Variability and misclassification of worker estimated hand force

Abstract

Ergonomic studies often use worker estimated hand force reproduced on a dynamometer to quantify force exposures but this method has not been well-studied in real work settings. This study evaluated the validity of worker estimates of hand force in a field study and determined the misclassification of worker estimated hand force exposures compared to directly measured forces. Eight experienced sheet metal assemblers completed ¼-inch diameter fastener installations using 6 different pneumatic tools. Grip forces were recorded by a pressure mat and were compared to worker estimated forces demonstrated on a dynamometer. Directly measured and worker estimated readings showed moderate correlations (0.53-0.67) for four installation tools and fair to moderate for two tools. The coefficient for variation of force estimates was 65% within repeated subject trials and 78% between averaged subject trials but 69% between subject trials during actual tool installations. Misclassification of worker estimated exposures varied by two cut-points: 29% using 4.0 kg and 49% using 6.0 kg. The force match procedure may provide adequate differentiation of high and low exposures in some settings, but is likely to result in substantial misclassification in other settings.

Figure 1

Testing set-up for directly measured trial with installation tool (a) and force matched trial with dynamometer (b).

Figure 1

Testing set-up for directly measured trial with installation tool (a) and force matched trial with dynamometer (b).

Figure 2

Simultaneous force readings from the dynamometer and the pressure mat that was applied to the dynamometer handle during maximum grip testing and force estimation trials.

Figure 3

Graphical distribution by boxplots of the directly measured peak force readings (dotted) and force estimated readings on the dynamometer (white). The median is the horizontal line in the box. The length of the box is the interquartile range (IQR). Outliers that are between 1.5 and 3 IQR’s are labeled as circles (o) and more than 3 IQR’s are labeled by an asterisk (*).