Assessment of the reliability of the motor unit size index (MUSIX) in single subject “round-robin” and multi-centre settings

James J.P. Alix*, Christoph Neuwirth, Lucy Gelder, Christian Burkhardt, José Castro, Mamede de Carvalho, Malgorzata Gawel, Stephan Goedee, Julian Grosskreutz, Timothée Lenglet, Cristina Moglia, Taha Omer, Maarten Schrooten, Sanjeev Nandedkar, Erik Stalberg, Paul E. Barkhaus, Jasna Furtula, Johannes P. van Dijk, Reto Baldinger, Joao CostaMarit Otto, Arne Sandberg, Markus Weber

*Corresponding author for this work
8 Citations (Scopus)


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.

Original languageEnglish
JournalClinical Neurophysiology
Issue number5
Pages (from-to)666-674
Number of pages9
Publication statusPublished - 05.2019
Externally publishedYes

Research Areas and Centers

  • Centers: Center for Neuromuscular Diseases


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