The linear electrode array: a useful tool with many applications.

In this review we describe the basic principles of operation of linear electrode arrays for the detection of surface EMG signals, together with their most relevant current applications. A linear array of electrodes is a system which detects surface EMG signals in a number of points located along a line. A spatial filter is usually placed in each point for signal detection, so that the recording of EMG signals with linear arrays corresponds to the sampling in one spatial direction of a spatially filtered version of the potential distribution over the skin. Linear arrays provide indications on motor unit (MU) anatomical properties, such as the locations of the innervation zones and tendons, and the fiber length. Such systems allow the investigation of the properties of the volume conductor and its effect on surface detected signals. Moreover, linear arrays allow to estimate muscle fiber conduction velocity with a very low standard deviation of estimation (of the order of 0.1-0.2 m/s), thus providing reliable indications on muscle fiber membrane properties and their changes in time (for example with fatigue or during treatment). Conduction velocity can be estimated from a signal epoch (global estimate) or at the single MU level. In the latter case, MU action potentials are identified from the interference EMG signals and conduction velocity is estimated for each detected potential. In this way it is possible, in certain conditions, to investigate single MU control and conduction properties with a completely non-invasive approach. Linear arrays provide valuable information on the neuromuscular system properties and appear to be promising tools for applied studies and clinical research.     

Influence of amplitude cancellation on the simulated surface electromyogram.

The purpose of the study was to quantify the influence of selected motor unit properties and patterns of activity on amplitude cancellation in the simulated surface electromyogram (EMG). The study involved computer simulations of a motor unit population with physiologically defined recruitment and rate coding characteristics that activated muscle fibers whose potentials were recorded on the skin over the muscle. Amplitude cancellation was quantified as the percent difference in signal amplitude when motor unit potentials were summed before and after rectification. The simulations involved varying the level of activation for the motor unit population, the recording configuration, the upper limit of motor unit recruitment, peak discharge rates, the amount of motor unit synchronization, muscle fiber length, the thickness of the subcutaneous tissue, and the motor unit properties that change with advancing age. The results confirmed a previous experimental report (Day SJ and Hulliger M, J Neurophysiol 86: 2144-2158, 2001) that amplitude cancellation in the surface EMG can reach 62% at maximal activation. A decrease in the range of amplitudes of the motor unit potentials, as can occur during fatiguing contractions, increased amplitude cancellation up to approximately 85%. Differences in the amount of amplitude cancellation were observed across all simulated conditions, and resulted in substantial changes in the absolute magnitude of the EMG signal. The most profound factors influencing amplitude cancellation were the number of active motor units and the duration of the action potentials. The effects of amplitude cancellation were minimal (<5%) when the EMG amplitude was normalized to maximal values, with the exception of variations in peak discharge rate and recruitment range, which resulted in differences up to 17% in the normalized EMG signal across conditions. These results indicate the amount of amplitude cancellation that can occur in various experimental conditions and its influence on absolute and relative measures of EMG amplitude.     

Motor unit acceleration maps and interference mechanomyographic distribution

The study analyses the two-dimensional distribution of surface mechanomyographic (MMG) signal generated by the activation of single motor units located in three transverse positions in the tibialis anterior muscle. In 12 healthy volunteers, surface MMG signals were recorded from the tibialis anterior muscle with a 3x4 grid of accelerometers spaced by 20 and 30mm in the transverse and longitudinal direction. Three intramuscular electromyographic (EMG) signals were recorded with wire electrodes inserted 20-mm apart, between the first and second most proximal accelerometers of each column of the grid. The subject was asked to activate three different motor units (target motor units) in three contractions with visual feedback from each of the three intramuscular recordings (three locations). The MMG signals from the 12 accelerometers were averaged using the intramuscular single motor unit action potentials as trigger in order to obtain surface motor unit acceleration maps (MUAMs). The peak-to-peak value of the averaged MMG depended on motor unit location (P<0.001) and on the transverse position of the accelerometer in the grid (P<0.05). Moreover, MUAM amplitude depended on the interaction between motor unit location and transverse accelerometer position (P<0.05), demonstrating an influence of motor unit location on the generated MUAM. The observed dependency of MUAMs on motor unit location provides a quantitative analysis of the effect of the volume conductor on the recorded surface MMG signal.     

Amplitude cancellation reduces the size of motor unit potentials averaged from the surface EMG.

The purpose of the study was to evaluate the influence of selected physiological parameters on amplitude cancellation in the simulated surface electromyogram (EMG) and the consequences for spike-triggered averages of motor unit potentials derived from the interference and rectified EMG signals. The surface EMG was simulated from prescribed recruitment and rate coding characteristics of a motor unit population. The potentials of the motor units were detected on the skin over a hand muscle with a bipolar electrode configuration. Averages derived from the EMG signal were generated using the discharge times for each of the 24 motor units with lowest recruitment thresholds from a population of 120 across three conditions: 1) excitation level; 2) motor unit conduction velocity; and 3) motor unit synchronization. The area of the surface-detected potential was compared with potentials averaged from the interference, rectified, and no-cancellation EMGs. The no-cancellation EMG comprised motor unit potentials that were rectified before they were summed, thereby preventing cancellation between the opposite phases of the potentials. The percent decrease in area of potentials extracted from the rectified EMG was linearly related to the amount of amplitude cancellation in the interference EMG signal, with the amount of cancellation influenced by variation in excitation level and motor unit conduction velocity. Motor unit synchronization increased potentials derived from both the rectified and interference EMG signals, although cancellation limited the increase in area for both potentials. These findings document the influence of amplitude cancellation on motor unit potentials averaged from the surface EMG and the consequences for using the procedure to characterize motor unit properties.     

Neuromuscular adjustments that constrain submaximal EMG amplitude at task failure of sustained isometric contractions

The amplitude of the surface EMG does not reach the level achieved during a maximal voluntary contraction force at the end of a sustained, submaximal contraction, despite near-maximal levels of voluntary effort. The depression of EMG amplitude may be explained by several neural and muscular adjustments during fatiguing contractions, including decreased net neural drive to the muscle, changes in the shape of the motor unit action potentials, and EMG amplitude cancellation. The changes in these parameters for the entire motor unit pool, however, cannot be measured experimentally. The present study used a computational model to simulate the adjustments during sustained isometric contractions and thereby determine the relative importance of these factors in explaining the submaximal levels of EMG amplitude at task failure. The simulation results indicated that the amount of amplitude cancellation in the simulated EMG (～ 40%) exhibited a negligible change during the fatiguing contractions. Instead, the main determinant of the submaximal EMG amplitude at task failure was a decrease in muscle activation (number of muscle fiber action potentials), due to a reduction in the net synaptic input to motor neurons, with a lesser contribution from changes in the shape of the motor unit action potentials. Despite the association between the submaximal EMG amplitude and reduced muscle activation, the deficit in EMG amplitude at task failure was not consistently associated with the decrease in neural drive (number of motor unit action potentials) to the muscle. This indicates that the EMG amplitude cannot be used as an index of neural drive.     

The methods of surface EMG analysis

The use of surface emg as a tool for quantification is described. First, the specific advantages of the surface emg are discussed. Techniques for analysis of the emg signal which estimate and detect the action potentials of the individual motor units and estimate some global properties of muscle activity are reviewed. A survey of data on relations between the properties of motor unit action potential, the properties of motor unit activity and the results of signal processing are given.     

Estimation of monopolar signals from sphincter muscles and removal of common mode interference

The detection of surface electromyogram (EMG) by multi-electrode systems is applied in many research studies. The signal is usually recorded by means of spatial filters (linear combination of the potential under at least two electrodes) with vanishing sum of weights. Nevertheless, more information could be extracted from monopolar signals measured with respect to a reference electrode away from the muscle. Under certain conditions, surface EMG signal along a curve parallel to the fibre path has zero mean (property approximately satisfied when EMG is sampled by an array of electrodes that covers the entire support of the signal in space). This property allows estimating monopolar from single differential (SD) signals by pseudoinversion of the matrix relating monopolar to SD signals. The method applies to EMG signals from the external anal sphincter muscle, recorded using a specific cylindrical probe with an array of electrodes located along the circular path of the fibres. The performance of the algorithm for the estimation of monopolar from SD signals is tested on simulated signals. The estimation error of monopolar signals decreases by increasing the number of channels. Using at least 12 electrodes, the estimation error is negligible. The method applies to single fibre action potentials, single motor unit action potentials, and interference signals.The same method can also be applied to reduce common mode interference from SD signals from muscles with rectilinear fibres. In this case, the last SD channel defined as the difference between the potentials of the last and the first electrodes must be recorded, so that the sum of all the SD signals vanishes. The SD signals estimated from the double differential signals by pseudoinvertion are free of common mode.     

Analysis of motor units with high-density surface electromyography

Although the behaviour of individual motor units is classically studied with intramuscular EMG, recently developed techniques allow its analysis also from EMG recorded in multiple locations over the skin surface (high-density surface EMG). The analysis of motor units from the surface EMG is useful when the insertion of needles is not desirable or not possible. Moreover, surface EMG allows the measure of motor unit properties which are difficult to assess with invasive technology (e.g., muscle fiber conduction velocity or location of innervation zones) and may increase the number of detectable motor units with respect to selective intramuscular recordings. Although some limitations remain, both the discharge pattern and muscle fiber properties of individual motor units can currently be analyzed non-invasively. This review presents the conditions and methodologies which allow the investigation of motor units with surface EMG.     

Estimation of muscle fiber conduction velocity of doublet discharges

A doublet is defined as two consecutive discharges of a motor unit occurring at short time interval between each other (e.g., <20 ms). In this paper, we propose a method for the estimation of muscle fiber conduction velocity (CV) from two partly overlapping action potentials generated by the same motor units. The method is based on the minimization of the mean square error between time-filtered versions of two surface EMG signals recorded along the direction of muscle fibers. The minimization is performed over the filter parameters that define the two propagation delays. The method was tested on simulated and experimental signals. Simulation results showed that the method is only in some cases superior to the simpler peak approach, due to limitations in the ideal model used for the algorithm development. However, application to experimental signals that mimic doublet motor unit discharges showed a substantial improvement in estimation quality of the new method with respect to the peak method.     

Short term bed-rest reduces conduction velocity of individual motor units in leg muscles

Space permanence simulations such as prolonged bed-rest can mimic some of the physiological modifications in the human body and provide study conditions that are more accessible than during space flight. A short term bed-rest experiment was organized to simulate the effects of weightlessness for studying the adaptation to this condition. Eight healthy young volunteers were studied before and immediately after the 14 day periods of strict bed-rest.Surface EMG signals were detected with linear electrode arrays from vastus medialis, vastus lateralis and tibialis anterior muscle during isometric voluntary contractions at 20% MVC. Motor unit action potentials (MUAPs) of individual motor units were extracted from the interference EMG signals with a partial decomposition algorithm and averaged.MUAP templates generated by the same motor unit could be retrieved before and after bed-rest period. Muscle fiber conduction velocity (CV) was estimated from each averaged MUAP template and from the global EMG signal. Both global and single MU conduction velocity was observed to decrease by about 10% after the bed-rest period (p < 0.05). Amplitude and power spectral parameters did not significantly change after the bed-rest period.It is concluded that a short term bed-rest reduces the CV of individual motor units without a significant effect on muscle force or on other electrophysiological parameters.     

Detection of individual motor units of the puborectalis muscle by non-invasive EMG electrode arrays

PURPOSE: The purpose of the study was to demonstrate that anatomical features of individual motor units of the puborectalis muscle can be detected with non-invasive electromyography (EMG) and to evaluate differences in electrophysiological properties of the puborectalis muscles in a small group of healthy and pathologic subjects. METHODS: Multichannel EMG was recorded by means of a flexible probe applied on the gloved index finger and carrying an array of eight equally spaced (1.15 mm) electrodes. A multichannel EMG amplifier provided seven outputs corresponding to the pairs of adjacent electrodes. Tests were performed in three different positions (dorsal, left and right) over the puborectalis muscle on 20 subjects (nine healthy, seven constipated and four incontinent patients). Motor unit action potentials (MUAPs) generated at the innervation zone of a MU and propagating along the muscle fibers generated repetitive characteristic patterns on the seven output channels allowing identification of anatomical features of the motor units. RESULTS: MUAPs were observed travelling in either one or both directions with the array in dorsal position, and mainly in ventral-to-dorsal direction in either lateral position. MUAP amplitude was lower in constipated and incontinent patients with respect to healthy subjects. The conduction velocity estimated on the identified MUAPs was lower for constipated patients with respect to healthy subjects suggesting different mechanical properties of the active motor units. CONCLUSIONS: This technique allows the extraction of relevant information about the anatomical features (innervation zone position and overlapping of motor unit branches) of the puborectalis muscle and its electrophysiological properties and maybe can be applied as an novel methodology for assessing the anorectal function in patients.     

Limitations of the surface electromyography technique for estimating motor unit synchronization

Motor unit synchronization was estimated from the surface electromyograms (EMG) of the first dorsal interosseus muscle of human volunteers by a simplified surface-EMG technique (Milner-Brown et al. 1973, 1975). Single motor units were identified from intramuscular recordings and were used to obtain a spike-triggered average of the surface-EMG. The discharge rate of a reference motor unit was controlled at two levels (high and low), and the effect of motor unit activity on the surface-EMG estimate of synchronization was studied in 56 motor units. The surface-EMG estimate of motor unit synchronization was significantly higher when the reference motor unit discharged at the high rate than when it discharged at the low rate. A regression analysis indicated that the synchronization ratio calculated from the surface EMG was significantly correlated with the level of EMG activity in the muscle. Motor unit synchronization was also estimated from surface-EMG measurements that were derived by computer simulation. The simulation permitted manipulation of motor unit activity (discharge rate and recruitment) with a complete absence of synchrony among the units in the pool. The stimulated surface-EMG index was influenced by an artifact associated with signal rectification, and this effect changed non-monotonically with motor unit activity. Furthermore, the increase in the motor unit activity reduced the signal-to-noise ratio of the spike-triggered surface EMG average, and consequently decreased the sensitivity of the surface-EMG index as an estimate of motor unit synchronization. We conclude that the simplified surface-EMG method (Milner-Brown et al. 1973, 1975) does not provide a useful index of motor unit synchronization due to its inability to accurately distinguish the synchronization from methodological effects related to a rectification artifact and variation in the signal-to-noise ratio.     

Motor unit number estimation and quantitative needle electromyography in stroke patients

ObjectiveTo evaluate the effect of upper motor neuron damage upon motor units’ function by means of two separate and supplementary electrophysiological methods.MethodsThe abductor digiti minimi muscle of the non-paretic and the paretic side was studied in forty-six stroke patients with (a) motor unit number estimation (MUNE) – adapted multiple point stimulation method and (b) computerized quantitative needle electromyography (EMG) assessing the configuration of voluntary recruited motor unit potentials. Main outcome comparisons were focused on differences between non-paretic and paretic side.ResultsOn the affected hands mean MUNE value was significantly lower and mean area of the surface recorded single motor unit potentials was significantly larger than the corresponding ones on the non-paretic hands. EMG findings did not reveal remarkable differences between the two sides. Neither severity nor chronicity of stroke was related to MUNE or EMG parameters.DiscussionMUNE results, which suggested reduced motor unit numbers in stroke patients, in conjunction with the normal EMG features in these same muscles has given rise to different interpretations. In a clinical setting, reinnervation type changes in the EMG similar to that occurring in neuronopathies or axonal neuropathies should not be expected in muscles with central neurogenic lesion.     

Spatial distribution of surface action potentials generated by individual motor units in the human biceps brachii muscle

This study analyses the spatial distribution of individual motor unit potentials (MUPs) over the skin surface and the influence of motor unit depth and recording configuration on this distribution. Multichannel surface (13 × 5 electrode grid) and intramuscular (wire electrodes inserted with needles of lengths 15 and 25 mm) electromyographic (EMG) signals were concurrently recorded with monopolar derivations from the biceps brachii muscle of 10 healthy subjects during 60-s isometric contractions at 20% of the maximum torque. Multichannel monopolar MUPs of the target motor unit were obtained by spike-triggered averaging of the surface EMG. Amplitude and frequency characteristics of monopolar and bipolar MUPs were calculated for locations along the fibers’ direction (longitudinal), and along the direction perpendicular (transverse) to the fibers. In the longitudinal direction, monopolar and bipolar MUPs exhibited marked amplitude changes that extended for 16–32 mm and 16–24 mm over the innervation and tendon zones, respectively. The variation of monopolar and bipolar MUP characteristics was not symmetrical about the innervation zone. Motor unit depth had a considerable influence on the relative longitudinal variation of amplitude for monopolar MUPs, but not for bipolar MUPs. The transverse extension of bipolar MUPs ranged between 24 and 32 mm, whereas that of monopolar MUPs ranged between 72 and 96 mm. The mean power spectral frequency of surface MUPs was highly dependent on the transverse electrode location but not on depth. This study provides a basis for the interpretation of the contribution of individual motor units to the interference surface EMG signal.     

Simulations of the alpha motoneuron pool electromyogram reflex at different preactivation levels in man

The alpha motoneuron pool and the surface electromyogram (EMG) of the human soleus muscle are modelled, respectively, by an alpha motoneuron pool model generating the firing patterns in the motor units of e muscle and by a muscle model using these discharge patterns to simulate the surface EMG. In the alpha motoneuron pool model, we use a population of motoneurons in which cellular properties like cell size and membrane conductance are distributed according to experimentally observed data. By calculating the contribution from each motor unit, the muscle model predicts the EMG. Wave forms of the motor unit action potentials in the surface EMG are obtained from experimental data. Using the model, we are able to give a quantitative prediction of the motoneuron pool activity and the reflex EMG output at different preactivation levels. The simulated data are consistent with experimentally obtained results in healthy humans. During static isometric muscle preactivations, the simulations show that the reflex strength is highly dependent on the intrinsic threshold properties of the alpha motoneuron pool. Received: 27 April 1993/Accepted in revised form: 8 September 1993     

Branched EMG electrodes for stable and selective recording of single motor unit potentials in humans.

Branched surface EMG electrodes are bipolar electrodes with the hot signal pole referenced to two or more short-circuited leading-off surfaces. This technique provides stable recording of single motor unit potentials during real movements, up to maximal muscle contractions. The selective characteristic of branched electrodes is based on the same principles as the double differential detection system and spatial filtering technique proposed later. Equi-weight calculations to assess the selectivity of different electrode types and their position are used. The main advantage of branched electrodes, especially high stability, is achieved by the wire electrode version. The design, manufacture, implementation, and application of wire electrodes are discussed in detail. During recording of motor unit potentials, electrodes are positioned subcutaneously over the muscle fascia. This positioning maximizes electrode stability. Appropriate orientation of the electrode relative to the muscle architecture ensures adequate selectivity for single motor unit recordings. Branched electrodes require ordinary EMG equipment (two or even one amplifier).     

Comparison of force and EMG measures in normal and reinnervated tibialis anterior muscles of the rat.

The relationship between motor unit force and the recorded voltage produced by activated muscle unit fibres (electromyogram, EMG) was examined in normal and reinnervated rat tibialis anterior muscles. The number, cross-sectional area, and radial distance from the recording electrode of muscle fibres in a given unit, obtained directly from a sample of glycogen-depleted motor units, were analysed in relation to the magnitude of the EMG signal produced by that unit. EMG peak to peak amplitude and area varied as approximately the square root of twitch force in both normal and reinnervated units. Furthermore, the EMG amplitude increased approximately as the total cross-sectional area of the motor unit (number of muscle fibres x the average cross-sectional area of the fibres) and inversely with approximately the square root of the distance of fibres from the recording electrodes on the surface of the muscle.     

Motor unit potentials with satellites in dystrophinopathies

IntroductionThe objective of this study was to analyze the motor unit potentials (MUPs) with satellite components i.e., delayed by at least 2 ms baseline from the main component, in the dystrophinopathies.MethodsThe parameters of the MUPs recorded from the biceps brachii muscle in the Duchenne and Becker Muscle Dystrophy (DMD, BMD) were analyzed. The origin of the MUP satellite components was studied using a computer simulation.ResultsAs compared with normal potentials, both the main and the satellite MUP components are smaller in size, while the main components are more irregular. The computer simulation allows the range of muscle fiber diameters to be determined, and suggests that the variability characterizing diameters within the motor unit is responsible for generating the delayed, satellite components, via the linear relationship between the fiber diameter and the conduction velocity of the action potential.DiscussionThe enhanced understanding of the origin of the MUP satellite components augments the knowledge about the relationship between muscle morphology and bioelectrical activity. The indirect evaluation of the range of muscle fiber diameters by means of a computer simulation may provide a new quantitative morphological data available from the EMG.     

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.     

Recent progress in the diagnostic use of surface EMG for neurological diseases.

The different techniques to measure and analyze surface EMG are summarized with an emphasis on the clinician's point of view. The application of surface EMG in neurological disease is hampered by many inherent problems, especially the difficulties in extracting features of single motor units. However, the evolution of surface EMG from single bipolar recordings via a linear array of multiple electrodes to densely packed, multi-channel electrode arrays could in principle solve this problem. The added value of using multiple channels (up to 128) with an interelectrode distance of a few millimetres to obtain more spatial information is emphasized. At least for some muscles it is now possible to extract information from the surface EMG, conventionally thought to belong to the domain of needle EMG (for example the "electrical size" of motor units). The use of analysis techniques such as the estimation of muscle fiber conduction velocity has already proven to be of diagnostic value in several myopathies characterized by a disturbed membrane function and in metabolic myopathies with abnormal fatigue profiles. Future research should be directed at the development of analysis techniques enabling the extraction of more relevant motor unit variables from surface EMG signals.