Trunk kinematic variability as a function of time during the early phase of a repetitive lifting task

Lift-to-lift variability occurs in repetitive lifting tasks due to alterations in the lifting techniques used by the lifter, resulting in variability in lower back tissue loading. Understanding how trunk variability changes with time in the initial phases of a lifting bout may provide insights into the risk of injury during work startup. The purpose of this study was to quantify the variation of lifting kinematics and kinetics during the initial phase of a lifting bout. Twenty participants performed a repetitive lifting task continuously for 30 min. The load was equivalent to 10% of each participant's body weight and lifting was done at a rate of six lifts/min. Kinematic variables (three-dimensional range of motion, angular velocity, and angular acceleration) of the trunk were measured using the Lumbar Motion Monitor and a dynamic biomechanical model estimated peak L5/S1 moment and spine compression. The variances of these variables were compared across 10-min intervals: 0-10 min, 10-20 min, and 20-30 min. Results indicate a significant reduction in the variance of the peak sagittal acceleration, the sagittal range of motion, the transverse range of motion, peak sagittal moment, and peak spine compression between the first and second time intervals, followed by no significant change in variance between the second and third intervals. The downward trend in variation of these kinematic and kinetic variables suggests an initial adjustment period as the lifters reach a steady state of their lifting technique. The reduced variance of spinal loading may reduce the probability that a tissue tolerance is exceeded.