Biomechanics- Shoulder

    Scapulohumeral Rhythm is the relationship of movement between the scapula and the humerus during upper extremity movements.  The clinical relevance of this rhythm is that in order to obtain the full 180 degrees of motion in the shoulder, there must be a synchronized movement between the scapulothoracic and glenohumeral joint. The standard ratio of the scapulohumeral rhythm is 2:1. This means that for every 2 degrees the humerus moves in the glenohumeral joint, the scapula moves 1 degree in the scapulothoracic joint. So even though the majority of movement occurs at the glenohumeral joint, without that 1 degree of movement of the scapula, one would not be able to obtain the full range of motion that our shoulders can achieve. The scapulohumeral rhythm also helps in maintaining length tension relationships, meaning that muscles do not shorten as much as they would without the upward rotation of the scapula. This allows the muscles to sustain their force production through a larger range of motion.  

    The scapula is a stable base for the humerus to move in various directions. There would be less control and overall function of the glenohumeral joint if the scapula were not functioning properly. This could lead to decreased range of motion, impingement, and pain. The scapulohumeral rhythm prevents impingement between the humerus and acromion. The simultaneous movement of the scapula and humerus during shoulder elevation limits the relative movement between the bones and, if the scapula does not move during this motion, there would be increased friction of the subacromial space and less efficient muscle movements. 

    Optimal function of the shoulder is reliant on the contribution of the scapula and humerus. Scapulohumeral rhythm is a common way to assess muscle function and can make one aware of injuries. By understanding the relationship between the scapula and humerus, you can be aware of what could be contributing to a dysfunction of the joint and, therefore, be able to fully address the issue.