Respiratory mechanical advantage of the canine external and internal intercostal muscles

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Abstract
The current conventional view of intercostal muscle actions is based on the theory of Hamberger (1749) and maintains that as a result of the orientation of the muscle fibres, the external intercostals have an inspiratory action on the lung and the internal interosseous intercostals have an expiratory action. This notion, however, remains unproved. In the present studies, the respiratory actions of the canine external and internal intercostal muscles were evaluated by applying the Maxwell reciprocity theorem. Thus the effects of passive inflation on the changes in length of the muscles throughout the rib cage were assessed, and the distributions of muscle mass were determined. The fractional changes in muscle length 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. The external intercostals in the dorsal third of the rostral interspaces were found to have a large inspiratory effect. However, this effect decreases 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 most caudal interspaces. The internal intercostals in the caudal interspaces have a large expiratory effect, but this effect decreases ventrally and rostrally, such that it is reversed to an inspiratory effect in the most rostral interspaces. These observations indicate that the canine external and internal intercostal muscles do not have distinct inspiratory and expiratory actions as conventionally thought. Therefore, their effects on the lung during breathing will be determined by the topographic distribution of neural drive.