Along- and cross-muscle fiber shear moduli in skeletal muscle

The material properties of muscle play a central role in how muscle resists joint motion, transmits forces, and repairs itself. While many studies have evaluated muscle's tensile properties, few have investigated muscle's shear properties. Our objective was to quantify the shear moduli along (along-muscle fiber) and transverse (cross-muscle fiber) to muscle fibers. We collected data from the extensor digitorum longus, tibialis anterior, and soleus from rat hindlimbs to evaluate the consistency of shear moduli across muscles. We applied strains and measured stress in three configurations (parallel, perpendicular, across) and used our measurements to determine the along- and cross-muscle fiber tensile and shear moduli. Our methods are the first to account for tensile strain during shear loading and compare measurements between muscles. The measured shear stress-strain slope was greater in the perpendicular compared with parallel and across configurations for all muscles. After accounting for both tensile and shear components, we found that the Young's modulus was greater in the along- relative to cross-muscle fiber direction while there was no significant difference between the along- and cross-muscle fiber shear moduli. Therefore, differences in the measured stress-strain slopes were due to differences in tensile strain applied during shear loading rather than the shear moduli. There were no significant differences in shear or Young's moduli between muscle groups, suggesting that material properties are consistent between muscles. Our results characterize shear properties across skeletal muscles, providing a novel dataset to assign muscle properties in computational models and compare with image-based estimates of stiffness.