Consensus for experimental design in electromyography (CEDE) project: Electrode selection matrix

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Abstract

The Consensus for Experimental Design in Electromyography (CEDE) project is an international initiative which aims to guide decision-making in recording, analysis, and interpretation of electromyographic (EMG) data. The quality of the EMG recording, and validity of its interpretation depend on many characteristics of the recording set-up and analysis procedures. Different electrode types (i.e., surface and intramuscular) will influence the recorded signal and its interpretation. This report presents a matrix to consider the best electrode type selection for recording EMG, and the process undertaken to achieve consensus. Four electrode types were considered: (1) conventional surface electrode, (2) surface matrix or array electrode, (3) fine-wire electrode, and (4) needle electrode. General features, pros, and cons of each electrode type are presented first. This information is followed by recommendations for specific types of muscles, the information that can be estimated, the typical representativeness of the recording and the types of contractions for which the electrode is best suited. This matrix is intended to help researchers when selecting and reporting the electrode type in EMG studies.

Introduction

The quality of electromyography (EMG) recordings and the validity of the interpretation of the data depend on many characteristics of the recording set-up and analysis procedures. The optimal features differ between applications based on the question to be addressed and the muscle under investigation. There are many issues to consider and the purpose of the Consensus for Experimental Design in Electromyography (CEDE) project is to provide expert consensus opinion of optimal features of recording set-up and analysis to address a range of experimental questions. EMG electrodes are the interface between the tissues and the recording system. The properties of the signal depend on their type (surface or intramuscular), configuration (e.g. bipolar, matrix), and materials/construction (e.g. Ag/AgCL, conductive ink). Selection of the appropriate type of electrode requires careful consideration of the signal that is to be recorded, and the way in which the recording is to be interpreted.

Electrodes for recording electromyography can be broadly defined as surface or intramuscular. Surface electrodes are often applied as a conventional surface electrode pair or an array (linear or matrix). Intramuscular electrodes can be either fine-wire or needles. In both cases, multiple versions are available, that vary in their design characteristics and recording properties. Less conventional surface electrodes (e.g., anal and vaginal probes) (Keshwani and McLean, 2015, Merletti, 2016, Merletti et al., 2016, Mesin et al., 2009a, Mesin et al., 2009b), and some innovative new electrodes that are currently under development/investigation (e.g. tattoo electrodes, high‐adhesion stretchable electrodes, wearable high-resolution facial array) (Ferrari et al., 2018, Inzelberg et al., 2018, Liu et al., 2017), generally share properties with those attached to the skin (i.e. conventional surface or matrix electrodes), but have different types of fixation and configuration, for specific contexts/situations. These can be considered according to the principles described for conventional surface or matrix electrodes.

  • a.

    Conventional surface electrode. Typically consists of a single recording channel using differential amplification of pairs of electrodes (either applied separately or as a pair integrated into a single device) placed on the skin overlying a targeted muscle. Other applications may involve more than two electrodes whose signals are combined to produce a single output channel (e.g. double differential amplification, Laplacian). (Merletti et al., 2016, Merletti, 2016, Merletti et al., 2009).

  • b.

    Linear array or matrix surface electrode (also known as electrode arrays or grids, multi-channel surface EMG, high-density surface EMG). Based on a multichannel detection system arranged in one- (1-D) or two-dimensional (2-D) electrode arrays (Merletti et al., 2016, Merletti, 2016, Merletti et al., 2009).

  • c.

    Fine-wire electrode. Consists of fine diameter insulated wire(s) placed in the muscle via a hypodermic needle. The insulation is removed from the tip (the length of which is a determinant of the electrode’s receptive area) and is bent to maintain its placement in the muscle, and the needle is withdrawn (Merletti and Farina, 2009).

  • d.

    Needle electrode. For these electrodes, the needle remains in the muscle. Configurations involve either the needle shaft or tip (electrically insulated from the remaining shaft) acting as a recording surface, or the recording surfaces can be mounted on the shaft or tip of the needle. Several configurations are available with different characteristics. These electrodes are commonly used to assess neurophysiological characteristics of neuromuscular disorders (Merletti and Farina, 2009).

Not all electrodes are suitable for all applications, and electrodes must be carefully selected with specific attention to the question to be answered and the desired properties of the recording (Mesin et al., 2009a, Mesin et al., 2009b). The decision to use an electrode type depends on the characteristics of the muscle under investigation and the purpose of the study. Each type of electrode has advantages and disadvantages that require consideration (Turker, 1993). Recommendations have been made for bipolar surface EMG including electrode shape and size, electrode placement, inter-electrode distance, electrode material, and sensor construction (Hermens et al., 2000), and have been recently updated (Afsharipour et al., 2019). Other factors require consideration when selecting electrodes for a specific application. These factors include the nature of the task (dynamic vs. static; maximal vs. submaximal), location of the muscle innervation zones, potential for crosstalk, potential sources of noise (e.g. motion artefact, electromagnetic radiation), depth of the muscle and thickness of the subcutaneous fat tissue (as the signal is attenuated in the subcutaneous tissues when recorded from surface EMG – an effect that differs between individuals). Specific issues relate to the recorded muscle (architecture, location and size), and the information to be estimated and interpreted (e.g. EMG amplitude vs. discrimination of single motor unit action potentials) (Merletti et al., 2016, Merletti, 2016, Disselhorst-Klug et al., 2009, Farina et al., 2014, Kuiken et al., 2003, Merlo and Campanini, 2016, Staudenmann et al., 2010).

Given the complexity, diversity, and variability of EMG research and the growth in research applications of EMG, recommendations to guide decision-making in recording, data analysis, and reporting of EMG studies are crucial for accurate interpretation of findings. This paper presents a guide to decision-making that can be used when selecting the most appropriate electrode for a proposed EMG application and the process undertaken to achieve consensus in developing these guidelines.

Section snippets

Project overview

The CEDE project is an international initiative which aims to develop consensus-based matrices to guide decision-making in recording, analysis, and interpretation of EMG data. Each design matrix considers specific study design features and the issues that need to be considered when designing and interpreting the results from an EMG experiment/study. The aim is to guide high-quality EMG research that enables valid and consistent interpretation of findings, to aid the review of research using

Results

After phase 1, thirteen experts (65%) provided additional comments regarding the content and format of the matrix. Four experts (out of nine) provided additional feedback on the section related to the analysis of EMG amplitude/frequency content for final refinement of the matrix.

From the 20 experts who agreed to participate in the Delphi process, 18 (80%) replied to the first-round questionnaire. After round one, five sections were ranked with insufficient consensus. Appendix 1 shows the

Discussion

The presented matrix represents the current state-of-art consensus for the selection of electrodes for EMG recording. Four electrode types were considered; conventional surface electrodes, array or matrix surface electrodes, fine-wire electrodes, and needle electrodes. This matrix is designed to aid decisions regarding the appropriateness of specific electrode types for specific applications, data analyses, and interpretations. This matrix includes general features for surface and intramuscular

Conclusion

In summary, the aim of the electrode selection matrix, developed by the CEDE project team, is to improve the quality of EMG recordings and enhance the validity of the interpretations drawn on the basis of these recordings. The authors wish to underline that the matrix is not intended to replace formal training or education for EMG practice, as this remains necessary. Rather, it may be used as a reference when planning studies, and when reporting (and justifying) the decisions that are made in

Statements

Funding

This research was funded by the National Health and Medical Research Council (NHMRC) of Australia (Program Grant: APP1091302), Australia. PWH is supported by an NHMRC Senior Principal Research Fellowship (APP1102905), Australia. MB is supported by the University of Queensland Research Training Scholarship, Australia. MCK was supported by a Program Grant (APP1132524), Partnership Project (APP1153439) and Practitioner Fellowship (APP1156093) from the NHMRC, Australia. AH is supported by

Declaration of Competing Interest

None declared.

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