Nonlinear surface EMG analysis to detect changes of motor unit conduction velocity and synchronization

Corticospinal drive during painful voluntary contractions at constant force output

In the voluntary contractions, output force can be maintained constant although the inhibitory influences exerted by pain on muscle activity. We investigated changes in the spontaneous and evoked activity of the abductor digiti minimi muscle (ADM) and the biceps brachii muscle (BIC) in healthy volunteers during constant force noxious contraction, resulting from chemically activated nociceptive afferents. EMG-force relationship, motor-evoked response (MEP) to transcranial magnetic stimulation and determinism (DET) of surface EMG signals during constant force contraction was analyzed before, during and after chemically induced tonic activation of their nociceptive afferents. Under constant force contraction, amplitude of surface EMG signal decreased in BIC and increased in ADM during pain with respect to control condition. In both muscles, the size of motor-evoked potential (MEP) induced by transcranial magnetic stimulation (TMS) of the primary motor cortex was significantly higher during pain than in control. Level of determinism extracted from surface EMG signal by non-linear method was similarly and significantly increased in both muscles during pain stimulation. Finally, nociceptive stimulation caused a decline in steadiness of the force exerted by ADM and BIC. These results are interpreted in terms of increased corticospinal synchronizing inputs. The possibility that it may play a role in governing force production to counteract pain inhibitory influences on motor system is considered.

Analysis of surface EMG spike shape across different levels of isometric force

This research evaluated changes in surface electromyographic (SEMG) spike shape across different levels of isometric force. Ninety-six subjects generated three 5-s isometric step contractions of the elbow flexors at 40, 60, 80, and 100% of maximal voluntary contraction (MVC). Force and bipolar SEMG activity were monitored concurrently. The mean spike amplitude (MSA) exhibited a linear increase across the four levels of force. The mean spike frequency (MSF) remained stable from 40 to 80% of MVC; it then decreased from 80 to 100% of MVC. There was a concomitant increase in mean spike slope (MSS) that indicates that the biceps brachii (BB) relied on the recruitment of higher threshold motor units (MUs) from 40 to 80% of MVC. However, there progressive decrease in the mean number of peaks per spike (MNPPS) that suggests that MU synchronization was additionally required to increase force from 80 to 100% of MVC. The spike shape measures, taken together, indicate that the decrease in frequency content of the signal was due to synchronization, not an increased probability of temporal overlap due an increase in rate-coding.

Recurrence quantification analysis of surface EMG detects changes in motor unit synchronization induced by recurrent inhibition

The systemic injection of L-Acetylcarnitine (L-Ac) induces a reversible increase in recurrent inhibition. In addition, L-Ac potentiation of recurrent inhibition has been found to increase the synchronous activity of single motor units, as detected by traditional linear analysis in the time domain. This result has been recently confirmed using a nonlinear method based on the analysis of embedded determinism (%DET) extracted from the surface EMG. The present study aimed at testing the general applicability of RQA methodology, as a viable tool for assessing motor unit synchronization, by extending the analysis of surface EMG, as revealed by changes in %DET induced by L-Ac, to many upper and lower limb muscles and to muscles that are not easily studied by needle electrodes, such as the orbicularis oculi. Subjects performed brief periods of tonic contractions, alternated to periods of rests to avoid muscle fatigue. Pharmacological enhancement of recurrent inhibition was obtained by a short-lasting intravenous injection of L-Ac. Control experiments were performed replacing L-Ac injection with saline injection. The average %DET showed a significant increase during L-Ac injection in the deltoid, biceps brachii, extensor carpi radialis, while no effect was observed in the opponens pollicis and abductor digiti minimi for the upper limb muscles. Similarly, the average %DET showed a significant increase during L-Ac injection in the quadriceps, soleus, and tibialis anterior, while no effect was observed in the abductor hallucis for the lower limb muscles. RQA of orbicularis oculi muscle activity showed no increase in %DET during L-Ac injection in analogy to what found in the intrinsic muscles of the hand and foot, known to be devoid of recurrent inhibition. The presence or absence of drug-induced increase in motor unit synchronization agrees with the known distribution of recurrent inhibition in the various motor nuclei. The overall significance of these findings is the potential application of RQA methodology as a reliable and independent tool for generally assessing motor unit synchronization from surface EMG under strictly controlled experimental condition.

Neuromuscular responses to exercise investigated through surface EMG

Physical exercise promotes a wide spectrum of short and long term responses of different organs and apparatuses. While skeletal muscle adaptations to the different training regimens are conveniently known and described, the neural counterpart of them are still to be described in full. In this paper, an attempt is made to fix the state of the art and particularly to point out the contribution derived from the analysis of the surface electromyographic signal. In this paper, some examples of sEMG applications in exercise physiology will be reported from studies where only strictly non-invasive techniques (or of very limited invasiveness) were applied. A consistent amount of space in this lecture will be dedicated to the advanced analysis of sEMG using non linear tools.

Motor unit synchronous firing as revealed by determinism of surface myoelectric signal

Information on motor strategies can be extracted from the surface electromyogram (EMG) by non-linear methods. The percentage of determinism (%DET) obtained from recurrence quantification analysis (RQA) may be a sensitive variable to detect synchronous motor unit behaviour. The purpose of the present study was to validate this methodology by comparing it with an established technique estimating the degree of synchronization of pairs of voluntary activated motor units from the correlation of their firing in the time-domain. Single motor unit activity was recorded in extensor carpi radialis (ECR) muscle by pairs of tungsten microelectrodes inserted into the muscle belly. Cross-correlation analysis was performed in order to determine synchronization peak area by computing synchronous impulse probability. Surface EMG activity was recorded in parallel by electrodes placed over the skin of the same muscle and %DET was used as a measure of synchronous activity. The %DET appeared to be a valid measure of synchronization yielding results comparable to those obtained with cross-correlation analysis. Increases in %DET (t = 64.59, P < 0.0001) highly correlated (r2 = 0.70, P = 0.0013) with pharmacologically induced increases in the synchronization activity of pairs of ECR motor units (t = 8.71, P < 0.0001). RQA may be used as an alternative methodology for testing synchronous motor unit behaviour from surface EMG under physiological and pathological conditions.

Models to explain fatigue during prolonged endurance cycling

Much of the previous research into understanding fatigue during prolonged cycling has found that cycling performance may be limited by numerous physiological, biomechanical, environmental, mechanical and psychological factors. From over 2000 manuscripts addressing the topic of fatigue, a number of diverse cause-and-effect models have been developed. These include the following models: (i) cardiovascular/anaerobic; (ii) energy supply/energy depletion; (iii) neuromuscular fatigue; (iv) muscle trauma; (v) biomechanical; (vi) thermoregulatory; (vii) psychological/motivational; and (viii) central governor. More recently, however, a complex systems model of fatigue has been proposed, whereby these aforementioned linear models provide afferent feedback that is integrated by a central governor into our unconscious perception of fatigue. This review outlines the more conventional linear models of fatigue and addresses specifically how these may influence the development of fatigue during cycling. The review concludes by showing how these linear models of fatigue might be integrated into a more recently proposed nonlinear complex systems model of exercise-induced fatigue.

The use of correlation integrals in the study of localized muscle fatigue of elbow flexors during maximal efforts

Innovative applications of non-linear time series analysis have recently been used to investigate physiological phenomena. In this study, we investigated the feasibility of using the correlation integral to monitor the localized muscle fatigue process in the biceps brachii during sustained maximal efforts. The subjects performed isometric maximum contractions until failure in elbow flexion (90 degrees from neutral). The median and the 70th percentile frequency of the Surface electromyography (SEMG) power spectrum, the integrated SEMG, and the Correlation Integral (CI) were evaluated during the trials. The linear correlation between these variables and the elbow torque production was used to quantify the ability of a parameter to follow the fatiguing process. The CI had the highest linear correlation with torque (0.77 (0.12SD)), while the spectral indices correlations with torque were much lower. The decreasing trend of the torque production was followed by the spectral indices for only the beginning part of the contraction, while the CI increased sharply after the torque production fell to about 0.60 of the MVC. This suggests that the CI is sensitive to different changes of the SEMG signal during fatigue than the spectral variables.

An EMG fractal indicator having different sensitivities to changes in force and muscle fatigue during voluntary static muscle contractions

During a sustained contraction, electromyographic signals (EMGs) undergo a spectral compression. This fatigue behaviour induces a shift of the mean and the median frequencies to lower frequencies. On the other hand, several studies conclude that the mean/median frequency can increase, decrease or remain constant with an increasing force level. Such inconsistency is embarrassing since the fatigue state may be influenced by the force level. In this paper, we propose a frequency indicator which is sensitive to the force level independently of the fatigue state evaluated at 70% of the maximal voluntary contraction. Ten healthy volunteers participated in the study and both surface EMGs (from the short head of the biceps brachii) and force signals were measured. This study compared force and fatigue effects on the EMGs during short (3-s) isometric contractions at different strength intensities and during a sustained isometric contraction until exhaustion. The EMGs partly show 1/falpha spectral behaviours since their power spectral densities may experimentally fit with two linear segments in a log-log representation. The measured "right" slope produces variations of force as 20 times the variations of fatigue. 1/falpha Behaviour may be related to stochastic fractals. This fractal indicator is a new frequency indicator that is thus complementary to other known classical frequency indicators when studying force during unknown fatigue states.

Influence of high motor unit synchronization levels on non-linear and spectral variables of the surface EMG

The aim of this study was to investigate the influence of high degrees of motor unit synchronization on surface EMG variables extracted by linear and non-linear analysis techniques. For this purpose, spectral and recurrent quantification analysis (RQA) were applied to both simulated and experimental EMG signals. Synthetic surface EMG signals were generated with a model of volume conductor comprising muscle, fat, and skin tissues. The synchronization was quantified by the percent of discharges of each motor unit synchronized with discharges of other motor units. The simulated signals presented degrees of synchronization in the range 0-80% (10% increments) and three mean values of motor unit conduction velocity distribution (3, 4 and 5 m/s). Experimental signals were collected from the first dorsal interosseous muscle of five patients with Parkinson disease during 10s of rest and 10s of isometric voluntary contraction at 50% of the maximal force. Mean power spectral frequency (MNF) and percent of determinism (%DET) of the surface EMG were computed from the simulated and experimental signals. In the simulated signals, %DET was linearly related to the level of synchronization in the entire range considered while MNF was sensitive to changes in synchronization in a smaller range (0-20%), outside which it levelled off. The experimental results indicated that %DET was significantly higher in the resting condition (with presence of tremor; mean +/- S.E., 85.4 +/- 0.8%) than during the voluntary contraction (which partly suppressed tremor; 60.0 +/- 2.3%; P < 0.01). On the contrary, MNF did not depend on the condition (114.3 +/- 1.5 Hz and 118.0 +/- 0.8 Hz for the resting and voluntary contraction, respectively), confirming the simulation results. Overall, these results indicated that linear and non-linear analyses of the surface EMG may have different sensitivities to the underlying physiological mechanisms in specific conditions, thus their joint use provides a more complete view of the muscle status than spectral analysis only.

The extraction of neural strategies from the surface EMG

This brief review examines some of the methods used to infer central control strategies from surface electromyogram (EMG) recordings. Among the many uses of the surface EMG in studying the neural control of movement, the review critically evaluates only some of the applications. The focus is on the relations between global features of the surface EMG and the underlying physiological processes. Because direct measurements of motor unit activation are not available and many factors can influence the signal, these relations are frequently misinterpreted. These errors are compounded by the counterintuitive effects that some system parameters can have on the EMG signal. The phenomenon of crosstalk is used as an example of these problems. The review describes the limitations of techniques used to infer the level of muscle activation, the type of motor unit recruited, the upper limit of motor unit recruitment, the average discharge rate, and the degree of synchronization between motor units. Although the global surface EMG is a useful measure of muscle activation and assessment, there are limits to the information that can be extracted from this signal.

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.

Surface electromyogram signal modelling

The paper reviews the fundamental components of stochastic and motor-unit-based models of the surface electromyogram (SEMG). Stochastic models used in ergonomics and kinesiology consider the SEMG to be a stochastic process whose amplitude is related to the level of muscle activation and whose power spectral density reflects muscle conduction velocity. Motor-unit-based models for describing the spatio-temporal distribution of individual motor-unit action potentials throughout the limb are quite robust, making it possible to extract precise information about motor-unit architecture from SEMG signals recorded by multi-electrode arrays. Motor-unit-based models have not yet been proven as successful, however, for extracting information about recruitment and firing rates throughout the full range of contraction. The relationship between SEMG and force during natural dynamic movements is much too complex to model in terms of single motor units.

EMG recurrence quantifications in dynamic exercise

This study was designed to evaluate the suitability of nonlinear recurrence quantification analysis (RQA) in assessing electromyograph (EMG) signals during dynamic exercise. RQA has been proven to be effective in analyzing nonstationary signals. The subject group consisted of 19 male patients diagnosed with low back pain. EMG signals were recorded from left and right paraspinal muscles during isoinertial exercise both before and after 12 weeks of regimented physical therapy. Autorecurrence analysis was performed between the left and right EMG signals individually, and cross-recurrence analysis was performed on the left-right EMG pairs. Spectral analysis of the EMG signals was employed as an independent, objective measure of fatigue. Increase in the RQA variable % determinism during the 90-s dynamic tests was found to be a good marker for fatigue. Before physical therapy, this nonlinear marker revealed simultaneous increases in motor unit recruitment within each pool and between left and right pools. After physical therapy, the motor unit recruitment was less within and between pools, indicative of increased fatigue resistance. Finally, fatigue resistance (less increase in % determinism) correlated well with subjective scores of pain relief. Taken together, these latter results indicate that recurrence analysis may be useful in charting the efficacy of a specific exercise therapy program in reducing low back pain by elevating the fatigue threshold.

Continuous wavelet transform in the evaluation of stretch reflex responses from surface EMG

OBJECTIVE

This is the first reported use of the continuous wavelet transform (CWT) of the surface EMG (sEMG) to extract the reflex response to muscle stretch. We used a modulus-based method to estimate instantaneous amplitude-envelopes from ridges of the CWT (referred in this work as sEMG intensity) to extract the dynamic reflex response from sEMG. We tested the method on tendon reflexes where excellent temporal resolution is required to identify the different latency components, and on the tonic stretch reflex (tonic SR) response to an ongoing perturbation that characteristically has a low signal to noise ratio.

METHODS

Eight subjects without neurological impairment were subjected to a series of archilles tendon taps and a 2 min continuous perturbation of the ankle using a pseudo-sinusoidal stretch profile containing frequencies from 0.1 to 8.0 Hz. The tendon reflexes were assessed in the soleus muscle at 10% of MVC and the tonic SR in tibialis anterior while the muscle was relaxed, at 5 and 10% of maximal voluntary contraction. Root mean square (RMS) and wavelet ridge extraction was applied to the sEMG signal to extract sEMG amplitudes (RMS) and intensities for all reflexes. To obtain the tonic SR, these estimates and those from the sEMG-RMS were subsequently cross-correlated with the perturbation record to yield 2 sets of estimates of reflex gain and coherence for comparison.

RESULTS

The sEMG intensities were highly correlated with the torques resulting from a ramped voluntary contraction. Following tendon taps, the method resolved the M1, M2, M3 response components at accurate latencies and with more complete reconstruction of the components than RMS-derived estimates. The wavelet ridge estimates extracted the tonic SR from resting and contracting muscles with significantly higher coherence than RMS estimates. Reflex gain, when estimated from sEMG intensity or sEMG-RMS, demonstrated similar relationships to the perturbation frequency and background contraction level. When the sEMG intensity reflex gain estimates from different subjects were pooled, they showed significantly lower variance about the mean than gain estimates derived from the rectified sEMG.

CONCLUSIONS

Wavelet-ridge extraction provides a valid approach to reflex evaluation from sEMG that does not depend on the absolute amplitude of the potentials measured at the EMG electrodes. This may have substantial advantages in more directly comparing responses between subjects on an absolute frequency scale without the need for normalisation against maximal contraction levels.

A Surface EMG Generation Model With Multilayer Cylindrical Description of the Volume Conductor

We propose a model for surface electromyography (EMG) signal generation with cylindrical description of the volume conductor. The model is more general and complete with respect to previous approaches. The volume conductor is described as a multilayered cylinder in which the source can be located either along the longitudinal or the angular direction, in any of the layers. The source is represented as a spatio-temporal function which describes the generation, propagation, and extinction of the intracellular action potential at the end-plate, along the fiber, and at the tendons, respectively. The layers are anisotropic. The volume conductor effect is described as a two-dimensional spatial filtering. Electrodes of any shape or dimension are simulated, forming structures which are described as spatial filters. The analytical derivation which leads to the signal in the temporal domain is performed in the spatial and temporal frequency domains. Numerical issues related to the frequency-based approach are discussed. The descriptions of the volume conductor and of the source are applied to the cases of signal generation from a limb and a sphincter muscle. Representative simulations of both cases are provided. The resultant model is based on analytical derivations and constitutes a step forward in surface EMG signal modeling, including features not described in any other analytical approach.

A novel approach for estimating muscle fiber conduction velocity by spatial and temporal filtering of surface emg signals

We describe a new method for the estimation of muscle fiber conduction velocity (CV) from surface electromyography (EMG) signals. The method is based on the detection of two surface EMG signals with different spatial filters and on the compensation of the spatial filtering operations by two temporal filters (with CV as unknown parameter) applied to the signals. The transfer functions of the two spatial filters may have different magnitudes and phases, thus the detected signals have not necessarily the same shape. The two signals are first spatially and then temporally filtered and are ideally equal when the CV value selected as a parameter in the temporal filters corresponds to the velocity of propagation of the detected action potentials. This approach is the generalization of the classic spectral matching technique. A theoretical derivation of the method is provided together with its fast implementation by an iterative method based on the Newton's method. Moreover, the lowest CV estimate among those obtained by a number of filter pairs is selected to reduce the CV bias due to nonpropagating signal components. Simulation results indicate that the method described is less sensitive than the classic spectral matching approach to the presence of nonpropagating signals and that the two methods have similar standard deviation of estimation in the presence of additive, white, Gaussian noise. Finally, experimental signals have been collected from the biceps brachii muscle of ten healthy male subjects with an adhesive linear array of eight electrodes. The CV estimates depended on the electrode location with positive bias for the estimates from electrodes close to the innervation or tendon regions, as expected. The proposed method led to significantly lower bias than the spectral matching method in the experimental conditions, confirming the simulation results.

Gender differences in skeletal muscle fatigability are related to contraction type and EMG spectral compression

The purposes of this study were 1) to evaluate gender differences in back extensor endurance capacity during isometric and isotonic muscular contractions, 2) to determine the relation between absolute load and endurance time, and 3) to compare men [n = 10, age 22.4 +/- 0.69 (SE) yr] and women (n = 10, age 21.7 +/- 1.07 yr) in terms of neuromuscular activation patterns and median frequency (MF) shifts in the electromyogram (EMG) power spectrum of the lumbar and hip extensor muscles during fatiguing submaximal isometric trunk extension exercise. Subjects performed isotonic and isometric trunk extension exercise to muscular failure at 50% of maximum voluntary contraction force. Women exhibited a longer endurance time than men during the isometric task (146.0 +/- 10.9 vs. 105.4 +/- 7.9 s), but there was no difference in endurance performance during the isotonic exercise (24.3 +/- 3.4 vs. 24.0 +/- 2.8 repetitions). Absolute load was significantly related to isometric endurance time in the pooled sample (R(2) = 0.34) but not when men and women were analyzed separately (R(2) = 0.05 and 0.04, respectively). EMG data showed no differences in neuromuscular activation patterns; however, gender differences in MF shifts were observed. Women demonstrated a similar fatigability in the biceps femoris and lumbar extensors, whereas in men, the fatigability was more pronounced in the lumbar musculature than in the biceps femoris. Additionally, the MF of the lumbar extensors demonstrated a greater association with endurance time in men than in women (R(2) = 0.45 vs. 0.19). These findings suggest that gender differences in muscle fatigue are influenced by muscle contraction type and frequency shifts in the EMG signal but not by alterations in the synergistic activation patterns.