<P> The range of motion at the shoulder may be limited by pain . A painful arc of movement may be present during forward elevation of the arm from 60 ° to 120 ° . Passive movement at the shoulder will appear painful when a downwards force is applied at the acromion but the pain will ease once the downwards force is removed . </P> <P> When the arm is raised, the subacromial space (gap between the anterior edge of the acromion and the head of the humerus) narrows, through which the supraspinatus muscle tendon passes . Anything that causes further narrowing has the tendency to impinge the tendon and cause an inflammatory response, resulting in impingement syndrome . This can be caused by bony structures such as subacromial spurs (bony projections from the acromion), osteoarthritic spurs on the acromioclavicular joint, and variations in the shape of the acromion . Thickening or calcification of the coracoacromial ligament can also cause impingement . Loss of function of the rotator cuff muscles, due to injury or loss of strength, may cause the humerus to move superiorly, resulting in impingement . Inflammation and subsequent thickening of the subacromial bursa may also cause impingement . </P> <P> The scapula plays an important role in shoulder impingement syndrome . It is a wide, flat bone lying on the posterior thoracic wall that provides an attachment for three different groups of muscles . The intrinsic muscles of the scapula include the muscles of the rotator cuff - the subscapularis, infraspinatus, teres minor and supraspinatus . These muscles attach to the surface of the scapula and are responsible for the internal and external rotation of the glenohumeral joint, along with humeral abduction . The extrinsic muscles include the biceps, triceps, and deltoid muscles and attach to the coracoid process and supraglenoid tubercle of the scapula, infraglenoid tubercle of the scapula, and spine of the scapula . These muscles are responsible for several actions of the glenohumeral joint . The third group, which is mainly responsible for stabilization and rotation of the scapula, consists of the trapezius, serratus anterior, levator scapulae, and rhomboid muscles and attach to the medial, superior, and inferior borders of the scapula . Each of these muscles has their own role in proper shoulder function and must be in balance with each other in order to avoid shoulder pathology . Abnormal scapular function is called scapular dyskinesis . One action the scapula performs during a throwing or serving motion is elevation of the acromion process in order to avoid impingement of the rotator cuff tendons . If the scapula fails to properly elevate the acromion, impingement may occur during the cocking and acceleration phase of an overhead activity . The two muscles most commonly inhibited during this first part of an overhead motion are the serratus anterior and the lower trapezius . These two muscles act as a force couple within the glenohumeral joint to properly elevate the acromion process, and if a muscle imbalance exists, shoulder impingement may develop . </P> <P> Impingement syndrome can usually be diagnosed by history and physical exam . On physical exam, the physician may twist or elevate the patient's arm to test for reproducible pain (Neer sign and Hawkins - Kennedy test). These tests help localize the pathology to the rotator cuff; however, they are not specific for impingement . Neer sign may also be seen with subacromial bursitis . </P>

Shoulder impingement involves compression of which of the following structures