Reproducibility of the motor unit number index (MUNIX) in normal controls and amyotrophic lateral sclerosis patients

Abstract

The motor unit number index (MUNIX) refers to an electrophysiologic technique that measures the approximate number of motor units using the surface electromyographic interference pattern (SIP) recorded during voluntary contraction. This study was done to assess the reproducibility of MUNIX performed on hypothenar muscles in 62 normal controls and 22 amyotrophic lateral sclerosis (ALS) patients. Inter- and intraoperator correlation coefficients for MUNIX were 0.74 and 0.86, respectively, in normal controls, and 0.95 and 0.93, respectively, in ALS patients (P < 0.01 in all). Inter- and intraoperator coefficients of variation for MUNIX measurements were 17.5% and 15.3%, respectively, in normal controls, and 23.7% and 24.0%, respectively, in ALS patients. This study shows a good correlation for MUNIX between intra- and interoperator results in both normal controls and ALS patients. The test-retest variability seems to be greater in ALS patients compared with normal controls, but this will need to be confirmed in future studies. Sources of variability should be identified and corrected for clinical use.

Applicability of the digital instrument to improve the reproducibility of motor unit number index

“…2,3 In conclusion, the mean values of MUNIX, the inter-operator COV (APB; 14.3, ADM; 13.6, Table 1), and the correlation coefficients (0.849 and 0.807, Table 1) indicate that the measurements after applying the digital dynamometer were much more reliable compared to those obtained in previous MUNIX and MUNE investigations. 2,3,[5][6][7][8][9][10][11] This technical improvement can be attributed to our efforts to reduce the test-retest variability while recording SIP signals by assuring close monitory through the digital dynamometer. In addition, our study revealed excellent test-retest reproducibility of MUSIX when assessing the average size of the motor units by using the digital instrument.…”

“…The MUNIX method involved assessing the APB and ADM muscles and the compound muscle action potential (CMAP) of the APB and ADM muscles were recorded using 10-mm flat disk electrodes with standard nerve conduction protocols stimulating median and ulnar nerve supramaximally, as has been previously reported. [1][2][3][4][5] The ADM and APB muscles were fully activated respectively by abduction of the little finger or by abduction of the thumb against a digital instrument calibrated by the manufacturer (Jamar Plus ® + Digital Hand Dynamometer, Sammons Preston Inc., Bolingbrook, IL, USA), and thereby the maximal muscle power could be digitally checked. Each muscle was gradually re-activated at five different levels of isometric force from minimum to maximum (slight, 25%, 50%, 75%, and 100%) using a digital instrument.…”

“…Unlike any previous study, each SIP signal of muscle contraction was digitally monitored to adjust the force levels based on the initial maximum value. This procedure was ingeminated to record CMAP values followed by ten SIP signals; we then analyzed the MUNIX results following the ideal case motor unit count (ICMUC) using the ''MUNIX'' program, which is a DOS-based shareware developed by Nandedkar et al [1][2][3] The MUSIX reflecting the average size of the motor units was calculated by dividing the CMAP amplitude by the MU-NIX value from the same muscles. To evaluate the reliability of a digital instrument, MUNIX was performed twice on the same day by different operators, both skilled EMG technicians.…”

“…The test-retest reproducibility was also expressed as the percent change in the difference by dividing the difference between the two measurements by the mean value (the coefficient of variation). 2,6 Results with p < 0.01 were considered significant. All statistical tests were performed using SPSS v19.0 (IBM Corp., Armonk, NY, USA).…”

“…[1][2][3] MUNIX and MUSIX depend on the surface electromyographic interference pattern (SIP) recorded during voluntary contraction instead of obtaining the mean single motor unit potential (SMUP) values which are critical in motor unit number estimation (MUNE) techniques. [1][2][3] In order to gain ten SIPs, the MUNIX software uses five different levels of isometric force to display voluntary surface electromyography (EMG) signals and consequently compute the results from the applicable muscles. Suk-Won Ahn.…”