Electromyographic recordings in shoulder muscles during eccentric movements.

The purpose of the present study was to investigate shoulder muscle activity during eccentric muscle movements, and to determine whether electromyograms in patients with joint laxity differed from those in normal subjects. Five normal subjects and 6 patients with generalized joint laxity and shoulder instability were studied. Both shoulders were investigated. Normalized electromyograms were recorded during eccentric loaded movements from 8 shoulder muscles in parallel. Intra-muscular fine wire electrodes were used for 3 muscles of the rotator cuff: subscapularis, supraspinatus, and infraspinatus. Surface electrodes were used for superficially located muscles: the anterior, middle, and posterior parts of the deltoid, pectoralis major, and latissimus dorsi. A general trend was an activation of several muscles rather than a single muscle during all movements investigated. Patients with generalized joint laxity activated their supraspinatus and subscapularis muscles to a higher level during flexion and adduction movements than normal subjects did. This might indicate a greater necessity for muscular activity to provide anterior shoulder stability in lax joints. Compared with concentric movements previously studied, results from this study showed that the magnitude of activation was significantly lower during eccentric movements in normal subjects and in patients with joint laxity.