The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during quiet breathing.

The elasticity of these muscles is crucial to the health of the respiratory system and to maximize its functional capabilities.

[3] There is no definitive list of accessory muscles, but the sternocleidomastoid and the scalenes (anterior, middle, and posterior) are typically included, as they assist in elevating the rib cage.

The levator labii superioris alaeque nasi muscle lifts the sides of the nostrils.

During quiet breathing, there is little or no muscle contraction involved in exhalation; this process is simply driven by the elastic recoil of the lungs.

These press the abdominal organs cranially (upward) into the diaphragm, reducing the volume of the thoracic cavity.