Forces at the glenohumeral joint in abduction.

The purpose of this study was to determine the forces in the glenohumeral joint for isometric abduction (including all directions of rotation) in the plane of the scapula. The lines of actions of muscles were obtained from 3 upper quarter specimens, electromyographic data was used, as well as geometrical parameters of shoulder motion. The main assumption made in the analysis was that the force in a muscle was proportional to its area times the integrated electromyographic signal. The resultant force reached a maximum of 0.89 times body weight at 90 degrees of abduction, while the shearing component up the face of the glenoid was a maximum of 0.42 times body weight at 60 degrees of abduction.