Scapulohumeral Rhythm

The interaction between the humerus and scapula is vital for optimal mobility of the shoulder. The coordinated movements of the arm occur through the interplay of articulation of the four joints in the shoulder complex making sure the movements between the scapula and humerus are synchronized. This synchronization prevents active insufficiency from occurring in the joint. The motion between the scapula and humerus is distributed between two joints making it a 2:1 ratio. Meaning, for every two degrees of movement at the glenohumeral joint there is one degree of movement at the scapulothoracic joint. Lateral rotation of the humeral head by the infraspinatus and teres minor allow for full range of motion between these two joints. This rhythm allows for an optimal length tension relationship and maintains the subacromial space. If the subacromial space is not maintained through coordinated movements of the scapula and humerus, there can be friction to form in the space resulting in shoulder impingement.