Because the pectoralis major is innervated by both the medial and lateral cords, both would need to be affected to result in absent motor strength there. Inspection for atrophy of the medial sternal fibers versus the lateral clavicular fibers and side to side comparison often helps to determine if the pectoralis major is affected. Weakness of the latissimus dorsi, triceps, and wrist extensors all point to a lesion at or proximal to the posterior cord. If necessary, revise your differential diagnosis based on the additional physical findings. Design your approach to the electrophysiologic examination based on the existing data.
Reflexes reflect the muscles and roots levels from which they are derived.
Absence of the triceps reflex suggests impairment of the radial nerve, posterior cord, middle trunk, or C7 root.