title: Two degrees of freedom quasi-static EMG-force at the wrist using a minimum number of electrodes.
abstract: Surface electromyogram-controlled powered hand/wrist prostheses return partial upper-limb function to limb-absent persons. Typically, one degree of freedom (DoF) is controlled at a time, with mode switching between DoFs. Recent research has explored using large-channel EMG systems to provide simultaneous, independent and proportional (SIP) control of two joints-but such systems are not practical in current commercial prostheses. Thus, we investigated site selection of a minimum number of conventional EMG electrodes in an EMG-force task, targeting four sites for a two DoF controller. In a laboratory experiment with 10 able-bodied subjects and three limb-absent subjects, 16 electrodes were placed about the proximal forearm. Subjects produced 1-DoF and 2-DoF slowly force-varying contractions up to 30% maximum voluntary contraction (MVC). EMG standard deviation was related to forces via regularized regression. Backward stepwise selection was used to retain those progressively fewer electrodes that exhibited minimum error. For 1-DoF models using two retained electrodes (which mimics the current state of the art), subjects had average RMS errors of (depending on the DoF): 7.1-9.5% MVC for able-bodied and 13.7-17.1% MVC for limb-absent subjects. For 2-DoF models, subjects using four electrodes had errors on 1-DoF trials of 6.7-8.5% MVC for able-bodied and 11.9-14.0% MVC for limb-absent; and errors on 2-DoF trials of 9.9-11.2% MVC for able-bodied and 15.8-16.7% MVC for limb-absent subjects. For each model, retaining more electrodes did not statistically improve performance. The able-bodied results suggest that backward selection is a viable method for minimum error selection of as few as four electrode sites for these EMG-force tasks. Performance evaluation in a prosthesis control task is a necessary and logical next step for this site selection method.
authors.affiliation: Worcester Polytechnic Institute (WPI), Worcester, MA, USA; Liberating Technologies, Inc., Holliston, MA, USA. Electronic address: ted@wpi.edu.
authors.forename: Edward A
authors.initials: EA
authors.lastname: Clancy
authors.affiliation: Liberating Technologies, Inc., Holliston, MA, USA.
authors.forename: Carlos
authors.initials: C
authors.lastname: Martinez-Luna
authors.affiliation: Worcester Polytechnic Institute (WPI), Worcester, MA, USA; Liberating Technologies, Inc., Holliston, MA, USA.
authors.forename: Marek
authors.initials: M
authors.lastname: Wartenberg
authors.affiliation: Worcester Polytechnic Institute (WPI), Worcester, MA, USA; University of North Carolina at Chapel Hill & North Carolina State University, Raleigh, NC, USA.
authors.forename: Chenyun
authors.initials: C
authors.lastname: Dai
authors.affiliation: Liberating Technologies, Inc., Holliston, MA, USA.
authors.forename: Todd R
authors.initials: TR
authors.lastname: Farrell
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D004566:Electrodes / Q000592:standards; D004576:Electromyography / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D014953:Wrist / Q000502:physiology*; D014955:Wrist Joint / Q000502:physiology
keywords: EMG; EMG signal processing; EMG-force; Electromyogram; Myoelectric control; Prosthesis; Prosthesis control