BIAXIAL MECHANICAL-PROPERTIES OF PASSIVE AND TETANIZED CANINE DIAPHRAGM

The architecture, vascular supply, and ease of tetanization make the d iaphragm an ideal structure in which to assess multidimensional mechan ical properties of active and passive striated muscle. We developed an isolated, perfused canine diaphragm preparation suitable for the asse ssment of biaxial stress-strain relations in both the resting state an d during tetanization. Each of 33 specimens had a wide, flat region (a pproximately 3 x 3 cm) wherein there was a single predominant fiber di rection. Simultaneous, equal stretchings were imposed in the fiber and perpendicular cross-fiber directions over the same strain ranges in b oth the passive state and during tetanic contraction. Highly nonlinear behavior was seen in the passive state with a limit of extensibility in both directions. The specimens were also markedly anisotropic, with the cross-fiber direction being stiffer than the fiber direction (slo pes of the regression line for the stresses in each direction averaged 3.97). Moreover, 31 of the 33 specimens were stiffer in the cross-fib er direction, one was isotropic, and one was stiffer in the fiber dire ction. During tetanization, the extent and distribution of anisotropy were significantly altered (regression slope averaged 1.08, and 18 spe cimens were now either isotropic or stiffer in the fiber direction). D isrupting the membranes covering each surface increased extensibility and decreased the anisotropy, thereby suggesting that these membranes bear most of the passive load and contribute greatly to the cross-fibe r stiffness and anisotropy of the intact diaphragm. Both before and af ter disruption of the surface membranes, there was still a consistent increase in cross-fiber stress during tetanization, implying active fo rce generation perpendicular to the fiber direction. In addition, the different passive and tetanized stress-strain relations imply that dif ferent forms of constitutive laws must be used to describe passive and active muscle.