  The purpose of this article is to discuss some aspects of the convergence
analysis performed in the paper [Design of momentum fractional LMS for
Hammerstein nonlinear system identification with application to electrically
stimulated muscle model, Eur. Phys. J. Plus (2019) \textbf{134}: 407]. It is
highlighted that the way the authors prove convergence suffers a lack of
correct and valid mathematical justifications.
