Changes in trunk dynamics and spine loading during repeated trunk exertions

Study Design. Trunk and hip kinematic and spinal loading changes were documented as experienced subjects performed a set of standard free-dynamic lifting tasks at various intervals throughout a 5-hour repetitive manual materials handling session. Objectives. To document how spine loading changes as a function of repetitive lifting during an extended period of lifting cycles. Summary of Background Data. Many studies have evaluated spine loading as a function of a specific lifting task, but no studies could be found in the literature that evaluated how spine loading may change with repeated exposure to a lifting task. Methods. Ten experienced material handlers were recruited. Each was required to transfer 11 pallets of 23 kg boxes at a rate of 125 lifts per hour. Before and after unloading each pallet, subjects were asked to perform a set of five standard free-dynamic lifting tasks. Electromyographic activities of 10 trunk muscles were recorded along with kinematic and kinetic data. An electromyography-assisted model was used to evaluate spine loading during the standard lifting tasks. Results. Subjects significantly changed their motion patterns throughout the lifting session. Trunk range of motion, velocity, and acceleration decreased in the sagittal plane, whereas these same measures increased for the hip. Trunk moment also decreased by 7% during the standard lifts. These changes were accompanied by a redistribution of muscle recruitment patterns, resulting in a decrease in spine compression and an increase in anterior/posterior shear. Conclusions. This study has shown that spine loading patterns do indeed change with repetition, suggesting that on needs to monitor these changes throughout repetitive lifting tasks if low back disorder risk is to be minimized.