Determinants of load carrying ability

Appfied Ergonomics 1988, 19.2, 111 - 121

Determinants of load carrying ability M.F. Haisman

Army Personnel ResearchEstablishment, Farnborough, Hants. GU14 6TD, UK The aim of this paper is to review the literature in respect of the main determinants of a person's load carrying ability. Possible determinants of load carriage ability include age, anthropometry, aerobic and anaerobic power, muscle strength, body composition and gender; other relevant factors are the subjective effects perceived during load carriage, the dimensions and placement of the load, biomechanical factors, nature of the terrain and the gradient, the effect of climate and protective clothing. It is important to distinguish between the maximum load carrying capacity and load carriage ability which enables the individual to retain the capability to perform other tasks - eg, observation and navigation, or industrial tasks. The soldier has been used as the worst case example of extremely heavy loads having to be carried for long durations; civilian examples are usually less demanding except in the case of mountaineers, explorers and some occupations. The energy cost of walking with loads has been found to depend primarily upon the walking speed, body weight and load weight, together with terrain factors such as gradient and surface type; equations exist which allow the prediction of energy expenditure from these variables, and they can provide a valuable guide in assessing the physical severity of proposed tasks involving load carriage. Other factors such as the degree of environmental heat stress and protective clothing worn would have to be taken into account, but the level of energy expenditure (or heat production) assumes central importance as it is related to physical exhaustion, heat exhaustion and also less directly to the efficiency of performance of occupational tasks involving load carriage. This review confirms that there is no obvious definition of a maximal load, because of the widely varying circumstances which might apply, but for healthy young males there appears to be some consensus for the traditional rule of thumb of one-third body weight, or 24 kg on an assumed mean body weight of 72 kg, or in terms of relative work load equivalent to one-third of the VO 2 max for a working day. Renbourn (1954c) considered that the load carried by the soldier will probably always be a compromise between what is physiologically sound and what is operationally essential. Load carriage in industrial and other civilian areas will also involve a similar compromise and may in some circumstances lead to important implications for health and safety.

Keywords: Physical exertion, performance assessment, load carrying

I ntroduction Interest in the military aspects of individual load carriage is longstanding and was recorded in such reports as the British Royal Commission of 1858 quoted by Renbourn (1954a). A number of reviews cover various aspects of load carriage - for example, energy expenditure studies (Passmore and Durnin, 1955; Redfearn et al, 1956); physiological limitations of the soldier and load carriage development (Kennedy, et al, 1973); the effects of load

Copyright ~) HMSO, London (1987)

carriage on military performance (Lotens, 1982). The manual handling and lifting review by Troup and Edwards (1985) is particularly concerned with the back and includes references to carrying problems. The scope of this review includes studies which are concerned with load carried on the trunk, hands or head, whereas lifting in a static position, (eg, I_egg and Pateman, 1984), and also the use of mechanical devices such as wheels, (Haisman et al, 1972) have not been considered. The aim is to draw together the main factors which affect load carriage itself, from civilian as well as military spheres. Although much of the early research was done from a military standpoint, more recent work has examined occupational

0003 6870/88/02 0111 - 11 $03.00 © 1988 Butterworth & Co (Publishers) Ltd

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load carriage, particularly in respect of occupational health problems, whereas other studies are related to the increase in popularity of leisure activities such as 'back-packing'.

Severe load carriage tasks The extent of the military load carriage problem may not be fully appreciated, either because it is believed that mechanised transport will always be available, or because the diversity of the equipments involved obscures the large weights involved - eg, an infantryman's load (Table 1). The basic clothing assembly weighs 7 kg and the Assault Dress equipment increases the total to 26 kg; with the addition of support weapons, radios and extra equipment the total weight carried can escalate to the very high figures quoted for military operations - eg, up to 68 kg in the Falklands operation (McCaig and Gooderson, 1986). United States infantrymen also have heavy loads (Kennedy et al, 1973) - a basic load is 17 kg, adding protective clothing and basic existence load increases it to 28 kg, radio operators carry loads of about 35 kg. Renbourn (1954c) has describe(i some of the medical conditions, such as march fracture, which can be caused by the carriage of heavy loads. Other military groups besides the infantry have demanding load carriage tasks; large quantities of ammunition weighing up to 45 kg may have to be manually handled (Legg and Patton, 1987). As an example of an industrial task, Magnusson et al (1987) investigated butchering; lifting and carrying boxes (up to 40 kg) and meat parts (up to 70 kg) were common tasks, and the total load was greater than 4000 kg per day. The authors suggested that these heavy loads contributed to a high incidence of physical disorders of the back and shoulders among butchers. Many other civilian occupations involve load carriage - for example, firemen (Louhevaara et al, 1985), postmen (Ilmarinen et al, 1984), brewery drivers (Astrand, 1958), mines rescue personnel (Lind and McNicol, 1968), and steel workers, refuse disposal operators and cable twisters (Garg et al, 1978). Cady et al (1979) showed that in a group of 1652 firefighters, strength, fitness and physical conditioning exerted a preventive effect on subsequent back injuries. Noro (1967) noted the importance of load carrying as a possible cause of injury and ill health in industrial workers.

Factors affecting load carrying ability Factors affecting load carriage have been examined m order to assess their importance, and to seek ways of minimising the overall strain on tile carrier. Maximal, safe and optimal load concepts will be examined. The direct effects of carrying excessive or unbalanced loads, such as musculo-skeletal disorders, may be more difficult to quantify. Carrying is one aspect of dynamic work, together with heavy manual handling and lifting, which have been implicated as risk factors in low back pain on the basis of epidemiological studies (Dul and Hildebrandt, 1987). Load factors 1. M a x i m u m weight of'load

There have been numerous attempts to define the upper limit of weight to be carried by the soldier. During World War 1 the soldier became grossly overloaded: Cathcart et al,(1923) described the problem of a heavy load of 27 kg increasing to 43 kg because of water and mud soaking into the clothing and equipment. This problem of water uptake has been minimised by using water-resistant materials but that extra load has now been replaced by other equipment (Table 1). Marshall (1950) cites training loads of about 27 kg for US troops being increased in combat. In respect of the maximal load carriage capacity of man, Soule et al (1978) showed that the constancy of measured energy expenditure per kg of load (or body weight) extended up to loads of 70 kg provided the load was well balanced and close to the centre of gravity of the body. Daniels (1956) cited reports of observations of loads of up to about 180 kg being carried using a Korean A frame. On the other hand there is evidence (Durnin and Passmore, 1967) that the physiological efficiency of load carriage falls at high load weights, especially at fast speeds and up inclines. Winsmann and Goldman (1976) compared load carriage systems and they showed that provided the weight is properly distributed over the body, weight per se is the most important factor in load carriage rather than the specific load-carriage system design. There is clearly a case for setting an upper limit to the weight carried, and if the load is not going to impair efficiency to a marked extent this weight limit ought to be less than 30 kg. While it is more logical to relate the load to the body weight, as it is obvious that a 30 kg load is a

Table 1: Weights of clothing and personal equipment carried by a British infantryman (kg)

112

A.

Dress

Clothing, boots and helmet

B.

Assault dress

Clothing etc as in A, weapon, ammunition, digging tool and equipment

C.

Combat order

Dress and equipment as in A'& B, food and warm clothing

D.

Marching order

Clothing and equipment as in A, B & C, spare clothing, rations, rucksack and sleeping bag

E.

Additional equipment to be added

There are a number of additional items which could have to be carried ranging in weight up to 16 kg

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Weight

Total weight

7.0

7"0

19.4

26.4

3"7

30-0

10"2

40.2

very heavy one for a 58 kg man (5th percentile for British Infantry, (Gooderson, 1976) but a reasonable load for a 91 kg man (95th percentile), it may, however, be quite impractical to tailor the load to body weight. In trying to apply these guidelines to an industrial workforce it should be remembered that this population may be older and less fit compared with young healthy soldiers. Gracovetsky (1986) has reviewed the biomechanical criteria used in the formulation of safe loads for lifting and carrying, and has considered the effect of load carriage on the spine. Troup and Edwards (1985) have drawn together limits and regulations for manual handling, although few are related to carrying per se. Lind and McNicol (1968) examined the blood pressure and heart rate responses to holding and carrying weights by hand and by shoulder harness in the context of the stretcher-carrying task of mine rescue personnel; they found a fatiguing response from hand carriage in excess of 10 kg, and shoulder carriage in excess of 80 kg. Borghols etal (1977) investigated loads up to 30 kg carried on the back and showed that in their subjects each extra kilogram of weight increased VO2 by 33.5 ml/min, heart rate by 1"1 beats/min and pulmonary ventilation by 0"6 1/min. 2. Dimensions o f the load

Morrissey and Liou (1984) varied the dimensions and weights of boxes carried in two hands by subjects walking on the level and showed that the width of the container can influence the metabolic cost of carrying; this dimension factor can be included in prediction equations for energy cost. Amor and Vogel (1974) compared three methods of carrying a missile 35 kg inweight and I "2m in length; they found no difference in energy cost between the methods but the subjects preferred to carry the missile in a horizontal position on the back. Torre (1973) investigated the effects of weight and length of a missile system on performance and found that the soldier was reluctant to carry loads longer than 0"79m (at 3"6 kg) when added to his existing load. 3. Load placement

Legg (1985) listed 12 different possibilities for load carriage on the body, and even that number could be increased by consideration of both high and low back-packs. (a) Load placed on the head, hands or feet

Soule and Goldman (1969) showed that energy costs of carrying a load on the head, hands and feet were, in comparison to a no load condition, in the ratio of l'2x for the head, 1"9x for the hands and 4 - 6 x for the feet (up to 6 kg on each foot). The maximum possible weight to be carried on the head of an Indian worker was defined as 30 kg (Datta et al, 1975). Strydom et al (1968) reported that provided the boot weight was no more than 1'8-2"9 kg per pair there was no increase in oxygen consumption. In contrast, Jones et al (1984) showed that subj ects wearing boots (1.78 kg per pair) had a higher energy cost for walking (except for the slowest walking speed) and running than for the same activity whenwearing atl-detic shoes (weight 0.62 kg per pair). Essentially similar results showing an increment of energy cost of 1 "0% per 100 g increase in weight of footwear was found.for females (Jones et al, 1986). Legg and Mahanty (1986) found that the increase in energy cost of carrying weight on the feet was 6"4x that of carriage on the trunk, in agreement with Soule and Goldman (1969). The fatiguing effects of carrying weight on the lower extremities (ankle spats) have been utilised to induce

physical conditioning in sedentary middle-aged men 0aandolf and Goldman, 1975). For hand carried weights (1"8 kg in each hand), Francis and Hoobler (1986) could only detect an increase in energy cost compared to a no load condition when running and not when walking. Using a biomechanical approach, Ghori and Luckwill (1985) investigated loads up to 20% body weight carried in either hand, and loads up to 50% carried on the back whilst subjects walked at a comfortable speed. Changes in several parameters were observed with the heavier loads on the back and in the hand but were not large considering the substantial loads involved. Burton (1986), using electromyographic analysis of lumbar and suprascapular muscle activity, found that clutching a shopping bag (US style) created a lower and more even load on the spinal musculature compared with carrying the load by handles (UK method). Again using electromyographical techniques, together with anaylsis of heart rate increments, Evans et al (1983) defined a hyperbolic relationship between time to exhaustion for load holding or for load carrying in the hands, and mass of the load up to 40 kg; the increments in heart rate at exhaustion were linearly related to load and were always greater for carrying than holding. (b) Alternative load carriage methods on the trunk

Datta and Ramanathan (1971) compared seven methods of carrying loads of 30 kg; a double pack (front and back) proved to be the best and the hands the worst in terms of physiological efficiency. Using biomechanical techniques, Kinoshita (1985) showed that compared with a back-pack system, a double pack (front and back) was more effective, especially for heavy loads (40% of body weight), because forward lean was reduced and the gait characteristics were closer to unloaded walking. I_egg and Mahanty (1985) compared five methods of carrying a load of 35% body weight on the trunk and found that there were no significant physiological differences between them. The front/back pack combination and a load carrying jacket were subjectively rated as more comfortable than the back packs (with or without frame), but the front/back pack was associated with a restrictive type of ventilatory impairment. Bobet and Norman (1984) compared two different load placements Oust below mid-back or just above shoulder level); heart rate analysis revealed no differences between the two but EMG analysis showed lower muscle tension associated with the lower, more stable, back load. Balogun et al (1986), found that the metabolic efficiencies (VO2 per kg total weight) of a headpack and a transverse yoke system of carriage were better than a front yoke system. Physical characteristics o f man 1. Body weight

That the maximum comfort load should be related to body weight is an idea of long standing. The weight carried by the soldier steadily increased during World War I up to 85% of body weight (Renbourn, 1954b). On the basis of the energy cost of load carriage, which rose steeply above 40% of body weight, Cathcart et al (1923) recommended that under laboratory conditions the maximum load for the maintenance of efficiency and health should be 40% of body weight, and for service conditions they accepted the traditional limit of one-third of body weight. Marshall (1950) cites the British and other studies to recommend an optimal marching load of not more than one-third of body weight.

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Kinoshita (1985) cites a number of studies to support the statement that the weight of the individual load should not exceed 40% of body weight for continuous carrying, and 50% for intermittent or occasional carrying. McFarland (1969), reviewing the safety of carrying objects in occupational tasks, considered that objects carried with hand grips become 'heavy' at about 35% of body weight. Individuals with a high body weight can therefore carry greater loads, but as will be discussed later the constituent proportions of body weight, whether muscle or fat, will be important. Indian porters appear to be an exception to any generalisations about the advantages of high body weight in load carriage; Nag et al (1978) studied porters of mean body weight 53 kg who carried loads up to 100 kg on the treadmill. These subjects were, however, very lean (body fat 8"3%) and had a high VO2 max relative to their body weight. 2. A n t h r o p o m e t r i c dimensions

The design of load carriage equipment must take into account the range of dimensions in key anthropometric variables in the population to be fitted, especially back length and waist circumference. Drury et al (1982) defined relationships between the height and weight of workers, box size and the hand positions adopted in industrial box handling.

Physiological factors 1. M a x i m u m aerobic p o w e r I V 0 2 max)

VO2 max is much used as an index of cardio-respiratory performance (Astrand, 1956; Shephard, 1968) and also as an index of ability to perform maximal work (Taylor et al, 1955; Mitchell et al, 1958). It follows that factors which will raise VO2 max will improve the ability to carry loads, and the converse will also be true. It has been shown by a large number of studies that aerobic physical training will increase VO2 max. Saltin (1969) showed that the absolute improvement in VO2 max is 33% starting from a post-3 weeks of bed-rest level, but the improvement is highly dependent upon the initial level of VO 2 max and may be about 20% for average, non-exercising individuals. A number of factors have been associated with a decrease in VO2 max - for example, increasing age (Hermansen, 1978); semi-starvation with consequent loss of lean body mass (Keys et al, 1950); high altitude (laugh et al, 1964); and dehydration (Buskirk et al, 1958). Factors such as these will tend to lower the maximal load carriage capacity, or slow the walking speed at which the load can be carried. Another important consideration is that a well-trained man cannot be expected to work all day at a work level equivalent to more than 50% of his VO2 max without becoming fatigued (Astrand, 1956), but Astrand (1960) showed that working at 50% could produce objective and subjective indications of fatigue, whereas the spontaneously chosen work load in building work corresponds with about 40% of individual VO2 max (Astrand, 1967). Edholm (1967) suggested a somewhat lower limit of 2000 kcal during work, equivalent to about 0.85 litres VO2/min for an 8 hour day. Saha et al (1979) proposed that an acceptable workload for average young Indian workers in comfortable thermal conditions should be 35% VO2 max, corresponding to 18"0 kJ/min energy expenditure (or 0"88 1/min VO2) and 110 beats/min for

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heart rate. Bink (1964) recommended (tematlvely) that ilk workload should not exceed 33% VO2 max during a 5013 minute work day, Shapiro et al (1973) recommended that adjusting the work intensity during prolonged work to the individual's VO2 max should minimise muscle enzyme leakage and muscle damage. To summarise, VO2 max will be a major determinan~ of the severity of load carriage tasks which can be sustained for a prolonged period. As VO2 max is usually correlated with body weight, and in particular with muscle mass, individuals with a high V Q max (l/min) and large load carriage capacity will also tend to have higher than average body weight or muscle mass. Shoenfeld et al (1977) used the size of the decrement in VO2 max after load carriage by young men to assess the maximum load which should be carried for 20 kin; they concluded that for individuals in good physical condition this should not exceed 25 kg - that is, just over one-third of the body weight of a 70 kg man. Davis (1983) reviewed the literature of fatigue caused by load carriage and suggested that long carries by males of more than 25 kg should be avoided. 2. M a x i m u m anaerobic power and muscle strength

Anaerobic power and muscle strength are important for activities of high intensity for brief periods of time - i e, less than 1 min. According to surveys of the tasks which soldiers undertake in the US Army, a considerable proportion require muscle strength, e g, handling heavy weights such as artillery shells, pulling, pushing and throwing (NATO, 1986). Methods of measurement of anaerobic power are available - e g, the Wingate ergometer test (Bar-Or et al, 1980) -and some of these methods have been compared recently (Patton and Duggan, 1987). Isometric muscle strength can be measured using strain gauge dynamometers (Hermansen et al, 1972), and by using an isokinetic dynamometer, dynamic muscle strength and muscle endurance can be measured (Thorstensson, 1976). Maximal isometric lifting strength has been included in test batteries designed for the pre-employment screening of" workers for jobs involving heavy manual handling (Griffin et al, 1984). In a recent study on 49 infantrymen carrying 18 kg on a maximal effort 10 mile road march, Dziados et al (1987) found that although VO2 max correlated with performance time, in a step-wise multiple regression analysis only hamstring muscle strength emerged as a significant predictor of march time. 3. B o d y composition

The main body composition factors to affect load carriage are, firstly, the size of the lean body mass (i e, total weight minus fat) and secondly, the proportion of the total weight which is fat. Lean body mass is highly correlated with VO2 max (Buskirk and Taylor, 1957) and is a positive factor in load carriage ability. Conversely, excess body fat is dead weight in the performance of work and degrades the performance of physical tasks involving movement of the body and external load; hence the utility of expressing VO 2 max on a body weight basis as an expression of aerobic fitness. A man weighing 85 kg with 25% body fat is carrying about 21 kg of body fat; assuming he need only have about 9 kg of fat (i e, about 10% of body weight) for good health, this represents about 12 kg less of external load which he can carry.

4. Gender

Snook and Ciriello (1974a) determined maximum weights and workloads for females on six carrying tasks; although women handled significantly less weight than men they experienced similar or higher heart rates. In general, women will be at a disadvantage in load carriage tasks because, compared with men, they tend to have lower body weight, higher body fat, lower VO2 max and lower muscle strength, particularly in the arm muscles (Vogel and Patton, 1978). Evans et al (1980) compared fit young males and females in a self-paced load carriage task of 1 - 2 hours duration. Although the males had a higher absolute energy expenditure than the females, in relative terms the two groups were very similar at about 45% of the V02 max. Females were shown to change their gait characteristics (stride length and swing time) more than males as the carried load was increased, and Martin and Nelson (1986) concluded that absolute loads for fcmales should be lower because of biomechanical as well as physiological considerations. Pierrynowski e t al (1981 a) also found that increases of load (up to 34 kg) produced virtually no alterations in gait pattern in male subjects, but in contrast, Kinoshita (1985) showed that both light (20% of body weight) and heavy (40%) loads substantially modified the normal walking gait pattern. Davis (1983) showed that stability decreases when loads are held at and above the waist level, higher and greater loads decreasing the stability, and the stability in females was always less than males. Some differences between the sexes are revealed during load carriage in hot climates, which is discussed later. 5. Age

It has already been mentioned that VO2 max decreases with age (Hermansen, 1978). This effect of ageing is likely to be associated with the decline in maximal heart rate with increasing age (Robinson, 1938). Furthermore, VO2 max (ml/kg/min) will decline as the body fat (as a percentage of body weight) increases with age (Durnin and Womersley, 1974). Astrand (1958) investigated a group of truck drivers aged 5 0 - 6 4 years (VO2 max 2'5 1/min at a heart rate of 160 beats/min). During their daily work they were engaged in carrying 5 0 - 8 0 cases of beer (43 kg), or 100-125 cases (I 9 kg), work which required oxygen intakes up to 2"5 1/min. The subjects could not be considered to be representative of the population age group because it is probable that physically weak workers left before reaching age 50 years. Samanta et al (1987) found that the maximum permissible load carried on the head (as defined by the load at 35% VO2 max) by groups of Indian porters ranging in age from 20 to over 50 years, decreased from 41 kg for the youngest group to 11 kg for the oldest.

Subjective aspects of load carriage Subjective reactions to the task can be considered in terms of the application of ratings of perceived exertion (RPE) and other rating scales as measures of the acceptability or the severity of the task, also in terms of the psychophysical approach to manual handling developed by Snook (1978). Borg (1970) developed a scale to elicit RPE from the relationships between the physiological responses and the subjective ratings to different levels of work. Although in some circumstances RPE correlates with the heart rate, Davies and Sargeant (1979) considered that heart rate has little influence on RPE and is not an important factor in the perception of effort.

RPE is a useful tool for evaluating the severity of a load carriage task, or for comparing different methods of carrying a load (Legg and Mahanty, 1985). Pandolf (1977) emphasised the importance of both local and central factors. When increments in RPE with increases in loads carried were compared with increases in heart rate and oxygen consumption, it was found that the perception of exertion increased faster than the cardio-respiratory measures (Goslin and Rorke, 1986); thus 'local' factors, if accentuated by load carriage, may dominate the overall perception of exertion. Borg (1982) described a new category scale with ratio properties, using numbers anchored by verbal expressions which were simple and understandable to most people; ratings according to this scale correlated with exercise blood lactate levels and may prove useful in future load carriage studies. The scale of Corlett and Bishop (1976), although aimed primarily at work postures when using industrial machines, can be adapted to assess local discomfort on a load carriage task. Using this scale, Randle and Legg (1985) showed that local discomfort was lower in subjects walking uphill in hot conditions than when additionally carrying 20 kg in the arms at the same external work rate, presumably because of the greater static component in the carrying task. Snook (1978) has undertaken a number of studies on manual handing using a psycho-physical approach and has published tables of maximum acceptable weights for males and females from an industrial population to carry over short distances (up to 8.5 m), (Snook et al, 1970; Snook and Ciriello, 1974a). This information can contribute to guidelines for the design of manual handling jobs and possibly reduce the back injuries attributable to such tasks.

Environmental factors 1. Climate

la.Hot climates Kamon and Belding (1971) found no difference in the metabolic costs of carrying loads up to 20 kg in the hands in hot climates (35 and 45°C) compared with those in a temperate climate, but heart rate was found to increase by 7--10 beats for each 10°C in air temperature. Based on indices of physiological cost, fatigue and the need for rest pauses, 15 kg loads appeared to be most suitable for the subjects. Snook and Ciriello (1974b) showed that load carrying ability was reduced by 11% in a hot environment (WBGT 27°C) with significantly higher rectal temperature and heart rate. Krajewski et al (1979) investigated a load carriage task demanding 30% and then 75% of VO2 max in men and women in warm-humid and hot-dry ambient conditions in order to validate the length of rest periods required. Although both sexes worked at the same relative load, the males carried 12 kg (in the hands) and the females 10 kg, and the males achieved higher blood lactate levels than the females. Judging by the heart rate plateau and limits of core temperature rise, only the rest period spent in neutral conditions proved adequate. Durnin et al (1966) found that the metabolic rate, heart rate, sweat rate and body temperature of acclimatised subjects carrying loads were all elevated in hot-wet and hot-dry climates, compared with similar work in a temperate climate. 1b. Cold climates In cold climates, the energy cost of walking at standard speeds with loads is increased compared with that predicted

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for temperate ambient conditions, but this is possibly not because of reduced air temperature per se (Haisman, 1977). It has been shown (Amor et al, 1973) that the energy cost of walking in multi-layer, cold-weather clothing is increased by up to 20% over the same task wearing shorts with the weight of multi-layer clothing carried on the belt; similar results were reported by Teitlebaum and Goldman (1972). Thus, should higher levels of energy expenditure be found in cold climates, the extra energy expenditure is more likely to be attributable to the weight, hobbling and restrictive effects of multi-layer clothing than to the effects of the cold itself. 2. Terrain

Strydom et al (1966) showed that carrying loads over sandy surfaces required an energy expenditure 80% greater than over a firm surface. Soule and Goldman (1972) investigated a variety of terrains including smooth and dirt roads, light and heavy brush, as well as swamp and sand, and they compared the results with the control conditions of walking on a tread-mill to show how the energy cost increased with increasing severity of the terrain. Pandolf et al (1976) examined the effects of walking in various snow depths and found that the energy cost was increased by 5x with a footprint depth of 45 cm in the snow. Thus, combining the snow depth effect with the effect of multi-layered coldweather clothing already mentioned, it can be seen that this is an activity which results in very high rates of energy expenditure and the production of large amounts of heat. 3. Grade and stair climbing Gordon et al (1983) compared the effects of added

load (up to 50% body weight) on walking subjects, with unloaded walking. They found that added loads brought about larger increases in heart rate and RPE than did unloaded walking on grades for equivalent increases in power. Borghols et al (I 977) compared the effects of carrying weight up to 30 kg at 25%, 50% and 75% of the subjects VO2 max; the effect of added weights was the same when walking with and without grade on the treadmill. Orsini and Passmore (1951) examined the carriage of loads (up to 38 kg) up and down stairs and concluded that much of the energy expended is used in maintaining body posture in between steps. The postal delivery workers studied by Ilmarinen et al (1984) exceeded 50% of VO2 max whilst carrying relatively light loads of mail upstairs at their own pace and it was recommended that such work be limited to 2 hours per day. Other factors affecting load-carrying ability 1. Sleep loss

In a series of studies on the effects of reduced sleep on military performance, Haslam (1984) has shown that the tasks worst affected are those requiring cognitive ability, especially sustained attention; physiological function, particularly in the performance of work, appears to be little affected. Some changes in physiological function during work have been found in other studies. Takeuchi et al (1985) examined a range of physical performance tests and found decrements after 64 hours of sleep loss only in vertical jump height and isokinetic strength. Martin and Chen (1982) found that after 50 hours of sleep loss, time to exhaustion by walking at 80% VO2 max was reduced by 20%. Soule and Goldman (1973) looked for changes over time in subjects carrying loads of 15 or 30 kg during one hour of self-paced work in every period of 6 hours for a total

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of 31 hours without sleep; there were no significant differences in walking speed for the 30 kg load and although the perceived exertion ratings showed a clear trend to increase, this only became significant with the 15 kg load. The combined effects of sustained manual work and partial sleep deprivatiorl (3 to 4 hours sleep per night over 8 days) on muscular strength and endurance was investigated by l.egg and Patton (1987). Isometric hand grip strength decreased and lower body anaerobic power increased over the trial period for all subjects, but in only the experimental group who handled heavy loads during tlie 8 days did the upper body anaerobic power fall. Murphyet a1(1984) investigated changes in anaerobic power capacity in infantrymen who engaged in a 5-day exercise allowing 4 hours sleep per day. They found a decrease in upper body/elbow flexor performance which may have been associated with the continuous load bearing of 28 kg over the 5-day period. 2. Protective clothing

A series of experiments has been conducted to study the effects of wearing protective clothing with loads in a range of temperate to hot environments (Gooderson, 1981). The results provide a guide which allows an appropriate work rate to be selected for a particular clothing assembly m different levels of climatic stress. As might be anticipated, the increased heat production associated with heavier loads exacerbates the heat stress when wearing such clothing. The effects of wearing multi-layer armoured vests (part of a total load of 25.6 kg) were investigated in hot-wet and hot-dry climates by Haisman and Goldman (1974). With impermeable garments, such as body armour, the amount of air movement under the garment associated with the movement of the wearer is important for the elimination of heat. Shvartz (1975) described methods of using conductive cooling for men wearing impermeable and other protective clothing in different work situations. Prediction of physiological strain involved in load carriage The energy cost of walking with loads has been found to be dependent primarily upon the speed of walking, the weight of the body and the load, and the gradient, and there are several equations available to predict energy cost from these variables. Goldman and Iampietro (1962) combined data from their own subjects with those from the literature to predict the energy cost of walking with loads up to 30 kg. They concluded that the energy cost per unit weight is essentially the same whether the weight is of the body or the load, but Myo-Thein et al (1985) showed that the increment in the energy expenditure of walking was lower for load weight (10% of body weight) than the energy expenditure per kg of body weight. Other equations have been derived for example, Durnin and Passmore (1967) derived an equation for walking on level ground; Workman and Armstrong (1963) and van der Walt and Wyndham (1973) predicted the energy cost of walking from body weight and speed; and the usefulness of other variables such as leg length, stride length and frequency has also been examined. Givoni and Goldman (1971) derived an equation using body and load weights, walking speed, slope and a terrain factor. A wide range of speeds and grades was included viz, walking 2"6-9 km/h and up to 25% grade, running from 8 17 km/h up to 10% grade, and loads up to 70 kg. Modifying coefficients were suggested for terrains other than the treadmill, for load placement if not carried on the trunk

Table 2: Metabolic rate (watts) as a function of march rate on a level tarmac road (for 70 kg man carrying 30 kg load including clothing) March rate (m/s)

0"69 0.83 0.97 1.11 1.25 1.39 1"53 1.67 1.81

Metabolic rate(watts)

213

245

283

327

376

431

493

560

633

Table 3: Energy cost of walking (watts) at a given speed (1.6 m/s) for various level terrains (Pandolf etal, 1977). (70 kg man with no load) Terrain

Tarmac road

Dirt road

Light brush

Hard snow

Heavy Swampy Loose brush bog sand

Soft snow

Soft snow

(15 cm) (25 cm) Energy cost (watts)

374

401

428

454

and for very heavy levels of work. The mean standard error of estimate over all conditions was 29 kcal/h. Soule and Goldman (I 972) investigated terrain coefficients, and Pandolf et al (1977) modified the equation to include walking speeds down to standing still. Most recently, Epstein et al (1987) developed an equation for predicting the metabolic cost of running with and without back-pack loads. The equation of Pandolf et al (1977) has been compared with observed data (Pimental and Pandolf, 1979) and was found to predict slightly high.for standing with loads, and low for slow speed walking; it is not useable for negative grades. Pimental et al (1982) showed that the equation was accurate for grades up to 10% at a speed of 1-12 m/s, but the predictions were too low (5-16%) for a slower speed on grades and for level walking (14-33%). Duggan and Ramsay (1987) found good agreement between the predicted values and observed energy expenditures of walking at 1 "67 m/s on a level treadmill with and without a 21 kg load; the predictions were on average 3% too high. Using this equation (Pandolf et al, 1977), the relationships between energy expenditure, march rate, grade, load weight and terrain can be examined. For example, Table 2 shows the effect of increasing speed for level walking with a load of 30 kg. In a previous section the effect of terrain was discussed. Table 3 shows the effect of different terrains at a standard speed for no load - for example, walking on loose sand is almost 80% more costly than walking on a tarmac road. Similarly, the effect of increasing grade and load is shown in Table 4. Other physiological parameters besides the energy expenditure of load carriage have also been estimated. Givoni and Goldman (1972) developed equations to predict the rectal temperature response to work, environment and clothing. This method of body temperature prediction has been compared with three other models (Haslam and Parsons, 1986). Predictions of rectal temperature have been used to estimate heart rate (Givoni and Goldman, 1973). The metabolic heat production is a major contributor to the problem of maintaining acceptable levels of deep body temperature and heart rate, particularly if evaporative skin cooling is limited by protective clothing in high ambient humidity.

508

589

669

785

1005

Prediction of physiological cost of industrial load carriage tasks The prediction models described in the previous sections were, in general, designed for steady load carriage. In the industrial setting it has been shown that most loads are carried intermittently (Drury et al, 1982). Randle (1987) compared four methods of predicting the metabolic cost of load carrying in the arms, including the Givoni and Goldman (1971) method; all four methods were inappropriate for an intermittent load carriage task and a revised model was proposed. Garg et al (1978) adopted an approach for estimating metabolic rates for manual materials handling jobs. Based on partitioning the jobs into component parts, the system has many applications, including investigation of the duration and frequency of rest periods and the comparison of alternative work methods. Load carriage and performance

An important consideration is how the load carried effects the performance of a task. Renbourn (1954b)noted the extreme example of the British infantry at Cambrai in 1917 who were so exhausted by their great loads that they were unable to take advantage of the first mass attack by tanks. Marshall (1950) described similar problems with American troops. Bensel and Lockhart (1975) found that load carriage equipment degraded body flexibility compared with a control condition. Lotens (1982) took the view that performance decrement due to carried load is dependent on weight and that the method of suspension (within limits) is of minor importance. The effect of increasing load weight on march-rate when self-pacing over 6"4 km can be seen (Table 5) (Hughes and Table 4: Energy cost of walking (watts) at a given speed (1"34 m/s) for loads of 0, 20 and 40 kg at various grades (Pandolf et al, 1977) (70 kg man) Grade

(%)

0

4

8

12

16

Load

0

294

425

556

687

819

20

362

531

700

868

1037

40

473

679

886

1092

(kg)

*

* outside the physiological range for young, healthy men

Applied Ergonomics

June 1988

117

Goldman, 1970). As the toad weight increases, the speed decreases proportionately, and the average energy cost per unit distance marched was found to be lowest for 30 40 kg of load. When the effects of carrying light and heavy loads (10 and 40% of body weight) at the same energy expenditure level were compared (Myles and Saunders, 1979), the heavier load was found to produce an extra strain on the cardiopulmonary system and was perceived by all subjects as harder work. There is, therefore, likely to be an effect on performance due to the weight carried, irrespective of an effect linked to total energy expenditure.

circumstances, but for healthy young males there appears t~ be some consensus for the traditional rule of thumb ,~t"onethird body weight, or 24 kg oll an assumed mean bod~ weight of 72 kg, or in terms of relative work load equivalent to one-third of the VO2 max for a working day. Renb~mr, (1954c) considered that the load carried by the soldier will probably always be a compromise between what is physiologically sound and what is operationally essential. Load carriage in industrial and other civilian areas will also involve a similar compromise between the person's capabilities and requirements of the task which may in some circumstances have important implications for health and safety.

Optimal load The optimal load is an elusive concept. Pierrynowski et al (1981 b) considered the minimum energy cost per unit of mass carried and distance covered, but they argued that the critical point concerned definition of the load (i e, the load or load + body mass). When the back-pack load only was taken into account, they recommended 40 kg as an optimal load decreasing to 7 kg when the defined load included the entire body mass. Legg (1985) examined six separate load carriage studies and considered that there was seldom a single 'best' way to carry a load. In the Hughes and Goldman (t970) self-pacing study quoted above, the lowest energy expended per kilogram-metre is for a rather high load in the range 44-59% of nude body weight. It may well be impossible to define an optimal load in isolation from other relevant factors such as the velocity, grade, climate, clothing and nature of the terrain. Total energy expenditure of the task integrates some of these factors and there is some evidence that fit male subjects will self-pace at an energy expenditure of 425 kcal/h (494 watts) -+ 10% (Hughes and Goldman, 1970; Levine et al, 1982). Other investigators such as Myles et al (1979) preferred to use relative workload when they found that fit young soldiers self-paced at 3 0 40% of VO2 max over a 6-day period, but in absolute terms the mean energy expenditure of 384 kcal/h was just within the 10% limits of 425 kcal/h. In 1950, Lippold and Naylor set out four essentials in the design of any load carriage equipment to ensure a minimum expenditure of energy: (1) elimination of local strain, (2) maintenance of normal posture, (3) maintenance of a normal and free gait, and (4) chest freedom. These essentials still hold today in the design of military or civilian equipment and should be considered in the definition of optimal load.

Acknowledgements It is a pleasure to thank colleagues at APRE and elsewhere who have advised during the evolution of this paper and to Dr R.F. Goldman for his suggestions on the original manuscripts. References Amor, A.F., and Vogel, J.A. 1974, Energy cost of manpacking the Swingfire missile. Army Personnel Research Establishment Report 5/74. Amor, A.F., Vogel, J.A., and Worsley, D.E. 1973, The energy cost of wearing multi layer clothing. Army Personnel Research Establishment, Tech Memo 18/73. Astrand, I. 1958, The physical work capacity of workers 5 0 - 6 4 years old. Acta Physiol Scan& 42, 73-86. Astrand, I. 1960, Aerobic work capacity in men and women with special reference to age. Acta Physiol Scan& 49, suppl 169. 1 92. Astrand, I. 1967, Degree of strain during building work as related to individual aerobic work capacity. Ergonomics, 10(3),293 303. Astrand, P.O. 1956, Human physical fitness with special reference to sex and age. Physiol Rev, 36,307-335.

Balogun, J.A., Robertson, R.J., Goss, F.L., Edwards, M.A., Cox, R.C., and Metz, K.F. 1986, Metabolic and perceptual responses while carrying external loads on the head and by yoke. Ergonomics, 29(12), 1623-1635.

Bar-Or, O., Dotan, R., Inbar, O., Rothstein, A., Karlsson, J., and Tesch, P. 1980, Anaerobic capacity and muscle fibre type distribution in man. Int J Sports Med. 1, 82-85. Bensel, C.K., and Lockhart, J.M. 1975, The effects of body armour and load carrying equipment on psychomotor performance. Natick Tech Report 75-92-CEMEL.

Conclusion

Bink, B. 1964, Additional studies on physical working capacity in relation to working time and age. Ergonomics, 83-86. Proceedings of 2nd International Ergonomics Congress, Dortmund.

Various aspects of load carriage have been reviewed. The literature confirms that there is no easy solution in the definition of a maximal load, because of widely varying

Table 5: Weight of load, energy cost and speed when self-pacing over 6"4 km. Compiled from

Hughes and Goldman (1970) with permission from the author and the J of Appl Physiol Weight of load (kg)

118

0

20

30

40

50

60

Speed km/h (approx)

8"0

6'5

5-8

5.2

4'3

3.7

Energy cost kcal/h

587

469

457

448

395

386

Energy cost per unit distance kcal/kg/m

1"04

0'83

0.79

0'79

0.84

0.84

Applied Ergonomics

June 1988

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