Shoulder-neck exposure from assembly work and the significance of rationalization

Abstract: The main purpose of this thesis was to investigate the implications of different rationalization levels of assembly work on shoulder-neck exposure. Three levels were studied: (1) a Chinese Tayloristic system, (2) a Swedish Tayloristic system, and (3) an assembly workplace rationalization in Sweden according to 'modern' rationalization principles including ergonomic purposes. A laboratory study was carried out to compare different electromyography (EMG) normalization procedures. It was found that different upper trapezius EMG normalization procedures may give widely differing load estimations. Translations between occupational EMG results normalized by different procedures may only be made at a very approximate level. A cross-sectional epidemiological study was carried out among 1603 workers among ten different Chinese occupational groups with an aim of providing prevalence data of musculoskeletal disorders. The results showed that the 12-month prevalence of musculoskeletal disorders, particularly in the low back and shoulder-neck regions, was high at many Chinese workplaces. Large differences of the prevalence rates occurred between different occupations. A more detailed follow-up study on one Chinese assembly line showed that, in general, the ergonomic working condition was not satisfactory according to general ergonomic principles. This appeared to cause poor working postures (upper arm flexion and abduction larger than 30° during a large proportion of the assembly cycle) and high 'static' upper trapezius load (= 5% Maximal Voluntary Electrical activation -- MVE). Further examination revealed that the Chinese assembly line was not well balanced and the assembly line organization was sensitive to production irregularities. This caused frequent breaks (idle time) between assembly operations and reduced total exposure time to the assembly operations. Compared with the Chinese line, the Swedish Tayloristic line had better ergonomically designed work stations which implied improved working postures on the assemblers. However, EMG amplitudes of the right upper trapezius and infraspinatus muscles did not decrease. This may be explained by a reduced idle time on the Swedish line due to an improved industrial engineering (better line balancing and less sensitivity to production irregularities). This hypothesis was supported by a further analysis of the EMG data, which showed that the Swedish subjects had less muscle activity of low amplitudes (< 2% Maximal Voluntary Electrical activation -- MVE), which may correspond to the idle periods, compared with the Chinese subjects. In the 'modern' Swedish assembly workplace, the realized rationalization led to only minor changes in most exposure parameters. Several expected exposure improvements were not observed possibly because a number of planned initiatives were never implemented after a policy shift of the company's management. For example, multi-skills were not provided to all assembly operators so that no actual job rotations were possible between different assembly stations. Only a few non-assembly tasks were introduced into the new assembly system and the operators still spent the majority of their time on assembly tasks, with the result that an expected varied exposure pattern was not seen. Based on these findings, it is concluded that ergonomic measures on work station design improvements may play an important role in improving mechanical exposure of assembly work. However, industrial engineering measures may also influence the mechanical exposure. Therefore, ergonomic implications of rationalization as well as the ergonomic potential of 'modern' rationalization should be considered in future studies of rationalization.