C5-T1 roots, schematic representation

Schematic representation of the left C5-T1 roots with the cervical spine straight (top) and with the head tilted markedly toward the right shoulder (bottom). This either stretches the nerve roots (dashed lines) or causes them to become avulsed. As can be seen from the illustration, the upper roots are most severely stretched with this, and hence, most likely to be damaged causing an Erb-Duchenne palsy. (Source: Fieux G. De la pathologie des paralysies obstétricales chez le nouveau-né. Paralysies obstétricales. Annales de Gynécolologie et d’Obstétrique 1897;47:52-64. Modified and edited by Douglas Lanska. Public domain.)