The effects of load weight and load starting height on variability of lifting kinematics and kinetics

Abstract

Trunk kinematic variables have been used to understand the risk of low back injuries in the workplace. Variability in the trunk kinematics as an individual performs a repetitive lifting task is an underexplored area of research. In the current study, it was hypothesized that workplace variables (starting height of lift and load weight) would have an impact on the variance in the kinematic and kinetic variables. Twenty participants performed 60 repetitions of an asymmetric lifting task under four different conditions representing two levels of load weight (5% or 10% of the participant's body weight) and two levels of starting height (80% or 120% of the participant's knee height). The Lumbar Motion Monitor was used to capture trunk kinematic variables from the concentric range of lifting motion while ground reaction forces were collected using a force platform. The primary dependent variables were the variance of kinematic and kinetic variables across these 60 repetitions. The results showed a significant effect of starting height on the variance of sagittal plane trunk kinematics with the lower starting height generating an increased variance (sagittal range of motion increased by 55%, average sagittal velocity increased by 95%, peak sagittal velocity increased by 105%, and peak sagittal acceleration increased by 130%). There was no consistent significant main effect of either independent variable on the variance of the transverse plane kinematics. Additionally, there was no significant effect of load weight on the variance of any trunk kinematic variables tested. In terms of ground reaction forces, it was shown that the starting height of the load had a significant effect on the variance of peak vertical ground reaction force, while the weight of the load had a significant effect on the variance of the peak shear force.