To date, studies of glenohumeral stress forces have been based on the notion of a unique center of rotation situated at the geometric center of the humeral head. Early work, e.g. Fischer et al in 1977, suggested that the instantaneous center of rotation can migrate during abduction producing muscle vectors with variable moments and consequently variable stress forces. We conducted a kinematic analysis of the shoulder forces during continuous movement.

An imaging device commonly used for angiography enabled acquisition of one image per second during a continuous abduction movement. Images were obtained for twenty healthy shoulders in ten subjects.

The instantaneous center of rotation was not situated at the geometric center of the humeral head. Although there was significant interindividual variability, the preferred positions for the instantaneous center of rotation implied a constant reproducible succession of gliding, rolling, and translation in variable proportions during the abduction movement. When the same movement was performed with a 3500g load held in the hand, the ICR migrated towards the upper part of the humeral head, implying a predominance of rolling motion over gliding and translation motions. Between 40° and 60° glenohumeral abduction, the instantaneous center of rotation exhibited a metaphyseal displacement, producing a longer lever arm for the supraspinatus and an abduction function for the infraspinatus and subscapularis, unrecognized to date. After 60° medialization of the instantaneous center of rotation lengthened the lever arm of the deltoid.

A valid analysis of the glenohumeral stress forces must take into consideration the localization of the instantaneous center of rotation. In a clinical setting, our findings demonstrate that the infraspinatus and subscapularis play an important role which must be considered both in surgery and rehabilitation.