Shoulder impingement is a common condition presumed to contribute to rotator cuff disease.
Impingement can occur externally with the coracoacromial arch or internally with the glenoid rim.
Normal scapulothoracic motions that occur during arm elevation include upward rotation, posterior
tilting, and either internal or external rotation. These scapulothoracic motions and positions are the
result of coupled interactions between sternoclavicular and acromioclavicular joints. The sternoclavicular
and acromioclavicular joints both contribute to scapulothoracic upward rotation. Posterior tilting is
primarily an acromioclavicular joint motion. The sternoclavicular and acromioclavicular joint motions
offset one another regarding final scapulothoracic internal/external rotation position. This manuscript
discusses these coupled interactions in relation to shoulder muscle function. Two case examples are
presented to demonstrate application of understanding these interactions and potential mechanisms of
movement abnormalities in targeting treatment interventions for movement based subgroups of
impingement patients.