Electromyography in voluntary posterior instability of the shoulder.

Electromyographic (EMG) activity of selected shoulder girdle muscles was analyzed during voluntary posterior subluxation of the glenohumeral joint in four subjects. Although there was a great deal of variation in the muscle activation patterns during subluxation, subjects either pushed the humeral head backwards with the anterior muscles (deltoid, biceps) or pulled the humeral head backwards with the infraspinatus and/or posterior deltoid. In all cases, inhibition of the scapular rotators and winging of the scapula accompanied this maneuver. The method used in this study is presented as an objective technique for quantifying EMG activity during subluxation and relocation phases of shoulder instability.