Muscle activation in the contralateral passive shoulder during isometric shoulder abduction in patients with unilateral shoulder pain

Studies have shown an increased muscle activation at the opposite passive s ide during unilateral contractions. The purpose of the present study was to examine the influence of pain on muscle activation in the passive shoulder during unilateral shoulder abduction. Ten patients with unilateral rotator tendinosis of the shoulder and nine healthy controls performed unilateral maximal voluntary contractions (MVC) and sustained submaximal contractions with and without subacromial injections of local anaesthetics of the afflic ted shoulder. Muscle activation was recorded by electromyography (EMG) from the trapezius, deltoid, infraspinatus and supraspinatus muscles in both sh oulders. During MVCs, the EMG amplitude from muscles of the passive afflict ed side was not different in patients and controls, and was not influenced by pain alterations. In contrast, the EMG amplitude from the muscles of the passive unafflicted side was lower in the patients and increased after pai n reduction. During the sustained submaximal contraction the EMG amplitude increased gradually in the passive shoulder to 15-30% of the EMG amplitude observed during MVC. This response was not influenced by differences in pai n. We conclude that muscle activation of the passive shoulder was closely r elated to the activation of the contracting muscles and thus related to cen tral motor drive, and not directly influenced by changes in pain. (C) 2000 Elsevier Science Ltd. All rights reserved.