A three-dimensional model of the shoulder girdle. Forces developed in deltoid and supraspinatus muscles during abduction.

The aim of this study was to design a three-dimensional model of the human shoulder girdle. Fourteen muscles were modelled geometrically using robotics software (Solid Dynamics System). A static study in reverse dynamics allowed the forces developed in seven muscles to be studied, particularly in the deltoid and supraspinatus during abduction of the arm with the forearm bent at 90 degrees, the scapula being fixed. After optimization by Excel, this model allowed simultaneous curves of forces developed in every modelled muscular bundle to be obtained during various degrees of abduction of the arm from 0 degrees to 100 degrees, in the scapular plane. The analysis of supraspinatus and deltoid curves revealed an interesting chronology of action. The first muscle developed efforts estimated at 140 N, quickly relieved by the deltoid. It is also interesting to note that a constant action of supraspinatus was found throughout abduction of the arm. Clinical applications seem to be in simulation of muscular pathology of the shoulder girdle. This study is continuing by releasing the fixed scapula and by simulating muscle transfers proposed for massive ruptures of the rotator cuff.