An insight on multiscale tendon modeling in muscle-tendon integrated behavior

This paper aims to highlight the need for a refined tendon model to reproduce the main mechanical features of the integrated muscle-tendon unit (MTU). Elastic non-linearities of the tendon, both at the nano and microscale, are modeled by a multiscale approach, accounting for the hierarchical arrangement (from molecules up to the fibers) of the collagen structures within the tissue. This model accounts also for the variation of tendon stiffness due to physical activity. Since the proposed tendon model is based on tissue structured histology, the training-driven adaptation laws are directly formulated starting from histological evidences. Such a tendon description is integrated into a viscoelastic Hill-type model of the whole MTU. A fixed-end contraction test is numerically simulated, and results based on both linear and non-linear tendon elastic model are compared. Sound and effective time-histories of muscle contractile force and fiber length are obtained only accounting for tendon elastic non-linearities, which allow to quantitatively recover some experimental data. Finally, proposed numerical results give clear indications towards a rational explanation of the influence of tendon remodeling induced by physical activity on muscular contractile force.