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Optimum load for carriage by Indian soldiers on different uphill gradients at specified walking speed
International Journal of Industrial Ergonomics 44 (2014) 260e265
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International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
Optimum load for carriage by Indian soldiers on different uphill gradients at specified walking speed Madhu Sudan Pal, Deepti Majumdar, Anilendu Pramanik, Bodhisattwa Chowdhury, Dhurjati Majumdar* Defence Institute of Physiology and Allied Sciences, Defence Research & Development Organisation, Min. of Defence, Govt. of India, Lucknow Road, Delhi 110054, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2012 Received in revised form 11 March 2013 Accepted 13 September 2013 Available online 5 February 2014
Physiological responses of soldiers while carrying different loads were studied to suggest maximum weight that can be carried by an Indian infantry soldier comfortably at different gradients at specific walking speed. Ten physically fit infantry soldiers walked at 4.5 km h1 on treadmill (0%, 5%, 10% and 15% gradient) without and with loads of 4.4, 10.7, 17.0 and 21.4 kg. At each gradient, all the loads including _ Þ, energy without load were experimented for 10 min. Heart rate (HR), oxygen consumption ðVO 2 _ expenditure (EE), respiratory frequency (RF) and minute ventilation ðVEÞ were determined using K4b2 _ , EE, RF and VE _ with increasing gradient and external load was system. A linear increase in HR, VO 2 _ observed. Based on physiological limit of 50%, 60% and 75% of VO 2max and linear regression equations optimum loads are suggested as permissible for carriage on different gradients by Indian soldiers at above speed. This combination of weight and gradient would improve the combat readiness of soldiers while carrying load. Relevance to industry: Most of the developing countries do not have load carriage standards, for either Industry or Military personnel and extrapolation of the data from developed countries do not seem feasible. Results of this study may be applicable in developing standards or in recommending optimal loads for similar populations under similar conditions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Load carriage Gradient Maximum permissible load Oxygen uptake Physiological cost Heart rate Soldier
1. Introduction The physiological cost of load carriage at level walking and different gradients has been investigated by many researchers (Sagiv et al., 2000; Pal et al., 2007; Abe et al., 2008; Bastien et al., 2005; Legg et al., 1992; Wu, 2006; Chow et al., 2009, 2011; Cho and Kima, 2012). However, in most of the studies load was placed as single compact unit and carried as backpack or rucksack. In military environment load is carried as multiple units such as combinations of haversack, backpack, web and rifle mainly in the upper torso as per their requirements. These units vary in shape, size and weight and may lead to unequal distribution of load over body and changes in gait pattern of an individual (Attwells et al., 2006; Ryu et al., 2006; Majumdar et al., 2010) which is known to cause increased energy demand and early onset of fatigue. To date, physiological studies of load carriage (as practised by soldiers) in military environment are limited and less reported, especially with
* Corresponding author. Tel.: þ91 11 23935745; fax: þ91 11 23914790. E-mail address: [email protected] (D. Majumdar). 0169-8141/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ergon.2013.09.001
varying gradients. Pal et al. (2009) suggested optimum load for carriage by Indian soldiers on level ground at specified walking speeds. No study of load carriage at varying gradients by Indian soldiers is reported. Similarly, load carrying standards, e.g., optimum loads are not available at different gradients. So far, physiological responses of Indian soldiers while carrying load carriage at varying gradients (uphill) have not been reported and no standard for optimum load carriage at different gradients is presently available. Carrying heavy loads over unpredictable terrain for long distances is a requirement common to military personnel (Liu, 2007). The platoon’s combat load varies by mission and includes the supplies physically carried into the fight. Present day soldiers need to carry more equipment, supplies, and ammunition than earlier days to enhance sustenance for longer duration. Manual load carrying by Indian soldiers is often necessary in the remote countryside and mountain areas of India as these areas are mostly inaccessible by vehicle. In these areas soldiers need to carry materials such as arms, ammunitions, rations, clothes and first-aid dressing for survival and at the same time ensuring the maintenance of their work capacity and combat readiness. However, the
M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
authors are unaware of any reported studies so far that have simultaneously analyzed the effect of both load placement and gradient on cardiorespiratory responses of soldier’s while carrying loads on different parts of the body. As load carrying capacity of an individual depends on walking speed and gradient, identification of the optimum load for a specific speed with different gradients is of utmost importance in deciding the combat readiness of soldiers. Carrying loads by troops is an important aspect of military operations that may prove to be very critical under some circumstances. Overloading of soldiers with ammunition and equipment may lead to discomfort, excessive fatigue, injuries and impair soldier’s ability to fight. Therefore, identification of optimum load is important in order to take preventive action to reduce the incidences of common injuries associated with load carriage, excessive fatigue and impairment of the soldier’s ability to fight. Scott and Christie (2004) proposed that military marches should be conducted under ‘steady state’ at an intensity that does not exceed _ 50% of maximum aerobic capacity ðVO 2max Þ, thus ensuring that soldiers are combat efficient. Vogel et al. (1980) recommended that _ if any task required a work rate of 60% VO 2max , it may be sustained for approximately 2 h, whereas tasks that require work rate of 75% _ VO 2max should not exceed approximately 30 min duration. If a soldier is exhausted from carrying a load, he may be unable to perform when he reaches the site of conflict. To ensure the most efficient use of energy, the planning for any load carrying should take into account the elimination of local strain, injuries and fatigue. The greatest concern of military researchers is to establish the conditions at which soldiers can perform prime functions of military exercises, maximally and efficiently, both during and after load carriage. Hence, present study hypothesized that load carriage on a positive gradient would increase cardiorespiratory responses compared to load carriage on a level gradient. The effect of gradient on cardiorespiratory responses of soldiers was assessed while carrying military loads and to estimate the optimum or maximum permissible load (within physiological limit of 50%, 60% and 75% _ VO 2max ) that can be carried comfortably on various gradients at a specific walking speed. 2. Methods 2.1. Participants Ten physically fit, nonsmoker, experienced (4þ years) male infantry soldiers from the Indian Army volunteered for the study. Mean age was 23.2 (SEM 0.83) years, height 172.6 (SEM 1.20) cm, _ weight 65.9 (SEM 2.24) kg and maximum aerobic capacity ðVO 2max Þ 47.5 (SEM 1.40) ml min1 kg1. They had no history of musculoskeletal or cardiovascular pathology. All volunteers provided informed consent to participate in this investigation. The study protocol was approved by the Institutional Ethical Committee on the use of Human as an Experimental subjects and experiment conforms to the principles outlined by the Declaration of Helsinki protocol, 1964. 2.2. Experimental details On the first day, soldiers were briefed about the purpose of the study. They were then habituated to walking on the motorized treadmill (Taeha, Intertrack 6025, Korea) at various gradients in the laboratory, with and without loads at 4.5 km h1 walking speed. Habituation process continued on the second day also. One day of rest was given on the next day. _ Maximum aerobic capacity ðVO 2max Þ, of the subjects was measured during treadmill exercise with regular increases in gradient (Harbor Protocol, Wasserman et al., 1994), while keeping
261
Table 1 Details of type, combination and placement of load for load carriage experiments. Load (kg)
Contents
% Body weight (BW)
0 4.4 10.7
No load (NL) 4.4 kg Haversack on back 4.4 kg Haversack on back þ 2.1 kg Web in front of waist þ 4.2 kg INSAS rifle in right hand 10.7 kg Backpack þ 2.1 kg Web in front of waist þ 4.2 kg INSAS rifle in right hand 10.7 kg Backpack þ 4.4 kg Haversack on left lateral side of the waist þ 2.1 kg Web in front of waist þ 4.2 kg INSAS rifle in right hand
0 6.7 16.2
17.0 21.4
25.8 32.5
_ 2max , the speed constant on fourth day. During measurement of VO each subject walked on treadmill at 0% gradient for 3 min at self selected comfortable walking speed. After 3 min of walking at comfortable speed chosen by the subjects, every minute the treadmill gradient was increased by 4%, so that the subject reaches _ _ VO 2max approximately in 10 min. A valid VO2max was obtained when at least two of these three criterions were met: i) maximum heart rate (HR) > 90% of age predicted maximum HR (220 beats min1 age), ii) Respiratory Exchange Ratio (RER) of at least _ , <200 ml min1 1.10 and iii) plateau in oxygen consumption (VO 2 change) with increasing work rate (You et al., 2004). Mean (SEM) _ values of RER and HR at maximum VO 2max were 1.24 (0.018) and 189.9 (1.038) beats min1, respectively. During the measurement of _ VO 2max , subjects wore a vest, underwear and physical training (PT) shoes. Subjects took rest on the fifth day. Load carriage experiments were carried out from sixth day onwards. The loads and gradients were assigned randomly to the subjects on different day. On the day of load carriage experiment the subjects reported to the laboratory at 8.00 a.m. after light breakfast and the experimental procedure was initiated after about 90 min of rest. They abstained from smoking or taking any food as long as they were in the laboratory. During the load carriage experiment all subjects wore full Infantry Uniform including combat boots and helmet. Load carriage experiments were carried out on each subject while carrying no load (NL, 0 kg) and loads of 4.4, 10.7, 17.0 and 21.4 kg at 0%, 5%, 10% and 15% gradient, respectively at medium pace (4.5 km h1) walking speed (Ganguli, 1973) on a treadmill. Details about load items and their placement on the body along with percentage of body weight (BW) are given in Table 1. At each gradient and load including NL, the experiment was carried out for 10 min duration. In this study, load carriage operations were carried out in the laboratory which simulated the operational loads carried under field situations. Carried load corresponded to 6.7% (for 4.4 kg load), 16.3% (for 10.7 kg load), 25.8% (for 17.0 kg load) and 32.5% (for 21.4 kg load) of the average body weight (BW) of the subjects studied. In total, each participant underwent 20 experiments (four gradients and five load conditions including no load), and was required daily to complete two exercise trials (10 min each) from 0930 h to 1300 h, with at least 90 min rest between two exercise trials to eliminate cumulative effect of first exercise trial on second trial. 2.3. Physiological measurements All load carriage experiments were conducted in a controlled laboratory environment of 22e25 C, 50e55% relative humidity and at the same hour of the day (between 0930 h and 1300 h) and every day for eliminating the specific dynamic action (SDA) of food for all practical purposes. The load carriage ensembles (LCe) used in this study was regularly used by Indian Infantry soldiers. Each subject used the same LCe and rifle during the experiments.
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_ Þ, heart rate (HR), energy expenditure Oxygen consumption ðVO 2 _ (EE), respiratory frequency (RF) and minute ventilation ðVEÞ, of each of the individual during rest and experiment were determined by the process of breath-by-breath gas analysis using K4b2 system _ (K4b2, Cosmed S.r.l, Italy). Before the VO 2max and each of the load carriage experiment on every day K4b2 system was calibrated with room air, volume calibration and standard gas mixture (16% O2 and 5% CO2). Each participant wore a polar chest belt before being fitted with appropriate load and portable unit and battery of the K4b2 system were fixed on the chest and back of the individual, _ respectively, with a harness, during measurement of VO 2max and while performing the load carriage experiments. A flexible face mask that covered the subject’s mouth and nose was secured with a nylon mesh hairnet and velcro straps. The face mask contained flowmeter and connected to the portable unit via capillary tube. After fitting the K4b2 system on the subject, both the data logger and telemetry mode of the system were turned on. The subject was then asked to breathe normally through the face mask for about 10 min sitting on a comfortable chair for the recording of resting physiological parameters. Telemetric recording in a personal computer indicated the state of recovery of the subjects between exercises. Physiological parameters were recorded in breath-bybreath mode. An average of the last 3-min physiological data from each exercise trial of 10 min was considered as individual value for further analysis. 2.4. Data analysis An average of the last 3 min of physiological data from each 10 min exercise trial was considered as individual value for further analysis. All data are presented as means SEM. To find out the significant effect of gradient (1st factor) and load (2nd factor) on _ 2 , EE, RF, VE _ and %VO _ 2max ), a various dependent variables (HR, VO two way repeated measure ANOVA was carried out followed by Bonferroni post hoc analysis, using Statistical Package for Social Sciences for Windows V16.0 (SPSS, Nikiski, AK 99635, USA). _ Regression analysis was performed between %VO 2max and load for different gradients to estimate the optimum or maximum permissible load. All differences were considered significant at p 0.05. 3. Results _ , EE, RF, VE _ and Cardiorespiratory responses, e.g. HR, VO 2 _ %VO 2max , during carriage of four different loads and no load with varying gradients at 4.5 km h1 walking speed are presented in Table 2 and Fig. 1aef. Two-way ANOVA with repeated measures showed a significant gradient load interaction on HR _ [F(3.125,28.128) ¼ 7.377, p < 0.001], VO 2 [F(4.514,40.626) ¼ 5.567, _ p < 0.001], EE [F(4.428,39.851) ¼ 9.855, p < 0.001], VE _ [F(4.036,36.323) ¼ 25.499, p < 0.001] and %VO 2max
[F(4.460,40.143) ¼ 5.455, p < 0.001]. No significant interaction was found in RF. _ The mean percentage of VO 2max (Y) was plotted against the magnitudes of loads (X) carried by the participants and four regression lines (Y on X) fitted through the points were drawn by the methods of least squares for 0%, 5%, 10% and 15% gradients (Fig. 2). The corresponding regression equations and correlation coefficients (r) have been presented. Significant (p < 0.01) positive correlations, r ¼ 0.97, 0.98, 0.99 and 0.99 were found at 0%, 5%, 10% and 15% gradients, respectively between Y and X. From regression equations, optimum loads were estimated for four different gra_ dients when 50%, 60% and 75% of VO 2max were considered as acceptable workloads. Graphic representation of acceptable loads is shown in Fig. 2 by drawing a perpendicular line on the X-axis, from
Table 2 Cardio-respiratory responses to treadmill walking at 4.5 km h1 (n ¼ 10) during carrying of different magnitudes of load at various gradients. Data presented as mean (SEM). SEM ¼ Standard Error of Mean. Parameters
Load Gradient (%) (kg) 0 5
Heart rate (beats min1)
0 93.8 (2.59) 110.6 4.4 95.1 (2.29) 115.6 10.7 100.8 (2.86) 123.9 17.0 103.6 (2.93) 128.6 21.4 107.5 (2.29) 136.7
Oxygen 0 consumption 4.4 1 1 (ml min kg ) 10.7 17.0 21.4
10
15
(3.52) (3.54) (2.70) (5.15) (4.21)
130.1 137.5 153.9 156.4 169.4
(3.50) (4.49) (3.76) (3.69) (2.56)
156.6 165.6 176.1 180.6 184.5
(3.33) (2.87) (2.76) (2.77) (2.08)
13.2 13.3 14.7 15.1 16.4
(0.32) (0.42) (0.39) (0.39) (0.65)
19.4 19.4 21.5 23.1 23.9
(0.50) (0.35) (0.62) (0.52) (0.66)
26.6 26.9 30.1 31.6 33.1
(0.88) (0.71) (0.84) (0.94) (1.33)
33.8 35.4 38.8 40.5 42.4
(1.16) (0.91) (0.87) (1.33) (1.66)
Energy expenditure (kcal min1)
0 4.4 10.7 17.0 21.4
4.3 4.4 4.8 4.9 5.3
(0.09) (0.14) (0.14) (0.16) (0.19)
6.3 6.4 7.0 7.5 7.9
(0.19) (0.18) (0.25) (0.24) (0.23)
8.8 8.9 9.8 10.3 10.9
(0.34) (0.30) (0.31) (0.32) (0.38)
11.0 11.6 12.8 13.5 14.3
(0.39) (0.49) (0.44) (0.51) (0.49)
Respiratory frequency (breath min1)
0 4.4 10.7 17.0 21.4
29.2 30.3 32.1 33.3 34.6
(1.27) (1.29) (1.38) (1.33) (1.09)
31.9 33.3 35.8 38.0 38.9
(1.34) (1.36) (1.67) (1.82) (1.25)
34.9 37.1 40.7 41.7 43.3
(1.66) (1.58) (1.85) (1.87) (1.64)
39.7 40.4 43.8 47.4 48.5
(1.85) (2.15) (1.22) (1.55) (1.86)
Minute ventilation (l min1)
0 4.4 10.7 17.0 21.4
28.3 29.8 32.8 32.4 35.8
(0.74) (1.16) (0.89) (0.94) (0.99)
39.1 41.0 45.4 48.0 50.4
(1.02) (1.27) (1.30) (1.09) (1.22)
52.2 55.5 62.5 65.0 72.0
(1.85) (1.85) (1.49) (2.42) (2.31)
68.3 74.4 84.0 94.3 99.8
(2.36) (3.17) (3.75) (3.82) (3.28)
_ %VO 2max
0 4.4 10.7 17.0 21.4
28.0 28.2 31.0 32.0 34.6
(0.63) (1.02) (0.68) (1.28) (1.23)
41.0 41.1 45.4 48.8 50.5
(1.21) (1.15) (1.45) (1.20) (1.65)
56.3 57.0 63.6 66.8 69.9
(1.95) (2.00) (1.85) (1.76) (2.39)
71.3 75.0 82.1 85.5 89.3
(2.33) (2.79) (1.99) (2.73) (2.65)
_ intercepts of acceptable workload (50%, 60% and 75% of VO 2max ) line and regression line at different gradients. 4. Discussion The present study was undertaken to determine the effect of gradients and load on cardiorespiratory responses during military load carriage on Indian infantry soldiers. It was observed that HR, _ , EE, VE, _ RF and %VO _ VO 2 2max gradually increased with increasing gradient and load. The findings of the present study are consistent with the work of Kirk and Schneider (1992), Todd and Scott (2002), Chung et al. (2005) and Crowder et al. (2007). Kirk and Schneider (1992) found that changes in treadmill gradient had a significant _ _ effect on HR, VO 2 and VE regardless of the type of pack carried. Todd and Scott (2002) studied the metabolic cost of South African soldiers during carrying of loads (50, 35 and 20 kg) under various conditions of gradient (10%, 0% and 10%) and speed (4, 5 and 6 km h1). They revealed that uphill marching showed significant increases in metabolic demands under all speed and load conditions. Chung et al. (2005) observed that climbing stairways involved an increased physiological burden as compared to level ground walking while carrying load. They also confirmed that heavier loads resulted in higher physiological cost than carrying of comparatively lighter loads. Crowder et al. (2007) examined metabolic cost during simulated road marching (speed 3.5 mile h1, grades 0, 5 and 10%) on military personnel while they carried a load of 27 kg. Results of their study showed significant differences in HR, _ 2 and VE _ across all gradients. Abe et al. (2008) observed a sigVO nificant interaction effect between load and gradient conditions for
4.4 kg 10.7 kg 17.0 kg 21.4 kg
45
0 kg
16
40
4.4 kg
14
10.7 kg
35
17.0 kg
30
21.4 kg
25 20 15 10
10
15
0
5
10
0 kg
50
10.7 kg
40
21.4 kg
30 20 10
VE (l/min)
17.0 kg
Gradient (%)
21.4 kg
8 6 4
0
100
0 kg 4.4 kg 10.7 kg
80
17.0 kg 21.4 kg
60 40 20
0 10
17.0 kg
10
5
15
0 0
5
10
Gradient (%)
10
15
Gradient (%)
120
4.4 kg
5
10.7 kg
Gradient (%)
60
0
4.4 kg
12
15
15
%VO2 max
5
0 kg
0
0 0
263
2
5
Gradient (%)
RF (b/min)
EE (kcal/min)
0 kg
200 180 160 140 120 100 80 60 40 20 0
VO2 (ml/min/kg)
HR (beats/min)
M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
100 90 80 70 60 50 40 30 20 10 0
0 kg 4.4 kg 10.7 kg 17.0 kg 21.4 kg
0
5 10 Gradient (%)
15
Fig. 1. Graphical representation of cardio-respiratory responses during treadmill walking at 4.5 km h1 with carrying different loads (0, 4.4, 10.7, 17.0 and 21.4 kg) at various _ , (ml kg1 min1), c. EE (kcal min1), d. RF (breath min1), e. VE _ (l min1), f. %VO _ gradients (0%, 5%, 10% and 15%). a. HR (beats min1), b. VO 2 2max .
energy cost of walking with load carriage. Similarly, Perrey and Fabre (2008) reported that during uphill walking, load and gradient significantly interacted for EE and HR. In the present study the authors have also found significant interactions effect between _ , EE, VE _ and %VO _ gradient and load for HR, VO 2 2max during load carriage operations. An interaction (gradient load) analysis on HR reveals that there is difference between the marginal means for the different levels of load across the different levels of gradient (122.8 vs 128.5 vs 138.7 vs 142.3 vs 149.5). The marginal means of gradient over levels of load (100.2 vs 123.1 vs 149.5 vs 172.7) are different with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (95.1 vs 115.6 vs 137.5 vs 165.6) and a similar pattern was observed while carrying 10.7 kg load (100.8 vs 123.9 vs 153.9 vs 176.1), 17.0 kg load (103.6 vs 128.6 vs 156.4 vs 180.6) and 21.4 kg load (107.5 vs 136.7 vs 169.4 vs 184.5). An interaction _ (gradient load) analysis on VO 2 observed that there is difference between the marginal means for the different levels of load across the different levels of gradient (23.25 vs 23.75 vs 26.27 vs 27.57 vs 28.95). The marginal means of gradient over levels of load (14.54 vs 21.46 vs 29.66 vs 38.18) are different with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (13.3 vs 19.4 vs 26.9 vs 35.4) and a similar pattern was observed while carrying 10.7 kg load (14.7 vs 21.5 vs 30.1 vs 38.8), 17.0 kg load (15.1 vs 23.1 vs 31.6 vs 40.5) and 21.4 kg load (16.4 vs 23.9 vs 33.1 vs 42.4). An interaction (gradient load) analysis on EE found that there is difference between the marginal means for the different levels of load across the different levels of gradient (7.6 vs 7.8 vs 8.6 vs 9.1 vs 9.6). The marginal means of gradient over levels of load (4.7 vs 7.0 vs 9.7 vs 12.6) are different with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (4.4 vs 6.4 vs 8.9 vs 11.6) and a similar pattern was observed while carrying 10.7 kg load (4.8 vs 7.0 vs 9.8 vs 12.8), 17.0 kg load (4.9 vs 7.5 vs 10.3 vs 13.5) and 21.4 kg load (5.3 vs 7.9 vs 10.9 vs _ reveals that 14.3). An interaction (gradient load) analysis on VE there is difference between the marginal means for the different
levels of load across the different levels of gradient (47.0 vs 50.2 vs 56.2 vs 59.9 vs 64.5). The marginal means of gradient over levels of load are different (31.8 vs 44.8 vs 61.4 vs 84.2) with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (29.8 vs 41.0 vs 55.5 vs 74.4) and a similar pattern was observed while carrying 10.7 kg load (32.8 vs 45.4 vs 62.5 vs 84.0), 17.0 kg load (32.4 vs 48.0 vs 65.0 vs 94.3) and 21.4 kg load (35.8 vs 50.4 vs 72.0 vs 99.8). An interaction (gradient load) analysis on _ %VO 2max found that there is difference between the marginal means for the different levels of load across the different levels of gradient (49.2 vs 50.3 vs 55.5 vs 58.3 vs 61.1). The marginal means of gradient over levels of load are different (30.8 vs 45.4 vs 62.7 vs 80.6) with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (28.2 vs 41.1 vs 57.0 vs 75.0) and a similar pattern was observed while carrying 10.7 kg load (31.0 vs 45.4 vs 63.6 vs 82.1), 17.0 kg load (32.0 vs 48.8 vs 66.8 vs 85.5) and 21.4 kg load (34.6 vs 50.5 vs 69.9 vs 89.3). Observations of the present study corroborated and extended the earlier research _ , EE, VE _ and %VO _ works. This increase in HR, VO 2 2max with the change in gradient and load may have been due to change in locomotion biomechanics as suggested by earlier researchers (Martin and Nelson, 1980). One of the objectives of the present study was to identify the optimum or maximal acceptable load to be carried by Indian soldiers at different gradients without undue fatigue. It was observed that at 4.5 km h1 walking speed, carrying of 21.4 kg load at 5% _ _ gradient demanded 50.5% of VO . Based on the 50% of VO 2max
2max
criteria (Vogel et al., 1980), it may be suggested that for an 8 h work day at 5% gradient, at walking speed of 4.5 km h1, the load should not exceed 21.4 kg (32.5% of body weight). At this walking speed, _ higher loads demanded more than 50% of VO 2max at 10% and 15% gradients. As India has steep and high mountain border areas in the Northern and Eastern parts of the country, soldiers cannot be restricted to carry loads up to only a 5% gradient. Therefore, at 10% and higher gradients soldiers should control either duration of working hours or walking speed in order to keep physiological or
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physical strain within acceptable limits. Vogel et al. (1980) sug_ 2max , it may gested that any task that required a work rate of 60% VO be sustained for approximately 2 h and tasks that required work _ rates of 75% VO 2max should not exceed approximately 30 min duration. According to the criterion of Vogel et al. (1980), the permissible magnitude of load from Fig. 2 is found to be approximately 6.6 kg (r ¼ 0.99, p < 0.01) for 2 h working duration, for the soldiers walking with 4.5 km h1speed at 10% gradient. At 15% gradient, the permissible magnitude of load was found to be approximately 4.0 kg (r ¼ 0.99, p < 0.001) for 30 min working _ duration only. When criterion of 50% VO 2max (Astrand, 1960) is followed, the permissible magnitude of load can be calculated from regression equations to be approximately 72.9 kg (r ¼ 0.97, p < 0.01) and 20.2 kg (r ¼ 0.98, p < 0.01) respectively for 0% and 5% (Fig. 2) gradient for 8 h working duration for the soldiers at the specified walking speed. The correlations between the variables are high and significant, allowing conclusions to be drawn based on either regression line. The estimated optimum loads at four different gradients in this study are as follows:
Work duration
Gradient (%)
Estimated optimum load in kg and (% of BW)
Eight hour Eight hour Two hour Thirty minutes Thirty minutes
0 5 10 10 15
72.9 (110.6% of BW) 20.2 (30.6% BW) 6.6 (10.1% BW) 28.9 (43.9% BW) 4.0 (6.1% BW)
where % BW ¼ body weight in kgs. Optimal load is an elusive concept. There is no easy solution for identification of an optimum or maximum permissible load. _ Different criteria, such as percentage of BW, percentage of VO 2max
etc. are used for identification of optimum load for carriage. Several earlier researchers (Astrand, 1960; Evans et al., 1980; Scott and _ Christie, 2004; Saha et al., 1979) stated percentage of VO 2max as a criteria for recommended acceptable work rate. They recom_ mended that 33e50% of VO 2max could be considered an acceptable work rate for an 8 h work day. On the other hand, Haisman (1988) and Knapik (1989) stated that for healthy young males, there appears to be some consensus for the traditional rule of thumb of the one-third BW, or 24 kg on an assumed mean BW of 72 kg for a
working day. Our recommendation, however, does not agree with _ 2max criteria or one-third of BW, particularly at higher the 50% of VO gradients (10% and 15%) for an 8 h work day. Sagiv et al. (2000) observed haemodynamic and cardiovascular responses during carrying of loads (0, 25 and 35 kg) at different gradients during _ , treadmill walking. They found significant increases in HR, VO 2 blood pressure and cardiac output with increasing gradient. Their study demonstrated that during isodynamic exercise, changes in the gradient of walking was the most important factor for cardiovascular and metabolic responses than the mass of the load carried. Laursen et al. (2000) found that carrying loads as light as 10 kg at a positive gradient of only 8% increased EE by 70%, while a load of 20 kg at the same gradient almost doubled EE. These increases in EE are due to added work required to displace the centre of mass (BW plus load carried) against gravity (Nagle et al., 1990). Todd (2001) stated that, under heavy load carriage conditions, 10% uphill gradient resulted in substantial increases in the demands placed on _ soldiers, to as high as 106% of predicted VO 2max . As per the nature of job, soldiers are supposed to carry loads for survival in field situations. Hence, they cannot be restricted to load carriage only at 0% or 5% gradients. They are required to carry loads at higher gradients, particularly at high altitudes. To carry external loads at 10%, 15% and higher gradients, one should control his walking speed or duration of carrying the external load. With reference to the South African soldiers, Scott and Christie (2000) recommended that soldiers could carry up to a 65 kg load, but walking speed should not exceed 4.5 km h1 at 0% gradient. This value is similar to our recommendation of carrying a 72.9 kg load at 0% gradient for an 8 h work day. On the other hand, Yu and Lu (1990) suggested an acceptable load for young Chinese men at 5 km h1 to be 20 kg (31.0% of BW) which was latter confirmed by Li et al. (1992), closely supporting the recommendations of this study which is about 20.2 kg (30.6% BW) at 5% gradient, i.e. about one third of the average body weight (65.9 kg) of the subjects this study. The present study is novel in its approach of recommending the optimum load for Indian infantry soldiers. The recommendations for permissible load for carriage at 0% and 5% gradients are 72.9 kg and 20.2 kg, respectively, for 8 h; 6.6 kg for 2 h at 10% gradient; and 28.9 kg and 4.0 kg for 30 min at 10% and 15% gradient, respectively, at walking speed of 4.5 km h1 However, the results of this study can be extrapolated for applying these recommendations on comparable civilian Indian populations. Also these recommendations can be used for deciding optimal load
_ Fig. 2. Relation between percentage of VO 2max and load carried by soldiers at different gradients.
M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
for carriage for similar populations with comparable anthropometric dimensions in other parts of the world. 5. Conclusion Present study concluded that with increasing positive gradient cardiorespiratory responses were significantly increased compared to level walking at medium pace. Based on physiological limits of _ 50%, 60% and 75% of VO 2max and linear regression equations, optimum loads or maximum permissible load are suggested for carriage on different uphill gradients at the specified walking speed without considering the external environmental factors, altitude and terrain factors. This combination of weight and gradient would improve the prime function of soldiers while carrying loads. Findings of the present study clearly demonstrate that during walking at 10% and above uphill gradient, one third of the body weight or _ 50% of VO 2max criteria could not be sufficient for identifying optimum load for being carried during 8 h working day. The present study was restricted to uphill gradient, medium pace walking, load up to 21.4 kg and in laboratory condition only. For recommendations on heavy loads, further detailed studies on biomechanical factors, prolonged duration, faster walking speed, altitude, different terrain and field conditions are required. Acknowledgements Authors would like to express their gratitude to volunteers for their participation in the study. They are thankful to Defence Research and Development Organisation, Ministry of Defence. Government of India for funding the project. References Abe, D., Muraki, S., Yasukouchi, A., 2008. Ergonomic effects of load carriage on the upper and lower back on metabolic energy cost of walking. Appl. Ergon. 39, 392e398. Astrand, I., 1960. Aerobic work capacity in men and women, with special reference to age. Acta Physiol. Scand. 49, 169. Attwells, R.L., Birrell, S.A., Hooper, R.H., Mansfield, N.J., 2006. Influence of carrying heavy loads on soldiers posture, movements and gait. Ergonomics 49, 1527e 1537. Bastien, G.J., Willems, P.A., Schepens, B., Heglund, N.C., 2005. Effect of load and speed on the energetic cost of human walking. Eur. J. Appl. Physiol. 94, 76e83. Cho, Y.J., Kima, J.Y., 2012. The effects of load, flexion, twisting and window size on the stationarity of trunk muscle EMG signals. Int. J. Ind. Ergon. 42, 287e292. Chow, D.H.K., Ting, J.M.L., Pope, M.H., Lai, A., 2009. Effects of backpack load placement on pulmonary capacities of normal schoolchildren during upright stance. Int. J. Ind. Ergon. 39, 703e707. Chow, D.H.K., Li, M.F., Lai, A., Pope, M.H., 2011. Effect of load carriage on spinal compression. Int. J. Ind. Ergon. 41, 219e223. Chung, M.K., Lee, Y.J., Lee, I., Choi, K.I., 2005. Physiological workload evaluation of carrying soft drink beverage boxes on the back. Appl. Ergon. 36, 569e574. Crowder, T.A., Beekley, M.D., Sturdivant, R.X., Johnson, C.A., Lumpkin, A., 2007. Metabolic effects of soldier performance on a simulated graded road march
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while wearing two functionally equivalent military ensembles. Milit. Med. 172, 596e602. Evans, W.J., Winsmann, F.R., Pandolf, K.B., Goldman, R.F., 1980. Self-paced hard work comparing men and women. Ergonomics 23, 613e621. Ganguli, S., 1973. Observations on the basic gait characteristics of Indian industrial workers. Indian J. Ind. Med. 14, 28e35. Haisman, M.F., 1988. Determinants of load carrying ability. Appl. Ergon. 19, 111e121. Kirk, J., Schneider, D.A., 1992. Physiological and perceptual responses to loadcarrying in female subjects using internal and external frame backpacks. Ergonomics 35, 445e455. Knapik, J., 1989. The army physical fitness test (APFT): a review of the literature. Milit. Med. 154, 326e329. Laursen, B., Ekner, D., Simonsen, E.B., Voigt, M., Sjogaard, G., 2000. Kinetics and energetics during uphill and downhill carrying of different weights. Appl. Ergon. 31, 159e166. Legg, S.J., Ramsey, T., Knowles, D.J., 1992. The metabolic cost of backpack and shoulder load carriage. Ergonomics 35, 1063e1068. Li, T., Liu, Z., Huang, H., Jin, S., Lu, S., 1992. A study on optimum load for physical work. J. Hum. Ergol. 21, 3e11. Liu, B.S., 2007. Backpack load positioning and walking surface slope effects on physiological responses in infantry soldiers. Int. J. Ind. Ergon. 37, 754e760. Martin, P.E., Nelson, R.C., 1980. The effect of carriage load on the walking pattern of men and women. Ergonomics 29, 1191e1202. Majumdar, D., Pal, M.S., Majumdar, D., 2010. Effects of military load carriage on kinematics of gait. Ergonomics 53, 782e791. Nagle, F.J., Webb, P., Wanta, D.M., 1990. Energy exchange in downhill and uphill walking: a calorimetric study. Med. Sci. Sports Exerc. 22, 540e544. Pal, M.S., Majumdar, D., Dhara, P.C., Majumdar, D., 2007. Cardiorespiratory responses of military load carriage with varying walking speeds and gradients. Indian J. Biol. Sci. 13, 1e9. Pal, M.S., Majumdar, D., Bhattacharyya, M., Kumar, R., Majumdar, D., 2009. Optimum load for carriage by soldiers at two walking speeds on level ground. Int. J. Ind. Ergon. 39, 68e72. Perrey, S., Fabre, N., 2008. Exertion during uphill, level and downhill walking with and without hiking poles. J. Sports Sci. Med. 7, 32e38. Ryu, T., Soon Choi, H., Choi, H., Chung, M.K., 2006. A comparison of gait characteristics between Korean and Western people for establishing Korean gait reference data. Int. J. Ind. Ergon. 36, 1023e1030. Sagiv, M., Ben-Gal, S., Ben-Sira, D., 2000. Effects of gradient and load carried on human haemodynamic responses during treadmill walking. Eur. J. Appl. Physiol. 83, 47e50. Saha, P.N., Datta, S.R., Banerjee, P.K., Narayane, G.G., 1979. An acceptable workload for Indian workers. Ergonomics 22, 1059e1071. Scott, P.A., Christie, C.J., 2004. Optimal speedeload combinations for military maneuvers. Int. J. Ind. Ergon. 33, 63e68. Scott, P.A., Christie, C., 2000. Cardiac responses under diverse combinations of marching speed and back pack load. Ergon. SA 12, 49e60. Todd, A.I., 2001. Selected Physiological and Perceptual Responses to Positive and Negative Gradients of SANDF Personnel (Unpublished M.Sc thesis). Department of Human Kinetics and Ergonomics, Rhodes University, Grahamstown, South Africa. Todd, A.I., Scott, P.A., 2002. Metabolic responses to various combinations of gradient, load and marching speed. Ergon. SA 1, 37e45. Vogel, J.A., Wright, J.E., Patton, J.F., 1980. Development of New Gender Free Physical Fitness Standards for US Army. US Army Research Institute of Environmental Medicine, USA, Natick, Massachusetts. Wasserman, K., Hansen, J.E., Sue, D.Y., Whipp, B.J., Casaburi, R., 1994. In: Principles of Exercise Testing and Interpretation, second ed. Lea and Febiger, Pennsylvania. Wu, S.P., 2006. Psychophysically determined 1 h load carrying capacity of Chinese female. Int. J. Ind. Ergon. 36, 891e899. You, T., Ryan, A.S., Nicklas, B.J., 2004. The metabolic syndrome in obese postmenopausal woman: relation to body compression, visceral fat and inflammation. J. Clin. Endocrinol. Metab. 89, 5517e5522. Yu, Y.Z., Lu, S.M., 1990. The acceptable load while marching at a speed of 5 km h1 for young Chinese males. Ergonomics 33, 885e890.
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International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
Optimum load for carriage by Indian soldiers on different uphill gradients at specified walking speed Madhu Sudan Pal, Deepti Majumdar, Anilendu Pramanik, Bodhisattwa Chowdhury, Dhurjati Majumdar* Defence Institute of Physiology and Allied Sciences, Defence Research & Development Organisation, Min. of Defence, Govt. of India, Lucknow Road, Delhi 110054, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2012 Received in revised form 11 March 2013 Accepted 13 September 2013 Available online 5 February 2014
Physiological responses of soldiers while carrying different loads were studied to suggest maximum weight that can be carried by an Indian infantry soldier comfortably at different gradients at specific walking speed. Ten physically fit infantry soldiers walked at 4.5 km h1 on treadmill (0%, 5%, 10% and 15% gradient) without and with loads of 4.4, 10.7, 17.0 and 21.4 kg. At each gradient, all the loads including _ Þ, energy without load were experimented for 10 min. Heart rate (HR), oxygen consumption ðVO 2 _ expenditure (EE), respiratory frequency (RF) and minute ventilation ðVEÞ were determined using K4b2 _ , EE, RF and VE _ with increasing gradient and external load was system. A linear increase in HR, VO 2 _ observed. Based on physiological limit of 50%, 60% and 75% of VO 2max and linear regression equations optimum loads are suggested as permissible for carriage on different gradients by Indian soldiers at above speed. This combination of weight and gradient would improve the combat readiness of soldiers while carrying load. Relevance to industry: Most of the developing countries do not have load carriage standards, for either Industry or Military personnel and extrapolation of the data from developed countries do not seem feasible. Results of this study may be applicable in developing standards or in recommending optimal loads for similar populations under similar conditions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Load carriage Gradient Maximum permissible load Oxygen uptake Physiological cost Heart rate Soldier
1. Introduction The physiological cost of load carriage at level walking and different gradients has been investigated by many researchers (Sagiv et al., 2000; Pal et al., 2007; Abe et al., 2008; Bastien et al., 2005; Legg et al., 1992; Wu, 2006; Chow et al., 2009, 2011; Cho and Kima, 2012). However, in most of the studies load was placed as single compact unit and carried as backpack or rucksack. In military environment load is carried as multiple units such as combinations of haversack, backpack, web and rifle mainly in the upper torso as per their requirements. These units vary in shape, size and weight and may lead to unequal distribution of load over body and changes in gait pattern of an individual (Attwells et al., 2006; Ryu et al., 2006; Majumdar et al., 2010) which is known to cause increased energy demand and early onset of fatigue. To date, physiological studies of load carriage (as practised by soldiers) in military environment are limited and less reported, especially with
* Corresponding author. Tel.: þ91 11 23935745; fax: þ91 11 23914790. E-mail address: [email protected] (D. Majumdar). 0169-8141/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ergon.2013.09.001
varying gradients. Pal et al. (2009) suggested optimum load for carriage by Indian soldiers on level ground at specified walking speeds. No study of load carriage at varying gradients by Indian soldiers is reported. Similarly, load carrying standards, e.g., optimum loads are not available at different gradients. So far, physiological responses of Indian soldiers while carrying load carriage at varying gradients (uphill) have not been reported and no standard for optimum load carriage at different gradients is presently available. Carrying heavy loads over unpredictable terrain for long distances is a requirement common to military personnel (Liu, 2007). The platoon’s combat load varies by mission and includes the supplies physically carried into the fight. Present day soldiers need to carry more equipment, supplies, and ammunition than earlier days to enhance sustenance for longer duration. Manual load carrying by Indian soldiers is often necessary in the remote countryside and mountain areas of India as these areas are mostly inaccessible by vehicle. In these areas soldiers need to carry materials such as arms, ammunitions, rations, clothes and first-aid dressing for survival and at the same time ensuring the maintenance of their work capacity and combat readiness. However, the
M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
authors are unaware of any reported studies so far that have simultaneously analyzed the effect of both load placement and gradient on cardiorespiratory responses of soldier’s while carrying loads on different parts of the body. As load carrying capacity of an individual depends on walking speed and gradient, identification of the optimum load for a specific speed with different gradients is of utmost importance in deciding the combat readiness of soldiers. Carrying loads by troops is an important aspect of military operations that may prove to be very critical under some circumstances. Overloading of soldiers with ammunition and equipment may lead to discomfort, excessive fatigue, injuries and impair soldier’s ability to fight. Therefore, identification of optimum load is important in order to take preventive action to reduce the incidences of common injuries associated with load carriage, excessive fatigue and impairment of the soldier’s ability to fight. Scott and Christie (2004) proposed that military marches should be conducted under ‘steady state’ at an intensity that does not exceed _ 50% of maximum aerobic capacity ðVO 2max Þ, thus ensuring that soldiers are combat efficient. Vogel et al. (1980) recommended that _ if any task required a work rate of 60% VO 2max , it may be sustained for approximately 2 h, whereas tasks that require work rate of 75% _ VO 2max should not exceed approximately 30 min duration. If a soldier is exhausted from carrying a load, he may be unable to perform when he reaches the site of conflict. To ensure the most efficient use of energy, the planning for any load carrying should take into account the elimination of local strain, injuries and fatigue. The greatest concern of military researchers is to establish the conditions at which soldiers can perform prime functions of military exercises, maximally and efficiently, both during and after load carriage. Hence, present study hypothesized that load carriage on a positive gradient would increase cardiorespiratory responses compared to load carriage on a level gradient. The effect of gradient on cardiorespiratory responses of soldiers was assessed while carrying military loads and to estimate the optimum or maximum permissible load (within physiological limit of 50%, 60% and 75% _ VO 2max ) that can be carried comfortably on various gradients at a specific walking speed. 2. Methods 2.1. Participants Ten physically fit, nonsmoker, experienced (4þ years) male infantry soldiers from the Indian Army volunteered for the study. Mean age was 23.2 (SEM 0.83) years, height 172.6 (SEM 1.20) cm, _ weight 65.9 (SEM 2.24) kg and maximum aerobic capacity ðVO 2max Þ 47.5 (SEM 1.40) ml min1 kg1. They had no history of musculoskeletal or cardiovascular pathology. All volunteers provided informed consent to participate in this investigation. The study protocol was approved by the Institutional Ethical Committee on the use of Human as an Experimental subjects and experiment conforms to the principles outlined by the Declaration of Helsinki protocol, 1964. 2.2. Experimental details On the first day, soldiers were briefed about the purpose of the study. They were then habituated to walking on the motorized treadmill (Taeha, Intertrack 6025, Korea) at various gradients in the laboratory, with and without loads at 4.5 km h1 walking speed. Habituation process continued on the second day also. One day of rest was given on the next day. _ Maximum aerobic capacity ðVO 2max Þ, of the subjects was measured during treadmill exercise with regular increases in gradient (Harbor Protocol, Wasserman et al., 1994), while keeping
261
Table 1 Details of type, combination and placement of load for load carriage experiments. Load (kg)
Contents
% Body weight (BW)
0 4.4 10.7
No load (NL) 4.4 kg Haversack on back 4.4 kg Haversack on back þ 2.1 kg Web in front of waist þ 4.2 kg INSAS rifle in right hand 10.7 kg Backpack þ 2.1 kg Web in front of waist þ 4.2 kg INSAS rifle in right hand 10.7 kg Backpack þ 4.4 kg Haversack on left lateral side of the waist þ 2.1 kg Web in front of waist þ 4.2 kg INSAS rifle in right hand
0 6.7 16.2
17.0 21.4
25.8 32.5
_ 2max , the speed constant on fourth day. During measurement of VO each subject walked on treadmill at 0% gradient for 3 min at self selected comfortable walking speed. After 3 min of walking at comfortable speed chosen by the subjects, every minute the treadmill gradient was increased by 4%, so that the subject reaches _ _ VO 2max approximately in 10 min. A valid VO2max was obtained when at least two of these three criterions were met: i) maximum heart rate (HR) > 90% of age predicted maximum HR (220 beats min1 age), ii) Respiratory Exchange Ratio (RER) of at least _ , <200 ml min1 1.10 and iii) plateau in oxygen consumption (VO 2 change) with increasing work rate (You et al., 2004). Mean (SEM) _ values of RER and HR at maximum VO 2max were 1.24 (0.018) and 189.9 (1.038) beats min1, respectively. During the measurement of _ VO 2max , subjects wore a vest, underwear and physical training (PT) shoes. Subjects took rest on the fifth day. Load carriage experiments were carried out from sixth day onwards. The loads and gradients were assigned randomly to the subjects on different day. On the day of load carriage experiment the subjects reported to the laboratory at 8.00 a.m. after light breakfast and the experimental procedure was initiated after about 90 min of rest. They abstained from smoking or taking any food as long as they were in the laboratory. During the load carriage experiment all subjects wore full Infantry Uniform including combat boots and helmet. Load carriage experiments were carried out on each subject while carrying no load (NL, 0 kg) and loads of 4.4, 10.7, 17.0 and 21.4 kg at 0%, 5%, 10% and 15% gradient, respectively at medium pace (4.5 km h1) walking speed (Ganguli, 1973) on a treadmill. Details about load items and their placement on the body along with percentage of body weight (BW) are given in Table 1. At each gradient and load including NL, the experiment was carried out for 10 min duration. In this study, load carriage operations were carried out in the laboratory which simulated the operational loads carried under field situations. Carried load corresponded to 6.7% (for 4.4 kg load), 16.3% (for 10.7 kg load), 25.8% (for 17.0 kg load) and 32.5% (for 21.4 kg load) of the average body weight (BW) of the subjects studied. In total, each participant underwent 20 experiments (four gradients and five load conditions including no load), and was required daily to complete two exercise trials (10 min each) from 0930 h to 1300 h, with at least 90 min rest between two exercise trials to eliminate cumulative effect of first exercise trial on second trial. 2.3. Physiological measurements All load carriage experiments were conducted in a controlled laboratory environment of 22e25 C, 50e55% relative humidity and at the same hour of the day (between 0930 h and 1300 h) and every day for eliminating the specific dynamic action (SDA) of food for all practical purposes. The load carriage ensembles (LCe) used in this study was regularly used by Indian Infantry soldiers. Each subject used the same LCe and rifle during the experiments.
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M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
_ Þ, heart rate (HR), energy expenditure Oxygen consumption ðVO 2 _ (EE), respiratory frequency (RF) and minute ventilation ðVEÞ, of each of the individual during rest and experiment were determined by the process of breath-by-breath gas analysis using K4b2 system _ (K4b2, Cosmed S.r.l, Italy). Before the VO 2max and each of the load carriage experiment on every day K4b2 system was calibrated with room air, volume calibration and standard gas mixture (16% O2 and 5% CO2). Each participant wore a polar chest belt before being fitted with appropriate load and portable unit and battery of the K4b2 system were fixed on the chest and back of the individual, _ respectively, with a harness, during measurement of VO 2max and while performing the load carriage experiments. A flexible face mask that covered the subject’s mouth and nose was secured with a nylon mesh hairnet and velcro straps. The face mask contained flowmeter and connected to the portable unit via capillary tube. After fitting the K4b2 system on the subject, both the data logger and telemetry mode of the system were turned on. The subject was then asked to breathe normally through the face mask for about 10 min sitting on a comfortable chair for the recording of resting physiological parameters. Telemetric recording in a personal computer indicated the state of recovery of the subjects between exercises. Physiological parameters were recorded in breath-bybreath mode. An average of the last 3-min physiological data from each exercise trial of 10 min was considered as individual value for further analysis. 2.4. Data analysis An average of the last 3 min of physiological data from each 10 min exercise trial was considered as individual value for further analysis. All data are presented as means SEM. To find out the significant effect of gradient (1st factor) and load (2nd factor) on _ 2 , EE, RF, VE _ and %VO _ 2max ), a various dependent variables (HR, VO two way repeated measure ANOVA was carried out followed by Bonferroni post hoc analysis, using Statistical Package for Social Sciences for Windows V16.0 (SPSS, Nikiski, AK 99635, USA). _ Regression analysis was performed between %VO 2max and load for different gradients to estimate the optimum or maximum permissible load. All differences were considered significant at p 0.05. 3. Results _ , EE, RF, VE _ and Cardiorespiratory responses, e.g. HR, VO 2 _ %VO 2max , during carriage of four different loads and no load with varying gradients at 4.5 km h1 walking speed are presented in Table 2 and Fig. 1aef. Two-way ANOVA with repeated measures showed a significant gradient load interaction on HR _ [F(3.125,28.128) ¼ 7.377, p < 0.001], VO 2 [F(4.514,40.626) ¼ 5.567, _ p < 0.001], EE [F(4.428,39.851) ¼ 9.855, p < 0.001], VE _ [F(4.036,36.323) ¼ 25.499, p < 0.001] and %VO 2max
[F(4.460,40.143) ¼ 5.455, p < 0.001]. No significant interaction was found in RF. _ The mean percentage of VO 2max (Y) was plotted against the magnitudes of loads (X) carried by the participants and four regression lines (Y on X) fitted through the points were drawn by the methods of least squares for 0%, 5%, 10% and 15% gradients (Fig. 2). The corresponding regression equations and correlation coefficients (r) have been presented. Significant (p < 0.01) positive correlations, r ¼ 0.97, 0.98, 0.99 and 0.99 were found at 0%, 5%, 10% and 15% gradients, respectively between Y and X. From regression equations, optimum loads were estimated for four different gra_ dients when 50%, 60% and 75% of VO 2max were considered as acceptable workloads. Graphic representation of acceptable loads is shown in Fig. 2 by drawing a perpendicular line on the X-axis, from
Table 2 Cardio-respiratory responses to treadmill walking at 4.5 km h1 (n ¼ 10) during carrying of different magnitudes of load at various gradients. Data presented as mean (SEM). SEM ¼ Standard Error of Mean. Parameters
Load Gradient (%) (kg) 0 5
Heart rate (beats min1)
0 93.8 (2.59) 110.6 4.4 95.1 (2.29) 115.6 10.7 100.8 (2.86) 123.9 17.0 103.6 (2.93) 128.6 21.4 107.5 (2.29) 136.7
Oxygen 0 consumption 4.4 1 1 (ml min kg ) 10.7 17.0 21.4
10
15
(3.52) (3.54) (2.70) (5.15) (4.21)
130.1 137.5 153.9 156.4 169.4
(3.50) (4.49) (3.76) (3.69) (2.56)
156.6 165.6 176.1 180.6 184.5
(3.33) (2.87) (2.76) (2.77) (2.08)
13.2 13.3 14.7 15.1 16.4
(0.32) (0.42) (0.39) (0.39) (0.65)
19.4 19.4 21.5 23.1 23.9
(0.50) (0.35) (0.62) (0.52) (0.66)
26.6 26.9 30.1 31.6 33.1
(0.88) (0.71) (0.84) (0.94) (1.33)
33.8 35.4 38.8 40.5 42.4
(1.16) (0.91) (0.87) (1.33) (1.66)
Energy expenditure (kcal min1)
0 4.4 10.7 17.0 21.4
4.3 4.4 4.8 4.9 5.3
(0.09) (0.14) (0.14) (0.16) (0.19)
6.3 6.4 7.0 7.5 7.9
(0.19) (0.18) (0.25) (0.24) (0.23)
8.8 8.9 9.8 10.3 10.9
(0.34) (0.30) (0.31) (0.32) (0.38)
11.0 11.6 12.8 13.5 14.3
(0.39) (0.49) (0.44) (0.51) (0.49)
Respiratory frequency (breath min1)
0 4.4 10.7 17.0 21.4
29.2 30.3 32.1 33.3 34.6
(1.27) (1.29) (1.38) (1.33) (1.09)
31.9 33.3 35.8 38.0 38.9
(1.34) (1.36) (1.67) (1.82) (1.25)
34.9 37.1 40.7 41.7 43.3
(1.66) (1.58) (1.85) (1.87) (1.64)
39.7 40.4 43.8 47.4 48.5
(1.85) (2.15) (1.22) (1.55) (1.86)
Minute ventilation (l min1)
0 4.4 10.7 17.0 21.4
28.3 29.8 32.8 32.4 35.8
(0.74) (1.16) (0.89) (0.94) (0.99)
39.1 41.0 45.4 48.0 50.4
(1.02) (1.27) (1.30) (1.09) (1.22)
52.2 55.5 62.5 65.0 72.0
(1.85) (1.85) (1.49) (2.42) (2.31)
68.3 74.4 84.0 94.3 99.8
(2.36) (3.17) (3.75) (3.82) (3.28)
_ %VO 2max
0 4.4 10.7 17.0 21.4
28.0 28.2 31.0 32.0 34.6
(0.63) (1.02) (0.68) (1.28) (1.23)
41.0 41.1 45.4 48.8 50.5
(1.21) (1.15) (1.45) (1.20) (1.65)
56.3 57.0 63.6 66.8 69.9
(1.95) (2.00) (1.85) (1.76) (2.39)
71.3 75.0 82.1 85.5 89.3
(2.33) (2.79) (1.99) (2.73) (2.65)
_ intercepts of acceptable workload (50%, 60% and 75% of VO 2max ) line and regression line at different gradients. 4. Discussion The present study was undertaken to determine the effect of gradients and load on cardiorespiratory responses during military load carriage on Indian infantry soldiers. It was observed that HR, _ , EE, VE, _ RF and %VO _ VO 2 2max gradually increased with increasing gradient and load. The findings of the present study are consistent with the work of Kirk and Schneider (1992), Todd and Scott (2002), Chung et al. (2005) and Crowder et al. (2007). Kirk and Schneider (1992) found that changes in treadmill gradient had a significant _ _ effect on HR, VO 2 and VE regardless of the type of pack carried. Todd and Scott (2002) studied the metabolic cost of South African soldiers during carrying of loads (50, 35 and 20 kg) under various conditions of gradient (10%, 0% and 10%) and speed (4, 5 and 6 km h1). They revealed that uphill marching showed significant increases in metabolic demands under all speed and load conditions. Chung et al. (2005) observed that climbing stairways involved an increased physiological burden as compared to level ground walking while carrying load. They also confirmed that heavier loads resulted in higher physiological cost than carrying of comparatively lighter loads. Crowder et al. (2007) examined metabolic cost during simulated road marching (speed 3.5 mile h1, grades 0, 5 and 10%) on military personnel while they carried a load of 27 kg. Results of their study showed significant differences in HR, _ 2 and VE _ across all gradients. Abe et al. (2008) observed a sigVO nificant interaction effect between load and gradient conditions for
4.4 kg 10.7 kg 17.0 kg 21.4 kg
45
0 kg
16
40
4.4 kg
14
10.7 kg
35
17.0 kg
30
21.4 kg
25 20 15 10
10
15
0
5
10
0 kg
50
10.7 kg
40
21.4 kg
30 20 10
VE (l/min)
17.0 kg
Gradient (%)
21.4 kg
8 6 4
0
100
0 kg 4.4 kg 10.7 kg
80
17.0 kg 21.4 kg
60 40 20
0 10
17.0 kg
10
5
15
0 0
5
10
Gradient (%)
10
15
Gradient (%)
120
4.4 kg
5
10.7 kg
Gradient (%)
60
0
4.4 kg
12
15
15
%VO2 max
5
0 kg
0
0 0
263
2
5
Gradient (%)
RF (b/min)
EE (kcal/min)
0 kg
200 180 160 140 120 100 80 60 40 20 0
VO2 (ml/min/kg)
HR (beats/min)
M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
100 90 80 70 60 50 40 30 20 10 0
0 kg 4.4 kg 10.7 kg 17.0 kg 21.4 kg
0
5 10 Gradient (%)
15
Fig. 1. Graphical representation of cardio-respiratory responses during treadmill walking at 4.5 km h1 with carrying different loads (0, 4.4, 10.7, 17.0 and 21.4 kg) at various _ , (ml kg1 min1), c. EE (kcal min1), d. RF (breath min1), e. VE _ (l min1), f. %VO _ gradients (0%, 5%, 10% and 15%). a. HR (beats min1), b. VO 2 2max .
energy cost of walking with load carriage. Similarly, Perrey and Fabre (2008) reported that during uphill walking, load and gradient significantly interacted for EE and HR. In the present study the authors have also found significant interactions effect between _ , EE, VE _ and %VO _ gradient and load for HR, VO 2 2max during load carriage operations. An interaction (gradient load) analysis on HR reveals that there is difference between the marginal means for the different levels of load across the different levels of gradient (122.8 vs 128.5 vs 138.7 vs 142.3 vs 149.5). The marginal means of gradient over levels of load (100.2 vs 123.1 vs 149.5 vs 172.7) are different with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (95.1 vs 115.6 vs 137.5 vs 165.6) and a similar pattern was observed while carrying 10.7 kg load (100.8 vs 123.9 vs 153.9 vs 176.1), 17.0 kg load (103.6 vs 128.6 vs 156.4 vs 180.6) and 21.4 kg load (107.5 vs 136.7 vs 169.4 vs 184.5). An interaction _ (gradient load) analysis on VO 2 observed that there is difference between the marginal means for the different levels of load across the different levels of gradient (23.25 vs 23.75 vs 26.27 vs 27.57 vs 28.95). The marginal means of gradient over levels of load (14.54 vs 21.46 vs 29.66 vs 38.18) are different with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (13.3 vs 19.4 vs 26.9 vs 35.4) and a similar pattern was observed while carrying 10.7 kg load (14.7 vs 21.5 vs 30.1 vs 38.8), 17.0 kg load (15.1 vs 23.1 vs 31.6 vs 40.5) and 21.4 kg load (16.4 vs 23.9 vs 33.1 vs 42.4). An interaction (gradient load) analysis on EE found that there is difference between the marginal means for the different levels of load across the different levels of gradient (7.6 vs 7.8 vs 8.6 vs 9.1 vs 9.6). The marginal means of gradient over levels of load (4.7 vs 7.0 vs 9.7 vs 12.6) are different with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (4.4 vs 6.4 vs 8.9 vs 11.6) and a similar pattern was observed while carrying 10.7 kg load (4.8 vs 7.0 vs 9.8 vs 12.8), 17.0 kg load (4.9 vs 7.5 vs 10.3 vs 13.5) and 21.4 kg load (5.3 vs 7.9 vs 10.9 vs _ reveals that 14.3). An interaction (gradient load) analysis on VE there is difference between the marginal means for the different
levels of load across the different levels of gradient (47.0 vs 50.2 vs 56.2 vs 59.9 vs 64.5). The marginal means of gradient over levels of load are different (31.8 vs 44.8 vs 61.4 vs 84.2) with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (29.8 vs 41.0 vs 55.5 vs 74.4) and a similar pattern was observed while carrying 10.7 kg load (32.8 vs 45.4 vs 62.5 vs 84.0), 17.0 kg load (32.4 vs 48.0 vs 65.0 vs 94.3) and 21.4 kg load (35.8 vs 50.4 vs 72.0 vs 99.8). An interaction (gradient load) analysis on _ %VO 2max found that there is difference between the marginal means for the different levels of load across the different levels of gradient (49.2 vs 50.3 vs 55.5 vs 58.3 vs 61.1). The marginal means of gradient over levels of load are different (30.8 vs 45.4 vs 62.7 vs 80.6) with the mean for 15% gradient being the highest. The cell means (Table 2) show an increasing pattern for levels of gradient during carrying of 4.4 kg load (28.2 vs 41.1 vs 57.0 vs 75.0) and a similar pattern was observed while carrying 10.7 kg load (31.0 vs 45.4 vs 63.6 vs 82.1), 17.0 kg load (32.0 vs 48.8 vs 66.8 vs 85.5) and 21.4 kg load (34.6 vs 50.5 vs 69.9 vs 89.3). Observations of the present study corroborated and extended the earlier research _ , EE, VE _ and %VO _ works. This increase in HR, VO 2 2max with the change in gradient and load may have been due to change in locomotion biomechanics as suggested by earlier researchers (Martin and Nelson, 1980). One of the objectives of the present study was to identify the optimum or maximal acceptable load to be carried by Indian soldiers at different gradients without undue fatigue. It was observed that at 4.5 km h1 walking speed, carrying of 21.4 kg load at 5% _ _ gradient demanded 50.5% of VO . Based on the 50% of VO 2max
2max
criteria (Vogel et al., 1980), it may be suggested that for an 8 h work day at 5% gradient, at walking speed of 4.5 km h1, the load should not exceed 21.4 kg (32.5% of body weight). At this walking speed, _ higher loads demanded more than 50% of VO 2max at 10% and 15% gradients. As India has steep and high mountain border areas in the Northern and Eastern parts of the country, soldiers cannot be restricted to carry loads up to only a 5% gradient. Therefore, at 10% and higher gradients soldiers should control either duration of working hours or walking speed in order to keep physiological or
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M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
physical strain within acceptable limits. Vogel et al. (1980) sug_ 2max , it may gested that any task that required a work rate of 60% VO be sustained for approximately 2 h and tasks that required work _ rates of 75% VO 2max should not exceed approximately 30 min duration. According to the criterion of Vogel et al. (1980), the permissible magnitude of load from Fig. 2 is found to be approximately 6.6 kg (r ¼ 0.99, p < 0.01) for 2 h working duration, for the soldiers walking with 4.5 km h1speed at 10% gradient. At 15% gradient, the permissible magnitude of load was found to be approximately 4.0 kg (r ¼ 0.99, p < 0.001) for 30 min working _ duration only. When criterion of 50% VO 2max (Astrand, 1960) is followed, the permissible magnitude of load can be calculated from regression equations to be approximately 72.9 kg (r ¼ 0.97, p < 0.01) and 20.2 kg (r ¼ 0.98, p < 0.01) respectively for 0% and 5% (Fig. 2) gradient for 8 h working duration for the soldiers at the specified walking speed. The correlations between the variables are high and significant, allowing conclusions to be drawn based on either regression line. The estimated optimum loads at four different gradients in this study are as follows:
Work duration
Gradient (%)
Estimated optimum load in kg and (% of BW)
Eight hour Eight hour Two hour Thirty minutes Thirty minutes
0 5 10 10 15
72.9 (110.6% of BW) 20.2 (30.6% BW) 6.6 (10.1% BW) 28.9 (43.9% BW) 4.0 (6.1% BW)
where % BW ¼ body weight in kgs. Optimal load is an elusive concept. There is no easy solution for identification of an optimum or maximum permissible load. _ Different criteria, such as percentage of BW, percentage of VO 2max
etc. are used for identification of optimum load for carriage. Several earlier researchers (Astrand, 1960; Evans et al., 1980; Scott and _ Christie, 2004; Saha et al., 1979) stated percentage of VO 2max as a criteria for recommended acceptable work rate. They recom_ mended that 33e50% of VO 2max could be considered an acceptable work rate for an 8 h work day. On the other hand, Haisman (1988) and Knapik (1989) stated that for healthy young males, there appears to be some consensus for the traditional rule of thumb of the one-third BW, or 24 kg on an assumed mean BW of 72 kg for a
working day. Our recommendation, however, does not agree with _ 2max criteria or one-third of BW, particularly at higher the 50% of VO gradients (10% and 15%) for an 8 h work day. Sagiv et al. (2000) observed haemodynamic and cardiovascular responses during carrying of loads (0, 25 and 35 kg) at different gradients during _ , treadmill walking. They found significant increases in HR, VO 2 blood pressure and cardiac output with increasing gradient. Their study demonstrated that during isodynamic exercise, changes in the gradient of walking was the most important factor for cardiovascular and metabolic responses than the mass of the load carried. Laursen et al. (2000) found that carrying loads as light as 10 kg at a positive gradient of only 8% increased EE by 70%, while a load of 20 kg at the same gradient almost doubled EE. These increases in EE are due to added work required to displace the centre of mass (BW plus load carried) against gravity (Nagle et al., 1990). Todd (2001) stated that, under heavy load carriage conditions, 10% uphill gradient resulted in substantial increases in the demands placed on _ soldiers, to as high as 106% of predicted VO 2max . As per the nature of job, soldiers are supposed to carry loads for survival in field situations. Hence, they cannot be restricted to load carriage only at 0% or 5% gradients. They are required to carry loads at higher gradients, particularly at high altitudes. To carry external loads at 10%, 15% and higher gradients, one should control his walking speed or duration of carrying the external load. With reference to the South African soldiers, Scott and Christie (2000) recommended that soldiers could carry up to a 65 kg load, but walking speed should not exceed 4.5 km h1 at 0% gradient. This value is similar to our recommendation of carrying a 72.9 kg load at 0% gradient for an 8 h work day. On the other hand, Yu and Lu (1990) suggested an acceptable load for young Chinese men at 5 km h1 to be 20 kg (31.0% of BW) which was latter confirmed by Li et al. (1992), closely supporting the recommendations of this study which is about 20.2 kg (30.6% BW) at 5% gradient, i.e. about one third of the average body weight (65.9 kg) of the subjects this study. The present study is novel in its approach of recommending the optimum load for Indian infantry soldiers. The recommendations for permissible load for carriage at 0% and 5% gradients are 72.9 kg and 20.2 kg, respectively, for 8 h; 6.6 kg for 2 h at 10% gradient; and 28.9 kg and 4.0 kg for 30 min at 10% and 15% gradient, respectively, at walking speed of 4.5 km h1 However, the results of this study can be extrapolated for applying these recommendations on comparable civilian Indian populations. Also these recommendations can be used for deciding optimal load
_ Fig. 2. Relation between percentage of VO 2max and load carried by soldiers at different gradients.
M.S. Pal et al. / International Journal of Industrial Ergonomics 44 (2014) 260e265
for carriage for similar populations with comparable anthropometric dimensions in other parts of the world. 5. Conclusion Present study concluded that with increasing positive gradient cardiorespiratory responses were significantly increased compared to level walking at medium pace. Based on physiological limits of _ 50%, 60% and 75% of VO 2max and linear regression equations, optimum loads or maximum permissible load are suggested for carriage on different uphill gradients at the specified walking speed without considering the external environmental factors, altitude and terrain factors. This combination of weight and gradient would improve the prime function of soldiers while carrying loads. Findings of the present study clearly demonstrate that during walking at 10% and above uphill gradient, one third of the body weight or _ 50% of VO 2max criteria could not be sufficient for identifying optimum load for being carried during 8 h working day. The present study was restricted to uphill gradient, medium pace walking, load up to 21.4 kg and in laboratory condition only. For recommendations on heavy loads, further detailed studies on biomechanical factors, prolonged duration, faster walking speed, altitude, different terrain and field conditions are required. Acknowledgements Authors would like to express their gratitude to volunteers for their participation in the study. They are thankful to Defence Research and Development Organisation, Ministry of Defence. Government of India for funding the project. References Abe, D., Muraki, S., Yasukouchi, A., 2008. Ergonomic effects of load carriage on the upper and lower back on metabolic energy cost of walking. Appl. Ergon. 39, 392e398. Astrand, I., 1960. Aerobic work capacity in men and women, with special reference to age. Acta Physiol. Scand. 49, 169. Attwells, R.L., Birrell, S.A., Hooper, R.H., Mansfield, N.J., 2006. Influence of carrying heavy loads on soldiers posture, movements and gait. Ergonomics 49, 1527e 1537. Bastien, G.J., Willems, P.A., Schepens, B., Heglund, N.C., 2005. Effect of load and speed on the energetic cost of human walking. Eur. J. Appl. Physiol. 94, 76e83. Cho, Y.J., Kima, J.Y., 2012. The effects of load, flexion, twisting and window size on the stationarity of trunk muscle EMG signals. Int. J. Ind. Ergon. 42, 287e292. Chow, D.H.K., Ting, J.M.L., Pope, M.H., Lai, A., 2009. Effects of backpack load placement on pulmonary capacities of normal schoolchildren during upright stance. Int. J. Ind. Ergon. 39, 703e707. Chow, D.H.K., Li, M.F., Lai, A., Pope, M.H., 2011. Effect of load carriage on spinal compression. Int. J. Ind. Ergon. 41, 219e223. Chung, M.K., Lee, Y.J., Lee, I., Choi, K.I., 2005. Physiological workload evaluation of carrying soft drink beverage boxes on the back. Appl. Ergon. 36, 569e574. Crowder, T.A., Beekley, M.D., Sturdivant, R.X., Johnson, C.A., Lumpkin, A., 2007. Metabolic effects of soldier performance on a simulated graded road march
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