EFFECT OF LOAD ASYMMETRY ON INTERNAL LOADING OF TRUNK

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In industry, carrying and lifting objects asymmetrically is the rule rather than the exception. This situation is hazardous to the musculoskeletal system due to an increase in coactivation of the musculature and an increase of the forces on the spine. The aim of this study is to quantify the activities of ten trunk muscles by using the surface electromyography (EMG) during maximal and submaximal isometric exertion under pure and combined loading conditions of the trunk. Combined loading is defined as the vectorial sum of moments in the sagittal and transverse planes. These planes are selected based on their prevalence in industrial tasks and low back injuries. To calculate the effect of internal loading, the muscle parameters are calculated from CT-scan images of each individual participant and individual muscle forces are estimated using an EMG-driven model. The compression and shear forces are calculated under suggested conditions. The effects of planar exertion (pure exertion) and combined exertion on the patterns of trunk muscle recruitment and compression and shear forces will be compared. The primary hypotheses of this study are: 1) The peak EMG activity of trunk muscles will be higher during the maximal combined exertion than during pure maximal exertion. 2) The mean of RMS-EMG of the ten selected trunk muscles will change significantly with the orientation of the net resultant moments of the trunk. 3) The compression and shear forces will be changed significantly by the orientation of the resultant moment. The present study will provide a better understanding of the effect of asymmetrical lifting on mechanical and neuromuscular performance and on the risk of injury to the back.