Biomechanics Blog 3

The scapulohumeral rhythm is significant in that it dictates the degree of motion between the scapula and the humerus. The shoulder has the greatest mobility of any joint in the body and therefore the degree of motion between the scapula and the humerus is vital for us to be able to utilize that range. 

For every 3 degrees of movement at the shoulder we have 1 degree of movement at the scapulothoracic joint, and 2 degrees of motion at the glenohumeral joint. 

This means, that at the shoulder which should have about 180 degrees of motion, the scapulothoracic joint will produce 60 degrees of motion and the glenohumeral joint will produce 120 degrees of motion. 

To utilize the full range of motion at the shoulder the humeral head must rotate laterally. The scapulohumeral rhythm permits optimal length tension relationships, which helps to prevent active insufficiency of the glenohumeral muscles. Additionally, the scapulohumeral rhythm helps to align the glenoid fossa with the head of the humerus. This is important because if these structures are not aligned it could have an effect on the measurements of range of motion. If the concave surface of the glenoid fossa is not aligned with the convex surface of the humeral head, range of motion may be compromised (less than 180).