Surface EMG mapping of the human trapezius muscle: the topography of monopolar and bipolar surface EMG amplitude and spectrum parameters at varied forces and in fatigue

Assessment of force and fatigue in isometric contractions of the upper trapezius muscle by surface EMG signal and perceived exertion scale

Quantifying muscle force and fatigue is important in designing ergonomic work stations, in planning appropriate work-rest patterns, and in preventing/assessing the progress of disorders. In 14 subjects (seven males, seven females), muscle force and fatigue were estimated by subjective perception (based on Borg scale CR10) and objective indexes extracted from surface electromyogram (EMG). The experimental protocol consisted of an isometric task selective for the upper trapezius muscle at different force levels (10-80% of maximal voluntary contraction--MVC, in steps of 10%MVC) and one fatiguing contraction (constant force level at 50%MVC until exhaustion). Surface EMG signals were detected by a two-dimensional (2D) array of electrodes placed half way between C7 and the acromion. The following variables were calculated from EMG signals: muscle fibre conduction velocity (CV), root mean square value (RMS), mean frequency of the power spectrum (MNF), fractal dimension (FD), and entropy. All detected signals were also used to build topographical maps of RMS. Both subjective and objective indications of force and fatigue can provide information on exerted force and endurance time (ET). In particular, Borg ratings, RMS, and entropy were significantly related to force, and the rate of change of CV, MNF, FD, and Borg ratings were predictive of the endurance time. Moreover, significant differences were found in Borg ratings between males and females. The correlation coefficient of pairs of topographical maps of RMS was high (of the order of 0.8). This reflects a characteristic spatial-temporal recruitment of upper trapezius motor units that is not affected by force levels or fatigue.

Muscle activation during a reach-to-grasp movement in sitting position: influence of the distance

The purpose of this study was to examine the influence of the reach distance on the muscle activation during a reach-to-grasp movement in the sitting position. Ten healthy male volunteers were tested. Surface EMG were recorded for the deltoid scapular, the deltoid clavicular, the triceps brachii, the biceps brachii and the brachioradialis. These muscles have been selected for their contribution to the cylindrical palmar prehension motion in the sagittal plane. Three distances have been tested: 20, 30 and 40 cm. For each distance, ten repeated measures have been recorded. From this in vivo data, the repeatability of the protocol has been tested. For this, relative (ICC) and absolute (SEM) reliabilities are determined in order to evaluate the intra operator repeatability of this protocol. It appears that the ICC values obtained are between 0.78 and 0.99 in all the conditions (15 conditions corresponding to three distances and five muscles). The intra operator repeatability is thus confirmed. From these surface EMG recordings the muscle activations have been calculated as the iEMG value. It appears that the muscle activation is greater with increased distances. The results contribute to the identification of the levels of muscle activation amplitude during a simple reach-to-grasp movement that is common in prehension research. This knowledge is essential in order to calculate the muscle forces and to integrate these forces in the prehension models developed nowadays in the robotic, rehabilitation, ergonomics field of research.

Spatial distribution of active muscle fibre characteristics in the upper trapezius muscle and its dependency on contraction level and duration

The aim of this study was to provide direct in vivo information of the physiological and structural characteristics of active muscle fibres from a large part of the upper trapezius muscle. Two-dimensional (2-D) multi-channel surface electromyography recordings were used, with 13 x 10 electrodes covering 6 x 4.5 cm of the skin's surface. A previously developed method was applied to detect individual propagating motor unit action potentials and to estimate their corresponding muscle fibre conduction velocity (MFCV) and muscle fibre orientation (MFO). Using these estimates, spatial distributions of MFCV and MFO were examined for five male subjects performing isometric shoulder elevation at different force levels. The main results were: (1) the general relationship between MFCV and force generation was non-systematic, with a positive relationship at the inferior part of the muscle, (2) the spatial distribution of MFCV at different force levels and fatigue was inhomogeneous and (3) the MFO was slightly different (6 degrees ) of the muscle fibres with origin superior compared to inferior to the C7 vertebra. These findings provide new information of the MFO of contracting muscle fibres and knowledge of the physiological characteristics of a large part of the upper trapezius muscle that previously was based on observations from human cadavers only.

Differential activation of regions within the biceps brachii muscle during fatigue

AIM

To examine the occurrence of repeated differential activation between the heads of the biceps brachii muscle and its relation to fatigue prevention during a submaximal contraction.

METHODS

Thirty-nine subjects carried out an isometric contraction of elbow flexion at 25% of maximal voluntary contraction (MVC) until exhaustion. A grid of 13 by 10 electrodes was used to record surface electromyographic signals from both heads of the biceps brachii. The root-mean-square of signals recorded from electrodes located medially and laterally was used to analyse activation differences. Differential activation was defined as periods of 33% different activation level between the two heads of the biceps brachii muscle.

RESULTS

Differential muscle activation was demonstrated in 30 of 33 subjects with appropriate data quality. The frequency of differential activation increased from 4.9 to 6.6 min(-1) at the end of the contractions with no change in duration of the differential activations (about 1.4 s). Moreover, the frequency of differential activation was, in general, negatively correlated with time to exhaustion.

CONCLUSION

The observed differential activation between the heads of the biceps brachii can be explained by an uneven distribution of synaptic input to the motor neurone pool. The findings of this study indicate that differential activation of regions within a muscle does not prevent fatigue at a contraction level of 25% of MVC.

The change in spatial distribution of upper trapezius muscle activity is correlated to contraction duration

The aim of the study was to confirm the hypothesis that the longer a contraction is sustained, the larger are the changes in the spatial distribution of muscle activity. For this purpose, surface electromyographic (EMG) signals were recorded with a 13 x 5 grid of electrodes from the upper trapezius muscle of 11 healthy male subjects during static contractions with shoulders 90 degrees abducted until endurance. The entropy (degree of uniformity) and center of gravity of the EMG root mean square map were computed to assess spatial inhomogeneity in muscle activation and changes over time in EMG amplitude spatial distribution. At the endurance time, entropy decreased (mean+/-SD, percent change 2.0+/-1.6%; P<0.0001) and the center of gravity moved in the cranial direction (shift 11.2+/-6.1mm; P<0.0001) with respect to the beginning of the contraction. The shift in the center of gravity was positively correlated with endurance time (R(2)=0.46, P<0.05), thus subjects with larger shift in the activity map showed longer endurance time. The percent variation in average (over the grid) root mean square was positively correlated with the shift in the center of gravity (R(2)=0.51, P<0.05). Moreover, the shift in the center of gravity was negatively correlated to both initial and final (at the endurance) entropy (R(2)=0.54 and R(2)=0.56, respectively; P<0.01 in both cases), indicating that subjects with less uniform root mean square maps had larger shift of the center of gravity over time. The spatial changes in root mean square EMG were likely due to spatially-dependent changes in motor unit activation during the sustained contraction. It was concluded that the changes in spatial muscle activity distribution play a role in the ability to maintain a static contraction.

Spatial dependency of trapezius muscle activity during repetitive shoulder flexion

The purpose of this study was to explore changes in spatial muscle activation within the three divisions of the trapezius muscle during a dynamic, cyclic task of the upper limb. Surface EMG signals were detected from thirteen healthy subjects from the upper, middle and lower divisions of the trapezius muscle at multiple electrode sites in the cephalad-caudal direction during a repetitive shoulder flexion task. Initial values and rate of change of average rectified value (ARV) and of instantaneous mean power spectral frequency (iMNF) were estimated at 45 degrees , 90 degrees and 120 degrees of shoulder flexion throughout the 5-min task. The location of the electrodes had a significant effect on initial EMG ARV for both the upper and middle division of the trapezius muscle (P<0.05). Both the rate of change and normalized rate of change of ARV were greatest for the most cranial muscle fibers of the upper division (P<0.05). Initial values and rates of change of iMNF were also affected by electrode location for the upper and lower divisions of the trapezius muscle (P<0.05). These results demonstrate that muscle activity and its changes over time depend on position within the three divisions of the trapezius muscle during a dynamic, cyclic task of the upper limb. This suggests non-uniform muscle fiber distribution and/or recruitment. The results also highlight the importance of multiple recording sites when investigating trapezius muscle function in dynamic tasks.

Visualization of lumbar muscle contraction synergy using surface electromyography (sEMG) streaming topography

Because of the difficulty in analysis and interpretation of surface electromyography (sEMG), the specific muscle contraction synergy associated with low back pain continues to be debated. Streaming topography is a novel method of continuously visualizing the distribution of sEMG signals during dynamic motion to provide a more comprehensive examination and subsequent insight into the synergy of muscle recruitment pattern. The purpose of this study was to assess the feasibility of streaming topography as a diagnostic tool. Ten healthy subjects were recruited to establish the normal pattern of lumbar muscle activity. An array of surface EMG electrodes was applied to the low back region and recorded during forward bending. The root mean square (RMS) of the sEMG signals were calculated as a function of both position and time to produce streaming topographical videos of the muscle activity in the lumbar region. In addition, a preliminary clinical study was carried out with 3 LBP patients. In normal subjects, RMS streaming topography was consistent, reproducible, and reliable. In clinical observation, the RMS streaming topography of LBP patients was obviously different from that of normal subjects. Some of LBP patients showed an asymmetric distribution during symmetric action. Streaming topography provides a dynamic analysis of lumbar muscle activities and illustrates the synergy of muscle contractions, which may be useful to improve physiotherapy management of LBP.

Muscle maintenance by volitional contraction against applied electrical stimulation

Muscle training exercises are needed for muscular endurance during spaceflight. This study was designed to investigate effects of volitional contraction against applied electrical stimulation on the muscular endurance of the proximal upper extremity. Thirteen healthy sedentary men were allocated into two groups. One group participated in a hybrid (HYB) exercise regimen in which the biceps brachii was stimulated as he volitionally extended his elbow, and the triceps brachii was stimulated as the volitionally flexed his elbow. The second group underwent a similar regimen in which the electrical stimulation (ELS) was alternatively delivered to the biceps brachii and then to the triceps brachii with the limb fixed. Forty-second surface electromyography (EMG) recordings at 50% maximum voluntary contraction (MVC) were made as baseline data at just before starting the training regimen, and again conclusion. The median frequency (MF) and mean power frequency (MPF) slopes with time were determined using power spectrum analysis. There were statistical significance only for the triceps in which the MF and MPF slopes in the HYB Group became less negative over the period of study (from -45.7+/-14.7 and -47.0+/-8.6%/min at baseline to -36.9+/-10.7 and -36.8+/-7.0%/min at the end of training, respectively). The corresponding values for these slopes in the ELS Group showed opposite tends with less marked changes of borderline significance for MF and of statistical significance for MPF. These results suggested that the HYB exercise regimen was capable of producing an improvement in triceps but not biceps brachii.

Clinical applications of high-density surface EMG: A systematic review

High density-surface EMG (HD-sEMG) is a non-invasive technique to measure electrical muscle activity with multiple (more than two) closely spaced electrodes overlying a restricted area of the skin. Besides temporal activity HD-sEMG also allows spatial EMG activity to be recorded, thus expanding the possibilities to detect new muscle characteristics. Especially muscle fiber conduction velocity (MFCV) measurements and the evaluation of single motor unit (MU) characteristics come into view. This systematic review of the literature evaluates the clinical applications of HD-sEMG. Although beyond the scope of the present review, the search yielded a large number of "non-clinical" papers demonstrating that a considerable amount of work has been done and that significant technical progress has been made concerning the feasibility and optimization of HD-sEMG techniques. Twenty-nine clinical studies and four reviews of clinical applications of HD-sEMG were considered. The clinical studies concerned muscle fatigue, motor neuron diseases (MND), neuropathies, myopathies (mainly in patients with channelopathies), spontaneous muscle activity and MU firing rates. In principle, HD-sEMG allows pathological changes at the MU level to be detected, especially changes in neurogenic disorders and channelopathies. We additionally discuss several bioengineering aspects and future clinical applications of the technique and provide recommendations for further development and implementation of HD-sEMG as a clinical diagnostic tool.

Treadmill locomotion in normal mice-Step related multi-channel EMG profiles of thigh muscles

Mouse models are increasingly used in current research on motor disorders. In mice, the myoelectrical activation of thigh muscles during locomotion has not yet, however, been investigated in depth. Especially intramuscular coordination has hardly been clarified. Therefore, the aims of this study were to characterize myoelectrical activity in the vastus lateralis (VL) and the biceps femoris (BF) muscle of the healthy mouse for reference purposes. The VL and the BF muscles of 12 healthy mice performing a total of 1985 steps during treadmill locomotion were investigated with two subcutaneous arrays each incorporating four electrodes. Eight-channel EMG was recorded simultaneously with high-speed videography. The EMG curves of each step were rectified and smoothed by calculating root mean square (RMS) profiles and then time-normalized for comparisons within and between animals. The EMG-activity of both muscles increased during late swing phase. The VL activity rose steeply and peaked during mid-stance phase, while the biceps activity reached a plateau during early stance phase. With increasing gait velocity, stance time decreased. The increase in gait velocity was also associated with greater EMG amplitudes. The results suggest that the BF lifts the lower hind leg during swing phase and stabilizes the leg during stance, while the VL bears the weight of the body during the stance phase.

Experimental muscle pain changes the spatial distribution of upper trapezius muscle activity during sustained contraction

OBJECTIVE

To investigate the effect of local excitation of nociceptive muscle afferents on the spatial distribution of muscle activity.

METHODS

Surface electromyographic (EMG) signals were recorded from the upper trapezius muscle of 10 healthy volunteers with a 5 x 13 electrode grid during 90-s isometric contractions before, during, 15 and 30 min after intramuscular injection of hypertonic (painful) or isotonic (non-painful) saline. From the multi-channel EMG recordings, two-dimensional maps of root mean square and mean power frequency were obtained. The centre of gravity of the root mean square map was used to quantify global changes in the spatial distribution of muscle activity.

RESULTS

During sustained contractions, average root mean square increased, average mean frequency decreased and the centre of gravity moved cranially. During experimental muscle pain, compared to before injection, the average root mean square decreased and there was a caudal shift of the centre of gravity. Fifteen minutes after the painful injection the centre of gravity returned to its original position.

CONCLUSIONS

Short-term dynamic reorganization of the spatial distribution of muscle activity occurred in response to nociceptive afferent input.

SIGNIFICANCE

The study furnishes an extension of the pain adaptation model indicating heterogeneous inhibition of muscle activity.

Assessment of muscle fatigue using sonomyography: muscle thickness change detected from ultrasound images

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. As the surface electromyography (SEMG) can be used to estimate the features of neuromuscular activations associated with muscle contractions, it has been widely employed as an objective tool to evaluate muscle fatigue. On the other hand, ultrasound imaging can inherently provide the morphological information of individual muscle, thus the architectural changes of muscles during fatigue can be obtained. In this study, we demonstrated the feasibility of using the dimensional change of muscles detected by ultrasound images, named as sonomyography (SMG), to characterize the behavior of muscles when they were in fatigue. The SEMG signals of the muscles were also recorded simultaneously and used for comparison. The right biceps brachii muscles of 8 normal young male adult subjects were tested for 30s under 80% of the maximal voluntary isometric contraction. The muscle fatigue was indicated by the change of the root-mean-square (RMS) and median frequency (MDF) of the SEMG signals. The results showed that the SEMG RMS had a linear increase with time with a rate of 2.9+/-1.9%/s (mean+/-S.D.), while the MDF decreased linearly with a rate of -0.60+/-0.26Hz/s. The muscle thickness, detected from the ultrasound images, continuously increased during the muscle fatigue but with a nonlinear increase with time, which was rapid during the initial 8.1+/-2.1s with a mean deformation rate of 0.30+/-0.19%/s and then became slower with a rate of 0.067+/-0.024%/s up to 20s after the contraction. The muscle deformation at 20s was 3.5+/-1.6%. The results demonstrated that the architectural change of muscles detected using SMG could potentially provide complementary information for SEMG for the muscle fatigue assessment.

Spatial and temporal changes of upper trapezius muscle fiber conduction velocity are not predicted by surface EMG spectral analysis during a dynamic upper limb task

The purpose of the study was to examine the temporal and spatial correlation between estimates of trapezius muscle fiber conduction velocity (CV) and surface EMG instantaneous mean power spectral frequency (iMPF) during dynamic movement of the upper limb. Surface EMG signals were detected from the upper division of the trapezius muscle in 13 healthy volunteers using linear arrays of eight electrodes at three locations in the cephalad-caudal direction. Subjects were asked to tap with their hands in a cyclic manner between targets positioned mid thigh and 120 degrees of shoulder flexion, to the beat of a metronome set at 88 beats per minute for 5 min. Muscle fiber CV and iMPF were estimated for each cycle at the time instant corresponding to 90 degrees of shoulder flexion. Non-significant correlations were identified between CV and iMPF initial values (R(2)=0.03-0.01), rate of change over time (R(2)=0.10-0.004) and normalized rate of change (R(2)=0.12-0.01) at all three locations on the upper trapezius muscle. These results demonstrate that both spatial and temporal variations in trapezius muscle fiber CV are not predicted by EMG spectral analysis during dynamic movement of the upper limb. This finding suggests that spectral analysis cannot be used to infer changes in the spatial and temporal behavior of muscle fiber CV during dynamic tasks.

EMG amplitude distribution changes over the upper trapezius muscle are similar in sustained and ramp contractions

AIM

To investigate whether global motor unit recruitment to compensate for muscle fatigue during sustained contraction and to regulate force increase during ramp contraction are controlled in similar manners in the upper trapezius muscle.

METHODS

Fourteen subjects performed a 10-s ramp contraction from 0% to 90% of maximal voluntary contraction (MVC) and a 3-min sustained contraction at 25% MVC. Both contractions involved isometric shoulder elevation with a multi-channel surface electromyographical grid placed on the skin above the muscle. To evaluate the global muscle activation pattern, the changes in spatial amplitude distribution of the sustained and the ramp contraction were examined and compared.

RESULTS

In both contraction types, the upper part of the trapezius muscle was spatially non-uniform (inhomogeneous) activated. Throughout the sustained contraction, the amplitude distribution of the upper trapezius muscle became more similar to the amplitude distribution at higher force levels.

CONCLUSION

These findings support the hypothesis that global motor unit recruitment to compensate for muscle fatigue during a sustained contraction, and to regulate force increase during a ramp contraction is controlled in a similar manner. Consequently, they confirm fundamental principles of motor unit activation based on recordings of limited motor unit samples.

Biomechanical and electromyographic evaluation of ankle foot orthosis and dynamic ankle foot orthosis in spastic cerebral palsy

This study evaluated the biomechanical and electromyographic effects of conventional ankle foot orthoses (AFOs) and dynamic ankle foot orthoses (DAFOs) on gait in patients with spastic cerebral palsy (CP). Thirteen patients with dynamic equinus underwent motion analysis with electromyography. Both AFOs and DAFOs provided longer stride length, permitted pre-positioning for initial contact, and successfully controlled the excessive plantarflexion during the swing phase. Median frequency (MF) of EMG signal indicated that extremely high firing was found in the patient's lower limbs compared to controls that resulted in tiredness. The DAFOs allowed a significantly larger total ankle range of motion than the AFOs. However, AFOs significantly reduced the MF while DAFOs did not. The reduced MF seen when wearing AFOs suggested an improvement of walking endurance. The DAFO had the advantage of less restriction on ankle movement, which avoids muscular atrophy and improves orthotic compliance.

On the errors in assessment of severity of involuntary movements using surface EMG

Surface electromyogram (SEMG) is a useful tool to depict involuntary movements, but evaluation of the intensity of such movements with SEMG over multiple recording instances requires awareness of factors influencing quantified SEMG signals. We investigated the differences in the amplitude of SEMGs due to electrode displacement in isometric voluntary contraction in the upper arm, forearm and lower leg in 8 healthy men. The SEMGs of gross muscle activity simultaneously recorded with 4 electrode pairs from the agonist and antagonist sides in 3 displacement conditions with respect to parallel position, interelectrode distance, and rotation were compared. The amount of EMG integration (equivalent to the average SEMG amplitude) of each electrode pair was compared to the reference electrode pair with interelectrode distance of 40 mm placed on the center of the tested muscles. The average EMG difference ratios ranged 1.1-2.2%/mm in parallel shift, 1.0-1.9%/mm in distance shift, and 0.3-0.6%/degree in rotation shift. Displacement error of electrodes in separate recording instances should be reduced using anatomical landmarks, when SEMG is applied as a quantitative method to evaluate change in the states of involuntary movements.

Inhomogeneities in muscle activation reveal motor unit recruitment

This paper presents a novel method to quantify spatial changes in muscle activation pattern by multi-channel surface electromyography (MCSEMG) in order to investigate motor unit recruitment variation. The method is based on non-uniform distributions of motor units that cause spatial inhomogeneous muscle activation. To evaluate the method, 15 subjects performed three isometric elbow flexion contractions consisting of slow sinusoidal changes in force ranging from 0% to 80% of the maximal voluntary contraction. MCSEMG electrodes were placed in a 10 x 13 grid over the biceps brachii muscle. From all channels, root mean square (RMS) values of bipolar leadings were computed over 0.5 s epochs over the whole recording. Thereafter, correlation coefficients were calculated between the RMS values at one epoch, with the RMS values at another epoch. Results showed consistent spatial changes in the distribution of RMS at different contraction levels up to 80% of maximal voluntary contraction and when comparing increasing and decreasing contractions at the same force level. These findings are reproducible within and between subjects, and in agreement with physiological phenomena and therefore indicate that the spatial inhomogeneities of motor unit properties in the biceps brachii muscle can be used to study changes in motor unit recruitment.

Simulation of Surface EMG Signals Generated by Muscle Tissues With Inhomogeneity Due to Fiber Pinnation

Surface electromyographic (EMG) signal modeling has important applications in the interpretation of experimental EMG data. Most models of surface EMG generation considered volume conductors homogeneous in the direction of propagation of the action potentials. However, this may not be the case in practice due to local tissue inhomogeneities or to the fact that there may be groups of muscle fibers with different orientations. This study addresses the issue of analytically describing surface EMG signals generated by bi-pinnate muscles, i.e., muscles which have two groups of fibers with two orientations. The approach will also be adapted to the case of a muscle with fibers inclined in the depth direction. Such muscle anatomies are inhomogeneous in the direction of propagation of the action potentials with the consequence that the system can not be described as space invariant in the direction of source propagation. In these conditions, the potentials detected at the skin surface do not travel without shape changes. This determines numerical issues in the implementation of the model which are addressed in this work. The study provides the solution of the nonhomogenous, anisotropic problem, proposes an implementation of the results in complete surface EMG generation models (including finite-length fibers), and shows representative results of the application of the models proposed.

Controller Adjustment of an Exoskeleton Robot for Shoulder Motion Assistance

We are developing exoskeleton robots to realize human shoulder motion assistance for the physically weak. In this paper, we propose controller adjustment for the controller of the exoskeleton robot for human shoulder motion assistance. Motion assistance in the entire movable range of the exoskeleton is realized with a few teaching motion patterns using the proposed controller adjustment. Muscle activity (electromyography) during shoulder motion and motion error between desired user shoulder motion and the measured assisted shoulder motion are evaluated.

Estimation of average muscle fiber conduction velocity from two-dimensional surface EMG recordings

We propose a novel method for the estimation of muscle fiber conduction velocity (CV) from surface EMG recordings. The approach is based on the analysis of signals detected along a number of linear electrode arrays parallel to the fiber direction, thus collected by a bi-dimensional (2-D) array (matrix) of electrodes. The information provided by the 2-D array is used to derive a maximum likelihood estimator which can be applied to any number of signals and which may account for missing channels in the matrix. An iterative technique in the frequency domain for the estimation of the propagation delay is proposed to reduce the computational time and avoid the limit of resolution due to signal sampling. The method proposed is applied to signals collected from the biceps brachii muscle of eight healthy subjects during isometric, constant force contractions at 50% of the maximal voluntary contraction torque. It is shown that CV estimation standard deviation and sensitivity to electrode displacements significantly decrease by the application of the method proposed with respect to classic CV estimation techniques. The method promises to be a useful tool when average CV is estimated for muscle assessment and diagnostic purposes.

A thin, flexible multielectrode grid for high-density surface EMG

Although the value of high-density surface electromyography (sEMG) has already been proven in fundamental research and for specific diagnostic questions, there is as yet no broad clinical application. This is partly due to limitations of construction principles and application techniques of conventional electrode array systems. We developed a thin, highly flexible, two-dimensional multielectrode sEMG grid, which is manufactured by using flexprint techniques. The material used as electrode carrier (Polyimid, 50 microm thick) allows grids to be cut out in any required shape or size. One universal grid version can therefore be used for many applications, thereby reducing costs. The reusable electrode grid is attached to the skin by using specially prepared double-sided adhesive tape, which allows the selective application of conductive cream only directly below the detection surfaces. To explore the practical possibilities, this technique was applied in single motor unit analysis of the facial musculature. The high mechanical flexibility allowed the electrode grid to follow the skin surface even in areas with very uneven contours, resulting in good electrical connections in the whole recording area. The silverchloride surfaces of the electrodes and their low electrode-to-skin impedances guaranteed high baseline stability and a low signal noise level. The electrode-to-skin attachment proved to withstand saliva and great tensile forces due to mimic contractions. The inexpensive, universally adaptable and minimally obstructive sensor allows the principal advantages of high-density sEMG to be extended to all skeletal muscles accessible from the skin surface and may lay the foundation for more broad clinical application of this noninvasive, two-dimensional sEMG technique.

Multichannel surface EMG: basic aspects and clinical utility

The generation of the surface electromyogram (sEMG) is described with regard to the properties of the single muscle fiber action potential as source, the physical aspects of volume conduction and recording configuration, and the properties and firing pattern of motor units (MUs). The spatial aspect of the motor unit action potential (MUP) is emphasized in relation to the results of high-density, multichannel sEMG measurements. The endplate zone, depth, size, and position of MUs can be estimated. The use of muscle fiber conduction velocity measurements in channelopathies and the changes in pathological fatigue are described. Using the unique patterns of spatial spread of MUPs over the skin (MU fingerprint), MU classification and the determination of firing moments is done noninvasively. Clinical applications of high-density sEMG measurements are reviewed. Emerging possibilities provided by MUP size and fingerprint measurements in neuromuscular disease and motor control are discussed. We conclude that multichannel sEMG adds unique, and sometimes indispensable, spatial information to our knowledge of the motor unit.

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.

Paraspinal muscle activities of patients with scoliosis after spine fusion: an electromyographic study

STUDY DESIGN

This prospective study compared the electromyographic activities of paraspinal muscles between normal subjects and patients with scoliosis before and after spine fusion.

OBJECTIVE

To investigate the functional changes of paraspinal muscles before and after spine fusion using surface electromyography.

SUMMARY OF BACKGROUND DATA

Idiopathic scoliosis is a common spine deformity. Surgical correction followed by spine fusion is the basic method of treatment, but the functional changes in paraspinal muscles after spine fusion still are unknown.

METHODS

For this study, 15 healthy subjects and 19 patients with idiopathic scoliosis were recruited. All 19 patients received posterior spine fusion from T2 to T5 to T12. The surface electromyography for normal subjects and for patients before and after spine fusion was recorded bilaterally along the thoracic and lumbar paraspinal muscles for different postures. Root mean square and median frequency values of electromyographic activities were used to compare results between normal subjects and patients with scoliosis before and after spine fusion.

RESULTS

Electromyographic activities showed significant differences between normal control subjects and patients with pre- or postoperative scoliosis. After spine fusion, electromyography showed lower root mean square activity in the thoracic region and higher root mean square activity in the lumbar region. Patients with preoperative scoliosis showed lower median frequency in the thoracic region and higher median frequency in the lumbar region than normal subjects and patients with postoperative scoliosis. The group with preoperative scoliosis showed the least symmetrical paraspinal muscle activity, followed by the postoperative and normal groups.

CONCLUSIONS

Patients with scoliosis present unbalanced electromyographic activity in the paravertebral muscles, which is diminished by spine fusion, but does not return to the values found in normal subjects. The paraspinal muscles in the thoracic region showed lower activity after spine fusion, probably because of atrophy. Higher muscular activity was found in the lumbar region, which may lead to muscle hypertrophy.

Influence of motoneuron firing synchronization on SEMG characteristics in dependence of electrode position

The frequency content of the surface electromyography (SEMG) signal, expressed as median frequency (MF), is often assumed to reflect the decline of muscle fiber conduction velocity in fatigue. MF also decreases when motor unit firings synchronize, and we hypothesized that this effect can explain the electrode-dependent pattern in our previous recordings from the trapezius muscle. An existing motoneuron (MN) model describes the afterhyperpolarization following a spike as an exponential function on which membrane noise is superimposed. Splitting the noise into a common and an individual component extended the model to a MN pool with a tunable level of firing synchrony. An analytical volume conduction model was used to generate motor unit action potentials to simulate SEMG. A realistic level of synchrony decreased the MF of the simulated bipolar SEMG by approximately 30% midway between endplate position and tendon but not above the endplate. This is in accordance with experimental data from the biceps brachii muscle. It was concluded that the pattern of decrease of MF during sustained contractions indeed reflects MN synchronization.