Biaxial constitutive relations for the passive canine diaphragm

Samples of the muscular sheet excised from the mid-costal region of dog dia phragms were subjected to biaxial loading. That is, stresses in the directi on of the muscle fibers and in the direction perpendicular to the fibers in the plane of the sheet were measured at different combinations of strains in the two directions. Stress-strain relations were obtained by fitting equ ations to these data. In the direction of the muscle fibers, for strains up to 0.7, stress is a modestly nonlinear function of strain and ranges up to similar to 60 g/cm. In the direction perpendicular to the fibers, the shee t is stiffer and more strongly nonlinear. At a strain in the perpendicular direction of similar to0.35, stress increases abruptly. The stress-strain r elation in the muscle direction is consistent with observations of passive muscle shortening in vivo. However, the stiffness in the perpendicular dire ction is not high enough to explain the observation that strains in the per pendicular direction in vivo are nearly zero. We conclude that, in the pass ive diaphragm in vivo, stress in the direction perpendicular to the muscle fibers is small.