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Tendons transmit muscle force to the skeleton. They are not rigid cables: they have a non-linear viscoelastic behavior, with an initial region of low stiffness (toe region, where the wavy collagen fibers are stretched) followed by a linear region (elastic region) and finally rupture. K_T tendon stiffness (N/mm) is key in sports biomechanics: stiffer tendons transmit force more quickly (greater joint RFD) but store less elastic energy; More elastic tendons act as springs and return energy with each step (Achilles tendon, stores ~35 J per stride in runners). The Young's modulus of tendon collagen is ~1.5 GPa. The tension on the Achilles tendon can reach 9 times the body weight in a jump. The Kelvin-Voigt (parallel spring with damper) or Maxwell (series) rheological model describes viscoelastic behavior: hysteresis in the loading-unloading cycle indicates energy loss (~10% in healthy tendon).

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