Effect of different multipliers and their interactions during manual lifting operations

The National Institute for Occupational Safety and Health (NIOSH) developed a lifting equation in 1981 to indicate ‘‘safe’’ occupational lifting limits. This equation was revised in 1991. The equation uses a series of lifting multipliers (parameters) to calculate corresponding recommended task weight limits. Due to the nature of risk factor interactions, the limits obtained from the NIOSH equation may not be appropriate for all lifting tasks. This laboratory experiment examined the effect of lifting parameters and their interactions as follows: lifting frequency, vertical lifting distance, and load weight. In this simulation study, 10 female building construction workers lifted weights in 48 different combinations of lifting parameters, in which four different lifting frequencies (1, 4, 7, and 14 lifts $min^{-1})$, three different load weights (5, 10, and 15 kg) and four different vertical lifting heights (knee, waist, shoulder, and maximum reach) were considered. The subjects did symmetric lifting for 10 min period in sagittal plane adopting free-style lifting technique. The recorded working heart rates were normalized based on the maximum heart rate obtained during maximum aerobic power measurement. MANOVA result showed that the main effects were significantly (p<0.0001) related with normalized working heart rate and the interaction effects of different lifting parameters contributed 10.01% of total variance of normalized working heart rate. Factorial design was applied to verify the interaction effects. Then, stepwise linear regression analysis was performed to identify the best predictive model using important parameters. It was observed that the contribution of interaction factors were not in similar pattern in case of different frequency responses. Finally, it is concluded that the interaction effects between different lifting parameters must be considered in addition to the effects of individual lifting parameters for further research as it implicates potential modifications to the currently recommended weight load estimation procedure.

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