The Importance of Scapulohumeral Rhythm in the Shoulder Complex

When referring to scapulohumeral rhythm, it is regarding the ratio of movement between the scapula and humerus at the scapulothoracic (ST) and glenohumeral (GH) joints and how they work together to allow complete ROM of the shoulder. For every 3˚ of shoulder movement, 1˚ should take place at the ST joint and 2˚ at the glenohumeral joint, so in a full 180˚ motion of the shoulder, 60˚ will be produced at the ST joint and 120˚ will be produced at the GH joint. This is a 2:1 ratio of humeral elevation to upward rotation of the scapula. The scapula must perform upward rotation when the humerus moves upward to prevent the humeral head from colliding with the acromion process. As the scapula rotates upward, it prevents this space from getting too small and allows full ROM. This is also important for the length-tension relationship of the muscles in this area so they do not shorten too much and can sustain their force for most of the movement.

The scapulohumeral rhythm is relevant in a clinical setting because it is important to understand how the shoulder complex should function, what signs to look for if it is not functioning properly, and how to treat specific conditions of the shoulder with muscle and joint knowledge. Dynamic stabilization is an important aspect of this rhythm because the shoulder is dependent on the muscles, ligaments, and tendons to maintain integrity of the shoulder complex. The rotator cuff (RTC) muscles: supraspinatus, infraspinatus, subscapularis, and teres minor align the humeral head with the smaller glenoid fossa. Injury to these muscles or the GH labrum may compromise stability of the GH joint. Shoulder impingement syndrome is a result of RTC weakness where the humeral head becomes elevated in the glenoid fossa during shoulder elevation causing pain, weakness at the shoulder, and less than full ROM. As future OT practitioners, it is important to be knowledgeable about scapulohumeral rhythm so we can treat conditions such as shoulder impingement in the most effective way.  

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