Electrical properties of commercial concentric EMG electrodes

Five electrical characteristics--impedance, broadband noise generation, line interference sensitivity, signal distortion, and common-mode conversion--were measured in five electromyographic (EMG) concentric needle electrodes (CNEs) from each of six commercial manufacturers. Untreated CNEs showed considerable variation in impedance and broadband noise characteristics, both within and among manufacturers. Electrolytic treatment reduced impedances by a factor between 1.5 and 4.0, and lessened within-manufacturer variability. Average post-treatment impedances at 100 Hz ranged from 31 to 436 kOhms, reflecting in part the range of core surface areas. Treatment also reduced the broadband noise to the level of the instrumentation noise for all but the highest impedance CNEs. Distortion and common-mode conversion were negligible for the lowest impedance CNEs. Line interference from a nearby power cord was completely suppressed only by those CNEs with fully shielded cables, and then only when the electromyographer also was grounded; there was no measurable benefit when the shield was driven, as opposed to grounded. We conclude that there are consistent differences in the properties of CNEs from different manufacturers, reflecting differences in materials, design, and construction; electrolytic treatment temporarily improves the performance of all CNEs, and tends to lessen the differences among them, most likely through a surface electrochemical effect; and both active and passive cable shields are effective in reducing extraneous line interference.