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 glenohumera l joint depends on compression of the humeral head into the glenoid, reacti on forces constitute an important aspect of shoulder biomechanics. The obje ctive of this study was to measure reaction forces at the glenohumeral join t during active scapula plane abduction. Furthermore, to clarify the relati onship 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 forcemoment sensor to di rectly 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 fav ored the supraspinatus tendon, whereas simulated paralysis of the supraspin atus 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 join t reaction Forces varies according to the ratio of forces between the supra spinatus and deltoid muscles. Thus, conditions characterized by either delt oid or supraspinatus dysfunction may result in abnormal loading mechanics a t the glenohumeral joint. Understanding the relationship between rotator cu ff function and glenohumeral reaction forces will aid in clarifying the imp ortance of muscular activity to shoulder stability and strength as it relat es to compression of the humeral head.