The Muscles of Inspiration
The muscles of inspiration include the diaphragm, the external intercostals, and the accessory muscles. The diaphragm and external intercostals are the most important muscles of breathing because the accessory muscles are only active with high ventilation rates. During inspiration, contraction of the diaphragm and external intercostal muscles enlarges the chest cavity. This action leads to an expansion and stretching of the lung. As the lung inflates, potential energy is stored in the stretched elastic structures of the lung such that with relaxation of inspiratory muscles, expiration occurs from elastic recoil of the lung-chest cage apparatus. Thus, at rest, expiration is normally considered to be passive because muscle contraction is not required.