Techniques for Assessing the Material Properties of Muscle and its Constituents

Reyna, William Eric

doi:https://doi.org/10.21985/n2-nv2k-ky92

Work

Techniques for Assessing the Material Properties of Muscle and its Constituents

Public

Download PDF

Download All Files (.zip)

We rely on the properties of our skeletal muscles to traverse our world, interact with objects, and complete everyday tasks. The macroscopic properties of muscles that endow us with these abilities arise from the material properties of muscle fibers and the surrounding extracellular matrix (ECM), as well as how they are arranged in muscles with different architectures. Consequently, changes in these primary constituents of muscle can contribute to some of the pathological conditions that arise from injury, disease, or aging. Unfortunately, we lack the knowledge of muscle’s material properties necessary to better understand how changes influence function. While we know the Young’s modulus of whole muscle, this fails to distinguish the independent contributions from muscle fibers and ECM. Furthermore, Young’s modulus is not sufficient to characterize the mechanical properties of muscle as it does not allow for estimation of shear, a property important for force transfer. The objective of this dissertation was to fill these two gaps by quantifying and evaluating techniques for assessing the material properties of muscle and its constituents. This objective was pursued in three parts. First, I quantified the three-dimensional (3D) shear modulus of whole muscle in muscles of differing architectures. I found no significant difference in shear moduli between muscles with different architectures. Muscle, when subjected to shear deformations, had a linearly increasing shear modulus with increasing strain. Further, shear modulus, when sheared perpendicular to muscle fibers was greater than any other direction. At the maximum predicted physiological strain (0.4), the shear modulus was 7 ±1 (mean ±CI), 6 ±1, and 4 ±1 kPa when measured perpendicular, parallel, and across muscle fibers, respectively. Second, I evaluated the efficiency and effect of three decellularization methods on isolating the ECM for direct testing. I found latrunculin B with osmotic shock was the most efficient method for removing cells compared to two other commonly used methods. Latrunculin B decellularization reduced DNA content to less than 10% of controls and substantially reduced the myosin and actin content to 15% and 23%, respectively. Additionally, following latrunculin B decellularization, the Young’s modulus of the remaining ECM was approximately half of the total passive stiffness. This result suggest that ECM carries approximately half of the passive load in muscle. Third, I discuss our understanding of how muscle structure may influence the measures from shear wave elastography (SWE), a noninvasive tool that has the potential to measure changes in the material properties of muscle’s constituents. Despite a plethora of studies using SWE, I found a lack of work that validated the assumptions of SWE in muscle against directly-measured material properties. There are a great deal of studies showing correlation between muscular diseases and shear wave speed, but none that have established how the underlying structural changes in muscle influence shear wave propagation. This unknown influence is partly due to the fact that we lack quantitative measures of the material properties of muscle at a level of detail on par with the shear waves we are inducing. Overall, this dissertation provides novel measures of muscle’s anisotropic shear moduli, a method to isolate ECM for mechanical testing, and future steps towards validating the measures of ultrasound elastography. These results provide insight into the anisotropic nature of muscle and parameters that can be used in muscle models to simulate 3D deformations.

Creator