In contrast, the innervation of muscles involved in flexion, the less impaired direction, are from the medial or lateral cords, with the exception of the deltoid (coracobrachialis, pectoralis major, and biceps brachii).26 It is generally thought that birth-related or obstetrical brachial plexus palsies involve the uppertrunk of the brachial plexus preferentially in contrast to BPP injuries in adults, which more often involve the lower trunk due to different injury mechanisms.4,27 Our results provide more detailed information related to the site of injury, emphasizing the key role of posterior cord injury leading to long-term shoulder dysfunction following birth injury.