The primary function of the lung is gas exchange. The mechanics of respiration are as important to adequate gas exchange as the properties inherent to the lung itself. The muscles that aid in inspiration and expiration play an important role in the mechanics of respiration. Often, an injury to these muscles or the nerves that control them can result into altered respiratory mechanics and an impairment of gas exchange.

The diaphragm is the primary muscle used during inspiration. It consists of a thin dome-shaped muscle that inserts into the lower ribs. It is innervated by the phrenic nerves arising from cervical segments 3, 4, and 5. During inspiration, the dome of the diaphragm contracts downward and forward, expanding the vertical dimension of the chest. In addition, there is an elevation of the lower ribs where the diaphragm inserts and the ribcage moves outward, resulting in an increase in the transverse diameter of the chest. Other accessory muscles of inspiration, not necessarily used during quiet respiration, include the sternocleidomastoid, scalenes, internal intercostal, and external intercostal muscles.

In contrast, expiration is passive during quiet breathing. After being expanded during inspiration, the elastic properties of the lung and chest wall allow them to return to their positions of equilibrium. Once expiration becomes active, as it does during hyperventilation, the muscles of the abdominal wall become the most important. When these muscles contract, the diaphragm is pushed upward. This return to the equilibrium position is further assisted by the internal intercostal muscles, which pull the ribs down and inward.