An affection of peripheral nerves, which would influence the shoulder girdle, is not so frequent. It is mostly secondary, connected with an iatrogenic affection at operations, or with traumas of the shoulder girdle. A base of a diagnostics of this disorder is a finding of a weakness of the muscular function of the affected muscle because the disorder of sensitiveness at peripheral nerves, which innervate the shoulder girdle, is not frequent ¹. Therefore an examination of the isometric contraction of muscles of the shoulder girdle is important. EMG brings a  confirmation of pathology of peripheral nerves. 

¹ we can meet a disorder of sensitiveness at affections of n. axillaris