Trunk muscle coactivation. The effects of load asymmetry and load magnitude

The present investigation describes the cocontraction of eight trunk muscles during the application of asymmetric loads to the torso. External moments of 10, 20, 30, 40, and 50 Nm were applied to the torso via a harness system. The direction of the applied moment was varied by 30 degree increments to the subjects' right side between and including sagittally symmetric orientations front and rear. Surface electromyography (EMG) was used to monitor the activities of the left and right Latissimus Dorsi, Erector Spinae, External Oblique, and Rectus Abdominus in 10 subjects. Analysis of Variance (ANOVA) procedures showed significant interactions between the moment magnitude and direction factors for 7 of the eight muscles. Further analyses were employed to determine which muscles were 'active' for each moment direction. This allowed the changes in muscle coactivation to be described as moment magnitudes and directions were varied.