Influences of task posture on physiological task responses

International Journal of Industrial Ergonomics, 1 (1987) 209-217 Elsevier SciencePublishers B.V., Amsterdam- Printed in The Netherlands

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INFLUENCES OF TASK POSTURE ON PHYSIOLOGICAL TASK RESPONSES Stephen J. Morrissey Industrial Engineering Department, Auburn University, AL 36849-3501 (U.S.A.)

(ReceivedJune 30, 1986; acceptedin revisedform October24, 1986)

ABSTRACT This paper reviews selected research on how task posture may effect task demands. It was found that task performance in stooped or nonerect task postures can result in higher task heart rates, metabolic costs, and ratings of discomfort and fatigue. However it was also found

that the type of task performed and the effective work rate wilt have the greatest impact on the subjective, biomechanical, and physiological strains experienced by the worker. Areas needing further research were identified and discussed.

1. INTRODUCTION

2. STATIC LOADS, STATIC POSTURES, AND TASK RESPONSES

In assessment of task demands there is available a considerable body of literature to help to evaluate whether a task or its elements are acceptable for a desired worker population. However, these reference data are based on operator performance in normal upright task postures. The influence of task posture on overall task demands and worker capacities is not as well documented. This paper will systematically review how a worker's physiological reactions to a task can be changed by workspaces in which the/worker cannot adopt a fully erect posture and must therefore bend, stoop or kneel to perform a task. Tasks performed in workspaces that are narrow but which have adequate head room are discussed by Mital (1986) and are not covered in this paper. 0169-8141/87/$03.50

Posture, the manner in which we carry or hold our bodies can influence task performance and work capacities. Numerious authors discuss changes in physiological responses caused by changes in body postures apart from the normal erect posture. These studies traditionally report that as the body posture changes from erect to lying down or sitting, there are corresponding changes in lung volumes, breathing rates, blood pressure and heart rates (Moreno and Lyons, 1961; Astrand and Rodahl, 1977); significantly lower strength capacities (Hafez et al., 1982; Chaffin andAndersson, 1984); and increases in reports of neck, back, and shoulder discomfort (Boussenna et al., 1982; Chaffin and Andersson, 1984).

© 1987 ElsevierSciencePublishers B.V.

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2.1 Measurement of task postures, task responses In the evaluation of a worker's responses to work in non-erect task postures, classification of posture as erect, sitting or lying down are inadequate since such classifications cannot adequately describe the range of bent or stooped postures caused by work environments with restricted headroom or workplace designs that force the worker to adopt a stooped posture. To describe these task postures, three primary approaches have been used. The first is to label a working posture in general as being stooped, kneeling, or lying down. The second approach is to identify the ceiling height under which workers must work and then describe the resulting task postures (e.g., the ceiling height was 132 cm, with workers kneeling on one knee). The third method is to describe the resulting task posture in terms of a percentage reduction from normal upright erect task posture or stature (e.g., workers had a stooped posture equivalent to a 20% reduction in stature). Each of these methods is used in reporting data, though findings are less than predictive whatever method is used. Contributing to the lack of overall predictability is that dif-

ferences in individual body structure and back strengths can result in two workers of approximately the same stature experiencing very different levels of biomechanical, physiological, and subjective strain during the "same posture" as defined by any of the above descriptive methods (Jorgensen, 1970; Boussenna et al., 1982). In assessing worker reactions to task postures, the following techniques have been used: Subjective opinion or rating of discomfort or fatigue; measurement of changes in task heart rate; changes in muscle electrical activity or electromyographic studies (EMG); measurement of the oxygen uptake, or the amount of metabolic energy needed to perform a task; and, changes in blood pressure. Each method has its advantages, but all are measuring the changes in the workers body due to the additional static strain of working in a non-erect task posture. 2.2 Reactions to static tasks A static load occurs whenever a limb or body posture is fixed for a period of time. A static posture tends to prevent the free flow of blood to the working muscles which increases anaerobic metabolic processes, and is

TABLE 1 Responses to erect and stooped task postures ~ Ceiling height

Sex

Heart rate (b/min)

Ventilation volume (1/min)

Adjusted oxygen uptake (ml/kg-min)

None

Male Female Male Female Male Female Male Female Male Female

89.2 89.7 96.0 107.5 86.8 92.0 82.2 89.9 88.5 100.5

10.6 9.6 12.8 12.4 13.9 12.0 13.2 11.0 17.0 16.2

5.0 4.4 5.7 5.8 6.8 5.8 6.8 6.0 8.3 8.1

90% of normal 80% of normal 70% of normal 60% of normal

Adapted from Morrissey et al. (1985). All subjects were accustomed to these postures and stood quietly with their arms behind their backs for 10 minutes.

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thus more fatiguing than dynamic postures in which motion occur. Static postures have been found to lead to higher blood pressures, heart rates and oxygen uptakes than are observed in dynamic activities (Wald and Harrison, 1975; Astrand and Rodahl, 1977). Further, whenever a worker is in a stooped posture, the back muscles which are used for general posture support have to maintain an additional static load to balance the upper body in the stooped posture. This results not only in greater forces in the back, but also greater torques which have to be sustained by the major body joints to help in posture support (Jorgensen, 1970; Boussenna et al., 1982). Jorgensen (1970) found that the workers' ability to maintain a stooped task posture such as that required with work stations where the worker has to lean forward, primarily depended upon their isometric back strengths. He concluded that most workers could maintain a stooped corresponding to a 20% reduction in height for extended periods of time (over 15 minutes) without excessive discomfort. Boussenna et ah (1982) studied subjective discomfort and ability to hold stooped task postures, and found significant negative correlations between the degree of stoop and reports of muscle discomfort and the maxim u m time the posture could be held. To study the physiological responses due to prolonged standing in stooped postures, Morrissey et al. (1985) had male and female subjects accustomed to working in stooped postures, stand quietly for 10 minutes under ceilings set at heights corresponding to 100%, 90%, 80%, 70%, and 60% of each subject's stature. Results of this study are summarized in Table 1 and show little overall change in the task heart rate with changes in posture. There were increases in ventilation volume and oxygen uptake as task postures became more stooped, but these changes were not significant nor highly correlated with posture. To examine how a combined task of holding loads while in stooped postures would

TABLE 2 Percentage increase in heart rates with posture Ceiling height 90% of normal 80% of normal

a

Load in kg 4.54

9.1

13.6

0.0

4.1

17.5

2.6

17.0

26.0

a Adapted from Morrissey (1986). Subjects were accustomed to standing in these postures while holding weights. Weights were compact, and held against body when in the postures for six minutes. Data are the precentage increase in the task heart rate compared to the task heart rate for the same load when held in a normal erect posture.

affect physiological responses, Morrissey (1986) had males who were accustomed to working in non-erect postures, hold compact loads against their bodies while standing under ceilings set at heights corresponding to 100%, 90% and 80% of their stature. Heart rate data resulting from these tasks is given in Table 2. It was found that when the load exceeded 4.54 kg within any posture, or the posture went below 90% of normal stature with a fixed load, there were large increases in the task heart rates.

2.3 Task postures and effective strengths The ability of humans to generate force depends upon the position of the body element or limb used (Williams and Stutzman, 1962; Hafez et al., 1982). Stooped body postures can also result in significantly different isometric and dynamic strenghts when compared to those measured in normal erect postures. Ridd and Davis (1980) measured the isometric arm strength of working coal miners in a normal erect posture and then in nonerect task postures. Compared to erect posture values, strengths decreased by 46% when the subjects were under ceilings set at a height that corresponded to 90% of their statures,

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and by 61% when under a ceiling set at 80% of their statures. When kneeling on one knee, with straight backs, the reduction in isometric strengths was found to be only 7%. Davis and Troup (1966) studied the intraabdominal pressures, or IAP (an index of the forces being exerted within the body) resulting from coal miners erecting heavy jacks (35 kg--56 kg) in work environments with ceilings at 107 cm or 137 cm. There were significant differences in the lAP's due to the task posture and ceiling heights. However it was suggested that the interaction between the subject's anthropometry and the ceiling height was probably as important in determining the results as the actual loads handled. Forces available for pushing or pulling tasks are also reduced in non-erect or stooped task postures. In these tasks reduction in posture is often due to handle placement which forces the worker to bend over while pushing or pulling. For these tasks, there are two primary forces involved: an initial force to get the object moving, and a sustaining force to keep it moving. In both cases, the optimal position for the handle is found to be about waist height, which requires a slight stoop.

The actual forces that can be exerted in these tasks also depends upon how often the load is moved, and for what distance. Extensive sets of data describing the forces that can be exerted in these tasks can be found in Snook (1978), Ayoub and McDaniel (1974), and in Chaffin et al. (1983).

3. POSTURE AND PERFORMANCE OF SIMPLE DYNAMIC TASKS 3.1 Stoopwalking-crawling tasks Stoopwalking is required whenever a worker walks with ceiling conditions that do not permit an erect posture. Two field studies of coal miners stoopwalking in a posture corresponding to a 20% reduction in standing height found increases in the metabolic cost of activity of 6% (Humphries et al., 1962) and 20% (Moss, 1935). The differences in metabolic cost were due to differences in the speed of walking and surface conditions between the studies. When stoopwalking was performed in a posture that corresponded to a 40% reduction in stature, increases in meta-

TABLE 3 Physiological responses to stoopwalking-crawling tasks ~ Ceiling height

Sex

Heart rate ( b / m i n )

Ventilation volume (l/rain)

Adjusted oxygen uptake (ml/kg-min)

Normal posture 90% of normal 80% of normal 70% of normal 60% of normal

Male Female Male Female Male Female Male Female Male Female

95.6 108.3 112.6 124.1 117.7 125.6 126.6 139.5 127.8 147.6

19.8 20.5 26.6 25.1 31.8 28.2 38.1 36.7 39.4 41.5

11.3 10.9 12.9 12.8 17.3 15.7 20.8 20.7 22.3 22.3

Adapted from Morrissey et. al (1985). Subjects were accustomed to working in these task postures. Speeds tested were from 5.6 km per hour to 2.4 km per hour in the no restriction to 70% of stature conditions; 2.4 km per hour to 1.2 krn per hour in the 60% condition. Data are the steady state values existing after at least four minutes of task performance. a

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bolic costs were found to be 60% (Humphries et al., 1962) and 62% (Moss, 1935). In a laboratory study of stoopwalking tasks, Morrissey et al. (1985) had male and female subjects who were accustomed to stoopwalking tasks, stoopwalk in a range of task postures with different walking speeds. The resuits (Table 3), found large and often significant increases in all measures of the physiological costs of the tasks. Analysis of the data found that it was not the task posture that led to significant increases in task demands, but it was instead, the speed at which subjects walked within each posture. Further evaluation of the results indicated that there were significant increases in all physiological measures when subjects were required to stoopwalk at "normal (posture) speeds" of from 1.34 metres per second to 1.56 metres per second (3 to 3.5 miles per hour), when ceilings were set at levels below 90 percent of their stature. The results from these three studies all indicate that if stoopwalking is a significant part of a job, then workers should not be expected to walk at normal posture speeds. If this cannot be done, then ad-

ditional time for recovery or rest must be provided if excessive fatigue is to be avoided.

3.2 Shovelling Shovelling in erect and non-erect task postures is a commonly performed task or job element. Studies of the physiological demands of shovelling are well established (Passmore and Durnin, 1955), but studies of shovelling in non-erect task postures are not as common. Humphreys et al. (1962) studied coal miners shovelling coal while on their feet, on their knees, and when lying down. The average metabolic costs of shovelling was uneffected by posture when miners shovelled at a selfpaced rate of about 25 scoops per minute. When shovelling at a rate of about 49 scoops per minute, the cost in the kneeling posture was 29% higher than in the erect posture. Morrissey et al. (1983) had male and female subjects, who were first accustomed to shovelling in non-erect task postures, shovel in a range of postures from upright erect to a stooped and kneeling posture. Results of this study are summarized in Table 4 and indicate

TABLE 4 Physiological responses to shovelling tasks

a

Ceiling height

Sex

Heart rate (b/rain)

Ventilation volume (1/rnin)

Adjusted oxygen uptake (ml/kg-min)

Normal posture 80% of normal 60% of normal Upright kneeling height 80% of kneeling height

Male Female Male Female Male Female Male Female

126.7 125.5 130.7 123.4 137.5 153.9 123.7 132.1

36.2 26,2 38.3 25,8 39.7 28.0 34.0 25.8

20.2 15.6 20.5 14.7 20.8 15.6 17.8 13.9

Male Female

127.9 122.8

33.2 24.6

16.9 11.1

Adapted from Morrissey et al. (1983). Subjects were accustomed to working in stooped task postures, and shovelling rate was fixed at 17 scoops per minute. Data are the steady state responses exsisting after at least five minutes of task performance. a

214 that while the resulting task heart rates were affected by the task postures, there was little change in the oxygen uptake with task postures. The finding of only small differences in the oxygen uptake across the task postures indicates that subjects were able to regulate their actual work load, which was the amount of material moved in each shovel load, to match the additional task strains due to the stooped postures. Observations of material shovelled by the subjects confirmed that there were differences in the amount of material shovelled as related to task posture. The dependence of the task heart rate on the task posture was due to the increased static loads required in posture control and the performance of heavy arm work, both of which are factors that may lead to increased task heart rates.

3.3 Materials carriage tasks Morrissey (1984) and Morrissey and Liou (1984) studied the effects of task posture on worker reactions to manual materials carriage tasks (MMC) while performed in stooped postures. In the first study, males conditioned to both MMC tasks and stoopwalking tasks, carried loads of varying weights (2.3 to 18.1 kg) for varying distances (1 to 5 meters) with varying rates of carriage (from one carry every 12 seconds to one carry every 90 seconds), under a range of ceiling heights set between 100% and 80% of each subject's stature. Analysis of the resulting task heart rate data found an overall lack of change in the task heart rates above the normal posture values. A large but not significant increase in task heart rate was found in only one condition which was at the fastest rate of load carriage (one carry every 12 seconds). The lack of heart rate elevation indicated that subjects were able to adjust their actual work rate through control of their walking speed, which compensated for the differences in task load, posture, and distance walked. The effect of this work rate

control was to give subjects additional recovery time between the actual task carriages. Only at the fastest rate of load carriage were the subjects unable to gain sufficient time to recover between actual carriages. In the second study of load carriage tasks, Morrissey and Liou (1984) had highly trained males perform a carriage task on a treadmill, so that a constant work load (constant task speed) could be generated at each of the task posture loadings. For the range of container widths (15.2 to 30.5 cm), postures (normal posture to stooping under a ceiling set at at 90% of stature), loads (2.3 to 18.1 kg) and treadmill speeds (4.0 to 6.4 km per hour), there were significant increases in the task heart rate and oxygen uptake, which were found to be a function of the work load and the task posture. It was suggested that if excessive physiological strains and the resulting fatigue were to be avoided in stooped and paced tasks, then work loads must be reduced.

3.4 Posture and more complex tasks In a study of a more complex task performed in non-erect postures, Voss (1973) used a simulted strawberry picking task to study how task posture influenced task performance and the metabolic costs. In the task, subjects picked up small metal tags from the floor and placed them in a bowl held in one hand while maintaining a fixed posture and rate of forward movement. Postures studied were bent at the waist with and without arm support, kneeling with one knee on the floor with and without arm support, and squatting. Performance, defined as the number of metal tags picked up per minute, was not affected by the task posture or the speed of forward movement. The metabolic cost of activity and task heart rates were significantly affected by the posture and to some extent, by the speed of forward movement. The bending and kneeling postures performed

215 without arm support required 25% to 55% more energy and bad significantly higher task heart rates than the same postures when postures when performed with arm support which lowered the effective static loadings. In this study, task heart rate was found to be the most sensitive index of task posture strain. In a simulation of an escape from an underground coal mine, Kamon, Doyle and Kovac (1983) had experienced miners crawl, stoopwalk and walk along a passageway which had ceiling heights ranging from no restrictions to crawling height only. It was found that this activity, even under non-emergency conditions and with subjects accustomed to performing the task, resulted in miners experiencing very high levels of physiological strain and considerable fatigue.

4. DISCUSSION AND CONCLUSIONS This paper has shown that the posture in which a task is performed can have a definite impact on the resulting physiological task demands and strains experienced by the worker. The factors that determined the total strain experienced by the worker and resulting physiological reactions are: (a) Type of task performed. (b) The work rate within the posture. (c) The interaction of the worker's anthropometry (body size) with the task posture. It was concluded that if the task to be performed is self-paced, that is the workers can adjust their effective work rates through changes in task pace or in the amount of work done per unit time, then the metabolic costs may stay relatively stable across most task postures. However, task heart rates will show some dependency upon the task posture if the tasks require arm and upper body work, such as are needed in shovelling, materials handling, force exertion, and materials carriage. The degree to which changes in the task heart rate will occur with non-erect task pos-

tures is not consistent, but it may be related to a percentage of the individual's maximum capacity to tolerate the biomechanical and physiological strains that are generated in these tasks and postures. In general, task heart rate appears to be the most sensitive index of the overall physiological and posture strain experienced in stooped posture tasks that involve upper body or arm work. In tasks that are paced, or have a fixed rate of task performance such as is required in a treadmill or in fixed rate load carriage tasks, stoopwalking at designated rates, or in some types of materials handling, then physiological costs of activity will increase significantly as the postures become more stooped. Based on the studies reviewed in this paper, the increases in the physiological cost of activity above the normal posture levels can be expected to be large, and often significant, when tasks are performed under ceilings that are below 90% of the workers stature, and which also have performance rates or standards based on performance in normal postures. With these fixed rate tasks, care must be used to insure that the non-fatigued work capacities of the workers are not exceeded. The reports by Ridd and Davis (1980) on changes in effective strengths; by Moss (1935), Humphreys and Lind (1962), and Morrissey et al. (1985) on stoopwalking tasks; by Morrissey (1984) and Morrissey et al. (1985) on materials carriage tasks can provide additional information on expected levels of stress as a function of task posture and performence rate. It is also possible that recovery from work in non-erect paced tasks may require more time than from the normal posture tasks due to the high static loads and greater lactic acid concentration in the tissues. While there are no direct tests of this question, studies of miners have suggested this to be true (Moss, 1935; Humphreys et al., 1962; Ayoub et al., 1981). In tasks that require the exertion of force, or strength, non-erect task postures have been

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shown to result in very large drops in the maximum available force that can be exerted. In their study of strengths in non-erect postures, Ridd and Davis (1980) noted that teaching workers to change their task postures, from stooped to kneeling, could lead to higher strength capacities and possibly, to lower rates of back injury. Such a suggestion is clearly useful, as it has been known for some time that there is an increased rate of back injury in worker populations that work in stooped task postures (Lawrence, 1955; Ayoub et al., 1981). In tasks that require the worker to maintain a force for an extended period of time, such as is required to support the body in stooped task postures, there is a significant relationship between the time for which the posture can be held and the workers isometric back strengths (Jorgensen, 1970). Finally, while not directly addressed in this review, the impact of the workplace environment on the worker's responses must also be considered. It is known that the use of personal protective equipment such as a respirator can markedly change the physiological demands present in a task (Raven et al., 1979; Harber et al., 1984). Further investigations examining the interactions of task posture and personal protective equipment are needed. Similarly, the interaction of heat and task postures should be investigated, as heat is known to change the physiological reactions to a task (Kamon, 1980), primarily by elevating the task heart rate.

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