A parallel mechanism of the shoulder—application to multi-body optimisation

This paper describes and evaluates a parallel mechanism of the shoulder girdle. This mechanism was a closed kinematic chain composed of three segments (humerus, scapula and thorax) and three kinematic constraints. The clavicle was modelled as a constant length constraint between the sternoclavicular and acromioclavicular joint centres. The second kinematic constraint was also a constant length between the glenoid cavity and the humeral head for the glenohumeral joint. The third constraint was a point-on-ellipsoid contact for the scapulothoracic joint. Geometrical data required to build this kinematic model were obtained from the Visible Human Project. The parallel mechanism was then introduced into a multi-body optimisation for the computation of the scapulothoracic joint angles from surface sensors during the abduction of the arm of six able-bodied subjects. The initial guess of this optimisation was obtained by an acromial method. Compared to palpation of scapula anatomical landmarks, the multi-body optimisation with the proposed parallel mechanism allows estimating the shoulder kinematics with a better accuracy than the acromial method alone.