André De Troyer

Mechanics of the Respiratory Muscles

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.

Mechanical coupling between the ribs and sternum in the dog

We measured the axial (cranio-caudal) displacements of the sternum and the second and seventh bony ribs using linear displacement transducers in five supine anesthetized dogs during passive inflation and deflation, during quiet breathing and static inspiratory efforts before and after bilateral phrenicotomy, and during tetanic stimulation of either the sternocleidomastoids or the sternal fibers of the rectus abdominis. Quiet inspiration before and after phrenicotomy was always associated with a caudal displacement of the sternum and a cranial displacement of the seventh rib; the second rib, however, was either motionless or also showed an inspiratory caudal displacement. During static inspiratory efforts, the second rib was always moving in concert with the sternum in the caudal direction, while the seventh rib, in particular after phrenicotomy, usually moved in the cranial direction. Finally, for any given axial (cranial or caudal) displacement of the sternum, stimulation of the sternocleidomastoid or rectus abdominis muscles invariably caused the second rib to move disproportionately more than the seventh. These results indicate that the upper ribs are more tightly linked to the sternum than the lower ribs. This presumably results from the fact that the costal cartilages increase in length from above downwards, and it implies that the upper portion of the rib cage behaves more as a unit with the sternum than the lower portion.