Theoretical basis for general lifting equations based on mechanical work performed during manual lifting

Abstract

Objective: The objective of this research is to develop and validate general lifting equations based on the concept of mechanical work performed during the manual handling of objects. Background: To the authors’ knowledge, the NIOSH lifting equation was based on an empirical approach. Therefore, there is a need for general lifting equations that are based on the laws of mechanics and physics. Methods: General lifting equations were developed as a function of weight, horizontal distance in the sagittal plane, vertical travel distance, vertical distance from the floor to the origin of lift and angle of twist. The equations were based on the mechanical work required against gravity/friction, supplemented with stress coefficients to account for task difficulty not traditionally embedded in the dot product of gravity/friction forces and distance. Validation of the equations was performed with respect to the NIOSH lifting equation. Results: Results indicate that the general lifting index (GLI) was comparable to the NIOSH lifting index (LI) for different values of horizontal distance, vertical travel distance, twisting angle and vertical height at origin of lift. On the other hand, the GLI is more sensitive (i.e. larger) to individual changes in vertical travel distance or vertical height at origin of lift in comparison to the NIOSH LI. Consequently, the effect of horizontal distance was dampened in the GLI. Conclusion: The empirical approach inherent in human expertise, which was used by NIOSH in the development of their lifting equation, appears to be a good approximation on the basis of physics and mechanics laws. As originally planned by NIOSH, the lifting equation appears to be protective of the general workforce. Furthermore, the lifting equations hold the promise of assisting in the estimation of peak and cumulative lifting stresses in the workplace.