The Scapulohumeral Rhythm

The scapulohumeral rhythm is defined as a ratio of movement between the scapula and humerus. It is the collective movement of the scapula to maintain congruency of the glenoid fossa and the head of the humerus. This rhythm consists of upward rotation, posterior tilt, and lateral rotation, and the movement between the glenoid fossa and humeral head has to be synchronous. The scapular rotation allows for the shoulder abductor muscles to maintain an ideal length-tension relationship. The upward scapular rotation helps sustain the subacromial space that is located between the subacromial arch and head of the humerus, preventing impingement. The scapulohumeral rhythm is needed for efficient arm movement, full ROM, and a good muscle length-tension between the shoulder abductor muscles, which are the deltoid and supraspinatus muscles. Additionally, the rhythm increases congruency of the glenoid fossa and the head of the humerus. For every 3° of shoulder movement, 1° is at the scapulothoracic joint, and 2° is at the glenohumeral joint. Likewise, the ratio of the scapulothoracic motion and glenohumeral motion is 1:2. For example, if the shoulder is flexed to 180°, 120° is produced at the glenohumeral joint and 60° is produced at the scapulothoracic joint. The scapulohumeral rhythm helps prevent active insufficiency of the glenohumeral joint. If there is dysfunction of the scapular movement, there will be an increase of friction at the glenohumeral joint, which will cause pain to the structures in the subacromial space. Lastly, the clavicle, scapula, and humerus all function together to produce a kinetic chain. If there is dysfunction in one joint, that could result in a loss of total motion, impingement, and pain.