Coactivation levels induced by trunk muscles under isometric complex moment loading conditions

Abstract

This study investigated the coactivation levels induced by trunk muscles when isometric-complex moments were loaded. Complex moments were defined as the combination of two (two axial complex moments) or three (three axial complex moments) orthogonal uniaxial moments: flexion/extension (X-axis: -40, -20, +20, +40 N m), lateral bending right (Y-axis: +20, +40 N m), and twisting clockwise/counter-clockwise (Z-axis: -40, -20, +20, +40 N m). Thirty-two two axial complex moments and 32 three axial complex moments were applied to 12 male subjects via a chest harness in a standing posture. During the tasks, electromyography (EMG) signals from 10 trunk muscles were collected and muscle forces were predicted by using an EMG assisted model. Antagonistic muscles were identified by the inner-dot product definition and antagonist coactivation was assessed using two coactivation measures: the sum of antagonist forces and the antagonistic moment. In both the two and three axial moments loading conditions, two main effects (direction and magnitude) were found to be statistically significant (p<0.01). Among two axial complex moments, &apos;extension + twisting&apos; induced high coactivation of more than 45% of the total muscle forces. The antagonistic moments were high (about 43% of the external moment) in the combination of &apos;extension + twisting&apos; and &apos;lateral bending right + twisting&apos;. When three axial complex moments were applied, extension + lateral bending right + twisting counter-clockwise&apos; induced high coactivation of about 349 N (about 50% of the total muscle force), and the magnitude of the antagonistic moment was 21.9 N m (about 41% of the external moment). (c) 2007 Elsevier B.V. All rights reserved.