The diaphragm is the primary muscle responsible for inspiration during quiet breathing because its contraction increases the vertical dimension of the thoracic cavity, creating negative intrathoracic pressure that draws air into the lungs. Unlike accessory muscles such as the external intercostals, sternocleidomastoid, and scalene muscles, which become active only during forced or labored breathing, the diaphragm functions continuously and efficiently at rest. Clinically, phrenic nerve injury leads to diaphragmatic paralysis and significant respiratory compromise, highlighting its critical role. Understanding the diaphragm’s anatomy and innervation is essential in surgeries involving the neck, thorax, and upper abdomen to avoid respiratory complications.

Reference: Gray's Anatomy, Standring S, 42nd Edition.