1. The deep posterior muscles of the neck are innervated by the posterior branches of spinal nerves, which branch off immediately after the root emerges from the intervertebral foramen. Electromyographic examination of these muscles permits a differential diagnosis to be made between intraforaminal and extraforaminal brachial plexus lesions.

2. The earlier diagnosis and prognosis thus achieved permit definitive treatment, in particular suture of the torn nerve trunks in recent extraforaminal cases.