Ankle Joint Mechanical Impedance during Locomotion and Implications for Prosthesis Control and Rehabilitation

Shorter, Amanda L

doi:https://doi.org/10.21985/n2-dxhn-jv17

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Ankle Joint Mechanical Impedance during Locomotion and Implications for Prosthesis Control and Rehabilitation

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The dynamics of human joints are fundamental characteristic of the human motor system, and altered joint impedance can hinder mobility. Individuals with transtibial amputation typically experience slower and energetically costly gait, while individuals with chronic stroke experience persisting gait deficits arising from spasticity, hypertonia and paresis. Investigating joint impedance of impaired and non-impaired populations during locomotion improves our understanding of gait biomechanics and could lead to innovations in assistive technology and therepeutic intervention. Using a single degree of freedom mechatronic platform to perturb the ankle, I estimate ankle impedance during terminal stance phase of walking by implementing a parametric model consisting of stiffness, damping, and inertia. The stiffness component of impedance decreased from 3.7 to 2.1 Nm/rad/kg between 75% and 85% stance. Quasi-stiffness—the slope of the ankle’s torque-angle curve—showed a similar decreasing trend but was significantly larger at the onset of terminal stance phase. The damping component of impedance was increased relative to values previously reported during early and mid-stance phase, indicating an increase in damping in preparation for toe-off. Ankle impedance is also estimated at four time points throughout the stance phase of running (30%, 50%, 70% and 85% of stance). I compare impedance estimates between running and walking of young healthy adults. Ankle stiffness during running reached a maximum of 10 Nm/rad/kg at the end of mid-stance, decreasing in terminal stance phase to values previously reported during swing phase. Quasi-stiffness values differed significantly from stiffness across the stance phase of running. Comparing ankle impedance estimates between walking and running showed differences in both magnitude, and temporal variation. Finally, this experimental protocol was applied to individuals with chronic stroke. Both the paretic and non-paretic ankle impedance were estimated at four time points during walking (30%, 50%, 70% and 85% of stance), and muscle electromyography was collected from both lower limbs. I characterized the relationship between ankle impedance impairment and the clinical measures of mobility and impairment. Stiffness of the paretic ankle was decreased during mid-stance as compared to the non-paretic ankle, a change independent of muscle activity. Inter-limb differences in ankle stiffness, but not ankle damping or passive clinical assessments, strongly predicted walking speed and distance. This doctoral work expands our understanding of human ankle impedance during locomotion. It provides new insight into how ankle impedance is regulated during regions when substantial mechanical energy is added, and novel information about the biomechanics of running. Finally, this work elucidates how stroke alters ankle impedance during walking, and how clinical assessments may not indicate true representations of ankle stiffness and damping characteristics. This dissertation offers a more complete understanding of how sagittal plane ankle impedance is regulated durring walking, may provide a foundation for assessment of neuromotor pathologies, and could enable the design and control of biomimetic assistive technologies.

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