Task variability and extrapolated cumulative low back loads

Abstract

The aim of this research was to examine the repeatability of task cycle loading to assess the feasibility of data extrapolation approaches for calculating a shift cumulative exposure. The number of trials of a lifting task that would be necessary to provide a stable estimate of the cumulative low back loads sustained during manual material handling tasks throughout a normal working day was examined. Both a lab-based experiment (n=10 males) with a highly constrained task, and an externally paced industrial sample performing their regular workday task (n=8 males), were studied. Both study groups were videotaped while performing lifting trials and 10 consecutive trials were chosen for biomechanical analysis. Cumulative loading variables were determined for each lift and accumulating means were calculated for comparison with the gold standard, or criterion measure, which was the average of all 10 trials. It was found that a minimum of 4 trials was required to provide a stable estimate of cumulative loading in the lab-based task. The lab data were tightly clustered with an overall average coefficient of variation of 5.7%. Data obtained in the industrial setting were more variable with an average coefficient of variation of 24.8%. When individual lifting cycles were extrapolated to an 8 hour shift exposure, cumulative compression for the industrial task varied by 6.3 MN·s (range=15.2 to 21.5 MN·s) for one subject and for the constrained laboratory task it varied by 5.0 MN·s (range=32.0 to 37.0 MN·s). These results highlight the importance of using multiple cycle samples to establish a stable estimate of cumulative loading prior to extrapolating for a shift exposure.