ANALYSIS OF THE KINEMATIC AND DYNAMIC BEHAVIOR OF THE SHOULDER MECHANISM

A finite element musculoskeletal model of the shoulder mechanism consi sting of the thorax, clavicula, scapula and humerus has been used for analysis of the kinematic and dynamic behavior. The model includes 16 muscles, three joints, three extracapsular ligaments and the motion co nstraints of the scapulothoracic gliding plane which turns the shoulde r girdle into a closed-chain mechanism. Simulations are inverse dynami c. Input variables are the positions of the shoulder girdle and humeru s which have been recorded in 10 subjects during unloaded and loaded h umeral abduction and anteflexion. Comparisons of muscle force predicti ons and EMG recordings are hampered by the unknown force-length relati onship and the length dependency of EMG amplitude. It is concluded tha t EMG amplitude cannot be used for validation of complex musculoskelet al models. Muscle function is analyzed with help of a force and moment balance of the three joints. The moment balance includes the contribu tions of ligaments and the reaction forces at the scapulothoracic glid ing plane. The scapulothoracic gliding plane is very important for the motions and the stabilization of the shoulder girdle. The direction a nd magnitude of joint reaction forces are calculated as well. It is co ncluded that the model provides good insight into the mechanics of the shoulder mechanism and that it enables an analysis of the function of morphological structures.