Fundamental Concepts of Bipolar and High-Density Surface EMG Understanding and Teaching for Clinical, Occupational, and Sport Applications: Origin, Detection, and Main Errors.

Surface electromyography (sEMG) has been the subject of thousands of scientific articles, but many barriers limit its clinical applications. Previous work has indicated that the lack of time, competence, training, and teaching is the main barrier to the clinical application of sEMG. This work follows up and presents a number of analogies, metaphors, and simulations using physical and mathematical models that provide tools for teaching sEMG detection by means of electrode pairs (1D signals) and electrode grids (2D and 3D signals). The basic mechanisms of sEMG generation are summarized and the features of the sensing system (electrode location, size, interelectrode distance, crosstalk, etc.) are illustrated (mostly by animations) with examples that teachers can use. The most common, as well as some potential, applications are illustrated in the areas of signal presentation, gait analysis, the optimal injection of botulinum toxin, neurorehabilitation, ergonomics, obstetrics, occupational medicine, and sport sciences. The work is primarily focused on correct sEMG detection and on crosstalk. Issues related to the clinical transfer of innovations are also discussed, as well as the need for training new clinical and/or technical operators in the field of sEMG.

Monitoring Involuntary Muscle Activity in Acute Patients with Upper Motor Neuron Lesion by Wearable Sensors: A Feasibility Study.

Sustained involuntary muscle activity (IMA) is a highly disabling and not completely understood phenomenon that occurs after a central nervous system lesion. We tested the feasibility of in-field IMA measuring at an acute rehabilitation ward. We used wearable probes for single differential surface EMG (sEMG), inclusive of a 3D accelerometer, onboard memory and remote control. We collected 429 h of data from the biceps brachii of 10 patients with arm plegia. Data quality was first verified in the time and frequency domains. Next, IMA was automatically identified based on the steady presence of motor unit action potential (MUAP) trains at rest. Feasibility was excellent in terms of prep time and burden to the clinical staff. A total of 350.5 h of data (81.7%) were reliable. IMA was found in 85.9 h (25%). This was often present in the form of exceedingly long-lasting trains of one or a few MUAPs, with differences among patients and variability, both within and between days in terms of IMA duration, root mean square (RMS) and peak-to-peak amplitude. Our results proved the feasibility of using wearable probes for single differential sEMG to identify and quantify IMA in plegic muscles of bedridden acute neurological patients. Our results also suggest the need for long-lasting acquisitions to properly characterize IMA. The possibility of easily assessing IMA in acute inpatients can have a huge impact on the management of their postures, physiotherapy and treatments.

HDsEMG Activity of the Lumbar Erector Spinae in Violin Players: Comparison of Two Chairs.

This study compared an ergonomic alternative chair (A-chair) with a standard orchestra chair (O-chair) used by a group of 9 violin players. The features of the high-density surface EMG (HDsEMG) of the lumbar erector spinae muscles were used for the comparison. The violinists played the same pieces of music for 2 hrs without interruption on each chair in 2 different days, 1 week apart. HDsEMG was recorded for 20 s every 5 minutes using two electrode arrays of 16 × 8 electrodes each, one on each side of the spine and placed between the T11 and L4 levels. The sEMG was non-stationary and burst-like patterns were observed on 8 out of 9 violinists. The mean root mean square (RMS) and mean spectral frequency (MNF) value over the region of activity (ROA), the centroid of the ROA, the rates of change in time of the spatial mean of the RMS and MNF values, and the burst frequencies associated with the two chairs were compared. Statistically significant reductions of RMS were observed in each violinist between the O-chair and A-chair (range 11.80-78.36%). No significant changes of other spatial or spectral sEMG features were globally observed versus time or between chairs but were demonstrated by some subjects. It is concluded that the A-chair is associated with a decrease of the sEMG amplitude of the ESM without changes of the spatial and temporal patterns of muscle activation.

Analysis of High-Density Surface EMG and Finger Pressure in the Left Forearm of Violin Players: A Feasibility Study.

Wrist and finger flexor muscles of the left hand were evaluated using high-density surface EMG (HDsEMG) in 17 violin players. Pressure sensors also were mounted below the second string of the violin to evaluate, simultaneously, finger pressure. Electrode grid size was 110x70 mm (12x8 electrodes with interelectrode distance=10 mm and Ø=3 mm). The study objective was to observe the activation patterns of these muscles while the violinists sequentially played four notes--SI (B), DO# (C#), RE (D), MI (E)--at 2 bows/s (one bow up in 0.5 s and one down in 0.5 s) and 4 bows/s on the second string, while producing a constant (CONST) or ramp (RAMP) sound volume. HDsEMG images obtained while playing the notes were compared with those obtained during isometric radial or ulnar flexion of the wrist or fingers. Two image descriptors provided information on image differences. Results showed that the technique was reliable and provided reliable signals, and that recognizably different sEMG images could be associated with the four notes tested, despite the variability within and between subjects playing the same note. sEMG activity of the left hand muscles and pressure on the string in the RAMP task were strongly affected in some individuals by the sound volume (controlled by the right hand) and much less in other individuals. These findings question whether there is an individual or generally optimal way of pressing violin strings with the left hand. The answer to this question might substantially modify the teaching of string instruments.

Compression of high-density EMG signals for trapezius and gastrocnemius muscles.

BACKGROUND: New technologies for data transmission and multi-electrode arrays increased the demand for compressing high-density electromyography (HD EMG) signals. This article aims the compression of HD EMG signals recorded by two-dimensional electrode matrices at different muscle-contraction forces. It also shows methodological aspects of compressing HD EMG signals for non-pinnate (upper trapezius) and pinnate (medial gastrocnemius) muscles, using image compression techniques. METHODS: HD EMG signals were placed in image rows, according to two distinct electrode orders: parallel and perpendicular to the muscle longitudinal axis. For the lossless case, the images obtained from single-differential signals as well as their differences in time were compressed. For the lossy algorithm, the images associated to the recorded monopolar or single-differential signals were compressed for different compression levels. RESULTS: Lossless compression provided up to 59.3% file-size reduction (FSR), with lower contraction forces associated to higher FSR. For lossy compression, a 90.8% reduction on the file size was attained, while keeping the signal-to-noise ratio (SNR) at 21.19 dB. For a similar FSR, higher contraction forces corresponded to higher SNR CONCLUSIONS: The computation of signal differences in time improves the performance of lossless compression while the selection of signals in the transversal order improves the lossy compression of HD EMG, for both pinnate and non-pinnate muscles.

Effect of vaginal delivery on the external anal sphincter muscle innervation pattern evaluated by multichannel surface EMG: results of the multicentre study TASI-2.

INTRODUCTION AND HYPOTHESIS: A correlation exists between external anal sphincter (EAS) damage during birth and the subsequent development of fecal incontinence. This study evaluated the effect of delivery-related trauma on EAS innervation by means of intra-anal EMG performed with a rectal probe with 16 silver electrodes equally spaced along the circumference, before and after delivery. METHODS: Pre-partum EMG measurements were performed on 511 women, by nine clinical partners from five European countries at the 28th to 34th gestational weeks and the 6th to 8th post-delivery weeks; 331 women returned, after delivery, for the second test. The innervation zones (IZ) of EAS single motor units were identified by means of an EMG decomposition algorithm. RESULTS: The subjects were divided into four groups according to the delivery mode (Caesarean, vaginal with no evident damage, spontaneous lacerations and episiotomies). The number of IZs before and after delivery was compared. In the 82 women who underwent right mediolateral episiotomy, a statistically significant reduction of IZs was observed, after delivery, in the right ventral quadrant of the EAS (side of the episiotomy). Women who had Caesarean section, spontaneous lacerations or lack of evident damage did not present any significant change in the innervation pattern. CONCLUSIONS: Right episiotomy reduces the number of IZs on the right-ventral side of the EAS. The fast and reliable test proposed indicates the sphincter innervation pattern before delivery and helps obstetricians to evaluate the risks and to choose the preferred side of the episiotomy, if deemed necessary at the time of delivery.

Metrology in sEMG and movement analysis: the need for training new figures in clinical rehabilitation.

A new educational curriculum for the next generation of physical and occupational therapists is urgent in order to manage the recent fast advances in sensors, measurement technologies and related instrumentation. This is required by the growing role of STEM in rehabilitation, kinesiology, and sport sciences. Surface EMG technology is used in this work as a representative example of similar problems present in movement analysis, exoskeletons, and many other fields. A review of the most relevant articles and international projects in the field of interfacing physical therapy with measurement technology for quantitative assessment of outcome is presented. It is concluded that a new generation of educators is needed as well as a Ph.D. and/or a clinical doctorate degree in physical therapy, still lacking in many countries. It is urgent to consider knowledge translation since it will take many years before any recommended change in teaching will be accepted and show some effect. A call for a "white paper" on rehabilitation metrology is highly auspicable.

Right-left sEMG burst synchronization of the lumbar erector spinae muscles of seated violin players.

Burst-like activation of postural muscles has been previously described and plays a crucial role in elucidating the strategies for postural control adopted by the central nervous system (CNS). A spatio-temporal descriptor of surface electromyographic (sEMG) bursts (STB) is proposed and applied to statistically quantify the burst-like activity of the right and left (R-L) lumbar erector spinae muscle of nine seated violinists playing for two hours. The STB signal is the number of pixels of the high density sEMG (HDsEMG) maps simultaneously showing sEMG amplitude above a given threshold. Burst activity was present in all nine subjects. Four of them met four stringent criteria allowing analysis of frequency, duration, and synchronization between the R-L bursts after 0, 15, 30, 60, 120 min of playing. Mean square coherence between STBs of the two muscles was > 0.75 within ⁓1 Hz bandwidth between 2.2 Hz and 4.5 Hz depending on subject. Non-parametric statistics was applied to compare, in time and space, the R-L features of the bursts. The mean STB width was significantly associated primarily to side and secondarily to time and ranged from 100 to 250 ms. The right STB signals led the left (p < 0.02) by 0 - 160 ms.The inverted pendulum composed by the upper body of a seated violinist is controlled in an intermittent way. The erector spinae of the selected subjects were active, on average, for less than 50% of the time. These findings demonstrate a CNS strategy of intermittent back muscle activation presumably aimed to reducing fatigue during hours of playing in seated violinists.

Consensus for experimental design in electromyography (CEDE) project: Checklist for reporting and critically appraising studies using EMG (CEDE-Check).

The diversity in electromyography (EMG) techniques and their reporting present significant challenges across multiple disciplines in research and clinical practice, where EMG is commonly used. To address these challenges and augment the reproducibility and interpretation of studies using EMG, the Consensus for Experimental Design in Electromyography (CEDE) project has developed a checklist (CEDE-Check) to assist researchers to thoroughly report their EMG methodologies. Development involved a multi-stage Delphi process with seventeen EMG experts from various disciplines. After two rounds, consensus was achieved. The final CEDE-Check consists of forty items that address four critical areas that demand precise reporting when EMG is employed: the task investigated, electrode placement, recording electrode characteristics, and acquisition and pre-processing of EMG signals. This checklist aims to guide researchers to accurately report and critically appraise EMG studies, thereby promoting a standardised critical evaluation, and greater scientific rigor in research that uses EMG signals. This approach not only aims to facilitate interpretation of study results and comparisons between studies, but it is also expected to contribute to advancing research quality and facilitate clinical and other practical applications of knowledge generated through the use of EMG.

Consensus for experimental design in electromyography (CEDE) project: Application of EMG to estimate muscle force.

Skeletal muscles power movement. Deriving the forces produced by individual muscles has applications across various fields including biomechanics, robotics, and rehabilitation. Since direct in vivo measurement of muscle force in humans is invasive and challenging, its estimation through non-invasive methods such as electromyography (EMG) holds considerable appeal. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, summarizes recommendations on the use of EMG to estimate muscle force. The matrix encompasses the use of bipolar surface EMG, high density surface EMG, and intra-muscular EMG (1) to identify the onset of muscle force during isometric contractions, (2) to identify the offset of muscle force during isometric contractions, (3) to identify force fluctuations during isometric contractions, (4) to estimate force during dynamic contractions, and (5) in combination with musculoskeletal models to estimate force during dynamic contractions. For each application, recommendations on the appropriateness of using EMG to estimate force and justification for each recommendation are provided. The achieved consensus makes clear that there are limited scenarios in which EMG can be used to accurately estimate muscle forces. In most cases, it remains important to consider the activation as well as the muscle state and other biomechanical and physiological factors- such as in the context of a formal mechanical model. This matrix is intended to encourage interdisciplinary discussions regarding the integration of EMG with other experimental techniques and to promote advances in the application of EMG towards developing muscle models and musculoskeletal simulations that can accurately predict muscle forces in healthy and clinical populations.

Motor unit firing pattern of vastus lateralis muscle in type 2 diabetes mellitus patients.

INTRODUCTION: We investigated the motor unit (MU) firing pattern in type 2 diabetes mellitus (T2DM) patients by means of multichannel surface electromyography (SEMG). METHODS: Eight T2DM patients and 8 age-matched, healthy men performed a ramp-up contraction to 20% of maximal voluntary contraction (MVC). They also performed a sustained contraction at 10% of MVC during isometric knee extension. Multichannel SEMG signals recorded from the vastus lateralis muscle were decomposed with the convolution kernel compensation technique to extract individual MU firing patterns. RESULTS: During the ramp contraction, the extent of MU firing modulation was significantly attenuated in T2DM. Variability of MU firing rate was significantly higher in T2DM at later periods during the sustained contraction. CONCLUSIONS: Our findings suggest that T2DM patients manifest characteristic MU activity patterns due possibly to some degree of neuromuscular impairment affecting the integrity of MU firing modulation.

Uneven spatial distribution of surface EMG: what does it mean?

The aim of this work is to show how changes in surface electromyographic activity (sEMG) during a repetitive, non-constant force contraction can be detected and interpreted on the basis of the amplitude distribution provided by high-density sEMG techniques. Twelve healthy male subjects performed isometric shoulder elevations, repeating five times a force ramp profile up to 25 % of the maximal voluntary contraction (MVC). A 64-electrode matrix was used to detect sEMG from the trapezius muscle. The sEMG amplitude distribution was obtained for the force levels in the range 5-25 % MVC with steps of 5 % MVC. The effect of force level, subject, electrode position and ramp repetition on the sEMG amplitude distribution was tested. The sEMG amplitude was significantly smaller in the columns of the electrode grid over the tendons (repeated measures ANOVA, p < 0.01). The barycentre of the distribution of sEMG amplitude was subject-specific (Kruskal-Wallis test, p < 0.01), and shifted caudally with the increase of force levels and cranially with the repetition of the motor task (both p < 0.01, repeated measures ANOVA). The results are discussed in terms of motor unit recruitment in different muscle sub-portions. It is concluded that the sEMG amplitude distribution obtained by multichannel techniques provides useful information in the study of muscle activity, and that changes in the spatial distribution of the recruited motor units during a force varying isometric contraction might partially explain the variability observed in the activation pattern of the upper trapezius muscle.

Tutorial. Surface electromyogram (sEMG) amplitude estimation: Best practices.

This tutorial intends to provide insight, instructions and "best practices" for those who are novices-including clinicians, engineers and non-engineers-in extracting electromyogram (EMG) amplitude from the bipolar surface EMG (sEMG) signal of voluntary contractions. A brief discussion of sEMG amplitude extraction from high density sEMG (HDsEMG) arrays and feature extraction from electrically elicited contractions is also provided. This tutorial attempts to present its main concepts in a straightforward manner that is accessible to novices in the field not possessing a wide range of technical background (if any) in this area. Surface EMG amplitude, also referred to as the sEMG envelope [often implemented as root mean square (RMS) sEMG or average rectified value (ARV) sEMG], quantifies the voltage variation of the sEMG signal and is grossly related to the overall neural excitation of the muscle and to peripheral parameters. The tutorial briefly reviews the physiological origin of the voluntary sEMG signal and sEMG recording, including electrode configurations, sEMG signal transduction, electronic conditioning and conversion by an analog-to-digital converter. These topics have been covered in greater detail in prior tutorials in this series. In depth descriptions of state-of-the-art methods for computing sEMG amplitude are then provided, including guidance on signal pre-conditioning, absolute value vs. square-law detection, selection of appropriate sEMG amplitude smoothing filters and attenuation of measurement noise. The tutorial provides a detailed list of best practices for sEMG amplitude estimation.

Translation of surface electromyography to clinical and motor rehabilitation applications: The need for new clinical figures.

Advanced sensors/electrodes and signal processing techniques provide powerful tools to analyze surface electromyographic signals (sEMG) and their features, to decompose sEMG into the constituent motor unit action potential trains, and to identify synergies, neural muscle drive, and EEG-sEMG coherence. However, despite thousands of articles, dozens of textbooks, tutorials, consensus papers, and European and International efforts, the translation of this knowledge into clinical activities and assessment procedures has been very slow, likely because of lack of clinical studies and competent operators in the field. Understanding and using sEMG-based hardware and software tools requires a level of knowledge of signal processing and interpretation concepts that is multidisciplinary and is not provided by most academic curricula in physiotherapy, movement sciences, neurophysiology, rehabilitation, sport, and occupational medicine. The chasm existing between the available knowledge and its clinical applications in this field is discussed as well as the need for new clinical figures. The need for updating the training of physiotherapists, neurophysiology technicians, and clinical technologists is discussed as well as the required competences of trainers and trainees. Indications and examples are suggested and provide a basis for addressing the problem. Two teaching examples are provided in the Supplementary Material.

Consensus for experimental design in electromyography (CEDE) project: Single motor unit matrix.

The analysis of single motor unit (SMU) activity provides the foundation from which information about the neural strategies underlying the control of muscle force can be identified, due to the one-to-one association between the action potentials generated by an alpha motor neuron and those received by the innervated muscle fibers. Such a powerful assessment has been conventionally performed with invasive electrodes (i.e., intramuscular electromyography (EMG)), however, recent advances in signal processing techniques have enabled the identification of single motor unit (SMU) activity in high-density surface electromyography (HDsEMG) recordings. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, provides recommendations for the recording and analysis of SMU activity with both invasive (needle and fine-wire EMG) and non-invasive (HDsEMG) SMU identification methods, summarizing their advantages and disadvantages when used during different testing conditions. Recommendations for the analysis and reporting of discharge rate and peripheral (i.e., muscle fiber conduction velocity) SMU properties are also provided. The results of the Delphi process to reach consensus are contained in an appendix. This matrix is intended to help researchers to collect, report, and interpret SMU data in the context of both research and clinical applications.

Recruitment of motor units in the medial gastrocnemius muscle during human quiet standing: is recruitment intermittent? What triggers recruitment?

The recruitment and the rate of discharge of motor units are determinants of muscle force. Within a motoneuron pool, recruitment and rate coding of individual motor units might be controlled independently, depending on the circumstances. In this study, we tested whether, during human quiet standing, the force of the medial gastrocnemius (MG) muscle is predominantly controlled by recruitment or rate coding. If MG control during standing was mainly due to recruitment, then we further asked what the trigger mechanism is. Is it determined internally, or is it related to body kinematics? While seven healthy subjects stood quietly, intramuscular electromyograms were recorded from the MG muscle with three pairs of wire electrodes. The number of active motor units and their mean discharge rate were compared for different sway velocities and positions. Motor unit discharges occurred more frequently when the body swayed faster and forward (Pearson R = 0.63; P < 0.0001). This higher likelihood of observing motor unit potentials was explained chiefly by the recruitment of additional units. During forward body shifts, the median number of units detected increased from 3 to 11 (P < 0.0001), whereas the discharge rate changed from 8 ± 1.1 (mean ± SD) to 10 ± 0.9 pulses/s (P = 0.001). Strikingly, motor units did not discharge continuously throughout standing. They were recruited within individual, forward sways and intermittently, with a modal rate of two recruitments per second. This modal rate is consistent with previous circumstantial evidence relating the control of standing to an intrinsic, higher level planning process.

Surface electromyography features in manual workers affected by carpal tunnel syndrome.

INTRODUCTION: Alterations in surface electromyographic (sEMG) signals of the abductor pollicis brevis muscle were evaluated in 24 non-manual workers and 40 manual workers (25 asymptomatic and 15 reporting CTS symptoms). METHODS: The initial value (IV) and the normalized rate of change (NRC) of average rectified value (ARV), mean frequency of the power spectrum (MNF), and muscle fiber conduction velocity (MFCV) were calculated during contractions at 20% and 50% of maximal voluntary contraction (MVC). Neuromuscular efficiency (NME) and kurtosis of the sEMG amplitude distribution were estimated. RESULTS: With respect to controls, manual workers showed higher NME, lower ARV IV, and reduced myoelectric manifestations of fatigue (lower MNF NRC for both contraction levels, and lower MFCV NRC at 50% MVC). Kurtosis at 20% MVC showed higher values in symptomatic manual workers than in the other two groups. CONCLUSIONS: Kurtosis seems to be a promising parameter for use in monitoring individuals who develop CTS.

Consensus for experimental design in electromyography (CEDE) project: High-density surface electromyography matrix.

High-density surface electromyography (HDsEMG) can be used to measure the spatial distribution of electrical muscle activity over the skin. As this distribution is associated with the generation and propagation of muscle fiber action potentials, HDsEMG is processed to extract information on regional muscle activation, muscle fiber characteristics and behaviour of individual motor units. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, summarizes recommendations on the use of HDsEMG in experimental studies. For each application, recommendations are included regarding electrode montage, electrode type and configuration, electrode location and orientation, data analysis, and interpretation. Cautions and reporting standards are also included. The steps of the Delphi process to reach consensus are contained in an appendix. This matrix is intended to help researchers when collecting, reporting, and interpreting HDsEMG data. It is hoped that this document will be used to generate new empirical evidence to improve how HDsEMG is used in research and in clinical applications.

Postural activation of the human medial gastrocnemius muscle: are the muscle units spatially localised?

In cat medial gastrocnemius (MG), fibres supplied by individual motoneurones (muscle units) distribute extensively along the muscle longitudinal axis. In the human MG, the size of motor unit territory is unknown. It is uncertain if the absolute size of muscle unit territory or the size relative to the whole muscle is most comparable with the cat. By comparing intramuscular and surface electromyograms we tested whether muscle units extend narrowly or widely along the human MG muscle. Due to the pennation of the MG, if individual motoneurones supply fibres scattered along the muscle, then action potentials of single motor units are expected to appear sparsely on the surface of the skin. In nine healthy subjects, pairs of wire electrodes were inserted in three locations along the MG muscle (MG60%, MG75% and MG90%). A longitudinal array of 16 surface electrodes was positioned alongside the intramuscular electrodes. While subjects stood quietly, 55 motor units were identified, of which, significantly more units were detected in the most distal sites. The surface action potentials had maximum amplitude at 4.40 ±1.67 (mean±S.D.), 8.02±2.16 and 11.63±2.09 cm (P <0.001) from the most proximal surface electrode, for motor units in the MG60%, MG75% and MG90% locations, respectively. Single motor unit potentials were recorded by five consecutive surface electrodes, at most, indicating that muscle units extend shortly along the MG longitudinal axis. It is concluded that relative to the whole muscle, and compared with the cat, muscle units in human MG are localised. The localisation of muscle units might have implications for the regional control of muscle activity.