Moment arms of the anatomical subregions of the rotator cuff muscles during shoulder rotation

Background: Rotator cuff muscles are responsible for humeral rotation. Moment arms of different regions of these muscles during humeral rotation were analyzed in neutral and abducted positions.Methods: In eight cadaveric shoulders, subregions of the rotator cuff muscles were identified and their excursion during humeral rotation was measured in neutral and abducted positions from an internal rotation of 30 degrees to an external rotation of 45 degrees, with 15 degrees increments, using a 3-D digitizing system. Statistical tests were used to assess differences between subregions within a single muscle.Findings: The posterior-deep subregion of the supraspinatus muscle had greater moment arms compared to the anterior-superficial and anterior-middle subregions in both positions (p < 0.001). The middle and inferior subregions of the infraspinatus muscle and the teres minor muscle showed differences in moment arms compared to the superior region in an abducted position (p < 0.042). The superior subregion of the subscapularis muscle showed differences in moment arms compared to the middle and inferior subregions in an abducted position (p < 0.001).Interpretation: The posterior-deep subregion of the supraspinatus muscle behaved similar to the infraspinatus muscle, as an external rotator. The anterior-superficial and anterior-middle subregions of the supraspinatus muscle showed a biphasic behavior during rotation at a neutral position, but acted as pure external rotators during rotation at an abducted position. Inferior subregions of the infraspinatus and subscapularis muscles showed the largest moment arms compared to superior subregions. These findings support distinct functional roles of the rotator cuff muscle subregions.