Kinematics and Muscle Activation in Subacromial Pain Syndrome Patients and Asymptomatic Controls

Background: Conflicting theories exist about the underlying cause of chronic subacromial pain in the middle-aged population. We aim to improve our understanding of kinematics and muscle activation in subacromial pain syndrome to provide insight in its pathophysiology. Methods: In a cross-sectional comparison of 40 patients with subacromial pain syndrome and 30 asymptomatic controls, three-dimensional shoulder kinematics and electromyography-based co-contraction in 10 shoulder muscles were independently recorded. Glenohumeral and scapulothoracic kinematics were evaluated during abduction and forward flexion. Co-contraction was expressed as an activation ratio, specifying the relative agonistic and antagonistic muscle activity in each muscle. Findings: During abduction and forward flexion, the contribution of glenohumeral motion to elevation and glenohumeral external rotation was lower in subacromial pain syndrome (at 1200 abduction: - 9 degrees, 95% CI -14 degrees- -3 degrees; and - 8 degrees, 95% CI -13 degrees -3 degrees, respectively), and was compensated by more scapulothoracic motion. The pectoralis major's activation ratio was significantly lower (Z-score: -2.657, P = 0.008) and teres major's activation ratio significantly higher (Z-score: -4.088, P < 0.001) in patients with subacromial pain syndrome compared to the control group. Interpretation: Reduced glenohumeral elevation and external rotation in subacromial pain syndrome coincided with less teres major antagonistic activity during elevation. These biomechanical findings provide a scientific basis for intervention studies directed at stretching exercises to reduce glenohumeral stiffness in the treatment of subacromial pain syndrome, and teres major strengthening to improve humeral head depressor function.