Shoulder abduction moment arms in three clinically important positions

Summary The abduction moment arms of 4 shoulder muscles were calculated in clinically important positions to evaluate the best test situation for the supraspinatus based on its mechanical advantage. Moment arms of the supraspinatus, infraspinatus, and middle and anterior deltoid in 18 individuals were computed using individual magnetic resonance imaging data and a computer-assisted design tool for simulation. Three tests with the arm in the neutral position (arm hanging on side), at 90° of scaption, and at 90° of scaption and full internal humeral rotation (Jobe test) were investigated. The supraspinatus has a greater mechanical advantage vs the other tested muscles in the neutral arm position. In the Jobe position, the supraspinatus' abduction moment arm is reduced with increased internal humeral rotation. Comparing these results with the literature indicates that this new method is adequate for calculation of moment arms and may be used in any desired joint configuration. Level of evidence Basic science study.