Posture-Dependent Neuromuscular Contributions to Three-Dimensional Isometric Shoulder Torque Generation

The execution of activities of daily living requires the generation of three-dimensional shoulder torques through the coordinated activations of twenty shoulder muscles. Changes in shoulder posture can influence the contributions of individual shoulder muscles to shoulder torque generation, but it is unclear how the coordinated activity of shoulder muscles changes with shoulder plane of elevation and elevation angle. The purpose of this study was to characterize how neuromuscular coordination underlying three-dimensional shoulder function varies with arm posture. Muscle activations were obtained using surface electromyography from sixteen shoulder muscles as twelve healthy participants repeated eight three-dimensional isometric shoulder torques in twenty arm postures. Non-negative matrix factorization revealed the muscle synergies underlying shoulder torque generation across each of the experimental postures while the normalized similarity index assessed changes in overall synergy structure and linear mixed effects models assessed changes in the weighted contributions of individual muscles to each synergy. Our analysis revealed three distinct muscle synergies underlie healthy three-dimensional shoulder function. The overall structure of these synergies remained more similar than is expected by chance despite changes in shoulder posture. However, the weighted contributions of five muscles comprising the first synergy were influenced by changes in shoulder plane of elevation angle and six muscles comprising the second synergy were influenced by elevation angle. The weighted contributions of individual muscles comprising the third synergy were unaffected by posture. These findings suggest that the neuromuscular control plan for healthy shoulder function consists of three distinct synergies, whose overall structure is fixed across shoulder posture.