Scapulohumeral Rhythm

The scapulohumeral rhythm is important for movement and ROM. For full range of motion, the humeral head rotates laterally. Motion is distributed between both the scapula and the humerus. With both of these joints moving synchronously, it permits good length-tension relationship. As the scapula is rotating upward, the glenoid fossa is also rotating upward to maintain connection with the head of the humerus. The ratio between the humerus and the scapula is 2:1. This means that for every 3° of shoulder movement there is 1° of movement at the scapulothoracic joint (ST) and 2° of movement at the glenohumeral joint (GH). For example, if the humerus is abducted 150° upward the ST joint moves 50° and the GH joint moves 100°. The movement between these two joints allows for full range of motion for the shoulder. If one joint is injured or not functioning properly, this will affect the other joint and the ROM for the shoulder altogether. Also, scapula stability is important for preventing shoulder impingement syndrome. This causes scapulothoracic muscle weakness and rotator cuff muscle weakness. This would also affect ROM for the two joints and for shoulder movement as a whole. In conclusion, scapulohumeral rhythm affects both the ST joint and GH joint and is vital for full ROM in the shoulder.