Interindividual variation of physical load in a work task

Objectives This study analyzed the variation in physical work load among su bjects performing an identical work task. Methods Electromyographs from the trapezius and infraspinatus muscles and w rist movements were recorded bilaterally from 49 women during a highly repe titive industrial work task. An interview and a physical examination were u sed to define 12 potential explanatory factors, namely, age, anthropometric measures, muscle strength, work stress, and musculoskeletal disorders. Results For the electromyographs, the means of the 10th percentiles were 2. 2% and 2.8% of the maximal voluntary electrical activity (%MVE) for the tra pezius and infraspinatus muscles, respectively. However, the interindividua l variations were very large [coefficients of variation (CV) 0.75 and 0.62, respectively]. Most of the variance could not be explained; only height, s trength, and coactivation of the 2 muscles contributed significantly (R-adj (2) 0.20-0.52). The variation was still large, though smaller (CV less than or equal to 0.63), for values normalized to relative voluntary electrical activity (RVE). For the wrist movements, the median velocity was 29 degrees per second, and the interindividual variations were small (CV less than or equal to 0.24). Six factors contributed to the explained variance (R-adj(2 ) 0.12-0.55). Conclusions The interindividual variation is small for wrist movements when the same work tasks are performed. In contrast, the electromyographic vari ation is large, even though less after RVE normalization, which reduces the influence of strength, than when MVE is used. Because of these variations, several electromyographs are needed to characterize the exposure of a spec ific work task in terms of muscular load, and individual electromyographs a re preferable when the worker's risk of myalgia is being studied.