MUSCLE-ACTIVITY AND LOW-BACK LOADS UNDER EXTERNAL SHEAR AND COMPRESSIVE LOADING

Study Design. This study analyzed anatomic and neural control characte ristics of the trunk musculature. Subjects were exposed to external sh ear and compressive toads with equivalent moments to evaluate activati on patterns and loading on the low back. Objectives. The migration of activity between the thoracic and lumbar erector spinae muscle groups was examined to determine whether the motor control system chooses to minimize joint loading by recognizing differences in moment, compressi on, and shear support requirements and assigning muscle activation in the most appropriate way. Summary of Background Data. Loads were appli ed either parallel or perpendicular to the low back to create compress ive or shear forces. No previous study has attempted to isolate the re sponse of the trunk musculature with the type of external load. Method s. Eleven male subjects isometrically held an external load that was a ltered to create either a compressive or an anterior shear load on the low back but with equal extensor (reaction) moments (experiment 1). I n a second experiment four men repeated the task with an increased ran ge of applied loads (5-25 kg) together with measurements of intra-abdo minal pressure. Results. The tasks with a compressive external load re sulted in significantly higher levels of activation for all seven elec tromyographic channels recorded. Intra-abdominal pressure, compressive and shear joint forces were all higher in the compression loading met hod when equal loads and low back moments were compared. Conclusions. It was concluded that the motor control system does not arrange muscle activation levels in a way to minimize lumbar spinal loading at least for the relatively low levels of this study. Biomechanical models tha t use the objective criterion of minimum joint load may not be represe ntative of the motor control system, at least in the low back.