This article examines the mechanics of the muscles that drive expansion or contraction of the chest wall during breathing. The diaphragm is the main inspiratory muscle. When its muscle fibers are activated in isolation, they shorten, the dome of the diaphragm descends, pleural pressure (P(pl)) falls, and abdominal pressure (P(ab)) rises. As a result, the ventral abdominal wall expands, but a large fraction of the rib cage contracts. Expansion of the rib cage during inspiration is produced by the external intercostals in the dorsal portion of the rostral interspaces, the intercartilaginous portion of the internal intercostals (the so-called parasternal intercostals), and, in humans, the scalenes. By elevating the ribs and causing an additional fall in P(pl), these muscles not only help the diaphragm expand the chest wall and the lung, but they also increase the load on the diaphragm and reduce the shortening of the diaphragmatic muscle fibers. The capacity of the diaphragm to generate pressure is therefore enhanced. In contrast, during expiratory efforts, activation of the abdominal muscles produces a rise in P(ab) that leads to a cranial displacement of the diaphragm into the pleural cavity and a rise in P(pl). Concomitant activation of the internal interosseous intercostals in the caudal interspaces and the triangularis sterni during such efforts contracts the rib cage and helps the abdominal muscles deflate the lung.
© 2011 American Physiological Society.