Muscle forces that compress the glenohumeral joint during midranges of motion may lead to increased translational forces in endrange positions, such as the apprehension position, where symptoms of anterior instability occur. 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. Lines of action were derived from a standard geometric model and muscle force magnitudes were estimated with electromyography-based techniques. Resultant muscle force vectors were calculated by summing individual muscle force vectors. Compared to mid-range positions, lines of action of resultant force vectors were more anteriorly-directed in end-range positions. The deviation angle in the anterior direction was greatest (35°) and, consequently, stability was lowest in the apprehension position. Based on a sensitivity analysis, lines of action of resultant force vectors vary up to 6° within the population. In the apprehension position, muscle forces may promote anterior humeral head translation, predisposing the glenohumeral joint to anterior instability when other joint stabilizers are not functioning normally.

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