Mechanical Impedance of the Ankle During the Terminal Stance Phase of Walking

Human joint impedance describes the dynamic relationship between perturbation induced change in position and the resulting response torque. Understanding the natural regulation of ankle impedance during locomotion is necessary to discern how humans interact with their environments, and provide a foundation for the design of biomimetic assistive devices and their control systems. This paper estimates ankle impedance during terminal stance phase of walking using a parametric model consisting of stiffness, damping, and inertia. The model accurately described ankle torque, accounting for 90% ± 7.7% of the variance. Stiffness was found to decrease 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 constant during terminal stance phase, and was increased relative to values previously reported during early and mid-stance phases, indicating an increase in damping in preparation for toe-off. Inertia estimates were consistent with previously reported inertia values for the human ankle. This paper bridges a gap in our understanding of ankle impedance during walking, and provides new insight into how ankle impedance is regulated during regions when substantial mechanical energy is added.