Structural mechanisms linking muscle fiber alignment to elastic modulus in skeletal muscle

A key requirement in the design of skeletal muscle biomaterials is to match the mechanical properties of native tissue, which are governed by its microstructural organization. However, the specific microstructural features of native skeletal muscle and their corresponding passive mechanical properties have not yet been systematically investigated. This study aimed to address these gaps by characterizing the mechanical properties and muscle fiber alignment of skeletal muscle under three conditions. We further sought to associate the biaxial stress–stretch response with load-dependent behaviors of microstructure to better understand the structural mechanisms driving changes in viscoelastic behavior. We found that highly aligned fibers led to a higher elastic modulus (239.6 ± 105 kPa, P < 0.0001), indicating a significant increase in tissue stiffness. In contrast, tissues with a more random fiber distribution or greater fiber waviness exhibited stiffness values comparable to those of fresh tissue (84.31 ± 48.59 and 76.86 ± 21.67 kPa, respectively). These findings demonstrate that muscle fiber alignment significantly affects the passive mechanical properties of skeletal muscle by modulating the tissue response to passive loading. Overall, our results provide new insights into how alterations in muscle fiber alignment influence passive mechanical behavior and offer valuable guidance for the design and development of skeletal muscle biomaterials.