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Assessment of physical work load in epidemiologicstudies: concepts, issues and operationalconsiderationsJØRGEN WINKEL a & SVEND ERIK MATHIASSEN aa National Institute of Occupational Health, Department of Physiology , Division of AppliedWork Physiology , Solna, S-171 84, SwedenPublished online: 31 May 2007.

To cite this article: JØRGEN WINKEL & SVEND ERIK MATHIASSEN (1994) Assessment of physical work load in epidemiologicstudies: concepts, issues and operational considerations, Ergonomics, 37:6, 979-988, DOI: 10.1080/00140139408963711

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Assessment of physical work load in epidemiologic studies: concepts, issues and operational considerations

JBRGEN WINKEL and SVEND ERIK MATHIASSEN

National Lnstitute of Occupational Health, Department of Physiology, Division of Applied Work Physiology, 5-171 84 Solna. Sweden

Keywords: Epidemiology; Musculoskeletal; Physical work load; Review; Terminology.

Ergonomic epidemiology is a rapidly increasing field of research providing data on the occurrence of musculoskeletal disorders and possible risk factors. The present paper states, on the basis of a literature overview, that physical work load (mechanical~exposure) is poorly defined and measured in most studies on ergonomic epidemiology. On this background the paper: (1) suggests adjustments of mechanical exposure concepts and terminology; (2) concludes that invalid exposure assessment may. to a large extent, explain the lack of quantitative data on relationships between mechanical exposures and musculoskeletal disorders; and (3) suggests some guidelines for future quantitative assessments of mechanical exposure in large populations.

1. Background Work-related musculoskeletal disorders constitute a major world-wide problem (Armstrong et al. 1993). Generally, three types of risk factors are investigated and discussed: physical, psychosocial and individual. As a whole, the individual factors (e.g., age, gender) have received most scientific attention (cf. Winkel and Westgaard 1992), but their predictive value seems to be low (e.g. BattiC 1989, Armstrong et al. 1993). During the 1980s the emphasis in ergonomic epidemiology was put on the physical factors. However, research failed to provide convincing evidence of the etiologic importance of these factors (e.g. Hadler 1990, Nachemson 1992). The main focus has now turned to the psychosocial domain, and several researchers claim such factors to be the major explanation to 'work-related' musculoskeletal disorders (e.g. Hadler 1990, Nachemson 1992).

This may, however, be a premature conclusion owing to the insufficient concepts and operational methods used in most epidemiologic studies of physical factors (Winkel and Westgaard 1992). The aim of the present paper is therefore to (1) suggest adjustments of concepts and tenninology regarding physical factors, (2) discuss their low predictive values in the light of the prevalent methods and procedures for exposure assessment in epidemiologic studies and (3) suggest some guidelines for future quantitative assessments of physical factors in large populations.

2. Exposure terminology and definitions

2.1. Conceptual definition of mechanical exposure 'Physical factors' causing work-related musculoskeletal diseases are presupposed to exert their effects through 'physical' (mechanical) forces arising in the body (i.e., the 'physical work load'). These forces may in turn initiate or contribute to

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the pathophysiological changes. Conceptually, physical work load ('mechanical exposure') may therefore be expressed in terms of biomechanical events occurring in the body (internal exposure, see below). Events occumng external to the individual are only important as long as they bring about mechanical forces in the body.

Conceptually, mechanical exposure may be expressed through an unlimited number of force vectors. one for each point in the body, and each one varying in magnitude and direction according to time. Thus, a complete description of mechanical exposure comprises continuous real time registrations of the magnitude and direction of all exposure vectors.

2.2. Fundamental operationalization of exposure A thorough operational docuinentation of mechanical exposure in a target tissue may comprise continuous real time registrations of representative forces (figure 1 (a)). This kind of exposure presentation contains an unlimited amount of information which

0 Time

Force 4

0

Duration

( 0 ) (b) Figure 1. (a) Real-time registration of mechanical exposure (force) containing an unlimited

amount of information. (b) Illustration of the three primary dimensions suggested to be used when reducing the exposure data presented in (a). Definitions in text.

I External Exposure I

. , Internal Exposure

Active Internal Exposure modifiers

I Chronic Effect Injury Improvement

Figure 2. An exposure-effect model focusing on mechanical exposure. Each step in the model may be modified by individual factors as well as the surrounding environment (e.g., temperature, lighting, psychosocial factors). The model shows some feed-back loops which may modulate the exposure.

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Assessment of physical work load in epidemiologic studies 98 1

cannot be described and evaluated statistically in a handy way. Further data reduction is required.

In traditional ergonomic epidemiology several exposure concepts are derived from chemical epidemiology and, ultimately, from the science of radiation. Exposure is here primarily quantified in terms of concentration (level) and dose (amount, i.e., the product of level and time). However, these two concepts do not consider the temporal pattern of exposure delivery (the repetitiveness), which seems to be of particular physiological significance (e.g., Mathiassen 1993a).

A sufficient operational quantification of mechanical exposure may therefore comprise three main dimensions: level (amplitude), repetitiveness (frequency), and duration (figure 1 (b)). Exposure level refers to the magnitude of the mechanical force, repetitiveness to the frequency of shifts between force levels, and duration to the time extension of the exposure. The fundamental SI units are Newton (N), s - ' and s respectively.

2.3. An exposure-effect model Figure 2 presents an exposure-effect model focusing specifically on mechanical exposure. External exposure refers to factors in the work environment which may give rise to mechanical exposure in the body, The external exposure is by definition independent of the operator. Examples of operational measures are weight of parts to be lifted into a machine (level), lifting frequency required to feed the machine (repetitiveness), and ordinary working hours of the machine (duration).

The internal exposure comprises the forces acting on and in the body. Examples of operational measures are the compression force on a discus in the low back during lifting (level), lifting frequency of the individual (repetitiveness), and working time for the individual in the lifting task (duration).

The active internal exposure is the fraction of the internal exposure causing biological responses and effects on the target organs, tissues, cells and molecules.

Acute responses develop in the body over time as a consequence of internal exposures. Examples are increases in oxygen consumption, heart rate and perceived exertion.

In the long run chronic effects may develop such as an improved oxygen transportation system or a musculoskeletal disorder. Little is known about the mechanisms leading from internal mechanical exposure to musculoskeletal injury. During the recent years a number of hypotheses have been suggested, e.g., that some types of mechanical exposure may change the ion homeostasis of the muscle cells, leading to cell injury (Sejersted and Vgllestad 1993).

Important feed-back loops modulating the exposure are illustrated in figure 2. Acute responses and chronic effects may imply that the individual changes hisher . working technique or working hours, thus changing the internal exposure. A high incidence of injury in a company may also cause the management to change work organization or technology, thus changing the external exposure.

Other exposure+ffect models have been presented during the years. They vary considerably according to their aim. A stress-strain model has been developed by German and Finnish research groups (e.g., Rutenfranz et al. 1990, Rohmert 1984). Unfortunately, this model does not offer a clear-cut discrimination between internal exposures and acute responses. Recently, a model was suggested for the pathogenesis of work-related musculoskeletal disorders (Annstrong et al. 1993). It seems appropriate for this purpose, but lacks a clear discrimination between exposure and response as

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Table 1. Translation of some important exposure concepts from the classical epidemiology (cf. Checkoway er al. 1989, Armstrong et al. 1992) into equivalent terms regarding mechanical exposure as suggested in this paper. Further details in text.

Chemical exposure Mechanical exposure

Exposure + External exposure Exposure concentration + External exposure level Cumulative exposure + External exposure level

times duration Burden. dose rate + Internal exposure level Dose + Internal exposure level

times duration

defined in this paper. In addition. its definition of dose does not aim at a selective evaluation of mechanical factors.

2.4. Present and previous exposure concepts The terminology of exposure varies between research fields such as pharmacology. chemical epidemiology and risk analysis. The sections 2.1 .-3. suggested a refined terminology specifically addressing mechanical events related to musculoskeletal disorders. Table 1 shows how this terminology regarding forces relates to the 'classical' terminology regarding chemicals as defined by Checkoway et al. (1992).

The main adjustment of the exposure terminology aims at the repetitiveness, which now is considered an exposure dimension by itself. The classical dose concept ignores the temporal pattern (repetitiveness) of exposure delivery (cf. Checkoway and Rice 1992). Considerable evidence occurs that the repetitiveness of mechanical load is a significant factor for acute and chronic reactions (for references, see Winkel and Westgaard 1992, and Mathiassen 1993b). Thus, it seems to be important that the 'internal exposure' is expressed by all its three primary dimensions, level, repetitive- ness, and duration. How to integrate these dimensions in operational terms remains to be solved.

2.5. Variation and monotony During the recent years 'variation' and 'monotony' have become frequently used descriptors of mechanical exposure in the scientific literature as well as in governmental publications (Kamwendo et al. 1991, Jonsson 1988, National Swedish Board of Occupational Safety and Health 1983). According to the Concise Oxford Dictionary (Sykes 1976) 'variation' and 'monotony' may convey several different meanings, including subjective perceptions. These two expressions should therefore be avoided in objective descriptions of exposures if they are not specifically defined in physical terms.

We suggest introduction of the expression variation pattern defined as the interaction between exposure level and repetitiveness. Thus, the variation pattern conveys simultaneously the attained exposure levels and the frequency of shifts between them. If the same short-cyclic exposure variation pattern is repeated during a long period, the work task may be classified as 'monotonous'. A more detailed quantification of 'monotonous' exposure should be related to the kind and degree of expected effect (musculoskeletal or psychological). The word 'variation: may be used to describe exposure which is not 'monotonous'.

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Assessment of physical work.load in epidemiologic studies 983

2.6. Exposure-efect relationships Some years ago a U-shaped curve was suggested to illustrate the relationship between mechanical exposure and risk for musculoskeletal disorders (Winkel 1987). The exposure axis of this curve reflects an expert evaluation mainly based on the exposure level. Accordingly, heavy manual materials handling as well as constrained sitting work may imply an increased risk for low back pain. But the seated work may only imply an increased risk if the exposure duration is long. Also the repetitiveness of sitting is assumed to be important. As all three dimensions of exposure ought to be considered in the assessment of exposureeffect relationships, the U-model may not be sufficient.

3. Operational measures of mechanical exposure An overview of the literature shows that the most common way of classifying mechanical exposure in epidemiologic studies is by job title (Burdorf 1992, Winkel and Westgaard 1992). Unfortunately, a job title may comprise a wide range of work tasks, and the distribution and duration of these may vary considerably between individuals. Thus, risk estimates in terms of job title offer no practicable basis for ergonomic interventions.

An appropriate job exposure assessment should consider all tasks (including breaks) constituting the job. We thus suggest a thorough description of job exposure to comprise quantitative assessment of (1) 'task distribution', i.e. occurrence and duration of the different tasks included in the job; (2) 'task exposure', i.e. exposure of the different body parts due to each task.

A review of 72 studies on low-back pain (Burdorf 1992) showed that 38 of these assessed exposure by job title only. Questionnaire was used in 27 studies, observational methods in seven and direct measurement techniques in six. The advantages and disadvantages of the three latter main categories of exposure instruments are illustrated in figure 3. Self-report (questionnaire or diary) offers the possibility of studying a large number of individuals at a modest cost. Furthermore, by this single instrument you may

Observation Direct Self-report methods measurements

Cost

Capacity

Versatility - Generality - Exactness

Figure 3. In epidemiologic studies mechanical exposures have been assessed by self-repon (questionnaire and interview), observation methods, and direct technical measurements. The figure illustrates some important differences between these three main strategies.

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984 J. Winkel and S. E. Mathiassen

collect information about a versatility of different exposure variables. In addition, the questions may be designed to aim at exposure in general while direct methods only convey exposure information about the recording period. Therefore, self-report may seem to be the most appropriate instrument in epidemiologic studies. However, recent studies have shown a too low validity (Burdorf and Laan 1991, Wiktorin et al. 1993) and reliability (Wiktorin etal. 1991, Wiktorin etal. unpublished) in relation to the needs for ergonom,ic interventions.

The most precise and accurate operational estimation of the internal force vectors (i.e., the conceptual exposure) may be obtained by biomechanical modelling. Biomechanics offer in theory calculations of internal forces based on real time observations ofjoint position, external forces and anthropometry of the subject. Muscle forces may also be estimated by electromyography (Basmajian and DeLuca 1985). In large populations relevant to epidemiologic studies, cruder exposure methods have to be used. However, they ought to be validated in relation to more accurate quantitative estimations of internal exposure, e.g. by electromyography or biomechanics.

In epidemiologic studies. imprecise estimates of mechanical exposures may imply that the calculated relative risks are underestimated. Other factors (e.g. individual or psychosocial factors) are often investigated in the same study. However, if they are correlated with the mechanical exposure and measured with a higher precision, their risk ratio may beoverestimated (Phillips and ~ & t h 1991). According to our knowledge, this has never been considered in ergonomic epidemiology.

One important cause of imprecise estimates of mechanical exposures in epidemiologic studies is that all three main dimensions of exposure (level, repetitiveness, duration) are rarely considered simultaneously. A review of 39 of the most prominent original papers on relationships between physical work load and chronic shoulder-neck disorders (Winkel and Westgaard 1992) showed that exposure level was considered by 31 papers, duration by I I papers and repetitiveness by 10 papers. Only one of the papers considered all three dimensions.

Riihimiiki (1991) concluded that 'generally accepted' risk factors for low-back pain include, e.g., heavy physical work, prolonged sitting, heavy manual handling, frequent lifting, trunk rotation, pushinglpulling, and vibration. This kind of information is not useful in workplace interventions; 'heavy manual lifting' may cause training effects or disorders depending on how the three main exposure dimensions are combined in the task.

Previous studies have estimated the three exposure dimensions by a variety of variables. Level has been estimated by, e.g., EMG, workstation design, productivity, posture, external force, and heart rate. Repetitiveness has been estimated by, e.g., cycle time, pause patterns, and changes in posture. Duration has been estimated as, e.g., hours spent in a given posture or task per day, total number of working hours per day or week, and total employment time (for references, see, e.g., Winkel and Westgaard 1992). These operational estimates of exposure comprise variables pertaining to external exposure, internal exposure as well as acute response (figure 2); meta-analyses are therefore complicated (see below).

Work organization is often described as 'a separate main exposure category (e.g., Hagberg 1992) or as a factor mainly related to the psychosocial work environment. Work organization concerns the distribution cf work tasks between individuals. Thus, work organization constitutes a significant determinant of the internal mechanical exposure. The consequences of a work organization in terms of exposure may be quantified by assessing 'task distribution' as well as 'task exposure' (see above). Thus,

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Assessment of physical work load in epidemiologic studies 985

work organization is not needed as a separate exposure category, which would, anyhow, be difficult to compare between industries.

In conclusion, invalid exposure assessment may, to a large extent, explain the lack of quantitative data on relationships between mechanical exposure and musculoskeletal disorders. Today, it is therefore not possible to determine the relative significance of mechanical, psychosocial and individual factors for the development of muscu- loskeletal disorders.

4. Some guidelines for future mechanical exposure assessment

4.1. Measures related to pathogenesis Conceptually, mechanical exposure comprises an unlimited amount of information. Operational methods should be limited to those quantifying mechanical events tying up to prevalent hypotheses for pathogenesis in the musculoskeletal tissues.

4.2. Internal or external exposure measures Internal exposure measures should be used in studies aiming at the aetiology of disorders. However, practical changes at the workplace aim at changes of the external exposure. Thus, it is necessary also to measure the,external exposure as well as to investigate its relationship to internal exposure. Insufficient knowledge is available about such relationships. For instance, productivity is a frequently used measure of external exposure (e.g., Kuorinka and Koskinen 1979). But its validity as an estimate of internal exposure level and repetitiveness seems to be low (Mathiassen 1993b).

4.3. Exact measures Exposure-effect relationships must be indicated with an accuracy and precision sufficient for practical interventions. It is of limited practical value to know that 8 h of data entry work imply a large relative risk for shoulder-neck disorders compared with less than 1 h. Information is needed about the course of the exposure-effect curve between these two extreme values. The required accuracy depends on the level and duration of exposure. It may be sufficient to assess an exposure such as hours of VDT work per day with an accuracy of & 30 min. However, this accuracy is not sufficient for hazardous exposures occumng at the most for half an hour per'day.

Direct technical methods offer more reliable and vali'd data than self-reports, and should therefore be preferred when recording present exposures. Assessments of task exposure should be based on direct recordings from several subjects, as individuals performing the same task may show large differences in internal exposure (cf. Jonsson et al. 1988). It may, however, be necessary to rely preferentially on self-reports to obtain data on retrospective exposures and seasonal variations.

4.4. Dejnition and standardization of operational measures For each main target tissue in the body appropriate operational exposure measures need to be defined. The basis for these measures is suggested to be real-time registrations of head, arm, hand, trunk and leg postures. They may be recorded by angle transducers indicating deviations from line of gravity or angle between body parts. Other important exposure variables, such as weight of handled objects or psychogenic tension should be considered by other methods, e.g., direct force measurement and EMG.

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4.5. Exposure measures generalizable across industries Meta-analyses of exposure-effect relationships may only be carried out on the basis of comparable exposure measures. Exposure estimates specific to an industry (e.g. productivity measures) or arbitrary exposure classifications, are examples of exposure estimates which cannot be used properly in meta-analyses (Kuorinka and Koskinen 1979, Herberts er al. 198 1, Hiinting et al. 198 1, Kvarnstrom 1983, Dimberg et al. 1989). Exposure duration expressed as 'full time' may vary from 4.5 to 8 h according to society and company culture (Winkel and Westgaard 1992).

4.6. Exposure measures generalizable across time Exposure recordings by observational and direct technical methods are usually carried out only during a short period for each individual, typically a few minutes or hours (Burdorf and Laan 1991, Wiktorin et al., 1993). It may be questioned if such data are representative for the individual when performing the task orjob in general. Recordings of, e.g., head inclination and rotation during a few minutes in a monotonous task at the assembly line, supplied with data on working hours and pause schedule, may be a sufficient mapping of the individual neck exposure caused by the job. Much longer and repeated recording periods may be needed for workers performing more varied tasks. Procedures for optimal recording schedules according to task and exposure variable need to bedeveloped. In principle. quantitative dataon 'taskexposures' may be weighed according to 'task distribution' to obtain an estimate of job exposure (cf. chapter 3).

4.7. Standardization of data analysis and presentation Comparison between studies is often impeded by non-comparable procedures for analysis, classification and presentation of data. For instance, arm elevation has been classified in different angle intervals by different authors (Hiinting et al. 1981, Jonsson et al. 1988, Bjelle et al. 1981). Thus, the time distribution of elevation angles cannot be compared between studies. Consensus is needed with regard to procedures for data analysis and presentation. For instance, the Exposure Variation Analysis method (EVA, Mathiassen and Winkel 1991) may appear to offer a suitable standardized expression of the variation pattern of exposure.

4.8. Consumer-adapted exposure information Ergonomic epidemiology may offer data for work injury considerations as well as for planning and evaluation of ergonomic changes. Thus, the researchers' methods for data acquisition, analysis, and presentation should be applicable also for the safety personnel at the workplace. In particular, equipment costs and educational needs should be considered.

Acknowledgements Financial support from the Swedish Work Environment Fund is gratefully acknowledged.

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