Scapulohumeral Rhythm

What is the clinical relevance of the scapulohumeral rhythm?

How can it affect our ROM measurements of the shoulder?

The shoulder has a great amount of mobility but with that great mobility comes instability. The head of the humerus is 2/3 the size of the glenoid fossa. That is pretty big compared to the small size of the glenoid fossa. This is where the body enlists the help of the scapula. The scapula helps to stabilize the head of the humerus at the glenoid fossa. In order for the scapula to be of assistance in stabilization at the glenohumeral joint it must move with it. The collaborative movement of the scapula and the glenohumeral joint is called scapulohumeral rhythm. The scapulohumeral rhythm is an important dance for if the scapula did not move with the glenohumeral joint it could not be there to maintain the  stabilization of the head of the humerus in the glennoid fossa, as well as perform a couple of other important duties.

When the arm reaches upward, as to reach a coffee mug out of the cabinet and the shoulder is in elevation, the scapula rotates upward and tilts posteriorly to help the glenoid fossa move upward as well and maintain connection with the head of the humerus. For every 3 degrees of movement at the shoulder there is  2 degrees of movement at the glenohumeral joint and 1 degree of movement at the scapulothoracic joint. Scapulohumeral rhythm allows for the head of the humerus to rotate laterally which is needed for full range of movement. It also allows for the abductor muscles of the shoulder to maintain an optimal length tension relationship. The scapulohumeral connection when maintained also decreases shearing forces on the shoulder muscles. Upward rotation of the scapula also helps to maintain the subacromion  space and prevent impingement at that area. If the scapula cannot move with the glenohumeral joint and the rhythm is effected it limits full rang of motion as well as can cause impingement injuries.