Samples of the muscular sheet excised from the midcostal region of dog diaphragms were subjected to biaxial loading. That is, stresses in the direction 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 equations 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 approximately 60 g/cm. In the direction perpendicular to the fibers, the sheet is stiffer and more strongly nonlinear. At a strain in the perpendicular direction of approximately 0.35, stress increases abruptly. The stress-strain relation in the muscle direction is consistent with observations of passive muscle shortening in vivo. However, the stiffness in the perpendicular direction is not high enough to explain the observation that strains in the perpendicular direction in vivo are nearly zero. We conclude that, in the passive diaphragm in vivo, stress in the direction perpendicular to the muscle fibers is small.

The diaphragmatic muscle tendon is a biaxially loaded junction in vivo. Stress-strain relations along and transverse to the fiber directions are important in understanding its mechanical properties. We hypothesized that 1) the central tendon possesses greater passive stiffness than adjacent muscle, 2) the diaphragm muscle is anisotropic, whereas the central tendon near the junction is essentially isotropic, and 3) a gradient in passive stiffness exists as one approaches the muscletendinous junction (MTJ). To investigate these hypotheses, we conducted uniaxial and biaxial mechanical loading on samples of the MTJ excised from the midcostal region of dog diaphragm. We measured passive length-tension relationships of the muscle, tendon, and MTJ in the direction along the muscle fibers as well as transverse to the fibers. The MTJ was slack in the unloaded state, resulting in a J-shaped passive tension-strain curve. Generally, muscle strain was greater than that of MTJ, which was greater than tendon strain. In the muscular region, stiffness in the direction transverse to the fibers is much greater than that along the fibers. The central tendon is essentially inextensible in the direction transverse to the fibers as well as along the fibers. Our data demonstrate the existence of more pronounced anisotropy in the muscle than in the tendon near the junction. Furthermore, a gradient in muscle stiffness exists as one approaches the MTJ, consistent with the hypothesis of continuous passive stiffness across the MTJ. respiratory muscles; mechanics; stress; strain THE MUSCLE-TENDINOUS JUNCTION (MTJ) is a physiologically vital tissue interface. Muscle force is transmitted between the muscle and tendon through the MTJ. Previous MTJ analyses and interpretations considered force applied to the MTJ as parallel to the myofibril direction (11). In vivo, the diaphragm, and therefore the diaphragmatic MTJ, is subjected to complex states of loading not seen with other MTJ of uniaxially loaded skeletal muscles. The effect of biaxial loading on the deformation of the diaphragmatic MTJ remains uncharacterized, because the diaphragmatic MTJ, unlike other skeletal muscle MTJ, is biaxially loaded in vivo. Therefore, the length-tension relationships of the MTJ should be measured not only along the fibers (AF) but also transverse to the fiber direction (TF). Furthermore, comparison of the MTJ loaded biaxially with one loaded uniaxially along the direction of the muscle fibers can help identify structural specialization of force-transmitting junction that experiences more complex loading patterns. Tidball (15) demonstrated the importance of morphological and molecular aspects of force transmission across cell membranes in their elegant paper, but correlation of mechanical and morphological data is necessary to fully understand force transmission at the MTJ.Our previous study of in vivo mechanics of canine diaphragm showed substantially smaller mechanical strain in the TF compared with AF direction (8). Furthermore, our in vitro mechanics studies...

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