Scapulothoracic movement is small and inconsistent. You can observe scapulohumeral rhythm by palpating the scapula's position as a person elevates the shoulder. Helpful scapular landmarks for palpation are the base of the spine and the inferior angle.
Scapulohumeral rhythm serves at least two purposes.
It prevents impingement between the humerus and the acromion. Because of the difference in size between the glenoid fossa and the humeral head, subacromial impingement can occur unless relative movement between the humerus and scapula is limited.