A FINITE-ELEMENT MUSCULOSKELETAL MODEL OF THE SHOULDER MECHANISM

The finite element method described in this study provides an easy met hod to simulate the kinetics of multibody mechanisms. It is used in or der to develop a musculoskeletal model of the shoulder mechanism. Each relevant morphological structure has been represented by an appropria te element. For the shoulder mechanism two special-purpose elements ha ve been developed: a SURFACE element representing the scapulothoracic gliding plane and a CURVED-TRUSS element to represent muscles which ar e wrapped around bony contours. The model contains four bones, three j oints, three extracapsular ligaments, the scapulothoracic gliding plan e and 20 muscles and muscle parts. In the model, input variables are t he positions of the shoulder girdle and humerus and the external load on the humerus. Output variables are muscle forces subject to an optim ization procedure in which the mechanical stability of the glenohumera l joint is one of the constraints. Four different optimization criteri a are compared. For 12 muscles, surface EMG is used to verify the mode l. Since the optimum muscle length and force-length relationship are u nknown, and since maximal EMG amplitude is length dependent, verificat ion is only possible in a qualitative sense. Nevertheless, it is concl uded that a detailed model of the shoulder mechanism has been develope d which provides good insight into the function of morphological struc tures.