Interpretation of the Directional Properties of Voluntarily Modulated Human Ankle Mechanical Impedance

This article presents the results of two in-vivo studies providing measurements of human static ankle mechanical impedance. Accurate measurements of ankle impedance when muscles were voluntarily activated were obtained using a therapeutic robot, Anklebot, and an electromyographic recording system. Important features of ankle impedance, and their variation with muscle activity, are discussed, including magnitude, symmetry and directions of minimum and maximum impedance. Voluntary muscle activation has a significant impact on ankle impedance, increasing it by up to a factor of three in our experiments. Furthermore, significant asymmetries and deviations from a linear two-spring model are present in many subjects, indicating that ankle impedance has a complex and individually idiosyncratic structure. We propose the use of Fourier series as a general representation, providing both insight and a precise quantitative characterization of human static ankle impedance. NOMENCLATURE θ Angular displacement of the foot F(θ) Directional Ankle Impedance Function IE Inversion and Eversion, rotation of the sole of the foot in a frontal plane towards or away from the center of the body, respectively. DP Dorsiflexion and Plantarflexion, rotation of the foot in a sagittal plane upwards or downwards from a mid position, respectively. INTRODUCTION Physical therapists employ motion therapy for a wide variety of physical ailments. The condition of the ankle (or other joints) is often characterized using a subjective scale of muscle tone called the Modified Ashworth Scale [1]. Here we describe an effort to analyze the features of a subject’s ankle impedance function (the relationship between the rotational motion of the foot relative to the shank and the torque exerted about the ankle joint) in a standardized and quantitative fashion using data collected from a therapeutic robot. With the advent of quantitative descriptions of ankle impedance, more understanding and insight into human physiology may emerge, describing not only the magnitude of ankle impedance, but also symmetry properties and directions of minimum and maximum impedance. Sophisticated tools for measuring ankle impedance may improve clinical treatments and produce data useful for designing human-robot interaction. With objective and precise quantitative measurement, databases containing individual treatment histories as well as data on the general population can be established. The effects of various physical ailments and treatments can be compared among subjects, potentially improving clinical practice. Human-robot interaction may also benefit from a better quantification of human ankle impedance, enabling mobility assist devices and external robots to better predict and model the consequences of interacting physically with the human about the ankle joint. Prior work has provided detailed quantitative assessment of multi-variable mechanical impedance about other joints in 1 Copyright © 2010 by ASME Proceedings of the ASME 2010 Dynamic Systems and Control Conference DSCC2010 September 12-15, 2010, Cambridge, Massachusetts, USA