However, analysis of strength data revealed no significant differences between the children with BPP who had prior orthopedic surgery and those who had not. Extensors and external rotators were the weakest muscles in this sample of children with BPP. Extension, external rotation, and internal rotation involve muscles innervated by the posterior cord of the brachial plexus (triceps, deltoid, teres minor, subscapularis, and teres major).