Load carrying : in situ physiological responses of an infantry platoon

Morphological diversity is the source of differential stress when heavy work must be done by groups, as in the carrying of military equipment by a platoon. In this study 10 infantrymen each carried 40.5kg at the same pace over a 12km route on one day and 37% of personal body weight on another occasion. Physiological and perceptual responses indicated less stress was experienced when loads were normalised for morphological differences between the troops. INTRODUCTION Load carriage is commonplace among humans in all walks of life, but there is probably no sector in which it is more physically demanding than in the army. During military engagement foot-soldiers often carry extremely heavy backpacks and march long distances, following which they are required to carry out critical military tasks in which speed and accuracy of execution are essential. As early as 1869, Parkes pointed out that for greatest carrying efficiency and locomotor stability the load should be kept as close to the carriers’ mass as possible and should not exceed about a third of body weight. Over a century later extensive research is still being conducted in order to establish an optimal weight to be carried. Ramaswamy and Sivaraman (1956) defined as optimal any load which will not appreciably impair operational efficiency at the completion of the march. Although over the years attempts have been made to unburden soldiers by providing them with no more than what Lothian (1922) identified as “essential” items, Knapik et al. (1990a) have argued that with technological advancements, particularly in terms of increased firepower and protection, loads have progressively increased. The result has been that loads in excess of 60 kg are often carried, representing weights that are close to the body weight of some of the soldiers themselves. Borghols et al. (1978) investigated backpack loads up to 30 kg and demonstrated that each kilogram of weight increased oxygen consumption by 33.5 mR.min, heart rate by 1.1beats.min and pulmonary ventilation by 600mR.min. Although Soule et al. (1978) and Charteris et al. (1989)(see next article) have argued that the energy cost of moving body mass plus an external load increases linearly with increments in load, Maloiy et al. (1986) proposed that with heavy loads there is a curvilinear relationship with energy expenditure. ergonomics SA, 2000 (1) 19 While much emphasis has been placed on the load carried, the focus of this paper is on the soldier required to carry the load and to be combat-ready at the end of a forced march. Although it is essential for a platoon to operate as a single efficient unit, cognizance must be taken of the fact that a platoon is comprised of individuals, each displaying a unique profile of physical and mental capability. Over the last five years we have conducted several studies on the South African National Defence Force (SANDF). It is evident that the demographic profile of the army is now reflective of the nation as a whole, comprising substantial diversity of ethnic groups. Figure 1 illustrates the range of morphologies of a small group of soldiers selected from the above-mentioned projects. The lowest mass recorded was 54 kg and the highest 104kg. It is clear that when two soldiers representing these extremes are required to carry the same load they must be differentially taxed; the combat effectiveness of one, relative to the other, must be compromised. Traditionally footsoldiers have been required to carry similar loads, yet as early as 1922 Lothian recognised that the load to be carried has a definite relationship to the subject’s strength, which normally is in turn related to body mass. It was evident even then that more attention should be paid to subject morphology. Even without a load, Wyndham et al. (1971) has shown that body mass is an important determinant of oxygen consumption in walking. It is therefore evident that consideration should be given to the mass of the subject when deciding on the optimal load to be carried. Rather than everyone in a platoon carrying the same absolute load irrespective of individual size, loads could feasibly be normalised to some functionally meaningful morphological factor commensurate with individual differences. Cathcart et al. (1923) recommended that under laboratory conditions the maximum load for maintenance of efficiency and health should be 40% of body mass, a suggestion supported by Legg and Mahanty (1985). However, others have more recently suggested a third of body mass (Haisman, 1988; Knapik, 1989 and Kirk and Schneider, 1992). Acknowledging the importance of the human factor in the military and the need for all soldiers to be equally taxed by the load carried in order to be operationally effective as a unit, the present project was undertaken to assess the energy cost and perceptual responses of a platoon of ten soldiers each carrying; i) an absolute load of 40 kg and ii) carrying a mass-relative load of 37% body mass. Figure 1: Morphological diversity in the SANDF. ergonomics SA, 2000 (1) 20 METHOD While research conducted under laboratory conditions is rigorously controlled, and the majority of variables not being investigated can be held constant, when investigation is this strictly standardised the results may be somewhat sterile and not truly reflective of what happens when the same activity is carried out under natural environmental conditions. On the other hand, during in situ investigations many factors cannot be as rigorously controlled, particularly environmental conditions and external distractions. Nevertheless, Oborne (1987) has argued that field studies may have greater validity in terms of relating back directly to the situation under investigation. Bearing in mind that Ergonomics is an applied science, it is a discipline in which there is a need to conduct both rigorous laboratory experiments as well as field tests in which every attempt must be made to standardise conditions. In the present project experiments were conducted in situ in order to establish an holistic profile under conditions in which soldiers routinely march. Figure 2: Soldier on right connected to Metamax for expired air collection. Route: Subjects were required to march a 12km route which was selected and marked out by instructors at the Oudtshoorn Infantry School. The overall distance was sub-divided into three 4km sections. The terrain was essentially flat for the first 4.5km, of which the first 2.5km was tarmac. The next section traversed rolling hills with a very rough texture. The last section consisted of a short, steep hill, on which the highest altitude was reached and the march ended off on a fairly flat, open plain. Markers every 100m assisted in the controlling of the marching pace. A field ambulance with medics aboard followed the troops over the course. The subjects marched at a speed of 4km.h and this was carefully monitored by two pace-keepers who used response counters to maintain a cadence of 112 steps.min . The route was also demarcated every 250m. The march was sub-divided into 3 one-hour marches with a 15-minute break between the first and second hours and the third and fourth hours. Subjects: Ten soldiers were randomly selected from a company of 60 for a detailed physiological assessment using a portable ergospirometry system,(Metamax). This (1.8kg) system provides all the data required for a complete functional analysis of cardio-respiratory and metabolic activity under stress. The demographic profile of the soldiers was as follows: ergonomics SA, 2000 (1) 21 The average age was 21.2 (± 2.57) years, mean stature was 1741 (± 88) mm and mean mass was 73.4 (±15.39) kg. The BMI of the subjects was 23.5 (± 3.99) with the percentage body fat averaging 15.6 (± 3.45) %. Experimental conditions: All soldiers marched the 12km route under two conditions separated by two days rest. Every effort was made to keep all conditions as similar as possible, only the load being controlled. Condition I involved an Absolute load of 40.5kg, while Condition II employed a Relative load in which each subject carried 37% of his own body weight. Clothing mass was 2.62kg in both conditions. The metabolic measures taken during the last five minutes of each hour included breathing frequency (fb), heart rate (bt.min ), oxygen consumption (VO2) and carbon dioxide production (VCO2). Energy expenditure (kcal.min -1 and kJ.min) and power output (W) were derived from VO2. Local (lower limbs) and central (cardiovascular) RPE responses were collected during the last minute of each of the three hours and Body Discomfort ratings were recorded during the rest period after each hour. RESULTS A ‘reference’ heartrate was recorded prior to preparation for both conditions in order to establish a base-line against which work stress could be measured. There was no difference between the reference heart rates taken before the Absolute Load Condition and the Relative Load Condition. Reference heart rate was recorded at 73 bt.min. Working heart rates were recorded throughout the march and the average working and highest heartrates for each of the three hours are reported in respect of the Absolute and Relative Load conditions. From the results presented in Table I it is evident that not only were the subjects less taxed when carrying a load of 37% of their body weight, but more importantly once the loads had been relativized there was on average a twofold drop in inter-subject variability. The reduction in variability was most evident (2.5-fold) during the most taxing leg of the march i.e. during the second hour. One could argue therefore that the sub-optimal effects of absolute various relative load-carrying are further exacerbated by increases in environmental stress factors such as terrain and positive gradients. There was a significant increase in the rate of breathing when carrying the absolute load of 40 kg over the steady hill climb during the second hour. This increas