On the Reuse of Motor Unit Filters in High Density Surface Electromyograms Recorded at Different Contraction Levels

Aljaz Francic,Ales Holobar

doi:10.1109/access.2021.3104762

Aljaz Francic, Ales Holobar

Open Access

https://doi.org/10.1109/access.2021.3104762

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Abstract

We analyzed the efficiency of Motor Unit (MU) tracking across different experimentally recorded contraction levels of Biceps Brachii (BB), Tibialis Anterior (TA), First Dorsal Interosseous (FDI) and Abductor Digiti Minimi (ADM) muscles and in simulated conditions. We used the Convolution Kernel Compensation (CKC) algorithm to estimate the MU filters from high-density electromyograms (HDEMGs) at contraction levels ranging from 5 % to 70 % of Maximum Voluntary Contraction (MVC), and applied these MU filters to all the recorded contractions levels of the same muscle. For each MU filter estimation-application pair we assessed the number of identified MUs, Pulse-to-Noise Ratio (PNR), Mean Discharge Rate (MDR) and MUAP peak-to-peak (P2P) amplitude, normalized by HDEMG P2P amplitude. In simulated conditions, we also calculated the Precision ( $Pr$ ) and Sensitivity ( $Se$ ) of MU firing identification. We also reported the number of jointly identified MUs for all possible contraction level pairs. The results in simulated conditions confirmed the efficiency of MU filter transfer from low to high contraction levels. The large majority of MUs identified at lower contraction levels were tracked successfully at higher contraction levels, though many of them were not directly identified at higher contraction levels. Sensitivities of identified MU firings were above 97 %, decreasing only by about 1 % when transferring the MU filters from low to high contraction levels. In the experimental conditions the efficiency of the MU tracking depended significantly on the investigated muscle. The highest efficiency was demonstrated in the FDI muscle, where about 90 % and 70 % of MUs identified at the contraction level of MU filter estimation were also tracked at 10 % and 20 % higher contraction level, respectively. These figures decreased to 47 % and 20 % of MUs in TA, and to 21 % and 18 % of MUs in the BB muscle. The observed differences between the MU filter transfer efficiencies are likely caused by the differences in the size and anatomy of the investigated muscles.

Highlights

I N humans, skeletal muscles consist of muscle fibers that are grouped into Motor Units (MUs)
The results on synthetic high-density electromyograms (HDEMGs) confirmed the efficiency of the MU filter transfer from low to high contraction levels (Figs. 4 and 6)
The Pulse-to-Noise Ratio (PNR) decreased with the contraction level (Fig. 6), but stayed well above 30 dB, suggesting an accuracy > 90 % in MU firing identification [20]

Summary

Introduction

I N humans, skeletal muscles consist of muscle fibers that are grouped into Motor Units (MUs). Each of them is composed of several tens to several hundreds of muscle fibers, and the motor neuron that innervates all the fibers within that MU. Each electrical pulse in the motor neuron triggers the Single Fiber Action Potentials (SFAPs) in the innervated muscle fibers and, their contraction. Since the SFAPs of the fibers constituting the MU appear synchronously in time, they can be summed up conceptually into so-called Motor Unit Action Potential (MUAP). The human motor system increases the muscle force by increasing the number of active MUs and the frequency of their activation, the so-called MU firing rate [1], [2]

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