Impedance measurement system for concentric needle electrodes

Electromyographic recordings are significantly affected by electrode characteristics and associated instrumentation. This paper describes a system developed for measuring the interface impedance of concentric needle electrodes in normal saline at constant current density and over a frequency range of 10 Hz to 10 kHz. Five commercially available electrodes, having platinum as the core wire material were tested and the impedance was found to reduce exponentially with frequency according to an empirical equation Z=Kf^-m. The value of K, a measure of magnitude, was 1309 kohm/Hz^m and that of m was 0.75. The system was used to study the effect of electrolytic treatment on the electrode impedance. Considerable reduction was observed in the impedance magnitude (K=245 kohm/Hz^-m ) and its variation with frequency (m=0.58), but only for short duration. The system can be effectively used to optimise various parameters for achieving desired impedance characteristics of the electrode