Tendon injuries remain a major clinical challenge due to their limited intrinsic healing capacity and the high prevalence of agerelated degenerative changes. Understanding how tendon mechanical properties evolve throughout the lifespan is essential for improving therapeutic strategies and for informing the design of biomimetic scaffolds for tissue repair. Tendons exhibit a hierarchical structure that enables efficient force transmission and energy storage. Their mechanical behaviour is inherently complex, involving both elastic and timedependent viscoelastic components [1]. While the elastic modulus has traditionally been regarded as a primary determinant of tendon function, recent studies have shown that viscoelasticity plays a crucial role in load transfer and energy dissipation, and that these properties can change with aging [2]. In this study, we examined the agedependent evolution of the mechanical properties of the mouse Achilles tendon, analysing both elastic and viscoelastic parameters. Using stressrelaxation tests at multiple strain levels, we compared tendons from young (1 month), adult (4 months), and aged (24 months) mice. These tests revealed a clear agedependent degradation of both the elastic and viscoelastic properties of the tendon, and the resulting data were used to fit analytical models to computationally simulate their impact on force transmission. To demonstrate this, a geometry of the mouse soleus muscle was implemented in COMSOL Multiphysics together with a previously developed active skeletal muscle model [3]. The agespecific viscoelastic constitutive model of the Achilles tendon was incorporated into the simulation, and under identical contraction patterns the mechanical response corresponding to the three age groups was evaluated. The coupled muscle–tendon simulations showed that agerelated changes in tendon viscoelasticity reduced effective force transmission and altered the temporal dynamics of contraction, with aged tendons exhibiting both delayed force buildup and diminished peak output.