Nonlinear viscoelastic effects of fascia: experimental findings and constitutive modelling.

This study provides a comprehensive biomechanical characterization of the fascia lata, integrating systematic stress-relaxation protocols and, for the first time, dynamic mechanical analysis to resolve its complex dissipative mechanisms. Experimental tests were conducted along both longitudinal and transverse fibre orientations across multiple strain levels and frequencies to assess the synergistic effects of anisotropy, strain magnitude and loading rate. To describe these phenomena, three visco-hyperelastic constitutive frameworks were developed and compared: a first-order linear model (n=1), a generalized Maxwell model with two internal variables (n=2) and a nonlinear strain-dependent formulation. Results reveal a marked anisotropic viscoelastic profile, with the transverse direction exhibiting a significantly higher relaxation capacity (ratio of 0.666±0.144 at 2.5% strain) compared to the longitudinal axis (0.817±0.049), driven by the viscous flow of the interfibrillar ground substance. Although the tissue demonstrated marginal nonlinear viscoelasticity, the linear n=2 model outperformed the nonlinear n=1 framework, achieving superior predictive fidelity (R2=0.7321, NRMSE=0.1216 versus R2=0.7200, RMSE=0.2949 MPa). Results indicate that spectral refinement outweighs nonlinear complexity in this context. Specifically, the dual-branch linear approach captures the tissue's fundamental dissipation more effectively than a single-branch nonlinear formulation, which tends towards over-parametrization.