The shoulder girdle is actually a series of three joints:
- Clavicle and sernum: sternoclavicular joint
- Acromion (top of the scapula) and the clavicle: acromioclavicular joint
- Front of the scapula and back of the ribs: scapulothoracic joint
- This is what we commonly think of when discussing the shoulder joint. It's the head of the humerus connecting with the scapula: Glenohumeral joint
But in a healthy shoulder joint we have a natural rhythm of movement among the scapula and the humerus. As the humerus moves, so does the scapula. This association is termed scapulohumeral rhythm: it's the rhythm between the scapula and the humerus.
You may have heard the adage "many hands make work light." Well, in an anatomical sense, this is exactly what is happening. The scapula and humerus function together to achieve the same result. Namely, to deliver the hand in a controlled and precise manner.
Taking a deeper look into the scapulhumeral rhythm we discover there to be a 2:1 relationship in movement. Specifically, for every 2 degrees movement of the humerus there is 1 degree movement for the scapula.
An example would include rotating the arm upward. If the humerus rotates 100 degrees up, the scapula will be rotating 50 degrees.
Andrew Kossow, CSCS