Radiopaque markers were attached at 1- to 2-cm intervals along three n
earby muscle bundles to cover rectangular regions of the mid-costal di
aphragms of seven dogs. The markers were tracked by biplane video fluo
roscopy during spontaneous breathing (SB), mechanical ventilation with
the same tidal volume (MV), and at inflation to total lung capacity (
TLC) in the prone and supine positions. The three-dimensional position
s of the markers at functional residual capacity (FRC), at end inspira
tion during SB and MV, and at TLC were determined, and the strains in
the plane of the diaphragm relative to FRC were calculated. The princi
pal strains were found to lie nearly along the muscle bundle direction
and perpendicular to it. The principal strains along the muscle bundl
es, which describe muscle shortening, were uniform among the three bun
dles and uniform along the bundle for MV. For SB, in the prone and sup
ine positions, shortening was similar to 30% greater in the middle of
the bundle than near the central tendon and chest wall. Although the t
idal volumes were the same for SB and MV, the shortening was larger fo
r SB. The strains perpendicular to the bundle direction were not signi
ficantly different from zero. It appears that, for the loads that occu
r during tidal breathing, the diaphragm is inextensible in the directi
on perpendicular to the muscle direction. There is a very small displa
cement of the costal diaphragm at its insertion on the chest wall. The
displacement at the central tendon is primarily a result of muscle sh
ortening and rotation of the are of the muscle around its insertion on
the chest wall.