Data on exposure variability is an important remedy for designing and interpreting studies of occupational ergonomics. The present study aimed at retrieving the within- and between-subjects variance of several mechanical exposure parameters in a repeated, short-cycle task. Seven experienced operators repeatedly secured joints using two types of hand-held nutrunners. The joints were placed in three different locations on a rack, simulating automotive assembly. Bilateral muscle activity from the upper trapezius and the lower arm extensors, as well as head and upper arm inclination was continuously monitored. Exposure levels and their variance components were assessed in several data subsets using ANOVA. The results were interpreted in terms of statistical precision and power, and discussed as markers of important ergonomic qualities. A substantial exposure variability was found within and between subjects in all joint locations and for both tools. For mixed work across tools and locations, the necessary number of subjects to arrive at a group mean exposure with 95% confidence limits corresponding to +/- 10% of the mean ranged between 8 and 158, with posture recordings tending to require smaller populations than muscle activity recordings. Within-subject variance increased 2- to 37-fold, depending on exposure parameter, when work was 'enlarged' from securings with a specified location and tool to a mix of all locations and tools. Systematic differences between subjects in variability and responsiveness to 'work enlargement' indicated individualized motor control strategies. The results illustrate the importance of exposure variability data to the design of proper measurement strategies. They also suggest that the sizes of exposure variability per se can be interpreted as operational indices of what is thought to be important ergonomic risk indicators, such as the 'sameness' of repeated operations and the allowance for flexible working techniques.

Cyclic assembly work is known to imply a high risk for musculoskeletal disorders. To have operators rotate between work tasks is believed to be one way of decreasing this risk, since it is expected to increase variation in mechanical and psychological exposures (physical and mental loads). This assumption was investigated by assessing mechanical exposure variability in three assembly tasks in an electronics assembly plant, each on a separate workstation, as well as in a 'job enlargement' scenario combining all three stations. Five experienced operators worked for 1 h on each station. Data on upper trapezius and forearm extensor muscle activity were obtained by means of electromyography (EMG), and working postures of the head and upper arms were assessed by inclinometry. The cycle-to-cycle variance of parameters representing the three exposure dimensions: level, frequency and duration was estimated using ANOVA algorithms for each workstation separately as well as for a balanced combination of all three. For a particular station, the variability of trapezius EMG activity levels relative to the mean was higher than for extensor EMG: between-cycles coefficients of variation (CV) about 0.15 and 0.10, respectively. A similar relationship between CV applied to the parameter describing frequency of EMG activity. Except for head inclination levels, the between-cycles CV was larger for posture parameters than for EMG. The between-cycles variance increased up to six fold in the job enlargement scenario, as compared to working at only one station. The difference in mean exposure between workstations was larger for trapezius EMG parameters than for forearm extensor EMG and postures, and hence the effect of job enlargement on exposure variability was more pronounced for the trapezius. For some stations, job enlargement even implied less cycle-to-cycle variability in forearm extensor EMG parameters than working at that station only. Whether the changes in exposure variability associated with job enlargement were sufficient to imply a decreased risk for musculoskeletal disorders is not known.

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Musculoskeletal disorders are common among workers who use powered hand held tools in assembly industry. The present study was carried out at a bus assembly plant where pneumatic nutrunners are the dominant powered hand tool. The activity in five muscles was measured for different phases of the securing, comparing soft and stiff joints, three different joint positions, and two powered tools. Tne aims were to characterize the muscular load during usage of the chosen tools, and to be able to provide relevant information to the production engineers, who were to design a new production system. The variance in muscular activity was large, both within subjects and between the mean CUIV~S of different subjects. Still, it was possible to obtain characteristic shapes of EMG curves sampled under different conditions. EMG peaks were seen in the hand extensor muscles for the right-angled tool. Joint position strongly contributed to differences in muscular activity in all muscles, which together with the fmdtig that the tools made very small differences, were regarded as the most valuable results for the production engineers. 5-21

The purpose of this product is td study the correlation between self-reported and measured mechanical workload of neck and shoulder on workstations level. Six persons participated in the study, each working at six different workstations. The workstations were ranked from "lowest" to "highest" workload according to different variables. The correlations were studied on ranking order. Different types of assembly work were mainly carried out in all stations. In this work (few high workload situations, and a high degree of monotonous, repetitive work situations) one situation with a high workload could in this case be experienced as positive. The methods lead to contradicting results as regards identifying work situations with high loads. The results indicate that the methods lead to similar results as regards identifying work situations with low loads.

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