Alix, James J. P.Neuwirth, ChristophGelder, LucyBurkhardt, ChristianCastro, JoséCarvalho, MamedeGawel, MalgorzataGoedee, StephanGrosskreutz, JulianLenglet, TimothéeMoglia, CristinaOmer, TahaSchrooten, MaartenNandedkar, SanjeevStalberg, ErikBarkhaus, Paul E.Furtula, Jasnavan Dijk, Johannes P.Baldinger, RetoCosta, JoãoOtto, MaritSandberg, ArneWeber, Markus2022-05-092022-05-092019Clin Neurophysiol . 2019 May;130(5):666-6741388-2457http://hdl.handle.net/10451/52802© 2019 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.Objective: The motor unit size index (MUSIX) is incorporated into the motor unit number index (MUNIX). Our objective was to assess the intra-/inter-rater reliability of MUSIX in healthy volunteers across single subject "round robin" and multi-centre settings. Methods: Data were obtained from (i) a round-robin assessment in which 12 raters (6 with prior experience and 6 without) assessed six muscles (abductor pollicis brevis, abductor digiti minimi, biceps brachii, tibialis anterior, extensor digitorum brevis and abductor hallucis) and (ii) a multi-centre study with 6 centres studying the same muscles in 66 healthy volunteers. Intra/inter-rater data were provided by 5 centres, 1 centre provided only intra-rater data. Intra/inter-rater variability was assessed using the coefficient of variation (COV), Bland-Altman plots, bias and 95% limits of agreement. Results: In the round-robin assessment intra-rater COVs for MUSIX ranged from 7.8% to 28.4%. Inter-rater variability was between 7.8% and 16.2%. Prior experience did not impact on MUSIX values. In the multi-centre study MUSIX was more consistent than the MUNIX. Abductor hallucis was the least reliable muscle. Conclusions: The MUSIX is a reliable neurophysiological biomarker of reinnervation. Significance: MUSIX could provide insights into the pathophysiology of a range of neuromuscular disorders, providing a quantitative biomarker of reinnervation.engAmyotrophic lateral sclerosisElectrophysiologyMotor neuronMotor unitMotor unit number indexMotor unit size indexjournal article10.1016/j.clinph.2019.01.0201558-1497