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The nervous system leverages the non-linear properties of the Achilles tendon to regulate ankle impedance during postural control

Regulating ankle impedance is essential for controlled interactions with the environment and rejecting unexpected disturbances. Ankle impedance in the sagittal plane depends strongly on the triceps surae and Achilles tendon, but their relative contributions remain unknown. It is commonly assumed that ankle impedance is controlled by changing muscle activation and, thereby, muscle impedance, but this ignores the fact that tendon impedance also changes with loading caused by increases in muscle activation. Thus, we sought to determine the relative contributions from the triceps surae and Achilles tendon during conditions relevant to postural control. We used a novel technique that combines B-mode ultrasound imaging with joint-level perturbations to quantify ankle, muscle, and tendon impedance simultaneously across activation levels from 0 – 30% of maximum voluntary contraction. We found that muscle and tendon stiffness, the static component of impedance, increased with voluntary plantarflexion contractions, but that muscle stiffness exceeded tendon stiffness at very low loads (21 ± 7 N). Above these loads, corresponding to 1.3% of maximal strength for an average participant in our study, ankle stiffness was determined predominately by Achilles tendon stiffness. Hence, the nervous system leverages the non-linear properties of the Achilles tendon to increase ankle stiffness during postural conditions.