Relationship between the size of the recording electrodes and morphology of the compound muscle action potentials

Abstract

To determine the effect of recording electrode size on the compound muscle action potentials (CMAPs), maximally evoked CMAPs were obtained from the abductor digiti minimi muscle in ten normal subjects with circular-shaped surface recording electrodes of different diameters (4, 6, 8, 10, 12, and 14 mm). The electrodes were concentrically located on the same recording point on the muscle at its endplate zones when obtaining the CMAPs. The CMAP onset latency remained unchanged for all recording electrode sizes. The CMAP amplitude decreased with increasing electrode size. The mean absolute amplitude difference between CMAPs recorded with a 4-mm diameter electrode and those with a 14-mm diameter electrode was 1.22 mV, representing an amplitude decrement of 10% with this increase in electrode size. The general shape or morphology of the CMAPs was relatively well preserved.