Circuit Models and Simulation Analysis of Electromyographic Signal Sources-I: The Impedance of EMG Electrodes

Abstract
The impedance of surface and intramuscular biopotential electrodes was measured during rest and muscle contraction in humans. A frequency-dependent parallel RC circuit model of the skin-electrode interface that captures the dependence of this impedance on the size and geometry of electrode placement was developed and its components were estimated. The model could explain between 86-97 percent of the variations in impedance (Z), 72-92 percent of the variations in resistance (R), and 34-93 percent of the variations in capacitance (C). The impedance and resistance of these electrodes decrease by about tenfold for a 20-fold increase in frequency, while the capacitance decreases by about twofold for the same change in frequency (f). Thus, the overall FRC factor of this parallel RC circuit model remains nearly unchanged over the range of frequencies studied (50-2000 Hz). A significant difference was found between the impedances of the electrodes comprising the differential electrode pair. This imbalance in impedance was between 8-14 percent for surface electrodes and 6-19 percent for wire electrodes. These data provide essential design criteria for the development and simulation of a system for the measurement of electromyographic activity.