Visual Scapular Dyskinesis: Kinematics and Muscle Activity Alterations in Patients With Subacromial Impingement Syndrome

Abstract Objective To characterize scapular kinematics and shoulder muscle activity in patients with subacromial impingement syndrome, with and without visually identified scapular dyskinesis. Design Cross-sectional study. Setting Laboratory. Participants Participants with subacromial impingement syndrome (N=38) were visually classified using a scapular dyskinesis test with obvious scapular dyskinesis (n=19) or normal scapular motion (n=19). Interventions Not applicable. Main Outcome Measures An electromagnetic motion capture system measured 3-dimensional kinematics of the thorax, humerus, and scapula. Simultaneously, surface electromyography was used to measure muscle activity of the upper, middle, and lower trapezius; serratus anterior; and infraspinatus during ascending and descending phases of weighted shoulder flexion. Separate mixed-model analyses of variance for the ascending and descending phases of shoulder flexion compared kinematics and muscle activity between the 2 groups. Shoulder disability was assessed with the Pennsylvania Shoulder Score (Penn). Results The group with obvious dyskinesis reported 6 points lower on Penn shoulder function (0–60 points), exhibited a main group effect of less scapular external rotation of 2.1° during ascent and 2.5° during descent, and had 12.0% higher upper trapezius muscle activity during ascent in the 30° to 60° interval. Conclusions Patients with obvious dyskinesis and subacromial impingement syndrome have reduced scapular external rotation and increased upper trapezius muscle activity, along with a greater loss of shoulder function compared with those without dyskinesis. These biomechanical alterations can lead to or be caused by scapular dyskinesis. Future studies should determine if correction of these deficits will eliminate scapular dyskinesis and improve patient-rated shoulder use.