Quantitative Characterization of Steady-state Ankle Impedance with Muscle Activation

Abstract

Characterization of multi-variable ankle mechanical impedance is crucial to understanding how the ankle supports lower-extremity function during interaction with the environment. This paper reports quantification of steady-state ankle impedance when muscles were active. Vector field approximation of repetitive measurements of the torque-angle relation in two degrees of freedom (inversion/eversion and dorsiflexion/plantarflexion) enabled assessment of spring-like and non-spring-like components. Experimental results of eight human subjects showed direction-dependent ankle impedance with greater magnitude than when muscles were relaxed. In addition, vector field analysis demonstrated a non-spring-like behavior when muscles were active, although this phenomenon was subtle in the unimpaired young subjects we studied. INTRODUCTION Ankle mechanical impedance, which we define as a functional that maps a time history of angular motion of the ankle joint onto a corresponding ankle torque time-history, plays a significant role in natural interaction of the lower extremity with the environment, including postural stabilization during standing and propulsion, energy-absorption, and lowerlimb joint coordination during locomotion. Most prior studies of human ankle mechanical impedance focused on the sagittal plane (the dorsiflexion/plantarflexion (DP) direction) [1−3]. To the best of our knowledge, only two studies have measured ankle impedance in the frontal plane (the inversion-eversion (IE) direction) [4−5]. Considering that single degree of freedom (DOF) movements at the ankle are rare under natural physiological conditions, characterization of ankle impedance in multiple DOFs promises deeper understanding of its roles in lower extremity function. In a previous work by the authors [6], the multivariable steady-state torque-angle relation at the ankle was measured with muscles maximally relaxed, showing that (as expected) the ankle behaved like a (nonlinear) spring under those conditions. Although that work provided a baseline for understanding ankle impedance, it is not directly applicable to normal lowerextremity actions since they involve muscle activation, either singly, synergistically or antagonistically (co-contraction). Here we extend the characterization of steady-state multi-variable ankle mechanical impedance to muscle active conditions. EXPERIMENTS AND ANALYSIS METHODS Human Subjects Eight human subjects with no history of neuromuscular disorders (4 males and 4 females; age range mid 20’s ~ mid 30’s) were recruited for this study. Participants gave informed consent as approved by MIT’s Committee on the Use of Humans as Experimental Subjects (COUHES). Experimental Setup Steady-state torque-angle data in IE−DP space were captured using a wearable ankle robot, Anklebot [7], the same device used in previous work [6]. It was mounted at the knee and operated by a simple impedance controller which enabled stable data capture even in high muscle activation conditions. 1 Copyright © 2010 by ASME Proceedings of the ASME 2010 Dynamic Systems and Control Conference DSCC2010 September 12-15, 2010, Cambridge, Massachusetts, USA

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Cite this paper

@inproceedings{Lee2010QuantitativeCO, title={Quantitative Characterization of Steady-state Ankle Impedance with Muscle Activation}, author={Hyunglae Lee and Patrick Ho and Mohammad A. Rastgaar and Hermano Igo Krebs and Neville Hogan}, year={2010} }