Abstract
In the current work, we investigate the effect of aging on the viscosity of tendon subunits. To that scope, we make use of experimental relaxation curves of healthy and aged tendon fascicles and fibers, upon which we identify the viscosity parameters characterizing the time-dependent behavior of each tendon subunit. We subsequently combine the obtained results with analytical viscoelastic homogenization analysis methods to extract information on the effective viscous contribution of the embedding matrix substance at the fiber scale. The results suggest that the matrix substance plays a significant role in the relaxation process of the upper tendon subunits both for aged and healthy specimens. What is more, the viscosity coefficients computed for the fibrillar components indicate that aging leads to a viscosity reduction that is statistically significant for both fascicles and fibers. Its impact is more prominent for the lower hierarchical scale of fibers. As such, the reduced stress relaxation capability at the tendon macroscale is to be primarily attributed to the modified viscosity of its inner fibrillar subunits rather than to the matrix substance.
Highlights
The multiscale structure of tendons plays a functional role in the transfer of forces from the muscles to the bones (Maceri et al, 2012; Ge et al, 2018)
The viscosity parameters computed in sections Relaxing Healthy and Aging Fascicles and Fiber Scale Aging Relaxation have provided primal, data-based quantitative estimates of the effect of aging on the time-dependent behavior of fascicles and fibers
It has been shown that the viscosity coefficients ηfasc and ηf are subject to statistically significant reductions as a result of the aging process
Summary
The multiscale structure of tendons plays a functional role in the transfer of forces from the muscles to the bones (Maceri et al, 2012; Ge et al, 2018). The fascicles contained within the tendon unit are composed of fibers immersed in a matrix substance (Figure 1A) (Goh et al, 2008). Viscoelasticity has provided the theoretical basis for the characterization of the tendon’s relaxation behavior (Elliott et al, 2003; Machiraju et al, 2006; Screen, 2008; Shen et al, 2011), mathematically described as a function of both elastic and viscous parameters (Taylor et al, 1970; Christensen, 1982). The tendon’s viscoelastic properties have been shown to differ among its lower and upper scales, with the embedding matrix substance to play a significant role in the multiscale effective relaxation behavior (Karathanasopoulos et al, 2019)
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