The unstable shoulder in arm elevation: A three-dimensional and electromyographic study in subjects with glenohumeral instability

Background. Although an altered scapulohumeral rhythm is believed to be related to some shoulder dysfunctions, there is scarce evidence on the scapulohumeral rhythm and shoulder muscle activation patterns in subjects with glenohumeral instability.Methods. Six subjects with glenohumeral instability performed arm elevation in trials of 8 successive stationary positions. Kinematics were recorded on each stationary position using a 6 DOF electromagnetic tracking device. Scapular kinematics was analysed by comparison between recorded scapular angles and those estimated by a linear regression model of the scapulohumeral rhythm which served as standard for normality. Surface electromyographic activity was recorded from the upper and lower trapezius, serratus anterior and deltoid.Findings. Differences in the scapulohumeral rhythm were observed between subjects. Significant differences were founded in scapular protraction and spinal tilt. No differences were found in scapular lateral-rotation in all subjects. Activity of the lower trapezius and serratus anterior seem to increase mostly during the second third of arm elevation.Interpretation. In a particular angle of arm elevation, changes in scapular protraction and/or spinal tilt rotation could introduce changes on the alignment of the humeral head and the glenoid, predisposing/contributing to shoulder instability. These kinematics scapular changes are perhaps due to an inadequate muscular activity.RelevanceThe results of this study reinforce the relevance of the scapulohumeral rhythm assessment in the rehabilitation of patients with shoulder instability.