Trunk Muscle Activations While Resisting Asymmetric Loads in a Laterally Bent Trunk Posture

Asymmetric material handling frequently results in lateral bending of the torso. Each of these factors have been linked via epidemiological investigations to the incidence of low back disorders (LBD). Very little literature is available which describes the response of the trunk muscles in situations which would be analogous to handling materials while bent to the side. Such activities are observed frequently in industrial settings, especially during the initial and final portions of a lift. The objective of the current study was to describe the internal response of the trunk muscles as asymmetric loads were applied to the laterally bent torso. Specifically, this investigation quantified the electromyographic activities (EMG) of 8 trunk muscles under conditions where the trunk was isometrically loaded while the trunk was maintained in a 20 degree laterally bent posture. Moments with a magnitudes of 20 and 40 Nm were applied to fifteen subjects. The direction of the external moments was varied in 30 degree increments completely around the subjects. The EMG data indicates that the muscles showed the greatest activity when they were in opposition to the load's sagittal and frontal plane moment. The muscle showing the largest response was the External Oblique. Significant activity was also observed under conditions in which muscles were creating an antagonistic moment in either the sagittal plane, the frontal plane, or in both planes.