Impact of reference electrode position on motor unit number estimation (MUNE) in the tibialis anterior muscle using MScanFit: test-retest reliability

Objectives This study aimed to assess the effect of varying the reference electrode position, specifically comparing position A3 (medial patella) to routine position 1 (R1) and the MScan multicenter protocol position (M1), on compound muscle action potential (CMAP) and motor unit number estimation (MUNE) in the tibialis anterior muscle of healthy participants. Methods Twenty healthy participants underwent repeated MScanFit MUNE assessments with a 7-14 day interval between tests. The reference electrode (E2) was placed in three positions at each visit (A3, R1, and M1), while the active electrode (E1) remained constant. An additional seventeen participants were included to establish the minimal detectable true change in MUNE values using MScanFit, with the reference electrode exclusively in the M1 position. Results The reference electrode position significantly influenced CMAP and MUNE, with R1 resulting in lower values. However, no significant difference was observed between M1 and A3 positions. Relative and absolute reliability indicators favored using the M1 position for reference in MScanFit MUNE. In a combined dataset of 37 healthy participants, the average tibialis anterior muscle motor unit count was estimated at 148 (SD 25.2), with a minimal detectable true change of 55 units. Conclusions The preference for the M1 position over the alternative A3 position is supported, particularly for MScanFit MUNE assessments in the tibialis anterior muscle. Clinically, a true change in MUNE should consider the minimal detectable change of 55 motor units, underscoring the reality that large changes in MUNE are required to conclude a genuine change beyond measurement error. Significance For MUNE examinations of the tibialis anterior muscle, adhering to the electrode positions outlined in the MScan multicenter protocol is advisable. Awareness of measurement error limitations in MScanFit MUNE underscores its applicability in making longitudinal clinical decisions for individual patients.