The effects of the innervation zone and interelectrode distance on the patterns of responses for electromyographic amplitude and mean power frequency versus isometric torque for the vastus lateralis muscle

The purpose of the present study was to examine the effects of the innervation zone (IZ) and differences in interelectrode distance (IED) on the patterns of responses and mean values for absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus isometric torque for the vastus lateralis. Ten men performed submaximal to maximal isometric muscle actions of the dominant leg extensors, and four separate bipolar surface EMG signals were detected from the vastus lateralis using a linear electrode array. Two electrode arrangements had IEDs of 10 and 30 mm and were located directly over the IZ. Two other electrode arrangements also had IEDs of 10 and 30 mm, but were distal to the IZ. The results indicated that in 50% of the cases, the IZ affected the patterns of responses for absolute and normalized EMG amplitude and MPF versus isometric torque. In 80% of the cases, however, there were no differences between the electrode arrangements with 10 and 30 mm IEDs for the absolute and normalized EMG amplitude and MPF versus isometric torque relationships. Both electrode placement over the IZ and differences in IED affected the mean values for absolute EMG amplitude, but not those for absolute EMG MPF In addition, there were no mean differences among the four electrode arrangements for normalized EMG amplitude and MPF values. Thus, these findings supported normalization, and indicated that it is useful for reducing the influences of the IZ and differences in IED on absolute EMG amplitude and MPF data.     

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.     

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.     

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.     

Motor unit substitution in long-duration contractions of the human trapezius muscle.

We examined the activity pattern of low-threshold motor units in the human trapezius muscle during contractions of 10 min duration. Three procedures were applied in sequence: 1) static contraction controlled by maintaining a constant low level of the surface electromyogram (EMG)-detected root-mean-square signal, 2) a manipulation task with mental concentration, and 3) copying a text on a word processor. A quadrifilar fine-wire electrode was used to record single motor unit activity. Simultaneously, surface electrodes recorded the surface EMG signal. During these contractions, low-threshold motor units showed periods of inactivity and were substituted by motor units of higher recruitment threshold. This phenomenon was not observed during the first few minutes of the contraction. In several cases the substitution process coincided with a short period of inactivity in the surface EMG pattern. Substitution was observed in five of eight experiments. These observations may be explained by a time-variant recruitment threshold of motor units, sensitive to their activation history and temporal variation in the activity patterns. We speculate that the substitution phenomenon protects motor units in postural muscles from excessive fatigue when there is a demand for sustained low-level muscle activity.     

Functional subdivision of the upper trapezius muscle during low-level activation

The electromyographic (EMG) amplitude was recorded using bipolar surface electrodes placed at different positions above the upper trapezius muscle of 16 healthy subjects. One of the aims of this study was to investigate the variation in EMG activity between electrode positions. For this purpose three tasks were performed: a mental activation test, a dynamic movement test and 90° arm abduction. The EMG signals were full-wave rectified and averaged within windows that were 0.2?s in length. Normalized EMG activity showed significantly different EMG amplitudes at different electrode positions for two of the three tasks. The second aim of this study was to investigate whether the upper trapezius muscle may be functionally subdivided. For this purpose the normalized EMG amplitudes of each task were compared with the EMG amplitude recorded during submaximal shoulder elevation. While the EMG level was similar at one electrode position, significant differences were found at some of the other electrode positions, indicating a functional subdivision of the muscle. The present results indicate that for comparisons of upper trapezius EMG activity levels between some tasks or between subjects, it is worthwhile to make EMG recordings at several electrode positions.     

Influence of the surface EMG electrode on the compound muscle action potential

The recording characteristics of surface EMG electrodes were investigated. Compound muscle action potential (CMAP) and surface recorded motor unit action potentials were recorded from different muscles, using different surface electrode shapes and sizes. The CMAP was smaller for larger surface electrodes. This was more pronounced in smaller muscles. The CMAP was minimally affected by the geometry of the recording surface. With larger surface electrodes, shunting contributes to the reduction in MUAP amplitude. This is offset by a larger uptake area which gives a much smaller reduction in the CMAP amplitude for the larger muscles.     

Single motor unit and spectral surface EMG analysis during low-force, sustained contractions of the upper trapezius muscle

Intramuscular and surface electromyographic (EMG) activities were recorded from the left and right upper trapezius muscle of eight healthy male subjects during 5-min long static contractions at 2% and 5% of the maximal voluntary contraction (MVC) force. Intramuscular signals were detected by wire electrodes while surface EMG signals were recorded with linear adhesive electrode arrays. The surface EMG signals were averaged using the potentials extracted from the intramuscular EMG decomposition as triggers. The conduction velocity of single motor units (MUs) was estimated over time from the averaged surface potentials while average rectified value and mean power spectral frequency were computed over time from 0.5 s epochs of surface EMG signal. It was found that (1) MUs were progressively recruited after the beginning of sustained contractions of the upper trapezius muscle at 2% and 5% MVC, (2) the conduction velocity of the MUs active since the beginning of the contraction significantly decreased over time, and (3) although the CV of single MUs significantly decreased, the mean power spectral frequency of the surface EMG did not show a consistent trend over time. It was concluded that spectral surface EMG analysis, being affected by many physiological mechanisms, may show limitations for the objective assessment of localized muscle fatigue during low force, sustained contractions. On the contrary, single motor unit conduction velocity may provide an early indication of changes in muscle fiber membrane properties with sustained activity.     

Optimal placement of bipolar surface EMG electrodes in the face based on single motor unit analysis

Locations of surface electromyography (sEMG) electrodes in the face are usually chosen on a macro‐anatomical basis. In this study we describe optimal placement of bipolar electrodes based on a novel method and present results for lower facial muscles. We performed high‐density sEMG recordings in 13 healthy participants. Raw sEMG signals were decomposed into motor unit action potentials (MUAPs). We positioned virtual electrode pairs in the interpolated monopolar MUAPs at different positions along muscle fiber direction and calculated the bipolar potentials. Electrode sites were determined where maximal bipolar amplitude was achieved and were validated. Objective guidelines for sEMG electrode placement improve the signal‐to‐noise ratio and may contribute to reduce cross talk, which is particularly important in the face. The method may be regarded as an important basis for improving the validity and reproducibility of sEMG in complex muscle areas.     

Reducing muscle fatigue during transcutaneous neuromuscular electrical stimulation by spatially and sequentially distributing electrical stimulation sources

Purpose

A critical limitation with transcutaneous neuromuscular electrical stimulation is the rapid onset of muscle fatigue. We have previously demonstrated that spatially distributed sequential stimulation (SDSS) shows a drastically greater fatigue-reducing ability compared to a single active electrode stimulation (SES). The purposes of this study were to investigate (1) the fatigue-reducing ability of SDSS in more detail focusing on the muscle contractile properties and (2) the mechanism of this effect using array-arranged electromyogram (EMG).

Methods

SDSS was delivered through four active electrodes applied to the plantarflexors, sending a stimulation pulse to each electrode one after another with 90° phase shift between successive electrodes. In the first experiment, the amount of exerted ankle torque and the muscle contractile properties were investigated during a 3 min fatiguing stimulation. In the second experiment, muscle twitch potentials with SDSS and SES stimulation electrode setups were compared using the array-arranged EMG.

Results

The results demonstrated negligible torque decay during SDSS in contrast to considerable torque decay during SES. Moreover, small changes in the muscle contractile properties during the fatiguing stimulation using SDSS were observed, while slowing of muscle contraction and relaxation was observed during SES. Further, the amplitude of the M-waves at each muscle portion was dependent on the location of the stimulation electrodes during SDSS.

Conclusion

We conclude that SDSS is more effective in reducing muscle fatigue compared to SES, and the reason is that different sets of muscle fibers are activated alternatively by different electrodes.     

Arm movement and EMG mean power frequency in the trapezius muscle: a comparison between surface and intramuscular recording techniques.

In a previous study it has been shown that there is a systematic variation of the EMG mean power frequency recorded under dynamic conditions with surface electrodes from non-fatigued trapezius muscle. The variation was most pronounced in relation to shoulder joint angle. The aim of the present study was to find out whether this variation was caused by geometric factors such as electrode displacement, or by neuromuscular factors within the muscle. A comparison has been made between surface and intramuscular recordings. EMG was recorded with standard surface electrodes and with a monopolar "hooked wire" intramuscular electrode. The recordings were analyzed for calculation of mean power frequency (MPF) and root mean squared (RMS) signal level. For MPF we found a low, but statistically significant correlation (r = 0.30) between surface and intramuscular recordings. For RMS the corresponding correlation was high (r = 0.92). Analysis of variance showed a high proportion of explained variance related to the shoulder joint angle for surface recordings, but not for intramuscular recordings. The results were confirmed in the regression analysis. The results indicate that the observed variation is largely caused by geometric displacement of the surface electrodes and not by intrinsic neuro-muscular factors.     

Evidence of Potential Averaging over the Finite Surface of a Bioelectric Surface Electrode

Most bioelectric signals are not only functions of time but also exhibit a variation in spatial distribution. Surface EMG signals are often “summarized” by a large electrode. The effect of such an electrode is interpreted as averaging the potential at the surface of the skin beneath the electrode. We first introduce an electrical equivalent model to delineate this principle of averaging. Next, in a realistic finite element model of EMG generation, two outcome variables are evaluated to assess the validity of the averaging principle. One is the change in voltage distribution in the volume conductor after electrode application. The other is the change in voltage across the high impedance double layer between tissue and electrode. We found that the principle of averaging is valid, once the impedance of the double layer is sufficiently high. The simulations also revealed that skin conductivity plays a role. High-density surface EMG provided experimental evidence consistent with the simulation results. A grid with 120 small electrodes was placed over the thenar muscles of the hand. Electrical nerve stimulation assured a reproducible compound muscle response. The averaged grid response was compared with a single electrode matching the surface of the high-density electrodes. The experimental results showed relatively small errors indicating that averaging of the surface potential by the electrode is a valid principle under most practical conditions.     

The reproducibility of test contractions for calibration of electromyographic measurements

Summary The main purpose of this study was to evaluate the reproducibility of electromyographic (EMG) measurements and specifically to test a calibration procedure with submaximal test contractions. Bipolar surface electrodes (20 mm fixed distance) were repositioned by a tracing sheet on both trapezius muscles, halfway between acromion and processus prominens. Submaximal test contractions were performed by keeping both arms straight abducted 90° and forward flexed 10° for 15-s periods. The arm position could be precisely reproduced in the frontal plane, but deviated forwards by 4° in the horizontal plane, where the sensitivity of the EMG response to arm position was lowest. The electrodes were repositioned within a radius of 3 mm with a probability of 90%. Large deviations in the EMG response were found within this radius and a significant depression of the EMG response was recorded over the middle part of the muscle (the innervation zone?). This change in sensitivity of the EMG response with electrode position occurred in parallel for the test and maximal contractions. The total coefficient of variation was estimated to be 23% for recurrent EMG measurements using the calibration procedure described.     

Surface and wire electromyographic. Recording during fatiguing exercise

Electromyographic (EMG) signals are routinely recorded during assessments of biomechanic activity for ergonomic and medical purposes. Surface and intramuscular electrodes (i.m.) are employed by different researchers and there is a continuing need to understand the indications for each. This research evaluated the reproducibility of the shift of the frequency content of the EMG signals during fatiguing exercise using simultaneous recording with these two electrode types. Using healthy adult volunteers, EMG recordings from the vastus lateralis (VL) and anterior tibial (AT) were made during repeated isokinetic exertions causing fatigue. The median power frequency (MPF) was calculated for a 0.5 sec sample of data taken during each concentric contraction. Test-retest, and multiple day repeat testing were performed to exclude the effects of training and fatigue upon the data. Surface electrode recordings demonstrated better reliability testing with values of 0.54 for VL and 0.68 for AT, while simultaneous i.m. recordings were -0.90 and 0.47, respectively. This finding of greater reliability, combined with the ease of use of surface electrodes suggest that they should be preferred for most studies of EMG analysis of muscle fatigue.     

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.     

Electrical stimulation of a denervated muscle promotes selective reinnervation by native over foreign motoneurons

The effect of electrical stimulation of the denervated posterior cricoarytenoid (PCA) muscle on its subsequent reinnervation was explored in the canine. Eight animals were implanted with a planar array of 36 electrodes for chronic stimulation and recording of spontaneous and evoked electromyographic (EMG) potentials across the entire fan-shaped surface of a muscle pair. Normative EMG data were recorded from each electrode site before unilateral nerve section, and from the innervated partner after nerve section. After randomizing the animals to experimental and control groups, the right recurrent laryngeal nerve innervating the PCA abductor muscle and its adductor antagonists was sectioned and reanastomosed. The PCA muscle in four experimental animals was continuously stimulated during the 11-mo experiment, using a 1-s, 30-pps, biphasic pulse train composed of 1-ms pulses 2-6 mA in amplitude and repeated every 10 s. The remaining four animals served as nonstimulated controls. Appropriate reinnervation by native inspiratory motoneurons was indexed behaviorally by the magnitude of vocal fold opening and electromyographically by the potential across all electrode sites. Inappropriate reinnervation by foreign adductor motoneurons was quantitated by recording EMG potentials evoked reflexly by stimulation of sensory afferents of the laryngeal mucosa. All four experimental animals showed a greater level of correct PCA muscle reinnervation (P < 0.0064) and a lesser level of incorrect reinnervation (P < 0.0084) than the controls. Direct muscle stimulation also appeared to enhance the overall magnitude of reinnervation, but the effect was not as strong (P < 0.113). These findings are consistent with a previous report and suggest that stimulation of a mammalian muscle may profoundly affect its receptivity to reinnervation by a particular motoneuron type.     

Motor unit differentiation and integrated surface EMG in voluntary isometric contraction

The role of the differentiation of motor units (MU's) in the relation between integrated surface electromyogram and force was studied here for the biceps brachii, in the course of static contraction in man. The global EMG of the biceps brachii was recorded by two silver electrodes fixed to the skin. The MU's activity was simultaneously lead off by three bipolar wire electrodes inserted in three points of the muscle. MU firing frequency and recruitment were estimated by counting the action potentials on the three elementary records. The comparison between the increase of the integrated surface EMG and MU recruitment as a function of force shows that MU recruitment in itself cannot account for the increase in the integrated global EMG, particularly for high values of force. The difference between MU recruitment and integrated global EMG, which can neither come from a possible MU's synchronisation as was shown in the discussion, is being suggested to be connected to the progressive firing of “phasic? MU's. This type of recruitment may also play an important role in the gradation of isometric force in normal man.     

Stimulation map for control of functional grasp based on multi-channel EMG recordings

Transcutaneous activation of muscles with electrical stimulation has limited selectivity in recruiting paralyzed muscles in stroke patients. However, the selectivity could be increased by the application of smaller electrodes and their appropriate positioning on the skin. We developed a method for selecting the appropriate positions of the stimulating electrodes based on electromyography (EMG). The EMG activity maps were estimated from signals recorded with two electrode arrays and two 24-channel wearable amplifiers positioned on the nonparetic and paretic forearms. The areas where the difference between the EMG maps obtained from the nonparetic and paretic arms was significant were identified as the stimulation sites. The stimulation was applied through array electrodes with magnetic holders and two wearable stimulators with four output channels each. The measures of functionality included joint angles measured with goniometers (hand opening) and grasp force measured with a multi-contact dynamometer (grasping). The stimulation protocol comprised co-activation of flexors and extensors to stabilize the wrist joint and prevent pronation/supination.