Ratio of active to passive muscle shortening in the canine diaphragm

Active and passive shortening of muscle bundles in the canine diaphragm wer e measured with the objective of testing a consequence of the minimal-work hypothesis: namely: that the ratio of active to passive shortening is the s ame for all active muscles. Lengths of six muscle bundles in the costal dia phragm and two muscle bundles in the crural diaphragm of each of four bred- for-research beagle dogs were :measured by the radiopaque marker technique during the following maneuvers: a passive deflation maneuver from total lun g capacity to functional residual capacity, quiet breathing, and forceful i nspiratory efforts against an occluded airway at different lung volumes. Sh ortening per liter increase in lung volume was, on average, 70% greater dur ing quiet breathing than during passive inflation in the prone posture and 40% greater in the supine posture. For the prone posture, the ratio of acti ve to passive shortening was larger in the ventral and midcostal diaphragm than at the dorsal end of the costal diaphragm. For both postures, active s hortening during quiet breathing was poorly correlated with passive shorten ing. However, shortening during forceful inspiratory efforts was highly cor related with passive shortening. The average ratios of active to passive sh ortening were 1.23 +/- 0.02 and 1.32 +/- 0.03 for the prone and supine post ures, respectively. These data, taken together with the data reported in th e companion paper (T. A. Wilson, M. Angelillo, A. Legrand, and A. De Troyer , J. Appl. Physiol. 87: 554-560, 1999), support the hypothesis that, during forceful inspiratory efforts, the inspiratory muscles drive the chest wall along the minimal-work trajectory.