The shoulder joint has the greatest mobility in the body, but sacrifices stability as a result. The scapulohumeral rhythm is what allows us to have a large ROM at the shoulder that acts in three planes. Without the scapulohumeral rhythm, we would lack the functional ROM needed to perform Activities of Daily Living made possible by the clavicle, scapula, and humerus working together to form kinetic chains. However, if there is dysfunction at any of the joints, there can be a decrease in overall ROM. The scapulohumeral rhythm includes a ratio for movement between the scapula and the humerus, and since the motion is divided between two joints—the scapulothroacic joint and the glenohumeral joint—there is a greater ROM possible. Such a rhythm allows for a good length-tension relationship of the glenohumeral muscles as well as a reduction in shear forces as a result of congruency of the joints to avoid pain. For every 3 degrees of shoulder movement, 1 degree happens at the scapulothoracic joint and 2 degrees occur at the glenohumeral joint. So, if the full ROM at the shoulder is 180 degrees, 60 degrees occur at the scapulothoracic joint while 120 degrees occurs at the glenohumeral joint. The scapulohumeral rhythm includes upward rotation of the scapula which prevents impingement of the contents of the subacromial space by allowing the space to be preserved. Additionally, the glenoid fossa of the scapula and the head of the humerus move synchronously in the rhythm to avoid dislocation of the humerus.
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Image: https://www.bsiphysiotherapy.com/post/understanding-the-scapulohumeral-rhythm
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