Radiopaque markers were attached to the peritoneal surface of three ne
ighboring muscle bundles in the midcostal diaphragm of four dogs, and
the locations of the markers were tracked by biplanar video fluoroscop
y during quiet spontaneous breathing and during inspiratory efforts ag
ainst an occluded airway at three lung volumes from functional residua
l capacity to total lung capacity in both the prone and supine posture
s. Length and curvature of the muscle bundles were determined from the
data on marker location. Muscle lengths for the inspiratory states, a
s a fraction of length at functional residual capacity, ranged from 0.
89 +/- 0.04 at end inspiration during spontaneous breathing down to 0.
68 +/- 0.07 during inspiratory efforts at total lung capacity. The mus
cle bundles were found to have the shape of circular arcs, with the th
ree bundles forming a section of a right circular cylinder. With incre
asing lung volume and diaphragm displacement, the circular arcs rotate
around the line of insertion on the chest wall, the arcs shorten, but
the radius of curvature remains nearly constant. Maximal transdiaphra
gmatic pressure was calculated from muscle curvature and maximal tensi
on-length data from the literature. The calculated maximal transdiaphr
agmatic pressure-length curve agrees well with the data of Road et al.