[Study of the tracking movement in dislocation of the acromioclavicular joint]

Tracking movement in the scapular plane and electromyograms from the upper trapezius, rhomboideus, middle deltoideus, and pectoralis major muscles were analyzed during the movement in ten subjects who had grade II dislocation of the acromioclavicular joint. The purpose of this study was to clarify whether or not the dislocation of the acromioclavicular joint affects the rhythmic movement of the shoulder joint, and to evaluate objectively fatigue on the shoulder girdle of the dislocated side. The dislocation of acromioclavicular joint affects the fast rhythmic movement of the shoulder joint from 90 degrees elevation, but does not affect the movement from hanging position. The amplitude of electromyograms of some muscles on the dislocated side is greater than that on the sound side in the same movement of the shoulder joint. This increase of action potential on the dislocated side is thought to produce fatigue.