HR as determined from the linear relationship between % Vo,muand HR: (2) the heat-induced increments in HR; and (3) the endurance limits imposed by the age-dependent H Rmall.' Each 30 min of work was followed by 30 min of rest either under the same ambient conditions as for the working period, or
under neutral ambient conditions. Judged by the levelling off of HR and by setting the limits of the 1;e rise to 38°C during the consecutive walking periods, only the resting under the neutral conditions proved adequate. The level of blood lactic acid was the same under all ambient conditions for each sex, but was higher for the males, who carried a load of 12
kg, as compared to 10 kg for the females.
1. Introduction Early attempts to formulate cycles of work in order to avoid undue fatigue during physically demanding tasks were based on the absolute metabolic cost (Spitzer, cited by Lehman, 1962, Murrell 1965). A more recent approach to the design of work and rest periods has been to use the relative intensity of the metabolic cost, that is, oxygen uptake (Vo , ) as a fraction of the maximal oxygen capacity (VO,max) (Astrand and Rodah11970, Bonjer 1971, Rohmert 1973, Kamon 1979). Since the metabolic cost ofwork is a function of the oxygen delivery to the muscles, expressing the metabolic demand as a fraction of VO,max (fVo,max) closely relates the workload to the functional capacity of the circulatory system (Rowell 1974). The strain experienced by the cardiovascular system is reflected by increments in heart rate (HR), which is a factor in utilizing the HR as an indicator for strain accrued by physically demanding tasks. When work is performed under hot ambient conditions a further increase in the HR occurs because of the need to increase skin blood flow for heat dissipation. The heat-induced increments in HR above the work-specific HR would reduce the reserve capacity of the heart because of the limits imposed by the maximal attainable HR. Heat-induced increments in HR were equated with work-induced strain, in terms of fVo,max s so that the expectedH'R during work under hot ambient conditions could be applied to the design of work-rest schedules (Kamon 1979). A similar approach was taken in the study reported here. The working periods were determined according to the expected HR responses listed below. Under temperate ambient conditions: (a) the working time is inversely related to fVo,max; (b) the HR is linearly related to fVo,max at least over the range of O' 5 VO,max to 1·0 VO,max; and (c) the expected HR max is a function of age, and according to the American Heart Association (1972) can be calculated as HR max = 220-Age. The OOI4-U139/79/2208 0975 S02·00
~
1979Taylor and Francis Ltd.
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Janet Torma Krajewski et al.
expected HRs used were 130 beats min - 1 at O' 5 V02max and at V02ma" for safety margin, 1·96 standard deviations below the mean HR max as calculated from the above equation. The standard deviation for HR max was taken as seven beats min -1. Under heat stress the increments in HR above the work-specific HR (as related to jVo2max) are expected to be: (a) one beat min - 1 per I?C rise in air temperature above 25°C for dry heat (vapour pressure below 2·7 k Pa); and (b) one or two beats min -1, depending on the work intensity, for each minute of exposure to warmhumid ambient conditions (above 2·7 k Pa) (Gonzalez et al. 1978). In the past, core body temperature (7;;) served as an additional indicator for physiological strain. 7;; was shown to be linearly related to jV02max within certain limits of ambient conditions (Saltin and Hermansen 1966, Kamon 1975). However, beyond these limits 7;; was expected to rise in proportion to the heat stress imposed either by high air temperatures (Kamon and Belding 1971 a) or by high humidities (Kamon and Belding I971 b). The rise in 7;; stimulates the increase in blood flow to the periphery for heat dissipation, which may result in a decrease in blood flow to the working muscles and ultimately in a reduction in oxygen delivery to the muscles. In such situations when the oxygen demands of the muscles are not met, anaerobic pathways are recruited for energy production leading to an increase in the formation of lactic acid. In addition to shunting blood from the working muscle it is possible that blood will be redistributed from the internal organs to the skin, thereby decreasing the available lactate that can be metabolized by the liver (Rowell 1974). Evidence of such an increase in plasma lactate during work under hot ambient conditions as compared to work under cold or temperate ambient conditions was reported by Fink et al. (1975), and Irondelle and Freund (1977). This investigation was undertaken in order to complement our previous observations on work-rest schedule for work intensities demanding 0·4 and 0·6 jV02max (Kamon 1979). The main task in this' investigation was chosen to demand 0·75 of jV02max and to represent practical field situations of load carrying. The main task involved load carrying, but it was preceded by moderate work load (0'3jV02max) and was followed by rest. Based on the expected increments in HR due to both the intensity of the muscular work and the hea t stress employed, the assigned total work period was 30 min; 25 min for the moderate work (0,3 fVo 2max) and five minutes for the heavy work (0,75 fV0 2max)' Assuming that the most accommodating practical rotational work situation would be between two workers, the 30 min of work was followed by 30 min of rest. The working ambient conditions were either hot but dry or warm but humid, the resting ambient conditions were either under the same as those for for the work or under cooler neutral ambient conditions.
2. Methods 2.I. Subjects Three male and three female, fully acclimatized college students participated in this study. Their physical characteristics are summarized in table I. After informed consent was obtained, a physician administered a thorough physical examination, and a progressive exercise treadmill test to obtain an exercise cardiogram along with a determination of maximal aerobic capacity.
Scheduling rest for consecutive light and heavy workloads
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Table I.
Anthropometric data, maximal heart rate (HR max ), aerobic capacity (Vo 1max) and the oxygen cost of work (Vo , ) for the subjects. Fat (%)
HR m llix beat min- 1
60·0 60·9 47-4 56·1 ±7-6
1·58 1·70 1-47 158 ±0·12
24·3 25·0 16·1 21·9 ±4·8
188 190 200 192·7 ±6·4
2·23 2·25 2-29 2·26 ±0·03
Walk 0·67 0·76 0·60 0·68 ±0'08
Carry 1·68 1·70 1'77 1·72 ±0'05
66'8 85·9 95-4 82·7 ±14·6
1·83 2·13 2·17 2·04 ±0·19
15·9 14·5 26·6 19·0 ±6·6
200 204 182 195'3 ±11·7
3·70 5·52 4·75 4·66 ±0·91
0·99 1-44 1·28 1·24 ±0·23
2-70 4·25 3·58 3·51 ±0·78
Height (em)
Weight (kg)
Women I 2 3 Mean s.d .•
20 23 22 21·67 ± 1'53
154·0 168·6 160·0 160·9 ±7·3
Men I 2 3 Mean s.d."
25 23 30 26·0 ±3-6
Ino 188·6 182-6 182·7 ±5·8
* Standard deviation about the
Notation
WH-I WH-2 HD-I HD-2 C
V0 1max
SA (m")
Age (yr)
Table 2.
977
(l min" ')
O 2 Cost of Work (I min-I)
mean.
The mean dry bulb (Td . ) and wet bulb (7;..) temperatures for the working and resting periods. Description Ambience Warm-humid Warm-humid Warm-humid Neutral Hot-dry Hot-dry Hot-dry Neutral
Control
Period Working Resting Working Resting Working Resting Working Resting Working Resting
Ambient Conditions" Work period Rest period TWb °C Tdb °C T...b °C 7db °C 36
31
36
31
50
25
50
25
23
16
36
31
23
16
50
25
23
16
23
16
• Temperatures were within a standard deviation of t "C. Mean air movement was l m s - I in the heated room and 0·18 m s - 1 in the cooler resting room.
2.2. Workloads During the walk at O' 3 VO,ma., the treadmill was level with an average speed of 80·5 m min - 1 for the women and 93·9 m min - I for the men. At the onset of the carrying the inclination of the treadmill was adjusted to raise the individual's oxygen consumption to 0·75 V02maxincluding the cost of load carrying. The uphill grade was 10-14% for the women and 12-15% for the men. The weight carried by the women was 10 kg, while the men carried 12 kg. It was equally distributed in a box 45 x 25 x 23 cm. The box was carried in front on the hands with the elbows bent at 90°. 2.3. Ambient conditions The environmental conditions for the working and resting periods are summarized in table 2. The ambient conditions for resting were either the same as the working conditions or in a neutral environment. Thus, during one session the subjects worked and rested in a hot-dry environment of 7db 50°C, TWb 25°C (HD-J), while another session consisted of warm-humid conditions of 7db 36°C, 'T."b 31°C (WH-l). Two more sessions were conducted which permitted the subjects to rest in a neutral environment of Td b 23°C, TWb 16°C following work periods in hot-dry conditions
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Janet Torma Krajewski et al.
(HD-2), and warm-humid conditions (WH-2). A control test (C) was designed for work and rest periods under the neutral ambient conditions. The wind velocity was I m s- I for all tests within the climatic room, but in the outer room used for resting, the only air movement was due to central air conditioning and was calculated to be 0·18 m s- I. The order of the five experimental conditions were selected at random.
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2.4. Clothing The clothing included aT-shirt, shorts, a long sleeve cotton shirt, trousers, tennis shoes and socks. The women also wore a halter top beneath the T-shirt. 2.5. Measurements Rectal temperature (1;,) was measured by a thermistor inserted 10 em beyond the anal sphincter. Mean skin temperature (T.) was calculated as the unweighted average of six skin temperatures measured with uncovered, copper constantan thermocouples located on the forehead, chest, back, forearm and both thighs. The temperatures from the thermistor and thermocouples were recorded continuously throughout the test period. Heart rate (HR), obtained from an electrocardiogram, was taken for 15-30 s every five minutes, during the walk and rest periods and once each minute during the carrying. Oxygen uptake (Vo , ) was determined by the open circuit method. Expired air, collected through a low resistance valve into a Douglas bag, was analysed for O 2 and CO 2 content with a Beckman E2 paramagnetic analyser and a MSA Lira Infrared Meter respectively. Volume was measured with a Parkinson Cowan dry gas meter. Total sweat production (S) was calculated as the change in nude body weight, measured prior to and after the experimental session, and corrected for water intake. Evaporation rate (E v ) ' measured every 30 min, was determined by the change in total body weight and water intake. No corrections were made for respiratory evaporation or metabolic gas exchange weight loss. Heat equivalent of sweat and evaporation was calculated using 0·67 wh g-l as latent heat. Lactic acid concentration was measured spectrophotometrically with the colour change induced by the generation of NADH as a result of adding excess NAD and lactic dehydrogenase to plasma containing lactic acid (Sigma Chemical Co., Kit No. 726-UV). 2.6. Heat acclimation Prior to the experimental session each subject was heat-acclimated by two-hours daily exposures to 7db 50°C, TWb 25°C for four consecutive days and to Td b 36°C, TWb 31°C for two-three days. Exposure included level treadmill walking requiring O· 3 VO,mox' Full acclimation was determined by the reduction and levelling off in 1;. and HR and by the improved tolerance time. 2.7. Experimental procedures Upon arrival at the laboratory the subject rested for 20-30 min prior to obtaining a nude body weight. He then inserted the rectal thermistor probe, and with help from the experimenter placed the bipolar electrodes for the electrocardiogram and the. thermocouples on the skin before donning his clothing. Pre-test resting HR, 1;. and t, were obtained before entering the climatic room. Once in the climatic room, the subject rested approximately five minutes before a 5 cm ' venous blood sample was drawn from an antecubital vein with a minimum
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Scheduling rest for consecutive light and heavy work loads
979
of stasis. The blood was immediately transferred to a tube containing sodium fluoride to prevent any increase in lactic acid concentration as a result of glycolysis. The subject was permitted to rest for several minutes, if desired, after the blood sample was drawn. Total body weight was taken prior to the onset of walking. The periods of work were determined according to the fVo,max of each workload (Karnon 1979). Thus the respective periods were 25 min at the lower intensity of walking followed by five minutes at the higher intensity of carrying. To allow a practical rotation between two workers a 30 min resting period followed the carrying period. This protocol of 30 min of work followed by 30 min of rest was identical for all the ambient temperatures. At 30 min, body weight was again taken and the subject then began the rest period either in the heat chamber or in the cool, outer room. After sitting quietly for 30 min, body weight was measured and the cycle was repeated. In all, at each condition, the subject underwent three walk-carry-rest cycles. The subjects were permitted to drink water at room temperature ad libitum. A two-minute expired air sample was collected and analysed at least once during each walk, carry and rest period. Five minutes after the last carry period, a second blood sample was drawn in a manner identical to that described above. Upon conclusion of the day's final rest period, a final nude body weight was obtained. 2.8. Statistical analysis The 1;" t; and HR at the end of each walk, carry and rest period for all test conditions were submitted to a three-factor analysis of variance with repeated measures. The lactic acid changes, sweat and evaporative rates were analysed with a two-factor analysis of variance. The LSD test was employed for pairwise comparisons. A level of 0·95 was required for significance.
3. Results 3.1. Rectal temperature (1;,) responses The end value for each of the walking, carrying and resting periods, averaged separately for each sex, was used to describe the time course of 1;, for each of the ambient conditions (figures I (a) and I (b)). The pattern and magnitude of the 1;, responses differed for the two sexes during the heat exposures (HD and WH), but not during the control exposure (C). The 1;,level during the hot-dry (HD) heat exposure was significantly higher than the control exposure (C) for both sexes. Resting under the same ambient condition as the working periods (HD-I ; WH-I; figure I (a) either did not permit the 1;, to drop, or allowed only a slight drop. This, along with the greater rise in 1;, during the carrying periods resulted in a progressive rise in 1;, from one working period to another. During the last two hours of exposure to the hot-dry conditions (HD-\), the men revealed significantly higher 1;, during carrying and lower 1;, during resting as compared to the women. Working and resting under the warm-humid conditions (WH-I ; figure I (a)) resulted in a sharper rise in 1;, as compared to the HD-I ambient conditions. After 150 min 1;, peaked at 38·4 and 38·3 for the women and men respectively. Although the average 1;, at the onset of the exposure was lower for the men, their rate of 1;, rise was faster so that towards the end of the exposure (after 100 min) there was no significant difference between the sexes. Resting under cooler, temperate ambient conditions allowed the 1;, to drop, particularly for the men (HD-2, WH-2; figure I (b)), and although 1;, increased from one working cycle to another it peaked just above 38°C during the last carrying
980
Janet Torma Krajewski et al. 382
,""---,, " ,, ,,
37.8
.
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,:
\ \
\.',-,J ,
37·6 (WH-2)
368
1
--Walk --Carry ----Rut
30
60
90
120
,>0
180
TIME. min
,.. ~
380
37.6 (HO-l)
372
TIME, min
Figure I. (a) The time course of the averaged rectal temperature (~,) for the men (e) and for the women (0) during work and rest under the hot-dry (HD-I), warm-humid (WH-I and control (C) ambient conditions. See table 2 for details. Bars represent one standard deviation. Arrows indicate significant sex differences at the 0·05 level. (b) The time course of the averaged rectal temperature (7;,) for the men (e) and for the women (0) for work under hot-dry (HD-2) and warm-humid (WH-2) and resting under neutral ambient conditions. Bars represent one standard deviation. Arrows indicate significant sex differences at the 0·05 level.
Scheduling rest for consecutive light and heavy work loads
981
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--Walk --C;rry
60
30
150
180
TIME. min
i" I~'"
:::~ _~= :_~---_J r __
1~r----2
;:-0:"--1--"91
360
350
iHO-I)
340
:~
36 11-'"
-:L
----
lli_,~ r----f? ~ Tl=~
~
3~
__
.•
,.
(WH-J}
34
30
60
90
120
150
190
TIME. min
Figure 2. (a) The time course of the averaged mean skin temperature (f,) for the men (e) and the women (0), during work and rest under hot-dry (HD-l), warm-humid (WH-l) and control (C) ambient conditions. See table 2 for details. Bars represent one standard deviation. Arrows indicate significant differences between sexes at the 0·05 level. (b) The time course of the averaged mean skin temperature (1;) for the men (e) and the women (0) for working under the hot-dry (HD-2) and warm-humid (WH-2) and resting under neutral ambient conditions. Bars represent one standard deviation. Arrows indicate significant sex differences at the 0-05 level.
982
Janet Torma Krajewski et al.
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periods. These peaks were significantly lower than those for the exposure to the HD-I and WH-I conditions (figure I (a». There was a marked difference in the 1;. response of the two sexes; the responses of the men fluctuated significantly more than the women's responses. The men experienced a 1;. decrease during rest which reached significantly lower values than the women, particularly under the HD-l ambient conditions. However, during each working period the men's 1;. sharply rose to reach the same levels as the women's 1;e '
3.2. Mean skin temperature (1;) responses The end values of 1; for each period were averaged for each sex and were used to describe the time course of 1; for each ambient condition (figures 2 (a) and 2 (b». The' pattern of the 1; responses differed for each ambient condition. While f, dropped during the working period and rose during the resting period of the control exposure «e), figure 2 (a», it rose during the working period and dropped during the resting period for work in the heat and rest in the neutral (HD-2, WH-2; figure 2 (b». No fluctuations were noticed in 1; for the work and rest under the same ambient conditions (HD-I, WH-I; figure 2 (a)). No sex differences were found for the pattern of the change in 1;. However, there were differences in the magnitude of 1;. The values for the women were significantly higher than for the men during some of the working periods under HD-2 and WH-2 conditions and throughout the control study.
3.3. Heart rate (HR) responses Because there were no significant sex differences in HRs, the mean end values of HR for all subjects are described in figure 3. Work and rest in the heat (HD-I and WH-I, figure 3) resulted in higher HRs than work in the heat followed by rest in the neutral (HD-2 and WH-2), which in tum produced higher HRs than the control exposure. Throughout the HD-I and WH-I exposures the HR gradually rose from one hour to the next such that the walk 'and rest end values of the third hour were significantly higher than the first hour. Little change occurred between one carrying period and the next since HRs were already near maximum during the first carrying period. During the HD-2 and WH-2 exposures, the HR remained practically the same from one hour to the next except for the walking HR ofthe WH-2 conditions which rose by 12 beats min - 1. The walking HRs of WH-2 were significantly lower than those of WH-I while no difference was found between HD-I and HD-2.
3.4. Lactic acid The mean values of the increase in plasma concentration above the pre-work resting values are summarized in table 3 for each of the five ambient conditions and separately for the men and women. The increase in the lactate concentration was not significanlly different for any of the exposures. The women's lactate increase was significantly less than the men's. The women also tended to reveal greater increases when they were exposed to work and rest in the heat (HD-I', WH-l) than when exposed to rest outside the heated room (HD-2, WH-2), but the difference was statistically insignificant.
Scheduling rest for consecutive light and heavy work loads 190
CARRY
170
0-
•
i=
£iC
180
983
a
~
160 140 'c
E on
.
:>
130 120
w,,~
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.c
'" WH-I • WH2 o HO·I • HD2 a Control
110 Ul
W
f
Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
Scheduling rest for consecutive light and heavy work loads under hot ambient conditions a
a
JANET TORMA KRAJEWSKI , ELIEZER KAMON & BARBARA AVELLINI
a
a
The Noll Laboratory for Human Performance Research, Pennsylvania State University , University Park, Pa, 16802, U.S.A Published online: 27 Mar 2007.
To cite this article: JANET TORMA KRAJEWSKI , ELIEZER KAMON & BARBARA AVELLINI (1979) Scheduling rest for consecutive light and heavy work loads under hot ambient conditions, Ergonomics, 22:8, 975-987, DOI: 10.1080/00140137908924671 To link to this article: http://dx.doi.org/10.1080/00140137908924671
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
ERGONOMICS, 1979, VOL. 22, No.8, 975-987
Scheduling rest for consecutive light and heavy work loads under hot ambient conditions By JANET TORMA KRAJEWSKI, ELIEZER KAMON and BARBARA AVELLINI The Noll Laboratory for Human Performance Research, Pennsylvania State University,
Downloaded by [The University of Manchester Library] at 13:41 08 October 2014
University Park, Pa. 16802, U.S.A. Three male and three female heat-acclimated subjects participated in a series of five testing sessions aimed at validating a resting period which was assigned to follow work under
Cr..
warm-humid 36°C, Tw • 31°C) and hot-dry (T". 50°C, Tw • 25°C) ambient conditions. Each working period consisted of 25 min of walking at 30% VO,mu followed by five minutes of carrying a load uphill at 75%
V02max'
The working period was based on the expected HR
as it could be derived from: (I) the work-specific HR as determined from the linear relationship between % Vo,muand HR: (2) the heat-induced increments in HR; and (3) the endurance limits imposed by the age-dependent H Rmall.' Each 30 min of work was followed by 30 min of rest either under the same ambient conditions as for the working period, or
under neutral ambient conditions. Judged by the levelling off of HR and by setting the limits of the 1;e rise to 38°C during the consecutive walking periods, only the resting under the neutral conditions proved adequate. The level of blood lactic acid was the same under all ambient conditions for each sex, but was higher for the males, who carried a load of 12
kg, as compared to 10 kg for the females.
1. Introduction Early attempts to formulate cycles of work in order to avoid undue fatigue during physically demanding tasks were based on the absolute metabolic cost (Spitzer, cited by Lehman, 1962, Murrell 1965). A more recent approach to the design of work and rest periods has been to use the relative intensity of the metabolic cost, that is, oxygen uptake (Vo , ) as a fraction of the maximal oxygen capacity (VO,max) (Astrand and Rodah11970, Bonjer 1971, Rohmert 1973, Kamon 1979). Since the metabolic cost ofwork is a function of the oxygen delivery to the muscles, expressing the metabolic demand as a fraction of VO,max (fVo,max) closely relates the workload to the functional capacity of the circulatory system (Rowell 1974). The strain experienced by the cardiovascular system is reflected by increments in heart rate (HR), which is a factor in utilizing the HR as an indicator for strain accrued by physically demanding tasks. When work is performed under hot ambient conditions a further increase in the HR occurs because of the need to increase skin blood flow for heat dissipation. The heat-induced increments in HR above the work-specific HR would reduce the reserve capacity of the heart because of the limits imposed by the maximal attainable HR. Heat-induced increments in HR were equated with work-induced strain, in terms of fVo,max s so that the expectedH'R during work under hot ambient conditions could be applied to the design of work-rest schedules (Kamon 1979). A similar approach was taken in the study reported here. The working periods were determined according to the expected HR responses listed below. Under temperate ambient conditions: (a) the working time is inversely related to fVo,max; (b) the HR is linearly related to fVo,max at least over the range of O' 5 VO,max to 1·0 VO,max; and (c) the expected HR max is a function of age, and according to the American Heart Association (1972) can be calculated as HR max = 220-Age. The OOI4-U139/79/2208 0975 S02·00
~
1979Taylor and Francis Ltd.
Downloaded by [The University of Manchester Library] at 13:41 08 October 2014
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Janet Torma Krajewski et al.
expected HRs used were 130 beats min - 1 at O' 5 V02max and at V02ma" for safety margin, 1·96 standard deviations below the mean HR max as calculated from the above equation. The standard deviation for HR max was taken as seven beats min -1. Under heat stress the increments in HR above the work-specific HR (as related to jVo2max) are expected to be: (a) one beat min - 1 per I?C rise in air temperature above 25°C for dry heat (vapour pressure below 2·7 k Pa); and (b) one or two beats min -1, depending on the work intensity, for each minute of exposure to warmhumid ambient conditions (above 2·7 k Pa) (Gonzalez et al. 1978). In the past, core body temperature (7;;) served as an additional indicator for physiological strain. 7;; was shown to be linearly related to jV02max within certain limits of ambient conditions (Saltin and Hermansen 1966, Kamon 1975). However, beyond these limits 7;; was expected to rise in proportion to the heat stress imposed either by high air temperatures (Kamon and Belding 1971 a) or by high humidities (Kamon and Belding I971 b). The rise in 7;; stimulates the increase in blood flow to the periphery for heat dissipation, which may result in a decrease in blood flow to the working muscles and ultimately in a reduction in oxygen delivery to the muscles. In such situations when the oxygen demands of the muscles are not met, anaerobic pathways are recruited for energy production leading to an increase in the formation of lactic acid. In addition to shunting blood from the working muscle it is possible that blood will be redistributed from the internal organs to the skin, thereby decreasing the available lactate that can be metabolized by the liver (Rowell 1974). Evidence of such an increase in plasma lactate during work under hot ambient conditions as compared to work under cold or temperate ambient conditions was reported by Fink et al. (1975), and Irondelle and Freund (1977). This investigation was undertaken in order to complement our previous observations on work-rest schedule for work intensities demanding 0·4 and 0·6 jV02max (Kamon 1979). The main task in this' investigation was chosen to demand 0·75 of jV02max and to represent practical field situations of load carrying. The main task involved load carrying, but it was preceded by moderate work load (0'3jV02max) and was followed by rest. Based on the expected increments in HR due to both the intensity of the muscular work and the hea t stress employed, the assigned total work period was 30 min; 25 min for the moderate work (0,3 fVo 2max) and five minutes for the heavy work (0,75 fV0 2max)' Assuming that the most accommodating practical rotational work situation would be between two workers, the 30 min of work was followed by 30 min of rest. The working ambient conditions were either hot but dry or warm but humid, the resting ambient conditions were either under the same as those for for the work or under cooler neutral ambient conditions.
2. Methods 2.I. Subjects Three male and three female, fully acclimatized college students participated in this study. Their physical characteristics are summarized in table I. After informed consent was obtained, a physician administered a thorough physical examination, and a progressive exercise treadmill test to obtain an exercise cardiogram along with a determination of maximal aerobic capacity.
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Table I.
Anthropometric data, maximal heart rate (HR max ), aerobic capacity (Vo 1max) and the oxygen cost of work (Vo , ) for the subjects. Fat (%)
HR m llix beat min- 1
60·0 60·9 47-4 56·1 ±7-6
1·58 1·70 1-47 158 ±0·12
24·3 25·0 16·1 21·9 ±4·8
188 190 200 192·7 ±6·4
2·23 2·25 2-29 2·26 ±0·03
Walk 0·67 0·76 0·60 0·68 ±0'08
Carry 1·68 1·70 1'77 1·72 ±0'05
66'8 85·9 95-4 82·7 ±14·6
1·83 2·13 2·17 2·04 ±0·19
15·9 14·5 26·6 19·0 ±6·6
200 204 182 195'3 ±11·7
3·70 5·52 4·75 4·66 ±0·91
0·99 1-44 1·28 1·24 ±0·23
2-70 4·25 3·58 3·51 ±0·78
Height (em)
Weight (kg)
Women I 2 3 Mean s.d .•
20 23 22 21·67 ± 1'53
154·0 168·6 160·0 160·9 ±7·3
Men I 2 3 Mean s.d."
25 23 30 26·0 ±3-6
Ino 188·6 182-6 182·7 ±5·8
* Standard deviation about the
Notation
WH-I WH-2 HD-I HD-2 C
V0 1max
SA (m")
Age (yr)
Table 2.
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(l min" ')
O 2 Cost of Work (I min-I)
mean.
The mean dry bulb (Td . ) and wet bulb (7;..) temperatures for the working and resting periods. Description Ambience Warm-humid Warm-humid Warm-humid Neutral Hot-dry Hot-dry Hot-dry Neutral
Control
Period Working Resting Working Resting Working Resting Working Resting Working Resting
Ambient Conditions" Work period Rest period TWb °C Tdb °C T...b °C 7db °C 36
31
36
31
50
25
50
25
23
16
36
31
23
16
50
25
23
16
23
16
• Temperatures were within a standard deviation of t "C. Mean air movement was l m s - I in the heated room and 0·18 m s - 1 in the cooler resting room.
2.2. Workloads During the walk at O' 3 VO,ma., the treadmill was level with an average speed of 80·5 m min - 1 for the women and 93·9 m min - I for the men. At the onset of the carrying the inclination of the treadmill was adjusted to raise the individual's oxygen consumption to 0·75 V02maxincluding the cost of load carrying. The uphill grade was 10-14% for the women and 12-15% for the men. The weight carried by the women was 10 kg, while the men carried 12 kg. It was equally distributed in a box 45 x 25 x 23 cm. The box was carried in front on the hands with the elbows bent at 90°. 2.3. Ambient conditions The environmental conditions for the working and resting periods are summarized in table 2. The ambient conditions for resting were either the same as the working conditions or in a neutral environment. Thus, during one session the subjects worked and rested in a hot-dry environment of 7db 50°C, TWb 25°C (HD-J), while another session consisted of warm-humid conditions of 7db 36°C, 'T."b 31°C (WH-l). Two more sessions were conducted which permitted the subjects to rest in a neutral environment of Td b 23°C, TWb 16°C following work periods in hot-dry conditions
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(HD-2), and warm-humid conditions (WH-2). A control test (C) was designed for work and rest periods under the neutral ambient conditions. The wind velocity was I m s- I for all tests within the climatic room, but in the outer room used for resting, the only air movement was due to central air conditioning and was calculated to be 0·18 m s- I. The order of the five experimental conditions were selected at random.
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2.4. Clothing The clothing included aT-shirt, shorts, a long sleeve cotton shirt, trousers, tennis shoes and socks. The women also wore a halter top beneath the T-shirt. 2.5. Measurements Rectal temperature (1;,) was measured by a thermistor inserted 10 em beyond the anal sphincter. Mean skin temperature (T.) was calculated as the unweighted average of six skin temperatures measured with uncovered, copper constantan thermocouples located on the forehead, chest, back, forearm and both thighs. The temperatures from the thermistor and thermocouples were recorded continuously throughout the test period. Heart rate (HR), obtained from an electrocardiogram, was taken for 15-30 s every five minutes, during the walk and rest periods and once each minute during the carrying. Oxygen uptake (Vo , ) was determined by the open circuit method. Expired air, collected through a low resistance valve into a Douglas bag, was analysed for O 2 and CO 2 content with a Beckman E2 paramagnetic analyser and a MSA Lira Infrared Meter respectively. Volume was measured with a Parkinson Cowan dry gas meter. Total sweat production (S) was calculated as the change in nude body weight, measured prior to and after the experimental session, and corrected for water intake. Evaporation rate (E v ) ' measured every 30 min, was determined by the change in total body weight and water intake. No corrections were made for respiratory evaporation or metabolic gas exchange weight loss. Heat equivalent of sweat and evaporation was calculated using 0·67 wh g-l as latent heat. Lactic acid concentration was measured spectrophotometrically with the colour change induced by the generation of NADH as a result of adding excess NAD and lactic dehydrogenase to plasma containing lactic acid (Sigma Chemical Co., Kit No. 726-UV). 2.6. Heat acclimation Prior to the experimental session each subject was heat-acclimated by two-hours daily exposures to 7db 50°C, TWb 25°C for four consecutive days and to Td b 36°C, TWb 31°C for two-three days. Exposure included level treadmill walking requiring O· 3 VO,mox' Full acclimation was determined by the reduction and levelling off in 1;. and HR and by the improved tolerance time. 2.7. Experimental procedures Upon arrival at the laboratory the subject rested for 20-30 min prior to obtaining a nude body weight. He then inserted the rectal thermistor probe, and with help from the experimenter placed the bipolar electrodes for the electrocardiogram and the. thermocouples on the skin before donning his clothing. Pre-test resting HR, 1;. and t, were obtained before entering the climatic room. Once in the climatic room, the subject rested approximately five minutes before a 5 cm ' venous blood sample was drawn from an antecubital vein with a minimum
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of stasis. The blood was immediately transferred to a tube containing sodium fluoride to prevent any increase in lactic acid concentration as a result of glycolysis. The subject was permitted to rest for several minutes, if desired, after the blood sample was drawn. Total body weight was taken prior to the onset of walking. The periods of work were determined according to the fVo,max of each workload (Karnon 1979). Thus the respective periods were 25 min at the lower intensity of walking followed by five minutes at the higher intensity of carrying. To allow a practical rotation between two workers a 30 min resting period followed the carrying period. This protocol of 30 min of work followed by 30 min of rest was identical for all the ambient temperatures. At 30 min, body weight was again taken and the subject then began the rest period either in the heat chamber or in the cool, outer room. After sitting quietly for 30 min, body weight was measured and the cycle was repeated. In all, at each condition, the subject underwent three walk-carry-rest cycles. The subjects were permitted to drink water at room temperature ad libitum. A two-minute expired air sample was collected and analysed at least once during each walk, carry and rest period. Five minutes after the last carry period, a second blood sample was drawn in a manner identical to that described above. Upon conclusion of the day's final rest period, a final nude body weight was obtained. 2.8. Statistical analysis The 1;" t; and HR at the end of each walk, carry and rest period for all test conditions were submitted to a three-factor analysis of variance with repeated measures. The lactic acid changes, sweat and evaporative rates were analysed with a two-factor analysis of variance. The LSD test was employed for pairwise comparisons. A level of 0·95 was required for significance.
3. Results 3.1. Rectal temperature (1;,) responses The end value for each of the walking, carrying and resting periods, averaged separately for each sex, was used to describe the time course of 1;, for each of the ambient conditions (figures I (a) and I (b)). The pattern and magnitude of the 1;, responses differed for the two sexes during the heat exposures (HD and WH), but not during the control exposure (C). The 1;,level during the hot-dry (HD) heat exposure was significantly higher than the control exposure (C) for both sexes. Resting under the same ambient condition as the working periods (HD-I ; WH-I; figure I (a) either did not permit the 1;, to drop, or allowed only a slight drop. This, along with the greater rise in 1;, during the carrying periods resulted in a progressive rise in 1;, from one working period to another. During the last two hours of exposure to the hot-dry conditions (HD-\), the men revealed significantly higher 1;, during carrying and lower 1;, during resting as compared to the women. Working and resting under the warm-humid conditions (WH-I ; figure I (a)) resulted in a sharper rise in 1;, as compared to the HD-I ambient conditions. After 150 min 1;, peaked at 38·4 and 38·3 for the women and men respectively. Although the average 1;, at the onset of the exposure was lower for the men, their rate of 1;, rise was faster so that towards the end of the exposure (after 100 min) there was no significant difference between the sexes. Resting under cooler, temperate ambient conditions allowed the 1;, to drop, particularly for the men (HD-2, WH-2; figure I (b)), and although 1;, increased from one working cycle to another it peaked just above 38°C during the last carrying
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Janet Torma Krajewski et al. 382
,""---,, " ,, ,,
37.8
.
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,:
\ \
\.',-,J ,
37·6 (WH-2)
368
1
--Walk --Carry ----Rut
30
60
90
120
,>0
180
TIME. min
,.. ~
380
37.6 (HO-l)
372
TIME, min
Figure I. (a) The time course of the averaged rectal temperature (~,) for the men (e) and for the women (0) during work and rest under the hot-dry (HD-I), warm-humid (WH-I and control (C) ambient conditions. See table 2 for details. Bars represent one standard deviation. Arrows indicate significant sex differences at the 0·05 level. (b) The time course of the averaged rectal temperature (7;,) for the men (e) and for the women (0) for work under hot-dry (HD-2) and warm-humid (WH-2) and resting under neutral ambient conditions. Bars represent one standard deviation. Arrows indicate significant sex differences at the 0·05 level.
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--Walk --C;rry
60
30
150
180
TIME. min
i" I~'"
:::~ _~= :_~---_J r __
1~r----2
;:-0:"--1--"91
360
350
iHO-I)
340
:~
36 11-'"
-:L
----
lli_,~ r----f? ~ Tl=~
~
3~
__
.•
,.
(WH-J}
34
30
60
90
120
150
190
TIME. min
Figure 2. (a) The time course of the averaged mean skin temperature (f,) for the men (e) and the women (0), during work and rest under hot-dry (HD-l), warm-humid (WH-l) and control (C) ambient conditions. See table 2 for details. Bars represent one standard deviation. Arrows indicate significant differences between sexes at the 0·05 level. (b) The time course of the averaged mean skin temperature (1;) for the men (e) and the women (0) for working under the hot-dry (HD-2) and warm-humid (WH-2) and resting under neutral ambient conditions. Bars represent one standard deviation. Arrows indicate significant sex differences at the 0-05 level.
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Janet Torma Krajewski et al.
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periods. These peaks were significantly lower than those for the exposure to the HD-I and WH-I conditions (figure I (a». There was a marked difference in the 1;. response of the two sexes; the responses of the men fluctuated significantly more than the women's responses. The men experienced a 1;. decrease during rest which reached significantly lower values than the women, particularly under the HD-l ambient conditions. However, during each working period the men's 1;. sharply rose to reach the same levels as the women's 1;e '
3.2. Mean skin temperature (1;) responses The end values of 1; for each period were averaged for each sex and were used to describe the time course of 1; for each ambient condition (figures 2 (a) and 2 (b». The' pattern of the 1; responses differed for each ambient condition. While f, dropped during the working period and rose during the resting period of the control exposure «e), figure 2 (a», it rose during the working period and dropped during the resting period for work in the heat and rest in the neutral (HD-2, WH-2; figure 2 (b». No fluctuations were noticed in 1; for the work and rest under the same ambient conditions (HD-I, WH-I; figure 2 (a)). No sex differences were found for the pattern of the change in 1;. However, there were differences in the magnitude of 1;. The values for the women were significantly higher than for the men during some of the working periods under HD-2 and WH-2 conditions and throughout the control study.
3.3. Heart rate (HR) responses Because there were no significant sex differences in HRs, the mean end values of HR for all subjects are described in figure 3. Work and rest in the heat (HD-I and WH-I, figure 3) resulted in higher HRs than work in the heat followed by rest in the neutral (HD-2 and WH-2), which in tum produced higher HRs than the control exposure. Throughout the HD-I and WH-I exposures the HR gradually rose from one hour to the next such that the walk 'and rest end values of the third hour were significantly higher than the first hour. Little change occurred between one carrying period and the next since HRs were already near maximum during the first carrying period. During the HD-2 and WH-2 exposures, the HR remained practically the same from one hour to the next except for the walking HR ofthe WH-2 conditions which rose by 12 beats min - 1. The walking HRs of WH-2 were significantly lower than those of WH-I while no difference was found between HD-I and HD-2.
3.4. Lactic acid The mean values of the increase in plasma concentration above the pre-work resting values are summarized in table 3 for each of the five ambient conditions and separately for the men and women. The increase in the lactate concentration was not significanlly different for any of the exposures. The women's lactate increase was significantly less than the men's. The women also tended to reveal greater increases when they were exposed to work and rest in the heat (HD-I', WH-l) than when exposed to rest outside the heated room (HD-2, WH-2), but the difference was statistically insignificant.
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CARRY
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