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-dy namic lifting tasks at various intervals throughout a 5-hour repetitiv e manual materials handling session. Objectives. To document how spine loading changes as a function of repetitive lifting during an extende d period of lifting cycles. Summary of Background Data. Many studies h ave evaluated spine loading as a function of a specific lifting task, but no studies could be found in the literature that evaluated how spi ne loading may change with repeated exposure to a lifting task. Method s. 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 perfo rm a set of five standard free-dynamic lifting tasks. Electromyographi c activities of 10 trunk muscles were recorded along with kinematic an d kinetic data. An electromyography-assisted model was used to evaluat e spine loading during the standard lifting tasks. Results. Subjects s ignificantly changed their motion patterns throughout the lifting sess ion. Trunk range of motion, velocity, and acceleration decreased in th e sagittal plane, whereas these same measures increased for the hip. T runk moment also decreased by 7% during the standard lifts. These chan ges were accompanied by a redistribution of muscle recruitment pattern s, resulting in a decrease in spine compression and an increase in ant erior/posterior shear. Conclusions. This study has shown that spine lo ading patterns do indeed change with repetition, suggesting that on ne eds to monitor these changes throughout repetitive lifting tasks if lo w back disorder risk is to be minimized.