Abstract
Motor Unit Number Index (MUNIX) is a technique that provides a susceptive biomarker for monitoring innervation conditions in patients with neurodegenerative diseases. A satisfactory repeatability is essential for the interpretation of MUNIX results. This study aims to examine the effect of channel number and location on the repeatability of MUNIX. In this study, 128 channels of high-density surface electromyography (EMG) signals were recorded from the biceps brachii muscles of eight healthy participants, at 10, 20, 30, 40, 50, 60, 70, 80, and 100% of maximal voluntary contraction. The repeatability was defined by the coefficient of variation (CV) of MUNIX estimated from three experiment trials. Single-channel MUNIX (sMUNIX) was calculated on a channel-specific basis and a multi-channel MUNIX (mMUNIX) approach as the weighted average of multiple sMUNIX results. Results have shown (1) significantly improved repeatability with the proposed mMUNIX approach; (2) a higher variability of sMUNIX when the recording channel is positioned away from the innervation zone. Our results have demonstrated that (1) increasing the number of EMG channels and (2) placing recording channels close to the innervation zone (IZ) are effective methods to improve the repeatability of MUNIX. This study investigated two potential causes of MUNIX variations and provided novel perspectives to improve the repeatability, using high-density surface EMG. The mMUNIX technique proposed can serve as a promising tool for reliable neurodegeneration evaluation.
Highlights
Motor Unit Number Index (MUNIX) has been accepted as a neurological tool for technically friendly indexing the number of functioning motor unit (MU) of target muscle [1]
The conventional MUNIX can be represented by the Single-channel MUNIX (sMUNIX) of the channel with largest compound muscle action potential (CMAP) response, which is equivalent to the multi-channel MUNIX (mMUNIX) when N = 1
Conventional MUNIX is reprensented by mMUNIX (N = 1)
Summary
Motor Unit Number Index (MUNIX) has been accepted as a neurological tool for technically friendly indexing the number of functioning motor unit (MU) of target muscle [1]. Easier and quicker to perform than motor unit number estimation (MUNE), MUNIX has been proved an as reliable biomarker for assessing MU loss in different patient populations, including amyotrophic lateral sclerosis (ALS) [2], spinal cord injury (SCI) [3], multifocal motor neuropathy (MMN) [4], post-polio syndrome [5], stroke [6] and spinal muscular atrophy (SMA) [7]. The repeatability of MUNIX can be affected by multiple factors, including the variation in electromyography (EMG) signals and electrode positioning. Variations in compound muscle action potential (CMAP) signals and surface EMG contraction signals, and randomness of surface interferential patterns (SIP) selection can affect MUNIX results. The repeatability of MUNIX has been reported in both healthy and patient subjects, measured by coefficient of variation (CV), interclass correlation coefficients (ICC) and/or correlation coefficients (CC) [8,9,10,11]. It is necessary to find solutions to improve the repeatability of MUNIX; limited effort has been made
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