Stochastic Estimation of Human Ankle Mechanical Impedance in Lateral/medial Rotation

This article compares stochastic estimates of human ankle mechanical impedance when ankle muscles were fully relaxed and co-contracting antagonistically. We employed Anklebot, a rehabilitation robot for the ankle to provide torque perturbations. Surface electromyography (EMG) was used to monitor muscle activation levels and these EMG signals were displayed to subjects who attempted to maintain them constant. Time histories of ankle torques and angles in the lateral/medial (LM) directions were recorded. The results also compared with the ankle impedance in inversion-eversion (IE) and dorsiflexionplantarflexion (DP). Linear time-invariant transfer functions between the measured torques and angles were estimated for the Anklebot alone and when a human subject wore it; the difference between these functions provided an estimate of ankle mechanical impedance. High coherence was observed over a frequency range up to 30 Hz. The main effect of muscle activation was to increase the magnitude of ankle mechanical impedance in all degrees of freedom of ankle. NOMENCLATURE τ Torque applied to the ankle Fright Force applied by the right actuator Fleft Force applied by the left actuator Dactuator Distance between the 2 actuators θ Angle of the ankle θoffset Initial offset of the ankle Xright Distance traveled by the right actuator Xleft Distance traveled by the right actuator INTRODUCTION The ankle is of fundamental importance in different tasks such as locomotion and standing, given that it is the first joint to transfer the reactions forces from the ground to the rest of the body. Understanding the mechanical impedance of the human ankle may help to explain its role in the locomotion. During gait, the ankle rotates in all three anatomical planes, suggesting the mechanical impedance modulation occurs in all three degrees of freedom (DOF) of ankle. Ankle mechanical impedance has been studied in sagittal and frontal planes; however, there has been no reported estimation of ankle mechanical impedance in lateral/medial direction in transverse plane, which is presented in this paper. Previous work estimated the multivariable mechanical impedance of ankle in IE and DP, when the muscles connected to the ankle joint were relaxed and in co-contraction [1-3]. Ankle impedance during muscle contraction is important since most activities are performed with varying levels of muscle activation. The muscle contraction can be monitored using Surface Electromyography (EMG). The sEMG signal amplitudes provide qualitative descriptions of the force and/or speed produced by the muscle and has a near monotonic relationship to muscle force in isometric contraction [4]. Stochastic identification method was used for estimation of multivariable mechanical impedance of ankle. The stochastic method does not require a priori assumption regarding the dynamic characteristics of the system, and so it is a suitable method to estimate the impedance of complex systems [5]. The mechanical characteristics of the ankle in the transverse plane are especially important during step turn since a torque is applied to the ankle to pivot the weight of the body when changing directions. Using a motion capture camera system, we determined the average ankle range of motion (Table 1) during straight steps and in sharp 90° contralateral step turns (Fig. 1) 1 Copyright © 2014 by ASME Proceedings of the ASME 2014 Dynamic Systems and Control Conference DSCC2014 October 22-24, 2014, San Antonio, TX, USA