The Journey from Structure to Function: Multiscale Mechanical Models and Experiments Reveal How Collagen Organization Influences Passive Muscle Mechanics

The skeletal muscle extracellular matrix (ECM) is a beautiful and complex three-dimensional scaffold that transmits physical and chemical signals and regulates passive muscle properties. Collagen fibers determine the ECMs’ structural and tensile properties and have unique arrangements surrounding (epimuscular) and within (intramuscular) muscle. An accumulation of collagen is often used to characterize the development of fibrosis in neuromuscular disorders such as Duchenne muscular dystrophy (DMD); however, increased collagen levels do not explain alterations in passive muscle properties, such as increased stiffness, that contribute to muscle dysfunction. Thus, we must consider how collagen organization influences passive muscle mechanics, especially in complex muscles such as the diaphragm, which is severely affected in DMD.

My thesis couples imaging, mechanical testing, and multiscale finite element modeling to examine the role of collagen microstructure on macroscopic muscle tissue properties. First, I characterized collagen organization in the epimuscular ECM of diaphragm muscle and found that collagen fibers were oriented perpendicular to muscle fibers (cross-muscle fiber direction), with greater collagen fiber alignment in mdx (dystrophin null) relative to WT mice. I then developed epimuscular micromechanical models to determine the mechanical implications of changes in collagen structure on ECM properties and predicted higher cross-muscle fiber stiffness in the mdx compared with WT models. Next, I developed micromechanical models of both epimuscular and intramuscular regions and coupled their predictions to determine bulk muscle tissue properties. I then performed biaxial mechanical tests to characterize along- and cross-muscle fiber tissue properties in mdx and WT diaphragm muscle to directly calibrate and validate the models. We predicted higher cross-muscle fiber collagen alignment and stiffness in the mdx compared with WT models, with nonuniform stresses between ECM and muscle regions. Further, collagen fiber distribution had a much more substantial impact on tissue stiffness than ECM area fraction. Taken together, we show that the primary orientation of collagen fibers relative to muscle fibers explains anisotropic tissue properties observed in diaphragm muscle, and that the distribution of collagen fibers explains discrepancies between measurements of collagen amounts and tissue stiffness. This work provides novel insights into collagen’s complex role on passive muscle mechanics and highlights the capability of mechanical modeling to fill gaps along the journey from structure to function.