Asymmetric loading of erector spinae muscles during sagittally symmetric lifting

Functional asymmetry is among the multitude of risk factors for low-back pain (LBP), the most common injury under general industrial and agricultural conditions. However, previous studies showed that normal healthy individuals exhibit some functional asymmetry, indicating that not all asymmetry causes LBP. Therefore, the threshold value that is able to discriminate between normal and pathological situations is used as critical information to predict LBP. As a preliminary study to find threshold, the purpose of this study is to quantify the magnitude of bilateral asymmetries of erector spinae muscle forces of a healthy group during sagittally symmetric lifting. Ten healthy male subjects with no history of back pathology participated in this study, which collected motion capture, force data, and electromyography signals from six infrared cameras (MCam2, Vicon), two force platforms (AMTI), and surface EMG (BME Korea). In order to quantify the magnitude of bilateral asymmetry in the trunk muscle forces, we used 3D linked segment and EMG-assisted modeling approaches, both of which were verified based on their recapitulation of previously-proposed models. The results indicated that each muscle force in the lumbar region exhibited asymmetry during the entire lifting process. In particular, the erector spinae muscle forces exhibited an approximate 24% difference between bilateral sites (p<0.05). The results of this study provided data from normal individuals by which to identify pathological situations and predict LBP incidence within general industrial and agricultural conditions.