The effects of initial lifting height, load magnitude, and lifting speed on the peak dynamic L5/S1 moments

The purpose of this study was to quantify the peak dynamic bending moments on the spine during sagittal plane lifting as a function of the load's initial height above the floor, the load's magnitude, and the lifting speed. Ten male subjects participated in a repeated measures experiment in which 24 lifts were performed. The boxes lifted by the subjects contained loads that were 20, 100, 200, and 300 N. The lifts originated from three vertical locations (7.5 cm or “floor” level, knee level, and knuckle level), and were lifted at two qualitatively defined lifting speeds (“normal” and “fast”). All lifts were symmetric about the body's mid-sagittal plane and the boxes were gripped with both hands using the handles. Kinetic and kinematic data were used in a “bottom-up” linked segment dynamic model to determine the peak sagittal bending moment experienced by the subjects during each lift. The peak moments were significantly greater when lifting from lower lift heights (p<0.001), at faster lifting speeds (p<0.001), and with heavier loads (p<0.001). Moreover, the peak moments were dependent upon the combination of these three factors. Based on these data, a regression model was developed to predict the peak dynamic moment given the load magnitude, the starting height, and a qualitative estimate of lifting speed. This model was then used to assess whether the current National Institute for Occupational Safety and Health's (NIOSH's) model can control the peak bending moments acting on the spine as the limits for safe lifting are adjusted according to changes in the initial lifting height. It was found that the current NIOSH guidelines under-represent the increased bimechanical load experienced during low-level lifting.