Regulation of 3D Human Arm Impedance Through Muscle Co-Contraction

Harshil Patel, Gerald O'Neill and Panagiotis ArtemiadisHuman-Oriented Robotics and Control LabSchool for Engineering of Matter, Transport and EnergyArizona State UniversityTempe, AZ 85287Email: fhpatel5, goneill, panagiotis.artemiadisg@asu.eduABSTRACTHumans have the inherent ability of performing highly dex-terous and skillful tasks with their arms, involving maintenanceof posture, movement, and interaction with the environment. Thelatter requires the human to control the dynamic characteristicsof the upper limb musculoskeletal system. These characteristicsare quantitatively represented by inertia, damping, and stiffness,which are measures of mechanical impedance. Many previousstudies have shown that arm posture is a dominant factor in de-termining the end point impedance on a horizontal (transverse)plane. This paper presents the characterization of the end pointimpedance of the human arm in three-dimensional space. More-over, it models the regulation of the arm impedance with respectto various levels of muscle co-contraction. The characteriza-tion is made by route of experimental trials where human sub-jects maintained arm posture while their arms were perturbedby a robot arm. Furthermore, the subjects were asked to con-trol the level of their arm muscles' co-contraction, using visualfeedback of their muscles' activation, in order to investigate theeffect of this muscle co-contraction on the arm impedance. Theresults of this study show a very interesting, anisotropic increaseofarmstiffnessduetomuscleco-contraction. Theseresultscouldlead to very useful conclusions about the human's arm biome-chanics, as well as many implications for human motor control-specically the control of arm impedance through muscle co-contraction.INTRODUCTIONThe wide range of applications involving physical interac-tionofrobotswithhumanshasreceivedincreasedattentioninthelast few decades. Since the late 80's, there has been a substan-tial amount of interest in measuring human arm two dimensional(2D) end-point stiffness characteristics, where the arm is sup-ported and constrained to movement within a horizontal (trans-verse)plane. Aperturbationmethodformeasuringhandstiffnesswasdevelopedbyusingamanipulandumtodisplacethesubject'shand during maintenance of a given posture in [1]. Stiffness val-ues were represented both numerically and as ellipses (graphi-cally). These showed that the human musculo-skeletal systemhas spring like properties that enable posture stabilization andinteraction with the environment.The perturbation method for estimating arm stiffness hasbeen used by many other studies as well [25]. In [6] and [7],the perturbation method was extended to include measurementof other dynamic components: inertia and damping in additionto stiffness. The rst attempt to characterize arm impedance inthree-dimensional (3D) space was described in [8]. However thestochastic methods used were not able to provide an insight intothe neuromuscular system and its interaction with the environ-ment.Although most of the past studies have focused on pertur-bations during maintained hand posture, there are only a fewstudies that focused on the effect of muscle activation on the armstiffness-againonlyonthehorizontal(transverse)plane[2,9,10].Since every-day tasks involve movement of the upper arm inthe 3D space, the characterization of human arm impedance inthis space and the contribution of muscle co-contraction towardschanging these characteristics is signicant.In this paper, a systematic method for characterizing humanarm impedance in the 3D space and its regulation through mus-cle co-contraction is presented. A 7 degrees-of-freedom (DoFs)1 Copyright © 2013 by ASMEProceedings of the ASME 2013 Dynamic Systems and Control Conference DSCC2013 October 21-23, 2013, Palo Alto, California, USA