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

Objective: We investigated how the active electrode placement site influences compound muscle action potential (CMAP) configuration of the upper trapezius muscle (TM).

Methods: 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-) and placement of the electrode 2 cm behind the belly (CMAP-), were compared. CMAPs were also obtained with the active recording electrode placed in the supraspinous fossa (CMAP-).

Results: All CMAPs were recorded from 21 healthy volunteers. The mean peak-to-peak amplitude of CMAP- was 3.4 mV higher than that of CMAP- (11.0 ± 4.0 mV vs. 14.4 ± 4.9 mV; P < 0.01). The mean peak-to-peak amplitude of CMAP- was 10.3 ± 5.0 mV.

Conclusions: CMAP of the upper TM was always higher when the active recording electrode was placed 2 cm behind the belly of the muscle.

Significance: When stimulating the accessory nerve, a current spread occurs to the C5 spinal nerve root and another CMAP originating from the supraspinatus muscle occurs in the supraspinous fossa. The volume conduction from the supraspinatus muscle affects the active recording electrode on the TM, resulting in an increase in CMAP amplitude.