Glenohumeral rhythm: In abduction, what is the pattern of movement up to 90 degrees, and after 90 degrees?

• A. In abduction, the humerus accounts for the movement up to 90 degrees, after which scapular rotation dominates
• B. Scapula accounts for most movement up to 90 degrees; humerus after 90
• C. Humerus and scapula contribute equally at all angles
• D. The clavicle drives abduction exclusively

Answer: (A)

Glenohumeral rhythm describes how the shoulder joint (humerus in the glenoid) and the scapula work together as the arm is lifted. In abduction, most of the movement up to about 90 degrees comes from the glenohumeral joint itself, with the scapula remaining relatively neutral. Once you pass 90 degrees, the scapula must rotate upward to allow the arm to continue rising, so scapular motion becomes the dominant contributor for the remaining range. The typical pattern is about two-thirds of the motion from the humerus and one-third from the scapula, yielding roughly 120 degrees at the glenohumeral joint and about 60 degrees of scapulothoracic upward rotation to reach a full 180-degree abduction. This is why the description that the humerus handles the initial lift and the scapula takes over after 90 degrees best matches the normal rhythm.