EMG response of shoulder muscles to acute subacromial pain
Changes in activation levels of the force couples acting on the scapulothoracic and glenohumeral joint are assumed to be either the cause or the consequence of subacromial pain (SAP). Via experimentally-induced pain, this study attempted to investigate the effects of SAP on EMG variables of periscapular and glenohumeral muscles in the concentric and the eccentric phase of an arm elevation task.
20 healthy subjects performed an arm elevation task in the scapular plane in two conditions: 1) with SAP, induced by a hypertonic saline injection; and 2) in a pain free control situation. Fine wire and surface EMG were used to record muscle activity of the subscapularis, supraspinatus, infraspinatus, deltoid, serratus anterior and upper and lower trapezius.
No significant differences in activation levels between the SAP and control condition in the concentric phase; during the 60 to 30 degrees of elevation during the eccentric phase of the exercise, a significant increase in serratus anterior and deltoid activity and a significant decrease in infraspinatus activity were present. Possibly adding external load in the exercise would have elicited more distinct activation patterns.