Topographical characteristics of motor units of the lower facial musculature revealed by means of high-density surface EMG

The objective of this study was to systematically characterize motor units (MUs) of the musculature of the lower face. MU endplate positions and principal muscle fiber orientations relative to facial landmarks were identified. This was done by the analysis of motor unit action potentials (MUAPs) in the surface electromyogram. Thirteen specially trained, healthy subjects performed selective contractions of the depressor anguli oris, depressor labii inferioris, mentalis, and orbicularis oris inferior muscles. Signals were recorded using recently developed, 0.3-mm thin and flexible high-density surface electromyography (sEMG) grids (120 channels). For each subject and each muscle and for different low contraction levels, representative MUAPs ("MU fingerprints") were extracted from the raw sEMG data according to their spatiotemporal amplitude characteristics. We then topographically characterized the lower facial MUs' endplate zones and main muscle fiber orientations on the individual faces of the subjects. These topographical MU parameters were spatially warped to correct for the different sizes and shapes of the faces of individual subjects. This electrophysiological study revealed a distribution of the lower facial MU endplates in more or less restricted, distinct clusters on the muscle often with eccentric locations. The results add substantially to the basic neurophysiologic and anatomical knowledge of the complex facial muscle system. They can also be used to establish objective guidelines for placement of conventional (surface or needle) EMG electrodes as well as for clinical investigations on neuromuscular diseases affecting the facial musculature. The localized endplate positions may also indicate optimal locations for botulinum toxin injection in the face.     

"Facial" and standard concentric needle electrodes are not interchangeable

Motor unit action potentials (MUAPs) were recorded using a standard concentric needle (SCN), a "facial" concentric needle (FCN) and a monopolar needle (MN) in the biceps brachii and first dorsal interosseous manus muscles of 7 normal subjects. For each muscle, the MUAP durations were similar for all electrodes. However, the FCN electrode more frequently registered high-amplitude MUAPs compared to the SCN electrode. Hence, the FCN and SCN needles should not be considered interchangeable in assessment of EMG signal amplitude. Although, the pattern of changes in amplitude and duration on FCN and MN recordings is similar, the electrodes have different uptake areas.     

Tension in the Two Frontales: Electrode Placement and Artifact in the Recording of Forehead EMG

The conventional placement of electrodes for recording forehead, or “frontalis” EMG attenuates forehead potentials by placing one electrode over each frontalis muscle. The placement also precludes exploration of differences in the activity of the two muscles, and is influenced by potentials arising in nearby muscles. An alternative unilateral bipolar lead placement is suggested and demonstrated.     

Effect of position of electrodes relative to the innervation zone onsurface EMG

We investigated the effect of the position of electrodes relative to the innervation zone (IZ) of the biceps brachii muscle during isometric elbow flexion using eight-channel surface array electrodes. We estimated the location of the IZ near the centre of the muscle in 20 male subjects. The pulse peaks from electromyogram (EMG) waveforms were detected for each channel and averaged, the triphasic pulse was determined, and the peak values of the first and third phases were compared. The results showed significantly greater pulse values for the first phase when the electrode placement was proximal to the estimated IZ, and for the third phase when the electrode placement was distal to the estimated IZ. Using this method, the positional relationship between electrodes and IZ can be determined using a surface EMG waveform recorded with a pair of bipolar electrodes. This method may be clinically useful in confirming the reliability of a recorded surface EMG.     

Median averaging of electromyographic motor unit action potentials: comparison with other techniques

A technique of extracting individual motor unit action potentials (MUAPs) from EMG signals by median averaging, a modification of an existing method, is presented. To compare different techniques of MUAP extraction, 89 MUAPs were recorded with a concentric needle electrode in the brachial biceps muscle of normal subjects and patients with nerve and muscle diseases. MUAPs were also extracted by another method, called split-sweep median averaging, in which alternate MUAP discharges are averaged independently in two computer buffers until the two averaged signals appear equal on visual inspection by the operator. The amplitude, area, area: amplitude ratio, duration and number of phases and turns of each extracted MUAP were determined by each technique. Overall, there was a strong correlation between all features of the MUAPs extracted by median and splitsweep averaging, although the latter method required, on average, twice as many MUAP discharges to produce acceptable signals. We thus conclude that median averaging is a fast and accurate method that requires relatively few MU discharges to extract MUAP signals from spurious background signals.     

MUAP Number Estimates in Surface EMG: Template-Matching Methods and Their Performance Boundaries

Estimates of the number of motor unit action potential (MUAP)s appearing in the surface electromyogram (EMG) signal, which offers potentially valuable information about motor unit recruitment and firing rates, are likely to provide a more accurate reflection of the neural command to muscle than are current EMG quantification methods. In this paper, we show that the basic shapes of surface MUAPs recorded from the first dorsal interosseous (FDI) muscle can ideally be represented by a small number of waveforms. On the basis of this, we seek to estimate the number of MUAPs present in standard surface EMG records, using template-matching techniques to identify MUAP occurrences. Our simulation study indicates that the performance of template-matching methods for MUAP number estimation is mainly constrained by the MUAP superposition in the signal, and the maximum number of MUAPs allowed in the signal for a good estimation is determined by the duration of MUAPs. To further explore this from experimental surface EMG signals, we compare the recordings from a selective multiple concentric ring electrode against those derived from a standard differential EMG electrode situated over the same muscle. We conclude that the ring surface electrode only slightly reduces the MUAP duration and the less MUAP superposition rate contained in the signal is mainly achieved by reducing the pick up area of the electrode. Using a template-matching method, although the number of MUAPs can be approximately estimated based on a very selective surface EMG recording at low force levels, the maximum number of MUAPs correctly estimated from the surface EMG is constrained by the MUAP duration.     

Simultaneous estimation of muscle fibre conduction velocity and muscle fibre orientation using 2D multichannel surface electromyogram

The paper presents a new approach for simultaneous estimation of muscle fibre conduction velocity (MFCV) and muscle fibre orientation (MFO) for motor units (MUs) in two-dimensional (2D) multichannel surface electromyography recordings. This is an important tool for detecting changes and abnormalities in muscle function and structure. In addition, simultaneous estimation of MFO and MFCV avoids the necessity of manual electrode alignment. The proposed method detected propagating MU action potentials (MUAPs) in a running time window as moving components in amplitude maps. Thereafter, estimations were obtained by fitting a three-dimensional function to these maps. The performance was evaluated using synthetic MU signals at 10 dB SNR and authentic biceps brachii measurements. Results demonstrated MFCV and MFO estimates with standard deviations of less than 0.05 m s⁻¹ and 1° for simulated signals, and less than 0.2 m s⁻¹ and 4° for experimental data. However, standard deviations as low as 0.12 m s⁻¹ and 1.6° from real signals were demonstrated. It was concluded that the method performs as well as, or better than, linear array multichannel methods when individual propagating MUAPs can be identified, even if electrodes are not aligned with fibre direction.     

Selectivity of electromyographic recording techniques: simulation study

A modified line source model was used to simulate single muscle fibre action potentials and their radial decline as measured using a single fibre (SF), concentric needle (CN) and macroelectromyography (EMG) electrode. The 90 per cent attenuation distance, defined as the distance from the fibre at which the peak-to-peak amplitude of the action potential declines to 10 per cent of its value measured near the fibre, was determined. For the CN and macro electrode this distance was 1·4 and 4·1 times, respectively, the distance measured for the SF electrode Based on the decline characteristics obtained from our simulations and experimental data reported in the literature, we inferred that in a normal human biceps muscle the spike component of CN MUAPs is produced mainly by the action potentials of the closest one-six muscle fibres. This inference was used to interpret the changes in MUAP amplitude seen by clinical EMG in neuromuscular diseases. The decline of action potential amplitude with distance from the electrode was least when recordings were made by the macro-EMG electrode. The three electrodes differed in their uptake area and thus recordings made with them provided complementary information about the motor unit architecture and the way it changes in neuromuscular diseases.     

Comparison of amplitudes of surface macro motor unit potentials recorded from various muscle sites

Peak-to-peak amplitudes and total areas of surface macro motor unit potentials (S-MMUPs) were measured in 19 healthy volunteers. While participants maintained minimal isometric muscle contraction of the left biceps brachii, motor unit potentials (MUPs) were recorded from a needle and surface electrodes. The largest MUP recorded by the needle electrode was designated the trigger source. Electrical activities from the surface electrodes, which emerged synchronously with the trigger-potential, were averaged by the spike-triggered averaging (STA) technique. When the surface electrodes were placed over the muscle belly at a right angle to the muscle fibers, the S-MMUP amplitude and area decreased gradually with the distance of the electrodes from the point of insertion of the needle electrode. In contrast, when the surface electrodes were arranged parallel to the muscle fibers, the S-MMUP amplitude and area did not always decrease. In addition, negative peak positions in individual S-MMUPs showed a time delay along the muscle fibers. The placement and size of the surface electrodes, as well as the depth of the needle electrode, must be carefully considered when MUPs are analyzed by the STA technique. Muscle fiber conduction velocity (MFCV) is measurable by the STA technique combined with surface electrodes.     

Methodology of surface electromyography in gait analysis: review of the literature

Gait analysis is a significant diagnostic procedure for the clinicians who manage musculoskeletal disorders. Surface electromyography (sEMG) combined with kinematic and kinetic data is a useful tool for decision making of the appropriate method needed to treat such patients. sEMG has been used for decades to evaluate neuromuscular responses during a range of activities and develop rehabilitation protocols. The sEMG methodology followed by researchers assessed the issues of noise control, wave frequency, cross talk, low signal reception, muscle co-contraction, electrode placement protocol and procedure as well as EMG signal timing, intensity and normalisation so as to collect accurate, adequate and meaningful data. Further research should be done to provide more information related to the muscle activity recorded by sEMG and the force produced by the corresponding muscle during gait analysis.     

Confounding factors in water EMG recordings: an approach to a definitive standard

This work presents a study on the influence of the aqueous environment on the surface EMG (sEMG) signal recorded in bipolar montage from the abductor pollicis brevis muscle, when only the forearm is immersed in water. Ten men, 30.1±4.0 (mean ± SD) years old, performed ten 2-s 40% MVC isometric contractions of the abductor pollicis brevis muscle in two controlled environments (air and water, at a temperature of 32°C). They were always equipped with electrodes protected with a waterproof adhesive tape. No significant variations (paired Wilcoxon test) due to the environments were observed in the median frequency of the power spectrum (MDF) and in the root mean square (RMS) value of the sEMG signal. These results allow us to assess the methodological criteria to properly record sEMG signals in water and provide the basis to explain different findings obtained by other authors.During part of the development of this work, W. H. Veneziano and A. F. Rocha were with LISiN.     

Experimental and modelling investigation of surface EMG spike analysis

A pattern classification method based on five measures extracted from the surface electromyographic (sEMG) signal is used to provide a unique characterization of the interference pattern for different motor unit behaviours. This study investigated the sensitivity of the five sEMG measures during the force gradation process. Tissue and electrode filtering effects were further evaluated using a sEMG model. Subjects (N=8) performed isometric elbow flexion contractions from 0 to 100% MVC. The sEMG signals from the biceps brachii were recorded simultaneously with force. The basic building block of the sEMG model was the detection of single fibre action potentials (SFAPs) through a homogeneous, equivalent isotropic, infinite volume conduction medium. The SFAPs were summed to generate single motor unit action potentials. The physiologic properties from a well-known muscle model and motor unit recruitment and firing rate schemes were combined to generate synthetic sEMG signals. The following pattern classification measures were calculated: mean spike amplitude, mean spike frequency, mean spike slope, mean spike duration, and the mean number of peaks per spike. Root-mean-square amplitude and mean power frequency were also calculated. Taken together, the experimental data and modelling analysis showed that below 50% MVC, the pattern classification measures were more sensitive to changes in force than traditional time and frequency measures. However, there are additional limitations associated with electrode distance from the source that must be explored further. Future experimental work should ensure that the inter-electrode distance is no greater than 1cm to mitigate the effects of tissue filtering.     

Effects of monopolar and bipolar electrode configurations on surface EMG spike analysis

This study compared the effects of monopolar and bipolar electrode configurations on interference pattern analysis of the surface electromyographic (sEMG). Twenty-four college-aged male participants performed isometric actions of the elbow flexors at 40, 60, 80, and 100 percent of maximal voluntary contraction (MVC). Separate (Ag/AgCl) electrodes were used for both configurations. There were five measures associated with “spike shape” analysis: mean spike amplitude (MSA), mean spike frequency (MSF), mean spike slope (MSS), mean spike duration (MSD) and mean number of peaks per spike (MNPPS). A load-cell and wrist-cuff assembly was used to record isometric elbow flexion forces. Both electrode configurations resulted in the same trends force changes in spike shape measures across force levels: there was a linear increase in MSA, MSS, and a quadratic decrease in MSF and the MNPPS (p's < 0.05). The MSD underwent a quadratic increase (p < 0.05). The spike shape measures had greater mean magnitudes and exhibited greater rates of changes across force levels for the monopolar electrode configuration (p's < 0.05). The monopolar electrode configuration was therefore more sensitive to changes in muscle activity with increases in isometric force. This is an important consideration because the rate at which muscle electrical activity develops into a full interference pattern is an important qualitative and quantitative diagnostic measure.     

Rise time or rise rate: which should be a criterion for analysing motor unit action potentials?

We measured the rise time (RT) and rise rate (RR) of motor unit action potentials (MUAPs) when sharp sounds are heard in concentric EMG in the first dorsal interosseus muscle (FDI) and biceps brachii muscle (BIC) of normal subjects. MUAPs from FDI muscle with an RT of less than 500 microseconds were 85%, and those of more than 500 microseconds were 15%. In contrast, MUAPs from BIC muscle with an RT of less than 500 microseconds were 65%, and those of more than 500 microseconds were 35%. Distributions of the RR for FDI and BIC were also determined. MUAPs from FDI muscle with an RR more than 0.3 mV/ms were 98.3%, and those of less than 0.3 mV/ms were 1.7%. In contrast, MUAPs from BIC muscle with an RR of more than 0.3 mV/ms were 93%, and those of less than 0.3 mV/ms were 7%. We conclude it is better to use RR than RT when accepting MUAPs in clinical EMG, because even when sharp sounds are heard, MUAPs do not always have an RT of less than 500 microseconds. The use of RT and the sharpness of MUAPS therefore need to be reconsidered, or RR should be used in clinical EMG by automatic program.     

EMG Measurement as a Function of Electrode Placement and Level of EMG

Frontal EMG was concurrently measured using two different electrode placements for each of 12 subjects. One electrode placement was the standard placement of one electrode above each eyebrow (referred to as the horizontal placement). For the other placement, electrodes were positioned vertically above only the left eyebrow (referred to as the vertical placement). Comparisons between the measures of EMG using the two placements were assessed for three different levels of biofeedback-influenced EMG activity. The horizontal and vertical EMG measures were found to be positively correlated both for the entire group and for individual subjects. Also, the two measures of EMG were positively correlated at three different levels of EMG, i.e., high, medium, and low. Comparisons of horizontal and vertical EMG indicated that horizontal EMG was consistently higher than vertical EMG at all levels of EMG. These findings suggest that placement of EMG electrodes is a very important variable in accurately measuring the magnitude of muscular electrical activity. However, if repeated measures of EMG are used to evaluate changes in muscle tension, and electrode placement is consistent across conditions, electrode placement is of only marginal importance.     

基于运动神经元激励的表面肌电信号仿真研究

以单纤维动作电位的仿真为基础,结合运动单位的生理结构特点,利用神经肌肉系统激励与运动单位募集、发放间的近似关系,建立一个比较符合生理学特性的表面肌电(sEMG)信号模型,以仿真不同激励情况的sEMG信号.仿真实验发现,肌纤维与电极间距离的增加将使皮肤表面检测到的动作电位峰值下降;随着激励水平的提高,与仿真sEMG信号相关的收缩力逐渐增大,且仿真sEMG信号的时域波形以及频谱也与真实sEMG信号相似.实验结果表明仿真sEMG信号能够较有效地逼近真实sEMG信号,可用于运动单位发放检测等相关研究领域.     

Effect of electrode dimensions on motor unit potentials

Different effects of longitudinal and transversal electrode dimensions on nerve or muscle single fibre action potentials detected monopolarly, were reported in the literature. The results were contradictory. We studied motor unit potentials (MUPs) detected at a large distance (typical of surface recording) on the basis of a mathematical model without source simplification. The MUPs were calculated as a single convolution of the first temporal derivative of a realistic intracellular action potential and MU impulse response. The spatial averaging of the MUPs by rectangular plate electrodes was performed through analytical integration of the MU impulse response over the electrode area. The effects of longitudinal dimension of the electrode were stronger than those of a transversal one. The effects were distance dependent. The longitudinal dimension of the electrode influenced the main phases (that reflected the excitation origin and propagation) more than the terminal phases (that reflected the excitation extinction at the muscle fibers' ends). This was due to differences in the character of the potential fields (quadrupole or dipole) during generation of individual MUP phases. It was shown that the relative weight of the individual MUP phases could be stressed or suppressed by a proper choice of electrode dimensions, position and orientation.     

Investigating the performance of an amplitude-independent algorithm for detecting the hand muscle activity of stroke survivors

Abstract

To make robotic hand devices controlled by surface electromyography (sEMG) signals feasible and practical tools for assisting patients with hand impairments, the problems that prevent these devices from being widely used have to be overcome. The most significant problem is the involuntary amplitude variation of the sEMG signals due to the movement of electrodes during forearm motion. Moreover, for patients who have had a stroke or another neurological disease, the muscle activity of the impaired hand is weak and has a low signal-to-noise ratio (SNR). Thus, muscle activity detection methods intended for controlling robotic hand devices should not depend mainly on the amplitude characteristics of the sEMG signal in the detection process, and they need to be more reliable for sEMG signals that have a low SNR. Since amplitude-independent muscle activity detection methods meet these requirements, this paper investigates the performance of such a method on people who have had a stroke in terms of the detection of weak muscle activity and resistance to false alarms caused by the involuntary amplitude variation of sEMG signals; these two parameters are very important for achieving the reliable control of robotic hand devices intended for people with disabilities. A comparison between the performance of an amplitude-independent muscle activity detection algorithm and three amplitude-dependent algorithms was conducted by using sEMG signals recorded from six hemiparesis stroke survivors and from six healthy subjects. The results showed that the amplitude-independent algorithm performed better in terms of detecting weak muscle activity and resisting false alarms.     

Effects of muscle fibre shortening on the characteristics of surface motor unit potentials

Traditionally, studies dealing with muscle shortening have concentrated on assessing its impact on conduction velocity, and to this end, electrodes have been located between the end-plate and tendon regions. Possible morphologic changes in surface motor unit potentials (MUPs) as a result of muscle shortening have not, as yet, been evaluated or characterized. Using a convolutional MUP model, we investigated the effects of muscle shortening on the shape, amplitude, and duration characteristics of MUPs for different electrode positions relative to the fibre–tendon junction and for different depths of the MU in the muscle (MU-to-electrode distance). It was found that the effects of muscle shortening on MUP morphology depended not only on whether the electrodes were between the end-plate and the tendon junction or beyond the tendon junction, but also on the specific distance to this junction. When the electrodes lie between the end-plate and tendon junction, it was found that (1) the muscle shortening effect is not important for superficial MUs, (2) the sensitivity of MUP amplitude to muscle shortening increases with MU-to-electrode distance, and (3) the amplitude of the MUP negative phase is not affected by muscle shortening. This study provides a basis for the interpretation of the changes in MUP characteristics in experiments where both physiological and geometrical aspects of the muscle are varied.