Non-linear properties of the Achilles tendon determine ankle impedance over a broad range of activations in humans

Abstract

Regulating ankle mechanics is essential for controlled interactions with the environment and rejecting unexpected disturbances. Ankle mechanics can be quantified by impedance, the dynamic relationship between an imposed displacement and the torque generated in response. 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 that tendon impedance also changes with activation-induced loading. 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. At approximately 20% MVC for an average participant, ankle stiffness was four times more sensitive to changes in tendon stiffness than muscle stiffness. We provide the first empirical evidence demonstrating that the nervous system, through changes in muscle activations, leverages the non-linear properties of the Achilles tendon to increase ankle stiffness during postural conditions.

Intrinsic ankle stiffness is associated with paradoxical calf muscle movement but not postural sway or age

“…Such differences may be genetic, or related to training and exercise. Another source of variation may be subtle differences in standing posture and baseline ankle torque (Jakubowski et al., 2023 ; Sakanaka et al., 2015 , 2021 ).…”

“…The stiffest subject's value was ∼−0. source of variation may be subtle differences in standing posture and baseline ankle torque (Jakubowski et al, 2023;Sakanaka et al, 2015Sakanaka et al, , 2021.…”

“…Hence, we can say that the tendency for the calf muscles to shorten during forward sway is prevalent, and very much associated with low ankle stiffness.What is the source of ankle stiffness variation? During relatively low ankle torque levels maintained during quiet standing, the muscle is by far the stiffest element contributing to ankle stiffness, with the limiting factor being the Achilles tendon mechanics and foot anatomy(Jakubowski et al, 2023;Loram & Lakie, 2002). Any variation in the location of the Achilles insertion point, or the size and structure of the tendon or feet might be responsible for changes in joint stiffness.…”

Estimating Biological Stiffness Without Relying on External Joint Perturbations: A Musculoskeletal Modeling Framework

“…In Chapter 4 we compared the estimations of our proposed model-based framework against reference joint torque and stiffness, muscle-and tendon-level stiffness, as well as the displacement of individual muscles [144]. Different calibrations of increasing complexity indicated that it was possible to accurately capture all these biomechanical variables using Hill-type muscle models.…”