Influence of muscle activity on the three-dimensional shoulder girdle motion pattern - a study using open MR imaging

The objective of this study was to analyze the influence of shoulder muscle activity on the three-dimensional motion pattern of the shoulder girdle in the living. 14 healthy volunteers were investigated with an open MRI system at 60-120 d egrees abduction - with and without activity of the shoulder abductors. The 3D motion pattern of the humerus, scapula, clavicle and M. supraspinatus w ere computed. Under muscle activity, the glenoid demonstrated a slightly reduced elevatio n at 60 degrees of abduction (20,5 degrees +8,2 degrees vs. 23,1 degrees 6, 3 degrees; not significant), and at 120 degrees a significantly higher degr ee of rotation under muscle activity (43,3 degrees +/-8,6 degrees vs. 36,1 degrees +/-5,2 degrees; p<0,01), whereas at 90<degrees> no difference was o bserved. This caused a significant reduction of the scapulo-humeral rhythm at 120 degrees (2,6 vs. 1,5; p<0,01),a significant increase of the supraspi natus angle (123,3<degrees>+/-6,7 degrees vs. 117,3 degrees +/- 6,5 degrees ; p<0,05) as well as a reduction of the supraspinato-humeral relationship ( 1,0 vs. 0,97). The study demonstrates that muscle activity leads to an alteration of shoul der girlde motion patterns at higher degrees of abduction, with increased r otation of the scapula, an altered spatial relationship between the suprasp inatus and humerus.