Experimental investigation of reaction forces at the glenohumeral joint during active abduction.

Reaction forces at the glenohumeral joint counterbalance the mass moment of the upper extremity during shoulder motion and are directly related to the activity of muscles across the joint. Because stability of the glenohumeral joint depends on compression of the humeral head into the glenoid, reaction forces constitute an important aspect of shoulder biomechanics. The objective of this study was to measure reaction forces at the glenohumeral joint during active scapula plane abduction. Furthermore, to clarify the relationship between the deltoid and supraspinatus muscles throughout abduction, this study investigated the effect of 4 variations of applied muscle forces on the magnitude and direction of glenohumeral reaction forces. We used a dynamic shoulder testing apparatus equipped with a force-moment sensor to directly measure reaction forces. Joint reaction forces increased throughout abduction and peaked at approximately 90 degrees for all testing conditions. The largest reaction forces occurred when the ratio of applied forces favored the supraspinatus tendon, whereas simulated paralysis of the supraspinatus resulted in a significant decrease in joint compression. There were no differences in direction of the reaction force between testing conditions. The results of this study indicate that the magnitude of glenohumeral joint reaction forces varies according to the ratio of forces between the supraspinatus and deltoid muscles. Thus, conditions characterized by either deltoid or supraspinatus dysfunction may result in abnormal loading mechanics at the glenohumeral joint. Understanding the relationship between rotator cuff function and glenohumeral reaction forces will aid in clarifying the importance of muscular activity to shoulder stability and strength as it relates to compression of the humeral head.