Variability in mechanical exposure within and between individuals performing a highly constrained industrial work task

Abstract

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.

Systematic evaluation of observational methods assessing biomechanical exposures at work

“…Most jobs and tasks show daily or seasonal variability in biomechanical exposures, and, therefore, the sampling strategy -in terms of the number of subjects and measurements per subject -is decisive for the precision and credibility of the obtained result (148)(149)(150)(151)(152)(153)(154)(155)(156). Some of the methods included in the present selection are based on sampling at fixed-time intervals (eg, OWAS, PATH), while others apply continuous observation for longer periods (eg, PEO, TRAC).…”

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

“…In most of these studies, the subjects are measured for one day only, and in that case between-subjects variability is overestimated, since it also includes between-days, withindays and method variability [21]. For example, using the same measuring method as in the present study, gross between-subjects variability of, on average for three tasks, 8.6°, 4.6° and 8.0°, have been reported for the 50 th percentiles of the head and right and left upper arm, respectively, for studying hand-held nutrunners in a mixed group of men and women, [22].…”

“…The chosen tasks were intended to be representative for manual work, and the mean values are in accordance with similar tasks recorded during occupational work [1,5,22,23,30].…”

“…The size of between-days and between-subjects variability are crucial for determining sampling strategies, e.g. in epidemiological studies [19,32] and for surveillance of TLVs [9,20,21,22,26].…”

Expo[not-so]sure... Reflections on a Tricky Issue in musculoskeletal research.

“…The loss may come about because work has to be performed under time pressure in constrained postures or with standardized movements. Degrees of freedom might also be reduced due to dysfunctional muscle fibers or altered sensory feedback from active muscles, and people may even differ-"don't ask me why, Rabbit"-with respect to their basic motor control strategies (11).…”