Scapulohumeral Rhythm

The Scapulohumeral Rhythm is the ratio of movement between the scapula and the humerus. It helps to make sure movement between the scapula and the humerus remain even. For every 3 degrees of shoulder movement, the scapulothoracic joint moves 1 degree, and the glenohumeral joint moves 2 degrees.

The Scapulohumeral Rhythm helps maintain the subacromial space, which helps avoid impingements of the supraspinatus. It also aligns the glenoid fossa with the head of the humerus for proper shoulder function.

Motion during the Scapulohumeral Rhythm is distributed between two joints. This allows joint congruency, which results in decreased shear forces. The Scapulohumeral Rhythm is also important in muscle function. It helps to prevent active insufficiency of the glenoidhumeral muscles by allowing optimal length-tension relationships. Therefore, those muscles are still able to produce force.

When testing range of motion, if the shoulder abducts 180 degrees, the humerus abducts 120 degrees while the scapula upwardly rotates 60 degrees. For the humerus to fully flex or abduct, the humeral head must laterally rotate and the scapula must upwardly rotate and posteriorly tilt. Limited range of motion at the scapulothoracic joint will result in limited motion at the glenohumeral joint and vise versa.  If the scapular muscles are weak, then the range of motion of the humerus may be limited and vise versa.