Electromyographic activity in the immobilized shoulder girdle musculature during contralateral upper limb movements

Abstract

The purpose of this study was to quantify electromyographic (EMG) activity in the immobilized shoulder girdle musculature at rest and during a battery of contralateral upper limb activities. Six asymptomatic men, aged 22 to 33 years, volunteered to participate. Fine-wire (supraspinatus, infraspinatus) and surface (deltoids, trapezii, biceps, serratus anterior) electrodes recorded the mean peak normalized (percent maximal voluntary contraction [%MVC]) EMG activity from each immobilized muscle at rest and during slow, fast, and incrementally resisted contralateral upper limb motions (5, 15, and 25 lb). EMG activity in all muscles was low during quiet immobilized standing (<1.5% maximal voluntary contraction [MVC]). During slow contralateral upper limb motions, activity ranged from 0.7% to 51.6% MVC (highest in trapezii) and was less than 15% MVC in the supraspinatus, infraspinatus, and anterior deltoid. Bimanual jar opening increased biceps activity from 7.8% to 16.1% MVC. During fast contralateral upper limb motions, peak infraspinatus activity increased to 56.7% during a fast straightforward reach. Supraspinatus activity was relatively high during all resisted backward-pulling motions (25.2%-32.1% MVC), whereas resisted forward reaching produced relatively little activity in the anterior deltoid, supraspinatus, infraspinatus, or biceps. Several slow and fast motions produced high trapezius activity (>45% MVC) with low supraspinatus, biceps, and anterior deltoid activities (<10% MVC). Our findings suggest that (1) immobilized shoulder girdle muscle EMG activity during quiet standing is negligible in asymptomatic individuals; (2) contralateral upper limb motions at self-selected speeds are not likely to be harmful to healing tissues; (3) during early healing periods, patients with biceps-labral injury should minimize bimanual activities, those with supraspinatus injury should avoid backward-pulling motions, and those with infraspinatus injury should avoid fast straightforward reaches; and (4) cross-body, straightforward, or downward reaches at either a slow or fast speed may be appropriately prescribed as rehabilitative exercises that can be initiated while the shoulder remains immobilized.