[Function of the supraspinatus muscle in a dynamic shoulder model]

Active glenohumeral joint motion was simulated in ten cadaveric shoulder specimens with hydrodynamic cylinder forces at the deltoid muscle and at the rotator cuff. Computerized regulation initiated standardized cycles of glenohumeral joint motion in which the isolated effect of the supraspinatus muscle could be studied. The effect of the supraspinatus muscle on the elevation of the glenohumeral joint was measured with an ultrasonic sensor system. Pressures underneath the coracoacromial arch as an indicator of the 'impingement' at the shoulder were recorded with capacitive sensors. Elimination of force of the supraspinatus muscle led to a significant 6% decrease in elevation (p < 0.05). The deltoid muscle was able to reverse this loss of elevation by an increase in force equal to only one third the force lost at the supraspinatus muscle. When no force was applied to the supraspinatus muscle average pressures under the coracoacromial vault decreased by 8% (p > 0.05). In conclusion, the supraspinatus muscle produces less torque and more glenohumeral joint compression than the deltoid muscle. However, the supraspinatus muscle has no significant effect on the centering of the humeral head on the glenoid during elevation.