The control of shoulder muscles during goal directed movements, an inverse dynamic analysis

Fast goal directed arm movements in the sagittal plane were analyzed with a three-dimensional shoulder model with 95 muscle elements. Dynamics of the muscle elements were described by a third-order nonlinear muscle model. Muscle forces and activation were estimated using the method of inverse muscular dynamics, an optimization scheme which uses only very limited computational power. Most model results were similar to the EMG but some differences between model results and EMG were found in muscles where the EMG activity was subject dependent. For the movement studied, the thoracoscapular muscles were shown to deliver about 40% of the energy required for the acceleration of the arm during anteflexion and about 22% during retroflexion. Activity of thoracoscapular muscles was also required to ensure contact between the thorax and the scapula which is important for the mechanical stability of the shoulder. The rotator cuff muscles were found to deliver about 19% of the energy required for the acceleration of the arm during anteflexion and about 8% during retroflexion.