Improving the reproducibility of motor unit number index.

Introduction: Reproducibility is an important aspect of any method intended to be a marker of disease progression. In this study we investigated approaches for improving motor unit number index (MUNIX) reproducibility.

Methods: We used the intraclass correlation coefficient (ICC) and the coefficient of variation (CV) to study reproducibility in healthy subjects. We tested reproducibility between test and retest of a single MUNIX from 3 different muscles (S-MUNIX) and also of the mean of a set of 3 measurements from these same muscles (M-MUNIX).

Results: M-MUNIX was more reproducible than S-MUNIX. The CV showed a greater improvement than the ICC in all 3 muscles.

Conclusions: M-MUNIX may be a valuable approach for following motor unit loss, because it is more replicable than MUNIX. This may be especially relevant in amyotrophic lateral sclerosis patients, in whom MUNIX variability is higher than in healthy individuals. Muscle Nerve, 2016 Muscle Nerve 55: 635-638, 2017.