Respiratory mechanical advantage of the canine external and internal intercostal muscles

1. The current conventional view of intercostal muscle actions is based on the theory of Hamberger (1749) and maintains that as a result of the orient ation of the muscle fibres, the external intercostals have an inspiratory a ction on the lung and the internal interosseous intercostals have an expira tory action. This notion, however, remains unproved. 2. In the present studies, the respiratory actions of the canine external a nd internal intercostal muscles were evaluated by applying the Maxwell reci procity theorem. Thus the effects of passive inflation on the changes in le ngth of the muscles throughout the rib cage mere assessed, and the distribu tions of muscle mass were determined. The fractional changes in muscle leng th during inflation were then multiplied by muscle mass and maximum active stress (3.0 kg cm(-2)) to evaluate the potential effects of the muscles on the lung. 3. The external intercostals in the dorsal third of the rostral interspaces were found to have a large inspiratory effect. However, this effect decrea ses rapidly both toward the costochondral junctions and toward the base of the rib cage. As a result, it is reversed to an expiratory effect in the mo st caudal interspaces. The internal intercostals in the caudal interspaces have a large expiratory effect, but this effect decreases ventrally; and ro strally such that it is reversed to an inspiratory effect in the most rostr al interspaces. 4. These observations indicate that the canine external and internal interc ostal muscles do not have distinct inspiratory and expiratory actions as co nventionally thought. Therefore, their effects on the lung during breathing will be determined by the topographic distribution of neural drive.