Precision of measurements of physical workload during standardised manual handling. Part II: Inclinometry of head, upper back, neck and upper arms

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Abstract

For measuring the physical exposure/workload in studies of work-related musculoskeletal disorders, direct measurements are valuable. However, the between-days and between-subjects variability, as well as the precision of the method per se, are not well known.

In a laboratory, six women performed three standardised assembly tasks, all of them repeated on three different days. Triaxial inclinometers were applied to the head, upper back and upper arms. Between-days (within subjects) and between-subjects (within tasks) variance components were derived for the 10th, 50th and 90th percentiles of the angular and the angular velocity distributions, and for the proportion of time spent in predefined angular sectors.

For percentiles of the angular distributions, the average between-days variability was 3.4°, and the between-subjects variability 4.0°. For proportion of time spent in angular sectors, the variability depended on the percentage of time spent in the sector; the relative variability was scattered and large, on average 103% between days and 56% between subjects. For the angular velocity percentiles, the average between-days variability was 7.9%, and the average between-subjects variability was 22%.

The contribution of the measurement procedure per se to the between-days variability, i.e., the imprecision of the method, was small: less than 2° for angles and 3% for angular velocity.

Introduction

Physical workload (e.g., excessive and prolonged muscular load, awkward and constrained postures, and repetitive movements) has been identified as a risk factor for developing work-related musculoskeletal disorders (WMSDs) [6], [24], [27]. Quantitative exposure-response relations are, however, known only to a very limited extent. This lack of knowledge hampers surveillance and regulation of these risk factors, and present standards and guidelines are often expressed in qualitative, process-oriented terms [10], [11]. So far, attempts to implement these standards, guidelines, and regulations have not led to a decrease in the occurrence of WMSDs.

Legislative regulation, analogue to threshold limit values (TLVs) for exposure to toxic chemicals, noise and vibration, might prove more successful. By using technical measurements, which show a better validity, accuracy and precision than observation methods [17], [29], [36], [37], the scientific basis of TLVs can be improved, and compliance surveyed.

For measuring postures, uniaxial and biaxial inclinometers have been extensively used. One major advantage of inclinometry is that definitions of head, upper back, neck and upper arm postures adhere to the ISO-standard ‘Ergonomics – Evaluation of static working postures’ [15]. However, excessive errors may occur during inclination in arbitrary directions even when biaxial inclinometers are used [12]. Moreover, most inclinometers are based on transducers comprising moving parts, which limits their frequency response to a few Hz, and, hence, reduces their accuracy during dynamic conditions. To overcome these limitations we have developed triaxial accelerometers for whole-day ambulatory inclinometry [12], which have an accuracy and reproducibility that is independent of the direction and the magnitude of the inclination. They provide valid data under the dynamic conditions that occur during ordinary occupational work [7], [12]. The instrumentation has been applied in studies of occupational work, e.g. [2], [5], [14], [17], [33].

When inclinometers are applied for characterising the physical workload, variability (in addition to that inherent in the instrument) will be introduced, e.g., due to the non-perfect reproducibility of the reference positions. Moreover, for a particular subject there will be between-days variability, due to actual differences in task requirements, as well as differences in work performance. In addition, different individuals will not perform the same task in an identical manner. The size of between-days and between-subjects variability is crucial for determining sampling strategies, e.g. in epidemiological studies [19], [32] and for surveillance of TLVs [9], [20], [21], [22], [26].

This study is one part of a larger investigation, which also evaluated the precision of electromyography [25] and goniometry (to be published). The specific aim of the present study was to evaluate the usability of inclinometry based on triaxial accelerometers for assessing industrial tasks, in terms of precision of the method per se, as well as between-days and between-subjects variability.

Section snippets

Subjects

Six healthy, right handed, female subjects from the department staff participated in the study. Their median age was 44 (range 36–54) years, height 168 (158–173) cm, and weight 64 (58–82) kg. The Ethics Committee of Lund University approved the study, and all participants gave their written informed consent.

Work tasks

At each trial, the subjects performed three standardised work tasks in a laboratory setting. The tasks were designed to give different levels of physical exposure. The work task ‘materials

Results

As intended, the three work tasks differed regarding postures. For example, the head was held in a considerably more flexed position during ‘materials picking’ (24°/41°/51° for the 10th/50th/90th percentiles, and 33% of the time flexed >45°) as compared to ‘light assembly’ (14°/23°/29° and 0.21% of the time) and ‘heavy assembly’ (13°/28°/38° and 3.7% of the time; Table 1 and 2; Fig. 1). Tasks also contrasted with respect to movements; the non-cyclic task ‘materials picking’ displayed higher

Discussion

The tasks differed in postures and movements; however, the variability was only to a minor extent influenced by the tasks performed. For posture percentiles, the overall average variability was small (SDBD 3.4°, and SDBS 4.0°). Time spent in angular sectors varied more, the normalised SDBD was 103%, and the normalised SDBS was 56%. For angular velocities, the average variability was 7.9% for CVBD and 22% for CVBS.

Acknowledgements

This study was supported by the Swedish Medical Research Council, the Swedish Council for Work Life and Social Research, the Swedish Council for Work Life Research (including Change@Work), the Swedish National Institute of Working Life, the Swedish Council for Planning and Coordination of Research, AFA Insurance, the Medical Faculty of Lund University and the County Councils of Southern Sweden. Ms. Lothy Granqvist and Ms. Jeannette Unge, M.Sc., gave skilful technical assistance. Svend Erik

Gert-Åke Hansson received his M.Sc. in Electrical Engineering from Lund Institute of Technology, Sweden in 1976, and his Dr.Med.Sc. in 2000 from the Department of Occupational and Environmental Medicine at Lund University Hospital, Sweden. His present research interest is development of methods for measuring and analysing physical workload with relevance to epidemiological studies of work-related upper extremity musculoskeletal disorders.

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    Gert-Åke Hansson received his M.Sc. in Electrical Engineering from Lund Institute of Technology, Sweden in 1976, and his Dr.Med.Sc. in 2000 from the Department of Occupational and Environmental Medicine at Lund University Hospital, Sweden. His present research interest is development of methods for measuring and analysing physical workload with relevance to epidemiological studies of work-related upper extremity musculoskeletal disorders.

    Inger Arvidsson received her BScPT from Lund University, Sweden in 1983, and her MScPT from Karolinska Institutet, Stockholm, Sweden in 1996. She has been working as an occupational physiotherapist for several years, and her main research interest is the physical workload and the risk of musculoskeletal disorders in computer work.

    Kerstina Ohlsson received her Dr.Med.Sc. in 1995 from the Department of Occupational and Environmental Medicine at Lund University Hospital, Sweden. Her thesis focused on musculoskeletal disorders of the neck and upper limbs among women with industrial repetitive work. She is now engaged in research on the relation between physical workload and the development of musculoskeletal disorders.

    Catarina Nordander received her M.D. in 1989 from the Faculty of Medicine, Lund University, Sweden, and her Ph.D. in 2004 from the department of Occupational and Environmental Medicine at Lund University Hospital, Sweden. Her research focuses exposure-response relationships concerning physical exposure at work and the risk of musculoskeletal disorders. She is especially interested in gender aspects concerning exposure and response.

    Svend Erik Mathiassen defended his Ph.D. in applied work physiology at the Karolinska Institute, Stockholm, Sweden, while being employed at the National Institute for Working Life. He is currently Professor at the Centre for Musculoskeletal Research, University of Gävle, Sweden. His research is focused on methods for collecting, analyzing and interpreting data on physical loads in working life, with a particular emphasis on assessment and implications of load variation.

    Staffan Skerfving received his M.D. from the Karolinska Institute in Solna, Sweden, in 1966 and his PhD in 1972. He is currently Professor of Occupational medicine at Lund University. His research is focused on, i.a., the relationship between occupational mechanical exposure and musculoskeletal disorders.

    Istvan Balogh received his Dr.Med.Sc. in 2001 from the Department of Occupational and Environmental Medicine at Lund University Hospital, Sweden. He has since 1977 worked with occupational exposure assessment in general. His focus in the latest 15 years is in the field of musculoskeletal workload.

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