THE EFFECT OF ACTIVE LOADING ON TRANSLATION OF THE HUMERAL HEAD DURING GLENOHUMERAL ABDUCTION

This study was conducted to determine the effect of passive and active muscle loading on humeral head translation during glenohumeral abduction. A shoulder simulator produced unconstrained active glenohumeral abduction using several sets of loading ratios. Significantly greater translations occurred in passive motion as compared to active motion between 30 and 70 degrees of elevation in three dimensions and in the anterosuperior plane. No difference was found between the active motions. Also, translations of the humeral head decreased with active simulation of abduction emphasizing the importance of the rotator cuff muscles in creating and maintaining the ball-and-socket kinematics of the shoulder. This in-vitro study was conducted to determine the effect of passive and active loading on humeral head translation during glenohumeral abduction. Five cadaveric shoulders were tested using a shoulder simulator designed to produce unconstrained abduction of the humerus. Forces were applied to simulate loading of the supraspinatus, subscapularis, infraspinatus/teres minor, anterior, middle, and posterior deltoid muscles using four different sets of loading ratios. These were based on: equal loads to all cables (Constant-Constant); average physiological cross-sectional areas (pCSAs) of the muscles (pCSA); constant (Constant EMG), and variable (Variable EMG) values of the product of electromyographic data and pCSAs. In three dimensions, significantly greater translations occurred in passive motion as compared to active motion between 30 and 70 degrees of elevation (p It was clearly shown that the translations of the humeral head decreased with active simulation of abduction. These findings are in agreement with other in-vivo and in-vitro investigations. This emphasizes the importance of the rotator cuff muscles in creating and maintaining the ball-and-socket kinematics of the shoulder.