Elucidation of extracellular matrix mechanics from muscle fibers and fiber bundles

Abstract

The importance of the extracellular matrix (ECM) in muscle is widely recognized, since ECM plays a central role in proper muscle development (Buck and Horwitz, 1987), tissue structural support (Purslow, 2002), and transmission of mechanical signals between fibers and tendon (Huijing, 1999). Since substrate biomechanical properties have been shown to be critical in the biology of tissue development and remodeling (Engler et al., 2006; Gilbert et al., 2010), it is likely that mechanics are critical for ECM to perform its function. Unfortunately, there are almost no data available regarding skeletal muscle ECM viscoelastic properties. This is primarily due to the impossibility of isolating and testing muscle ECM. Therefore, this note presents a new method to quantify viscoelastic ECM modulus by combining tests of single muscle fibers and fiber bundles. Our results demonstrate that ECM is a highly nonlinearly elastic material, while muscle fibers are linearly elastic.

Figure 1

Schematic illustration of the arrangement of the three specimen types. Single fibers (pink lines) were isolated from the muscle and either tested individually or secured in groups. Bundles of a similar number of fibers embedded in ECM (light pink) were isolated and similarly secured.

Figure 2

Quadratic moduli for fibers, fiber groups and fiber bundles. Fiber bundles have a significantly higher modulus than either individual fibers or fiber groups. Fiber and fiber group moduli were not significantly different from each other (p>0.1). Fibers: n=17, Fiber Groups: n=6, Fiber Bundles: n=10. Bars indicate p<0.05.

Figure 3

Illustration of the potential sources of nonlinearity in fiber bundles. (A) Experimental data from 10 fibers is plotted in green (each line represents an individual fiber) with composite fiber behavior shown in blue. In this case, all fibers develop tension at the same sarcomere length and the composite behavior is linear. Since nonlinear bundle behavior is observed, ECM elasticity must be nonlinear. (B), Same experimental data as in (A) are plotted but now each fiber has been shifted to develop tension at different sarcomere lengths, which yields a nonlinear composite behavior. In this case, nonlinear bundle elasticity could be obtained with a linearly or nonlinearly elastic ECM.