Active stability of the glenohumeral joint decreases in the apprehension position

Background. Muscle forces that compress the glenohumeral joint during mid-ranges of motion may lead to increased translational forces in end-range positions, such as the apprehension position, where symptoms of anterior instability occur. Objective. The objective of this study was to quantify active stability provided by eight shoulder muscles in mid-range and end-range positions through muscle force vector analysis. Methods. Lines of action were derived from a geometric model and muscle force magnitudes were estimated with electromyography-based techniques. Resultant muscle force vectors were calculated by summing individual muscle force vectors. Results. Compared to mid-range positions, lines of action of resultant force vectors were more anteriorly directed in end-range positions compared to 15° of abduction, up to 26°. Consequently, anterior stability was lowest in the apprehension position. The magnitudes of the resultant force vectors were comparable to other studies. Based on a sensitivity analysis, lines of action of resultant force vectors vary up to 6° within the population. Conclusions. Data obtained from this model will improve conservative management, post-surgical rehabilitation, and strength training protocols.