Influence of placement sites of the active recording electrode on CMAP configuration in the trapezius muscle

•The active recording electrode site influences the CMAP waveform of the trapezius muscle (TM).•CMAP becomes high by placement of the active recording electrode 2 cm behind the belly of the TM.•Volume conduction from the supraspinatus muscle affects the CMAP waveform of the TM. We investigated how the active electrode placement site influences compound muscle action potential (CMAP) configuration of the upper trapezius muscle (TM). A nerve conduction study of the accessory nerve was performed, and the CMAPs obtained with two different placement sites, i.e., placement of the active recording electrode on the belly of the upper TM (CMAP-A) and placement of the electrode 2 cm behind the belly (CMAP-B), were compared. CMAPs were also obtained with the active recording electrode placed in the supraspinous fossa (CMAP-C). All CMAPs were recorded from 21 healthy volunteers. The mean peak-to-peak amplitude of CMAP-B was 3.4 mV higher than that of CMAP-A (11.0 ± 4.0 mV vs. 14.4 ± 4.9 mV; P