Measuring trunk motions in industry: variability due to task factors, individual differences, and the amount of data collected

The focus of this study was to determine the amount of data needed to ensur e sufficient accuracy in estimating mean trunk motions of employees perform ing industrial manual materials handling tasks. Over 450 tasks were selecte d, in which the load weight and the vertical start and destination heights of the activity remained constant throughout the task. Data were collected as employees did their work at the job site, using the Lumbar Motion Monito r. Variance components were estimated in a hierarchical design and used to compute standard errors of mean trunk kinematic measures. These analyses fo und task-to-task variation to be much larger than the variability due to ei ther multiple employees performing the same task or to repetitive movements within a task. Also, it was found that no significant reduction in the sta ndard errors occurred when data were gathered for more than three employees and three repetitions of each task by an employee. This study indicates th at the vast majority of variability in mean trunk motions is accounted for by the design of work tasks, and variations due to repeated cycles of a tas k or to employees are rather minor. It is also important as a basis for fut ure work on modelling low-back disorder risk based on a job's trunk kinemat ic measures.