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Energy expenditure while standing or walking slowly uphill or downhill with loads a
NANCY A. PIMENTAL & KENT B. PANDOLF
a
a
U.S. Army Research Institute of Environmental Medicine , Natick, Massachusetts, 01760, U.S.A Published online: 27 Mar 2007.
To cite this article: NANCY A. PIMENTAL & KENT B. PANDOLF (1979) Energy expenditure while standing or walking slowly uphill or downhill with loads, Ergonomics, 22:8, 963-973, DOI: 10.1080/00140137908924670 To link to this article: http://dx.doi.org/10.1080/00140137908924670
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ERGONOMICS, 1979, VOL. 22, No.8, 963-973
Energy expenditure while standing or walking slowly uphill or downhill with loads By NANCY A. PIMENTAL AND KENT B. PANDOLF
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U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts 01760, U.S.A. Eight fit male subjects (Z4 yr, 176 em, 79 kg) stood, or walked at speeds of 0·5 or 0·9 m S-1 for'ZO-min periods on grades of - 10 to + 25% with loads of ZO or 40 kg. Energy expenditure (watt) was not significantly different in any of the standing conditions; grade and load increased energy expenditure while standing but not significantly. Although all the standing energy expenditure means were relatively low, high perceived exertion ratings suggest limits to tolerance time in some of these conditions. All the standing means were significantly lower than the walking means. Walking 0·9 m S-I on a -10% grade was significantly lower than walking 0·5 m s-· on a + 10% grade, which was significantly lower than walking 0·9 m S-1 on a + 10% grade. In the walkingconditions therewas a significantdifference between loads: means for the 20 kg loads were lower than means for the 40 kg loads. As the condition became more strenuous by increasing load, speed, and/or grade (while walking), energy expenditure became more sensitive to changes in these variables. The current energy expenditure prediction formula (Pandolf el al. 1977) was found to predict slightly high for the standing conditions, low for walking 0·5 m S-l on a + 10% grade. and accurately for walking 0·9 m S-I on a + 10% grade. In the standing conditions the deviation between predicted and measured was higher at the 40 kg load than at the 20 kg load. The formula in its present form is not equipped to predict for negative grades. The results of this study suggest that the prediction formula may place too much emphasis on the effects of speed and load while standing and walking slowly.
1. Introduction Earlier, Cathcart and Orr (1919) did an extensive study on the energy expenditure of the infantry recruit in training in order to determine the food requirements of the soldier, because at that time food supplies were limited and a matter of supreme importance. Cathcart et al. (1920) also studied the rate of marching and energy expenditure to determine the optimal rate of marching, and represented the relation between energy cost per unit of time and speed by a parabola: y = ax". + bx
+
c
where y is the energy cost per unit time and x the distance covered per unit time, implying that the cost of work is at first great and diminishes to an optimal value, thereafter again increasing. Cathcart et al. (1923) also did further work with energy expenditure as a function of load in order to find the maximal economic load to be carried by the soldier. Since Cathcart's work there have been many attempts to develop an equation for predicting the energy expenditure of walking (BobbertI960, Cotes and Meade 1960, Durnin and Passmore 1967, Givoni and Goldman 1971, Goldman and Iampietro 1962, Margaria et al. 1963, Workman and Armstrong 1963). Most of these equations have proven invalid in some part of their scope: for having limited ranges of speed, grade, load; for not making provisions for the different types of walking surfaces; for not examining the effects of external load and subject weight separately (i.e. calculating the energy cost per unit total weight instead of separating the weight of the body from the external load). OOI4--{)1 ]9/79/22080963 502'00 If) 1979 Taylor and Francis Ltd.
N. A. Pimental and K. B. PandoIf
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Terrain coefficients were developed to correct for the type of surface traversed (Soule and Goldman 1972). Single coefficients were found to fit all measured values except for soft snow (Pandolf et al. 1976). The coefficient for soft snow was found to increase as the snow footprint depth increased. The location of the external load (head, hands, feet, or back) is another consideration. Soule and Goldman (1969) have studied this and found the torso to be the most economical mode of carrying loads. Most of the prediction equations previously mentioned were derived from studies in which the subjects carried loads in backpacks. Givoni and Goldman (1971) developed an empirical formula for energy expenditure prediction as a function of speed, external load, body weight, grade, and terrain: M = 1/(171,)[(2·7 + 3'2(V - 0'7)1'65) + G(0'23 + 0'29(V - 0,7))] where
M = metabolic rate, watt
= terrain factor, defined as 1·0 for treadmill walking 171, = body weight + clothing + external load, kg
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1/
V = velocity, m
G
=
S-1
slope (grade),
%
However, this equation has its limits: a speed range from 0·7 to 2·5 m s - I ; the productofthe load and the velocity must not exceed the numerical value of I00; the subject weight and external load are not treated separately. Studies were done walking at speeds below this limit of 0·7 m S-1 and also at zero velocity (standing) in order to develop a more encompassing equation (Pandolf et al. 1977). The current revised energy expenditure prediction formula (Pandolf et al. 1977), extrapolated and interpolated from previous studies (Goldman and Iampietro 1962, Hughes and Goldman 1970, Soule and Goldman 1969), is: M = 1·5W+ 2'0(W+ L)(L{W)2 + I/(W+ L)(I·5V 2 + 0·35VG) where
M = metabolic rate, watt W = subject weight, kg
L
= external load, kg
1/ = terrain factor (1/ = 1·0 for treadmill) V = velocity, m s - 1 G = grade (slope), %
This revised formula was developed to include standing, or walking at any speed up to running ( - 2·4 m s - I), on grades from 0 to + 25%, with loads up to 70 kg (including 0 kg). The equation does not need to include loads of greater than 70 kg because it has been shown (Soule et al. 1978) that when walking at 1·8 m S-I with 60 to 70 kg loads, individuals are working at 91 to 93% of their maximal oxygen uptake, with performance times reduced to only 8·3 min while carrying 70 kg. The validity of the new predictive formula was checked by comparing predicted with measured energy expenditure values from a study by Goldman and Iampietro (1962) with a resulting correlation coefficient (r) of 0·96. The new formula is also in very good agreement with the old one (Givoni and Goldman 1971), which was derived from the results of a large number of studies. . The energy expenditures of walking at very slow speeds (0,2 to 1·0 m S-1) were all taken on horizontal surfaces (grade = 0%). It is the purpose of this study to validate
Energy expenditure for standing and slow walking
965
and/or suggest revisions of the energy expenditure formula for walking at very slow speeds on a grade and also for simply standing on a grade. Grades used will be both positive and negative (uphill and downhill).
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2. Methods A study was conducted in which eight fit, adult males volunteered to be subjects. These subjects had an average age (mean ± SE) of 23·8 ± 0·9 yr; height (nude), 176·2 ± 2·8 em; weight (nude), 78·7 ± 5-4 kg; body fat, 17·5 ± 1·4% as determined by the method of Durnin and Womersley (1974); lean body mass, 64·5 ± 3·5 kg; body surface area, 1·95 ± 0·08 m 2 ; and resting heart rate (standing), 72·0 ± 4·0 beats min - 1. The subjects were asked to stand, or walk at speeds of O' 5 m s- 1 ( - 1·1 mph) or 0·9 m S-1 (-2'0 mph) for 20-min periods with loads of 20 or 40 kg, on a treadmill on grades from - 10 to + 25%. The design of the study called for ten conditions: (a) (b) (c) (d) (e) (f)
(g) (h) (i) (j)
Standing, + 10% grade; 20 kg load. Standing, + 10% grade; 40 kg load. Standing, + 25% grade; 20 kg load. Standing, + 25% grade; 40 kg load. Walking 0·9 m S-1, + 10% grade; 20 kg load. Walking 0·9 m s-1, + 10% grade; 40 kg load. Walking 0·9 m S-1, -10% grade; 20 kg load. Walking 0·9 m s- 1, - 10% grade; 40 kg load. Walking 0·5 m s-1, + 10% grade; 20 kg load. Walking O' 5 m s- 1, + 10% grade; 40 kg load.
Each subject was tested for one 20-min session per day for ten days. The order of the conditions was. randomly assigned to minimize any training effects. Subjects reported at the same time every day in order to minimize the possibility of any time effects. Four subjects reported in the morning and four in the afternoon. Subjects wore the standard U.S. Army fatigue uniform with combat boots (weight = 2·9 kg). In the standing test conditions subjects were free to shift load position during the 20 min, but during collections were asked to stand as quietly as possible. After 5 min, subjects were assumed to be in a steady state. Energy expenditure was determined by 3-min Tissot collections (min 5-8, 11-14, and 17-20) and subsequent analysis of expired O 2 and CO 2 (Beckman E-2 and LB-2 analysers, respectively). The method of Weir (1949)was used to calculate oxygen uptake (Vo ,). Heart rates were measured by radial artery palpation at minutes 6, 12, and 18. Subjects were asked to rate their perception of the work on the Borg Scale (Borg 1970) for ratings of perceived exertion just prior to the measurement of their heart rates. Local (primarily involving the working muscles and joints), central (involving cardiorespiratory systems), and overall (integrating local and central ratings with appropriate weightings) ratings were measured (pandolf 1978). All loads were carried with standard U.S. Army load carriage systems (plus padding on the shoulders to prevent chafing), properly balanced and located high on the back. The loads were lifted by the experimenter onto the subject's back so that the subject did not expend any additional energy. 2.1. Statistical treatment A four-way analysis of variance (Lindquist 1953) was used to evaluate measured
N. A. Pimental and K. B. Pando If
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energy expenditure (watt). Experimental variables were subject, load, condition, and time. A three-way analysis of variance (subject, load, and condition) was used to evaluate the difference between predicted (Pandolf et ill. 1977) and measured energy expenditure values. The same design was used to evaluate final heart rates and final differentiated ratings of perceived exertion. When significant differences were found in the analyses of variance, critical differences (c.d.) were calculated (Cicchetti 1972, Li 1964) to determine where the significant differences were. Paired t-tests were used to locate significant differences between predicted and measured energy expenditure values for all conditions. A correlation coefficient was calculated between all individual predicted and measured energy expenditure values (watt). 3. Results In the first analysis (energy expenditure, watt) the two interactions found to be significant at the 0·01 level were load by condition and time by condition. In the load by condition interaction, no significant differences were found among any of the measured energy expenditure means while standing. (See table I for measured and predicted energy expenditure means ± SE for all ten conditions.) All of the standing means were, however, significantly lower than the walking means. For both 20 and 40 kg loads, the walking energy expenditure means were significantly different from each other: walking at 0·9 m s - 1 on a - 10% grade was lower than walking O' 5 m S-l on a + 10% grade, which was lower than walking 0·9 m S-1 on a + 10% grade. In each of these walking conditions there was a significant difference between loads: means for the 20 kg loads were lower than means for the 40 kg loads. Tobie I.
Measured and predicted energy expenditure means ± one SE for standing and walking slowly on grades with loads Measured energy expenditure
Predicted energy expenditure
Velocity
(wall)
(m s-')
Load (kg)
Grade
(wall)
112·8 ± 4·8' 131·7 ± 5·0'
131·5 ± 7·1 182-8 ± 3·1
0·0 0·0
20 40
+10 +10
123·1 ±4·!' 136·4 ± 5·3'
131·5 ± 7·1 182-8 ± 3·1
0·0 0·0
20 40
+25 +25
253-3 ± 7·1 325·1 ± 5·7
t t
0·9 0·9
20 40
-10 -10
385·1 ± 8·2' 462·8 ± 8·7'
345·9 ± 19·0 440·9 ± 14-4
0·5 0·5
20 40
+10 +10
550·3 ± 15·8 691·4 ± 12-6
558·5 ± 30·6 696·5 ± 26·1
0·9 0·9
20 40
+10 +10
(%)
• Measured values significantly different from predicted values. unable to predict for negative grades.
t Formula
In the second interaction found to be significant at the 0·01 level (time by condition), two significant differences were found. The corresponding energy expenditure mean for the sample taken at min 11-14 was lower than the means for samples taken at min 5-8 and min 17-20 when walking 0·9 m s" 1 on a + 10% grade (load means were 626,4,609'1, and 627·0W for samples taken at min 5-8,11-14, and 17-20, respectively). The sample taken at min 5-8 was lower than the sample taken at min 17-20 when walking 0·9 m S-1 on a -10% grade (load. means were 281'9,289·4, and 296·2 W). All other trends in the time by condition interaction were similar to those in the load by condition interaction.
Energy expenditure for standing and slow walking
967
Paired t-tests found differences between predicted and measured energy expenditure values. In the standing conditions, the predicted values were significantly higher than the measured (p < 0·01 for 20 kg loads, p < 0·00 I for 40 kg loads). For walking O'5 m s - 1 on a + 10% grade, the predicted values were significantly lower than the measured (p < 0·01 for 20 kg loads, p < 0·02 for 40 kg loads). For walking 0·9 m S-1 on a + 10% grade, measured values were not significantly different from predicted values. In the standing conditions the deviation between predicted and measured was higher at the 40 kg load than at the 20 kg load. Although the formula over predicted for standing and under predicted for walking 0·5 m S-1 (on a + 10% grade), when all individual predicted values were correlated with individual measured values, a strong relationship (r = 0,99) was found (see figure 1).
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00.0 ms- l
'"cr::>
l-
• 0.0 t:. 0.0 .. 0.0 D 0.5 • 0.5 0.9
o
+ 10'-., 20kg
ms· 1 + 10%, 40kg m~·l
+ 25%, 20kg + 25%. 40kg
ms·.
+ 10%,20kQ
11"\$-\
ms-!
+
rna-I
+ 10%.20kQ
10'%,40kQ
• 0.9 ma- + to%, 40kQ l
e z a.
'"x '">Cl
cr z
'"'"
r =0.99
e
'"
I-
U
e
LlJ
cr a.
Figure I. The relationship between individual predicted (Pandolf et at. 1977) and measured energy expenditure values for standing and walking slowly (m s - I) on positive grades (%) with loads (kg).
In the heart rate analysis, the load by condition interaction was found to be significant. Among the standing conditions, no significant differences were found. (See table 2 for final heart rate means ± SE.) All the standing condition means were, however, significantly lower than the walking condition means at the 0·05 level with one exception: the mean for standing on a + 25% grade with a 20 kg load was not significantly different from walking 0·9 m s - 1 on a - 10% grade with a 20 kg load. For both loads, walking 0·9 m S-1 on a -10% grade and walking 0·5 m S-1 on a + 10% grade were significantly lower than walking 0·9 m S-1 on a + 10% grade (critical difference at the 0·05 level). Walking 0·9 m S-1 on a -10% grade and walking 0·5 m S-l on a + 10% grade were not significantly different. The final heart rate mean for walking 0·5 m S-l on a + 10% grade with a 20 kg load was significantly lower than the mean with a 40 kg load (p < 0'05). This was also true for walking 0·9 m S-1 on a + 10% grade. There were no significant differences between loads for the standing conditions or walking 0·9 m s -Ion a - 10% grade.
N. A. Pimental and K. B. Pandolf
968
In the analysis of perceived exertion ratings, the 40 kg load means were significantly higher than the 20 kg load means for local, central, and overall ratings (p .< 0'001). (See table 3 for final ratings of perceived exertion means ± SE.) For central and overall ratings, means for standing on a +25% grade were significantly lower than means for standing on a + 10% grade (p < 0·0 I for central, p < 0·05 for overall).
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Table 2.
Heart rate means ± one SE after 20-min periods of standing and walking slowly on grades with loads Heart rate (beats min -I)
Velocity (m S-I)
Load (kg)
Grade
80·6 ± 4-4 88·9 ± 5·0
0·0 0·0
20 40
+10 +10
82·1 ± 5·2 83·2 ± 5·8
0·0 0·0
20 40
+25 +25
93-4 ± 8-4 104·0 ± 7·2
0·9 0·9
20 40
-10 -10
101·5 ± 6·4 116·2 ± 9·4
0·5 0·5
20 40
+10 +10
123·8 ± 4·7 146·2 ± 8·5
0·9 0·9
20 40
+10 +10
(%)
c.d o.o, = 12·3. Table 3.
Load means ± one SE for local, central, and overall ratings of perceived exertion following 20-min periods of standing and walking slowly on grades with loads Velocity
(m
S-I)
0·0 0·0 0·9 0·5 0·9
Grade (%)
+10 +25 -10 +10 +10
20 kg load mean 40 kg load mean
Local RPE 11·9 ± 0·7 10·9 ± 0-4 11·9 ± 0·4 11·7 ± 0·5 12-6 ± 0·6
Central RPE
Overall RPE
± ± ± ±
0·5 0·5 0·4 0-4 ± 0·5
12-8 ± 0·6 11·4 ± 0·5 12·0 ± 0·4 11·9 ± 0·4 13·1 ±0·5
10·5 ± 0·3 13·1 ± 0·2
10·8 ± 0·2 13·7 ± 0·2
10·7 ± 0·2 13·7 ± 0·2
4.
12·6 11·2 12·0 \2·0 13·3
Discussion
It was expected that there would be no significant differences among the energy
expenditure values for the samples taken at the three times during each run (min 5-8, 11-14, and 17-20), since after 5 min subjects were assumed to be in a steady state. Two discrepancies were found among the samples in this study. The first was during the most strenuous condition (walking 0'9 m S-1 on a + 10% grade). In this run, the sample taken at min 11-14 had a corresponding energy expenditure value significantly lower than the other two samples. This may be because during the sample taken at min 5-8 energy expenditure was increased due to the difficulty in balancing the load . while working at such a high level. By min 11-14 the subjects may have adjusted to the conditions, but by the end ofthe run and the sample taken at min 17-20, were fatigued and required increased energy expenditure due to inefficiency of walking and recruitment of additional primary muscle fibres and auxiliary muscles. When walking downhill the increased energy expenditure during min 17-20 may have been because the subjects were unaccustomed to walking on a negative grade and by the end of the run were recruiting additional primary muscle fibres as well as auxiliary muscles.
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Energy expenditure for standing and slow walking
969
In the standing conditions in this study (velocity = 0·0 m s - 1), changes in grade made no significant difference in energy expenditure. This was true for standing on + 10% and + 25% grades. Although the increase in grade increased energy expenditure, it was not statistically a significant difference. Heart rates were actually lower for standing on a + 25% grade than for standing on a + 10% grade (with a 40 kg load). On the scale for ratings of perceived exertion, standing on a + 25% grade was rated significantly lower than standing on a + 10% grade for central and overall ratings, possibly because standing on a + 25% grade simulates standing on the level and leaning forward with a load placed on the back, moving the centre of gravity so as to counteract the effect of the load. Standing on negative grades has not been investigated; it may be that for standing on a downhill slope changes in grade significantly affect energy expenditure due to the additional energy expended by the leg and back muscles in resisting the downward pull of gravity on the body as balance is maintained. If this proves to be the case, a factor may have to be added to the energy expenditure prediction equation for standing (and possibly walking) on negative grades. At this point, when velocity is zero, grade does not enter into the prediction equation, It was found in this study that, while standing, as loads increased from 20 to 40 kg, energy expenditure increased but not significantly. In another standing study (PandoIf et al. 1977), increases in load while standing did increase energy expenditure significantly. This may be due to a slightly larger sample size in the earlier study (N = 10) and a greater range of loads (0 to 50 kg). Whatever the case may be, energy expenditure while standing is relatively low, and changes in load do not affect energy expenditure while standing nearly as much as they do while walking. It is important to note that, although in all the standing conditions energy expenditure was low, and increased only slightly with increased load, this is not an indication that these conditions can be safely maintained for long periods of time; muscle strain and fatigue are -two factors other than measured energy expenditure which affect the well-being of the man and limit tolerance time. In fact, even though the measured energy expenditure for standing on a + 10% grade with a 40 kg load was not significantly different from the lowest energy expenditure value in this study, it was rated second highest on the perceived exertion scale for local, central, and overall ratings (walking 0·9 m S-1 on a + 10% grade with a 40 kg load was rated highest). Because increases in load may make significant increases in energy expenditure while standing, while changes in grade do not seem to, it may be suggested that energy expenditure is more sensitive to changes in load than in grade (while standing). The results of this study, supported by the results of previous studies (Pandolf et al. 1977), indicate that energy requirements for walking (dynamic work) are very different from the energy requirements for standing (static work). Once velocity increases above zero, there are significant increases in energy expenditure. The results of this study show that even when comparing the most strenuous standing condition (carrying 40 kg on a + 25% grade) to the least strenuous walking condition (carrying 20 kg on a -10% grade), there was a large difference in energy expenditure, the walking condition requiring 117 W more than the standing. While walking at slow speeds ( - O· 2 to 1·0 m s - 1), changes in grade affect energy expenditure significantly, contrary to when velocity is zero. These changes are not linear, however; the same increase in grade will increase energy expenditure by a greater amount in a more strenuous condition. The condition may be made more strenuous by increasing speed, grade, or load, or a combination of these. A point
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970
N. A. Pimental and K. B. Pandolf
may be mentioned concerning the results of this study on downhill walking. It was hypothesized that there would be a savings in energy expenditure over walking on the level when walking on a negative grade. This did not prove to be the case. It was found that the energy cost of walking downhill on a -10% grade (0'9 m S-I, 40 kg load) was higher than the cost of walking at approximately the same speed and load (1,0 m S-I, 40 kg) on the level. This could be due to the involvement of the back and leg muscles in resisting the downward pull of gravity. The effect of negative grades while walking has not been studied adequately. At present the prediction model is not equipped to consider negative grades. While walking at slow speeds there are significant increases in energy expenditure with increasing load. The data of this study and the data of other studies (Pandolf et al. 1977) suggest that, within the range of 0·0 to 1·0 m s -1, the effect of increasing speed on energy expenditure is not linear. Following a sharp increase in energy expenditure as velocity increases above zero, there is a steady increase between 0·2 and 0·6 m s -1, and at approximately 0·8 m s - 1 another sharp rise can be observed. As with grade and load, the same increase in speed will increase energy expenditure by a greater percentage in a more stressful condition. When this study compared measured to predicted energy expenditure (Pandolf et al. 1977) values, it was found that the formula predicted high for the standing conditions (the Pandolf et al. (1977) standing data support this). For increasing load while standing, the formula predicts increases in energy expenditure of a greater magnitude than this study has shown. As loads increase from 0 to 50 kg, the formula predicts energy expenditure values that are from 10 to 35% higher than the measured values. When velocity is 0·2 to 0·6 m s -I, the formula predicts slightly low for loads of approximately 0 to 30 kg, accurately for approximately a 35 kg load, and high for loads above 35 kg. For speeds of 0·8 to 1·0 m s - 1 the difference between predicted and measured also becomes greater as loads increase, but at these higher speeds there is a higher initial baseline (the formula predicts accurately for a 30 kg load). These observations suggest that for slow speed walking the prediction formula places too much emphasis on the effects of speed, and is also too sensitive to changes in load. Concerning grade, the prediction formula was accurate for a + 10% grade at a speed of 0·9 m s - I. For this same grade at a speed of O' 5 m s - I, the formula predicted low. More studies concerning the effect of grade on slow speed walking will have to be done before any conclusions can be drawn. When comparing predicted to measured energy expenditure values, the downhill data could not be evaluated because the formula is unable to make predictions for negative grades. In other words, the model does not consider negative, or eccentric, muscular work. When negative grade values are put into the prediction formula, it is possible for negative energy expenditure values to arise. In this study's downhill condition (0,9 m S-I on a -10% grade), the difference between measured and predicted energy expenditure was not only 313 W, but the predicted value was negative (see figure 2). In summary, in the standing conditions changes in load and/or grade change energy expenditure only slightly (at least for positive grades), much less than the changes in energy expenditure these variables produce while walking, which is a very different condition from standing. Once walking, increases in load, grade, and/or speed increase energy expenditure significantly, and as the condition becomes more strenuous, energy expenditure becomes more sensitive to changes in load, grade, and
Energy expenditure for standing and slow walking
971
800
l&J
a:
::>
I-
400
oZ l&J
Q.
~ >o a:
200
VELOCITY· 0.9 ms-'
LOAO·20 kg
l&J
5
z
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l&J
Figure 2.
10
15
("!o)
Predicted energy expenditure values (Pandolf et al. 1977) forwalking on negative grades.
speed. The current energy expenditure formula (Pandolf et al. 1977) predicts high for standing conditions, and is too sensitive to the effects ofIoad while standing. For slow speed walking the formula may place too much emphasis on the effects of load and grade. For higher walking speeds the formula may predict accurately because energy expenditure is affected more by grade and load in these more strenuous conditions than in the slow speed walking conditions. The formula needs to be revised to be able to predict for negative grades since man goes both uphill and downhill. Indeed, man must walk downhill once he has walked uphill. Before the formula is revised further work should be done to provide information to fill in the-gaps in the data; particularly studies on the effects of grade and light loads on slow speed walking, the effects of load on standing, and downhill standing and walking. The authors wish to acknowledge the assistance of Fred R. Winsmann in the collection of the data and Ella H. Munro in the statistical analysis. The views, opinions, and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy, or decision, unless so designated' by other official documentation. Human subjects participated in these studies after giving their free and informed voluntary consent, Investigators adhered to AR 70-25 and USAMRDC Regulation 70-25 on Use of Volunteers in Research.
Huit sujets masculins (24 ans, 176 em, 79 kg) se sont tenus immobiles ou bien ont rnarche a des vitesses de 0·5 ou 0·9 m S-I pendant 20 mn sur une pente de -10% il + 25% avec des charges de 20 ou 40 kg. La depense energetique (en watts) eta it sensiblement la meme dans toutes les conditions en posture immobile. La pente et la charge ont, dans ces conditions, augmente la depense energetique, mais pas significativement. Bien que ies rnoyennes des depenses energetiques en posture immobile aient ete relativement basses, les jugements de l'effort percu montrent, par leur degre eleve, qu'il y a des limites au temps d'endurance pour certaines de ces conditions. Toutes les moyennes en imrnobilite sont significativement plus basses que celles en deplacernent. La marche a0·9 m S-l sur une pente de -10% fournit une moyenne significativerncnt plus basse que la marc he a 0·5 m s " J, sur une pente de + 10%, ellemerne significativement plus basse que la marche a0·9 m S-1 sur une pente de + 10%. Dans les conditions de marche, on a observe des differences significatives entre les charges: les moyennes sont plus basses avec les charges de 20 kg qu'avec celles de 40 kg. En augmentant l'astreinte par l'accroissement des charges, des vitesses et/ou des pentes (pour la marche) on note des modifications dans la sensibilite de la depense energetique. La formule classique de prediction de la depense energetique (Pandolf et coli. 1977) surestime legerernent la posture immobile, sousestime la marche a 0·5 m S-1 sur une pente de + 10%, mais estime correctement la marche 0·9 m S-1 sur une pente de + 10%. Dans les conditions d'immobilite. la difference entre la depense predite et la depense mesuree est plus elevee pour la charge de 40 kg que pour celle de 20 kg. La formule actuelle n 'est pas concue pour des predictions dans Ie cas de pentes negatives. Les resultats de cette etude
a
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suggerent quela formule de prediction favorise leseffcts de Ia vitesse et de la charge pour la posture immobile et pour la marche it allure leote. Acht rnannliche Vpn (24 Jahre, 176 em, 79 kg) standen oder ringen 20 Minuten lang mit Geschwindigkeiten von 0·5 oder 0·9 tn!» auf schiefen Ebenen mit Steigungen von -10 bis +25% und trugen dabei Lasten von 20 oder 40 kg. Fur die varia bien Versuchsbedingungen beim Stehen konnten keine signifikanten Unterschiede des Energie-umsatzes (EU) festgestellt werden: zunehmende Steigung und Last fuhrten lediglich zu einem nicht signifikanten Anstieg des EU. Trotz der relativ niedrigen Mittelwerte des EU beim Stehen lassen die Ergebnisse der subjektiven Einstufungen der Anstrengung den SchluB auf eine zeitlich begrenzte Ertraglichkeit unter bestimrnten Belastungskombinationen zu. Aile Mittelwerte des EU beim Stehen waren signifikant niedriger als diejenigen beim Gehen. Der EU beirn Gehen auf einer negativen Steigung von -10% mit 0·9 rnjs war signifikant niedriger als beim Gehen auf einer Steigung von + 10% mit 0·5 mis, und dieser war wiederum signifikant niedriger als bei einer Geschwindigkeit von 0·9 m/s bei gleicher Steigung. Beim Gehen ergab sich ein signifikanter Unterschied zwischen den Lasten: Die Mittelwerte bei Lasten von 20 kg waren niedriger als bei 40 kg. Mit einem Belastungsanstieg durch Erhohung der Last, Geschwindigkeit und/oder der Neigung (beim Gehen) erhohte sich auch die Empfindlichkeit des EU aufweitere Veranderungen der genannten Variabien. Es wurde festgestellt, daB die gangige Formel zur Voraussage des EU (Pandolf et 01. 1977) fur Steben zu leicht erhohten, fur Gehen mit 0·5 m/s auf einer Steigung von + 10% zu niedrigeren, und beim Gehen mit 0·9 rnjs auf + 10% iger Steigung zu genaueren Ergebnissen fuhrt. Beirn Stehen war die Abweichung zwischen vorausberechneten und gemessenen EU grofser bei der Last von 40 kg als bei derjenigen von 20 kg. Die Forme1 ist in ihrer gegenwartiben Form nicht geeignet fur Voraussagen bei negativen Steigungen. Die Ergebnisse vorliegender Untersuchung lassen vermuten, daf die erwahnte Formel zu stark den Einflufl der Gehgeschwindigkeit und der Last beim langsamen Gehen bzw. beim Stehen beriicksichtigt.
References BOOOERT, A. C., 1960" Energy expenditure in level and grade walking. Journal of Applied Physiology, IS, 1015-1021. BORG. G., 1970, Perceived exertion as an indicator of somatic stress. Scandinavian Journal of Rehabilitation Medicine, 2,92-98. CATHCART, E. P., LOTHIAN, N. V., and GREENWOOD, M., 1920, A note on the rate of marching and expenditure of energy in man. Journal of the Royal Army Medical Corps, 34, 297-305. CATHCART, E. P., and ORR, J. B" 1919, The Energy Expenditure ofthe Infantry Recruit in Training (London: H.M. STATIONERY OFFICE). CATHCART, E. P., RICHARDSON, D. T., and CAMPOELL, W., 1923, On the maximum load to he carried by , the soldier. Journal of the Royal Army Medical Corps, 40, 435-443; 41,12-24.87-98,161-178. CICCHL?1TI, D. V., 1972, Extension of multiple-range tests to interaction tables in the analysis of variance. Psychological Bulletin, 77, 40.5:....408. COTES, J. E., and MEADE, F .. 1960, The energy expenditure and mechanical energy demand in walking. . Ergonomics, 3, 97-119. . DURNtN, J. V. G. A., and PASSMORE,'R., 1967, Energylwork, and Leisure (London: HEINEMANN EDUCATIONAL BOOKS). DURNIN.J. V. G. A., and' WOMERSLEY, J., 1974. Body fat assessed from total body density and its estimate from skinfold thickness: measurements on 481 men and women aged 16-72 years. British Journal
of Nurritian,.32, 77-97. B., and GOLDMAN, R. F., 1971, Predicting metabolic energy cost. Journal of Applied Physiology,
GIVONI,
30,429-433. GOLDMAN, R. F., and lAM PIETRO, P. F., 1962, Energy cost of load carriage. Journal of Applied Physiology. 17, 675-676, HUGHES, A. L., and GOLDMAN, R. F" 1970, Energy cost of' hard work '. Journal of Applied Physiology, 29, 570-572. Lt, C. C., 1964, Introduction to Experimentul Statistics (New York: MCGRAw-HILL BOOK COMPANY). LINDQUIST, E. F., 1953, Design and Analysis of Experiments in Psychology and Education (Boston: HOUGHTON MIFFUN COMPANY).
MARGARIA. R., CERRETELLI, P., AGHEMO, P., and SASSI, G., 1963, Energy cost of running. Journal ofApplied Physiology, 18, 367-370. PANDOLF, K. 8., 1978, Influence of local and central factors in dominating rated perceived exertion during physical work. Perceptual and Motor Skills, 46, 683-698. PANDOLF, K. B., GIVONI, B" and GOLDMAN, R. F., 1977, Predicting energy expenditure with loads while standing or walking very slowly. Journal of Applied Physiology, 43, 577-581. PANDOLF, K. B., HAISMAN, M. F., and GOLDMAN, R. F., 1976, Metabolic energy expenditure and terrain coefficients for walking on snow. Ergonomics, 19, 683-690. SOULE, R. G., and GOLDMAN, R. F., 1969, Energy cost of loads carried on the head, hands, or feet. Journal of Applied Physiology, 27, 687-690.
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SOULE, R. G., and GOLDMAN, R. F., 1972, Terrain coefficients for energy cost prediction. Journal of Applied Physiology, 32, 706-708. SOULE, R. G., PANDOLF, K. B., and GOLDMAN, R. F.. 1'l7X. Energy expenditure of heavy load carriage. Ergonomics, 21, 373-381. WEIR, J. B. DeV., 1949, New methods for calculating metabolic rate with special reference to protein metabolism. Journal of Physiology, (London), 109, 1-9. WORKMAN, J. M., and ARMSTRONG, B. W., 1963, Oxygen cost of treadmill walking. Journal of Applied Physiology, 18, 798-803.
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Manuscript received 18 September 1978. Revised manuscript received 8 January 1979.
Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
Energy expenditure while standing or walking slowly uphill or downhill with loads a
NANCY A. PIMENTAL & KENT B. PANDOLF
a
a
U.S. Army Research Institute of Environmental Medicine , Natick, Massachusetts, 01760, U.S.A Published online: 27 Mar 2007.
To cite this article: NANCY A. PIMENTAL & KENT B. PANDOLF (1979) Energy expenditure while standing or walking slowly uphill or downhill with loads, Ergonomics, 22:8, 963-973, DOI: 10.1080/00140137908924670 To link to this article: http://dx.doi.org/10.1080/00140137908924670
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ERGONOMICS, 1979, VOL. 22, No.8, 963-973
Energy expenditure while standing or walking slowly uphill or downhill with loads By NANCY A. PIMENTAL AND KENT B. PANDOLF
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U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts 01760, U.S.A. Eight fit male subjects (Z4 yr, 176 em, 79 kg) stood, or walked at speeds of 0·5 or 0·9 m S-1 for'ZO-min periods on grades of - 10 to + 25% with loads of ZO or 40 kg. Energy expenditure (watt) was not significantly different in any of the standing conditions; grade and load increased energy expenditure while standing but not significantly. Although all the standing energy expenditure means were relatively low, high perceived exertion ratings suggest limits to tolerance time in some of these conditions. All the standing means were significantly lower than the walking means. Walking 0·9 m S-I on a -10% grade was significantly lower than walking 0·5 m s-· on a + 10% grade, which was significantly lower than walking 0·9 m S-1 on a + 10% grade. In the walkingconditions therewas a significantdifference between loads: means for the 20 kg loads were lower than means for the 40 kg loads. As the condition became more strenuous by increasing load, speed, and/or grade (while walking), energy expenditure became more sensitive to changes in these variables. The current energy expenditure prediction formula (Pandolf el al. 1977) was found to predict slightly high for the standing conditions, low for walking 0·5 m S-l on a + 10% grade. and accurately for walking 0·9 m S-I on a + 10% grade. In the standing conditions the deviation between predicted and measured was higher at the 40 kg load than at the 20 kg load. The formula in its present form is not equipped to predict for negative grades. The results of this study suggest that the prediction formula may place too much emphasis on the effects of speed and load while standing and walking slowly.
1. Introduction Earlier, Cathcart and Orr (1919) did an extensive study on the energy expenditure of the infantry recruit in training in order to determine the food requirements of the soldier, because at that time food supplies were limited and a matter of supreme importance. Cathcart et al. (1920) also studied the rate of marching and energy expenditure to determine the optimal rate of marching, and represented the relation between energy cost per unit of time and speed by a parabola: y = ax". + bx
+
c
where y is the energy cost per unit time and x the distance covered per unit time, implying that the cost of work is at first great and diminishes to an optimal value, thereafter again increasing. Cathcart et al. (1923) also did further work with energy expenditure as a function of load in order to find the maximal economic load to be carried by the soldier. Since Cathcart's work there have been many attempts to develop an equation for predicting the energy expenditure of walking (BobbertI960, Cotes and Meade 1960, Durnin and Passmore 1967, Givoni and Goldman 1971, Goldman and Iampietro 1962, Margaria et al. 1963, Workman and Armstrong 1963). Most of these equations have proven invalid in some part of their scope: for having limited ranges of speed, grade, load; for not making provisions for the different types of walking surfaces; for not examining the effects of external load and subject weight separately (i.e. calculating the energy cost per unit total weight instead of separating the weight of the body from the external load). OOI4--{)1 ]9/79/22080963 502'00 If) 1979 Taylor and Francis Ltd.
N. A. Pimental and K. B. PandoIf
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Terrain coefficients were developed to correct for the type of surface traversed (Soule and Goldman 1972). Single coefficients were found to fit all measured values except for soft snow (Pandolf et al. 1976). The coefficient for soft snow was found to increase as the snow footprint depth increased. The location of the external load (head, hands, feet, or back) is another consideration. Soule and Goldman (1969) have studied this and found the torso to be the most economical mode of carrying loads. Most of the prediction equations previously mentioned were derived from studies in which the subjects carried loads in backpacks. Givoni and Goldman (1971) developed an empirical formula for energy expenditure prediction as a function of speed, external load, body weight, grade, and terrain: M = 1/(171,)[(2·7 + 3'2(V - 0'7)1'65) + G(0'23 + 0'29(V - 0,7))] where
M = metabolic rate, watt
= terrain factor, defined as 1·0 for treadmill walking 171, = body weight + clothing + external load, kg
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1/
V = velocity, m
G
=
S-1
slope (grade),
%
However, this equation has its limits: a speed range from 0·7 to 2·5 m s - I ; the productofthe load and the velocity must not exceed the numerical value of I00; the subject weight and external load are not treated separately. Studies were done walking at speeds below this limit of 0·7 m S-1 and also at zero velocity (standing) in order to develop a more encompassing equation (Pandolf et al. 1977). The current revised energy expenditure prediction formula (Pandolf et al. 1977), extrapolated and interpolated from previous studies (Goldman and Iampietro 1962, Hughes and Goldman 1970, Soule and Goldman 1969), is: M = 1·5W+ 2'0(W+ L)(L{W)2 + I/(W+ L)(I·5V 2 + 0·35VG) where
M = metabolic rate, watt W = subject weight, kg
L
= external load, kg
1/ = terrain factor (1/ = 1·0 for treadmill) V = velocity, m s - 1 G = grade (slope), %
This revised formula was developed to include standing, or walking at any speed up to running ( - 2·4 m s - I), on grades from 0 to + 25%, with loads up to 70 kg (including 0 kg). The equation does not need to include loads of greater than 70 kg because it has been shown (Soule et al. 1978) that when walking at 1·8 m S-I with 60 to 70 kg loads, individuals are working at 91 to 93% of their maximal oxygen uptake, with performance times reduced to only 8·3 min while carrying 70 kg. The validity of the new predictive formula was checked by comparing predicted with measured energy expenditure values from a study by Goldman and Iampietro (1962) with a resulting correlation coefficient (r) of 0·96. The new formula is also in very good agreement with the old one (Givoni and Goldman 1971), which was derived from the results of a large number of studies. . The energy expenditures of walking at very slow speeds (0,2 to 1·0 m S-1) were all taken on horizontal surfaces (grade = 0%). It is the purpose of this study to validate
Energy expenditure for standing and slow walking
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and/or suggest revisions of the energy expenditure formula for walking at very slow speeds on a grade and also for simply standing on a grade. Grades used will be both positive and negative (uphill and downhill).
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2. Methods A study was conducted in which eight fit, adult males volunteered to be subjects. These subjects had an average age (mean ± SE) of 23·8 ± 0·9 yr; height (nude), 176·2 ± 2·8 em; weight (nude), 78·7 ± 5-4 kg; body fat, 17·5 ± 1·4% as determined by the method of Durnin and Womersley (1974); lean body mass, 64·5 ± 3·5 kg; body surface area, 1·95 ± 0·08 m 2 ; and resting heart rate (standing), 72·0 ± 4·0 beats min - 1. The subjects were asked to stand, or walk at speeds of O' 5 m s- 1 ( - 1·1 mph) or 0·9 m S-1 (-2'0 mph) for 20-min periods with loads of 20 or 40 kg, on a treadmill on grades from - 10 to + 25%. The design of the study called for ten conditions: (a) (b) (c) (d) (e) (f)
(g) (h) (i) (j)
Standing, + 10% grade; 20 kg load. Standing, + 10% grade; 40 kg load. Standing, + 25% grade; 20 kg load. Standing, + 25% grade; 40 kg load. Walking 0·9 m S-1, + 10% grade; 20 kg load. Walking 0·9 m s-1, + 10% grade; 40 kg load. Walking 0·9 m S-1, -10% grade; 20 kg load. Walking 0·9 m s- 1, - 10% grade; 40 kg load. Walking 0·5 m s-1, + 10% grade; 20 kg load. Walking O' 5 m s- 1, + 10% grade; 40 kg load.
Each subject was tested for one 20-min session per day for ten days. The order of the conditions was. randomly assigned to minimize any training effects. Subjects reported at the same time every day in order to minimize the possibility of any time effects. Four subjects reported in the morning and four in the afternoon. Subjects wore the standard U.S. Army fatigue uniform with combat boots (weight = 2·9 kg). In the standing test conditions subjects were free to shift load position during the 20 min, but during collections were asked to stand as quietly as possible. After 5 min, subjects were assumed to be in a steady state. Energy expenditure was determined by 3-min Tissot collections (min 5-8, 11-14, and 17-20) and subsequent analysis of expired O 2 and CO 2 (Beckman E-2 and LB-2 analysers, respectively). The method of Weir (1949)was used to calculate oxygen uptake (Vo ,). Heart rates were measured by radial artery palpation at minutes 6, 12, and 18. Subjects were asked to rate their perception of the work on the Borg Scale (Borg 1970) for ratings of perceived exertion just prior to the measurement of their heart rates. Local (primarily involving the working muscles and joints), central (involving cardiorespiratory systems), and overall (integrating local and central ratings with appropriate weightings) ratings were measured (pandolf 1978). All loads were carried with standard U.S. Army load carriage systems (plus padding on the shoulders to prevent chafing), properly balanced and located high on the back. The loads were lifted by the experimenter onto the subject's back so that the subject did not expend any additional energy. 2.1. Statistical treatment A four-way analysis of variance (Lindquist 1953) was used to evaluate measured
N. A. Pimental and K. B. Pando If
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energy expenditure (watt). Experimental variables were subject, load, condition, and time. A three-way analysis of variance (subject, load, and condition) was used to evaluate the difference between predicted (Pandolf et ill. 1977) and measured energy expenditure values. The same design was used to evaluate final heart rates and final differentiated ratings of perceived exertion. When significant differences were found in the analyses of variance, critical differences (c.d.) were calculated (Cicchetti 1972, Li 1964) to determine where the significant differences were. Paired t-tests were used to locate significant differences between predicted and measured energy expenditure values for all conditions. A correlation coefficient was calculated between all individual predicted and measured energy expenditure values (watt). 3. Results In the first analysis (energy expenditure, watt) the two interactions found to be significant at the 0·01 level were load by condition and time by condition. In the load by condition interaction, no significant differences were found among any of the measured energy expenditure means while standing. (See table I for measured and predicted energy expenditure means ± SE for all ten conditions.) All of the standing means were, however, significantly lower than the walking means. For both 20 and 40 kg loads, the walking energy expenditure means were significantly different from each other: walking at 0·9 m s - 1 on a - 10% grade was lower than walking O' 5 m S-l on a + 10% grade, which was lower than walking 0·9 m S-1 on a + 10% grade. In each of these walking conditions there was a significant difference between loads: means for the 20 kg loads were lower than means for the 40 kg loads. Tobie I.
Measured and predicted energy expenditure means ± one SE for standing and walking slowly on grades with loads Measured energy expenditure
Predicted energy expenditure
Velocity
(wall)
(m s-')
Load (kg)
Grade
(wall)
112·8 ± 4·8' 131·7 ± 5·0'
131·5 ± 7·1 182-8 ± 3·1
0·0 0·0
20 40
+10 +10
123·1 ±4·!' 136·4 ± 5·3'
131·5 ± 7·1 182-8 ± 3·1
0·0 0·0
20 40
+25 +25
253-3 ± 7·1 325·1 ± 5·7
t t
0·9 0·9
20 40
-10 -10
385·1 ± 8·2' 462·8 ± 8·7'
345·9 ± 19·0 440·9 ± 14-4
0·5 0·5
20 40
+10 +10
550·3 ± 15·8 691·4 ± 12-6
558·5 ± 30·6 696·5 ± 26·1
0·9 0·9
20 40
+10 +10
(%)
• Measured values significantly different from predicted values. unable to predict for negative grades.
t Formula
In the second interaction found to be significant at the 0·01 level (time by condition), two significant differences were found. The corresponding energy expenditure mean for the sample taken at min 11-14 was lower than the means for samples taken at min 5-8 and min 17-20 when walking 0·9 m s" 1 on a + 10% grade (load means were 626,4,609'1, and 627·0W for samples taken at min 5-8,11-14, and 17-20, respectively). The sample taken at min 5-8 was lower than the sample taken at min 17-20 when walking 0·9 m S-1 on a -10% grade (load. means were 281'9,289·4, and 296·2 W). All other trends in the time by condition interaction were similar to those in the load by condition interaction.
Energy expenditure for standing and slow walking
967
Paired t-tests found differences between predicted and measured energy expenditure values. In the standing conditions, the predicted values were significantly higher than the measured (p < 0·01 for 20 kg loads, p < 0·00 I for 40 kg loads). For walking O'5 m s - 1 on a + 10% grade, the predicted values were significantly lower than the measured (p < 0·01 for 20 kg loads, p < 0·02 for 40 kg loads). For walking 0·9 m S-1 on a + 10% grade, measured values were not significantly different from predicted values. In the standing conditions the deviation between predicted and measured was higher at the 40 kg load than at the 20 kg load. Although the formula over predicted for standing and under predicted for walking 0·5 m S-1 (on a + 10% grade), when all individual predicted values were correlated with individual measured values, a strong relationship (r = 0,99) was found (see figure 1).
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00.0 ms- l
'"cr::>
l-
• 0.0 t:. 0.0 .. 0.0 D 0.5 • 0.5 0.9
o
+ 10'-., 20kg
ms· 1 + 10%, 40kg m~·l
+ 25%, 20kg + 25%. 40kg
ms·.
+ 10%,20kQ
11"\$-\
ms-!
+
rna-I
+ 10%.20kQ
10'%,40kQ
• 0.9 ma- + to%, 40kQ l
e z a.
'"x '">Cl
cr z
'"'"
r =0.99
e
'"
I-
U
e
LlJ
cr a.
Figure I. The relationship between individual predicted (Pandolf et at. 1977) and measured energy expenditure values for standing and walking slowly (m s - I) on positive grades (%) with loads (kg).
In the heart rate analysis, the load by condition interaction was found to be significant. Among the standing conditions, no significant differences were found. (See table 2 for final heart rate means ± SE.) All the standing condition means were, however, significantly lower than the walking condition means at the 0·05 level with one exception: the mean for standing on a + 25% grade with a 20 kg load was not significantly different from walking 0·9 m s - 1 on a - 10% grade with a 20 kg load. For both loads, walking 0·9 m S-1 on a -10% grade and walking 0·5 m S-1 on a + 10% grade were significantly lower than walking 0·9 m S-1 on a + 10% grade (critical difference at the 0·05 level). Walking 0·9 m S-1 on a -10% grade and walking 0·5 m S-l on a + 10% grade were not significantly different. The final heart rate mean for walking 0·5 m S-l on a + 10% grade with a 20 kg load was significantly lower than the mean with a 40 kg load (p < 0'05). This was also true for walking 0·9 m S-1 on a + 10% grade. There were no significant differences between loads for the standing conditions or walking 0·9 m s -Ion a - 10% grade.
N. A. Pimental and K. B. Pandolf
968
In the analysis of perceived exertion ratings, the 40 kg load means were significantly higher than the 20 kg load means for local, central, and overall ratings (p .< 0'001). (See table 3 for final ratings of perceived exertion means ± SE.) For central and overall ratings, means for standing on a +25% grade were significantly lower than means for standing on a + 10% grade (p < 0·0 I for central, p < 0·05 for overall).
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Table 2.
Heart rate means ± one SE after 20-min periods of standing and walking slowly on grades with loads Heart rate (beats min -I)
Velocity (m S-I)
Load (kg)
Grade
80·6 ± 4-4 88·9 ± 5·0
0·0 0·0
20 40
+10 +10
82·1 ± 5·2 83·2 ± 5·8
0·0 0·0
20 40
+25 +25
93-4 ± 8-4 104·0 ± 7·2
0·9 0·9
20 40
-10 -10
101·5 ± 6·4 116·2 ± 9·4
0·5 0·5
20 40
+10 +10
123·8 ± 4·7 146·2 ± 8·5
0·9 0·9
20 40
+10 +10
(%)
c.d o.o, = 12·3. Table 3.
Load means ± one SE for local, central, and overall ratings of perceived exertion following 20-min periods of standing and walking slowly on grades with loads Velocity
(m
S-I)
0·0 0·0 0·9 0·5 0·9
Grade (%)
+10 +25 -10 +10 +10
20 kg load mean 40 kg load mean
Local RPE 11·9 ± 0·7 10·9 ± 0-4 11·9 ± 0·4 11·7 ± 0·5 12-6 ± 0·6
Central RPE
Overall RPE
± ± ± ±
0·5 0·5 0·4 0-4 ± 0·5
12-8 ± 0·6 11·4 ± 0·5 12·0 ± 0·4 11·9 ± 0·4 13·1 ±0·5
10·5 ± 0·3 13·1 ± 0·2
10·8 ± 0·2 13·7 ± 0·2
10·7 ± 0·2 13·7 ± 0·2
4.
12·6 11·2 12·0 \2·0 13·3
Discussion
It was expected that there would be no significant differences among the energy
expenditure values for the samples taken at the three times during each run (min 5-8, 11-14, and 17-20), since after 5 min subjects were assumed to be in a steady state. Two discrepancies were found among the samples in this study. The first was during the most strenuous condition (walking 0'9 m S-1 on a + 10% grade). In this run, the sample taken at min 11-14 had a corresponding energy expenditure value significantly lower than the other two samples. This may be because during the sample taken at min 5-8 energy expenditure was increased due to the difficulty in balancing the load . while working at such a high level. By min 11-14 the subjects may have adjusted to the conditions, but by the end ofthe run and the sample taken at min 17-20, were fatigued and required increased energy expenditure due to inefficiency of walking and recruitment of additional primary muscle fibres and auxiliary muscles. When walking downhill the increased energy expenditure during min 17-20 may have been because the subjects were unaccustomed to walking on a negative grade and by the end of the run were recruiting additional primary muscle fibres as well as auxiliary muscles.
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In the standing conditions in this study (velocity = 0·0 m s - 1), changes in grade made no significant difference in energy expenditure. This was true for standing on + 10% and + 25% grades. Although the increase in grade increased energy expenditure, it was not statistically a significant difference. Heart rates were actually lower for standing on a + 25% grade than for standing on a + 10% grade (with a 40 kg load). On the scale for ratings of perceived exertion, standing on a + 25% grade was rated significantly lower than standing on a + 10% grade for central and overall ratings, possibly because standing on a + 25% grade simulates standing on the level and leaning forward with a load placed on the back, moving the centre of gravity so as to counteract the effect of the load. Standing on negative grades has not been investigated; it may be that for standing on a downhill slope changes in grade significantly affect energy expenditure due to the additional energy expended by the leg and back muscles in resisting the downward pull of gravity on the body as balance is maintained. If this proves to be the case, a factor may have to be added to the energy expenditure prediction equation for standing (and possibly walking) on negative grades. At this point, when velocity is zero, grade does not enter into the prediction equation, It was found in this study that, while standing, as loads increased from 20 to 40 kg, energy expenditure increased but not significantly. In another standing study (PandoIf et al. 1977), increases in load while standing did increase energy expenditure significantly. This may be due to a slightly larger sample size in the earlier study (N = 10) and a greater range of loads (0 to 50 kg). Whatever the case may be, energy expenditure while standing is relatively low, and changes in load do not affect energy expenditure while standing nearly as much as they do while walking. It is important to note that, although in all the standing conditions energy expenditure was low, and increased only slightly with increased load, this is not an indication that these conditions can be safely maintained for long periods of time; muscle strain and fatigue are -two factors other than measured energy expenditure which affect the well-being of the man and limit tolerance time. In fact, even though the measured energy expenditure for standing on a + 10% grade with a 40 kg load was not significantly different from the lowest energy expenditure value in this study, it was rated second highest on the perceived exertion scale for local, central, and overall ratings (walking 0·9 m S-1 on a + 10% grade with a 40 kg load was rated highest). Because increases in load may make significant increases in energy expenditure while standing, while changes in grade do not seem to, it may be suggested that energy expenditure is more sensitive to changes in load than in grade (while standing). The results of this study, supported by the results of previous studies (Pandolf et al. 1977), indicate that energy requirements for walking (dynamic work) are very different from the energy requirements for standing (static work). Once velocity increases above zero, there are significant increases in energy expenditure. The results of this study show that even when comparing the most strenuous standing condition (carrying 40 kg on a + 25% grade) to the least strenuous walking condition (carrying 20 kg on a -10% grade), there was a large difference in energy expenditure, the walking condition requiring 117 W more than the standing. While walking at slow speeds ( - O· 2 to 1·0 m s - 1), changes in grade affect energy expenditure significantly, contrary to when velocity is zero. These changes are not linear, however; the same increase in grade will increase energy expenditure by a greater amount in a more strenuous condition. The condition may be made more strenuous by increasing speed, grade, or load, or a combination of these. A point
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970
N. A. Pimental and K. B. Pandolf
may be mentioned concerning the results of this study on downhill walking. It was hypothesized that there would be a savings in energy expenditure over walking on the level when walking on a negative grade. This did not prove to be the case. It was found that the energy cost of walking downhill on a -10% grade (0'9 m S-I, 40 kg load) was higher than the cost of walking at approximately the same speed and load (1,0 m S-I, 40 kg) on the level. This could be due to the involvement of the back and leg muscles in resisting the downward pull of gravity. The effect of negative grades while walking has not been studied adequately. At present the prediction model is not equipped to consider negative grades. While walking at slow speeds there are significant increases in energy expenditure with increasing load. The data of this study and the data of other studies (Pandolf et al. 1977) suggest that, within the range of 0·0 to 1·0 m s -1, the effect of increasing speed on energy expenditure is not linear. Following a sharp increase in energy expenditure as velocity increases above zero, there is a steady increase between 0·2 and 0·6 m s -1, and at approximately 0·8 m s - 1 another sharp rise can be observed. As with grade and load, the same increase in speed will increase energy expenditure by a greater percentage in a more stressful condition. When this study compared measured to predicted energy expenditure (Pandolf et al. 1977) values, it was found that the formula predicted high for the standing conditions (the Pandolf et al. (1977) standing data support this). For increasing load while standing, the formula predicts increases in energy expenditure of a greater magnitude than this study has shown. As loads increase from 0 to 50 kg, the formula predicts energy expenditure values that are from 10 to 35% higher than the measured values. When velocity is 0·2 to 0·6 m s -I, the formula predicts slightly low for loads of approximately 0 to 30 kg, accurately for approximately a 35 kg load, and high for loads above 35 kg. For speeds of 0·8 to 1·0 m s - 1 the difference between predicted and measured also becomes greater as loads increase, but at these higher speeds there is a higher initial baseline (the formula predicts accurately for a 30 kg load). These observations suggest that for slow speed walking the prediction formula places too much emphasis on the effects of speed, and is also too sensitive to changes in load. Concerning grade, the prediction formula was accurate for a + 10% grade at a speed of 0·9 m s - I. For this same grade at a speed of O' 5 m s - I, the formula predicted low. More studies concerning the effect of grade on slow speed walking will have to be done before any conclusions can be drawn. When comparing predicted to measured energy expenditure values, the downhill data could not be evaluated because the formula is unable to make predictions for negative grades. In other words, the model does not consider negative, or eccentric, muscular work. When negative grade values are put into the prediction formula, it is possible for negative energy expenditure values to arise. In this study's downhill condition (0,9 m S-I on a -10% grade), the difference between measured and predicted energy expenditure was not only 313 W, but the predicted value was negative (see figure 2). In summary, in the standing conditions changes in load and/or grade change energy expenditure only slightly (at least for positive grades), much less than the changes in energy expenditure these variables produce while walking, which is a very different condition from standing. Once walking, increases in load, grade, and/or speed increase energy expenditure significantly, and as the condition becomes more strenuous, energy expenditure becomes more sensitive to changes in load, grade, and
Energy expenditure for standing and slow walking
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800
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400
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Figure 2.
10
15
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Predicted energy expenditure values (Pandolf et al. 1977) forwalking on negative grades.
speed. The current energy expenditure formula (Pandolf et al. 1977) predicts high for standing conditions, and is too sensitive to the effects ofIoad while standing. For slow speed walking the formula may place too much emphasis on the effects of load and grade. For higher walking speeds the formula may predict accurately because energy expenditure is affected more by grade and load in these more strenuous conditions than in the slow speed walking conditions. The formula needs to be revised to be able to predict for negative grades since man goes both uphill and downhill. Indeed, man must walk downhill once he has walked uphill. Before the formula is revised further work should be done to provide information to fill in the-gaps in the data; particularly studies on the effects of grade and light loads on slow speed walking, the effects of load on standing, and downhill standing and walking. The authors wish to acknowledge the assistance of Fred R. Winsmann in the collection of the data and Ella H. Munro in the statistical analysis. The views, opinions, and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy, or decision, unless so designated' by other official documentation. Human subjects participated in these studies after giving their free and informed voluntary consent, Investigators adhered to AR 70-25 and USAMRDC Regulation 70-25 on Use of Volunteers in Research.
Huit sujets masculins (24 ans, 176 em, 79 kg) se sont tenus immobiles ou bien ont rnarche a des vitesses de 0·5 ou 0·9 m S-I pendant 20 mn sur une pente de -10% il + 25% avec des charges de 20 ou 40 kg. La depense energetique (en watts) eta it sensiblement la meme dans toutes les conditions en posture immobile. La pente et la charge ont, dans ces conditions, augmente la depense energetique, mais pas significativement. Bien que ies rnoyennes des depenses energetiques en posture immobile aient ete relativement basses, les jugements de l'effort percu montrent, par leur degre eleve, qu'il y a des limites au temps d'endurance pour certaines de ces conditions. Toutes les moyennes en imrnobilite sont significativement plus basses que celles en deplacernent. La marche a0·9 m S-l sur une pente de -10% fournit une moyenne significativerncnt plus basse que la marc he a 0·5 m s " J, sur une pente de + 10%, ellemerne significativement plus basse que la marche a0·9 m S-1 sur une pente de + 10%. Dans les conditions de marche, on a observe des differences significatives entre les charges: les moyennes sont plus basses avec les charges de 20 kg qu'avec celles de 40 kg. En augmentant l'astreinte par l'accroissement des charges, des vitesses et/ou des pentes (pour la marche) on note des modifications dans la sensibilite de la depense energetique. La formule classique de prediction de la depense energetique (Pandolf et coli. 1977) surestime legerernent la posture immobile, sousestime la marche a 0·5 m S-1 sur une pente de + 10%, mais estime correctement la marche 0·9 m S-1 sur une pente de + 10%. Dans les conditions d'immobilite. la difference entre la depense predite et la depense mesuree est plus elevee pour la charge de 40 kg que pour celle de 20 kg. La formule actuelle n 'est pas concue pour des predictions dans Ie cas de pentes negatives. Les resultats de cette etude
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suggerent quela formule de prediction favorise leseffcts de Ia vitesse et de la charge pour la posture immobile et pour la marche it allure leote. Acht rnannliche Vpn (24 Jahre, 176 em, 79 kg) standen oder ringen 20 Minuten lang mit Geschwindigkeiten von 0·5 oder 0·9 tn!» auf schiefen Ebenen mit Steigungen von -10 bis +25% und trugen dabei Lasten von 20 oder 40 kg. Fur die varia bien Versuchsbedingungen beim Stehen konnten keine signifikanten Unterschiede des Energie-umsatzes (EU) festgestellt werden: zunehmende Steigung und Last fuhrten lediglich zu einem nicht signifikanten Anstieg des EU. Trotz der relativ niedrigen Mittelwerte des EU beim Stehen lassen die Ergebnisse der subjektiven Einstufungen der Anstrengung den SchluB auf eine zeitlich begrenzte Ertraglichkeit unter bestimrnten Belastungskombinationen zu. Aile Mittelwerte des EU beim Stehen waren signifikant niedriger als diejenigen beim Gehen. Der EU beirn Gehen auf einer negativen Steigung von -10% mit 0·9 rnjs war signifikant niedriger als beim Gehen auf einer Steigung von + 10% mit 0·5 mis, und dieser war wiederum signifikant niedriger als bei einer Geschwindigkeit von 0·9 m/s bei gleicher Steigung. Beim Gehen ergab sich ein signifikanter Unterschied zwischen den Lasten: Die Mittelwerte bei Lasten von 20 kg waren niedriger als bei 40 kg. Mit einem Belastungsanstieg durch Erhohung der Last, Geschwindigkeit und/oder der Neigung (beim Gehen) erhohte sich auch die Empfindlichkeit des EU aufweitere Veranderungen der genannten Variabien. Es wurde festgestellt, daB die gangige Formel zur Voraussage des EU (Pandolf et 01. 1977) fur Steben zu leicht erhohten, fur Gehen mit 0·5 m/s auf einer Steigung von + 10% zu niedrigeren, und beim Gehen mit 0·9 rnjs auf + 10% iger Steigung zu genaueren Ergebnissen fuhrt. Beirn Stehen war die Abweichung zwischen vorausberechneten und gemessenen EU grofser bei der Last von 40 kg als bei derjenigen von 20 kg. Die Forme1 ist in ihrer gegenwartiben Form nicht geeignet fur Voraussagen bei negativen Steigungen. Die Ergebnisse vorliegender Untersuchung lassen vermuten, daf die erwahnte Formel zu stark den Einflufl der Gehgeschwindigkeit und der Last beim langsamen Gehen bzw. beim Stehen beriicksichtigt.
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30,429-433. GOLDMAN, R. F., and lAM PIETRO, P. F., 1962, Energy cost of load carriage. Journal of Applied Physiology. 17, 675-676, HUGHES, A. L., and GOLDMAN, R. F" 1970, Energy cost of' hard work '. Journal of Applied Physiology, 29, 570-572. Lt, C. C., 1964, Introduction to Experimentul Statistics (New York: MCGRAw-HILL BOOK COMPANY). LINDQUIST, E. F., 1953, Design and Analysis of Experiments in Psychology and Education (Boston: HOUGHTON MIFFUN COMPANY).
MARGARIA. R., CERRETELLI, P., AGHEMO, P., and SASSI, G., 1963, Energy cost of running. Journal ofApplied Physiology, 18, 367-370. PANDOLF, K. 8., 1978, Influence of local and central factors in dominating rated perceived exertion during physical work. Perceptual and Motor Skills, 46, 683-698. PANDOLF, K. B., GIVONI, B" and GOLDMAN, R. F., 1977, Predicting energy expenditure with loads while standing or walking very slowly. Journal of Applied Physiology, 43, 577-581. PANDOLF, K. B., HAISMAN, M. F., and GOLDMAN, R. F., 1976, Metabolic energy expenditure and terrain coefficients for walking on snow. Ergonomics, 19, 683-690. SOULE, R. G., and GOLDMAN, R. F., 1969, Energy cost of loads carried on the head, hands, or feet. Journal of Applied Physiology, 27, 687-690.
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SOULE, R. G., and GOLDMAN, R. F., 1972, Terrain coefficients for energy cost prediction. Journal of Applied Physiology, 32, 706-708. SOULE, R. G., PANDOLF, K. B., and GOLDMAN, R. F.. 1'l7X. Energy expenditure of heavy load carriage. Ergonomics, 21, 373-381. WEIR, J. B. DeV., 1949, New methods for calculating metabolic rate with special reference to protein metabolism. Journal of Physiology, (London), 109, 1-9. WORKMAN, J. M., and ARMSTRONG, B. W., 1963, Oxygen cost of treadmill walking. Journal of Applied Physiology, 18, 798-803.
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Manuscript received 18 September 1978. Revised manuscript received 8 January 1979.
