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Biomedical subjects

R Merletti

Publications and source records attributed to R Merletti.

At least 55 records · Page 3Linked to original sources
PubMed1999

Modeling of surface myoelectric signals--Part II: Model-based signal interpretation.

Experimental electromyogram (EMG) data from the human biceps brachii were simulated using the model described in [10] of this work. A multichannel linear electrode array, spanning the length of the biceps, was used to detect monopolar and bipolar signals, from which double differential signals were computed, during either voluntary or electrically elicited isometric contractions. For relatively low-level voluntary contractions (10%-30% of maximum force) individual firings of three to four-different motor units were identified and their waveforms were closely approximated by the model. Motor unit parameters such as depth, size, fiber orientation and length, location of innervation and tendonous zones, propagation velocity, and source width were estimated using the model. Two applications of the model are described. The first analyzes the effects of electrode rotation with respect to the muscle fiber direction and shows the possibility of conduction velocity (CV) over- and under-estimation. The second focuses on the myoelectric manifestations of fatigue during a sustained electrically elicited contraction and the interrelationship between muscle fiber CV, spectral and amplitude variables, and the length of the depolarization zone. It is concluded that a) surface EMG detection using an electrode array, when combined with a model of signal propagation, provides a useful method for understanding the physiological and anatomical determinants of EMG waveform characteristics and b) the model provides a way for the interpretation of fatigue plots.

Action Potentials↗
PubMed1998

Repeatability of electrically evoked EMG signals in the human vastus medialis muscle.

The repeatability of spectral and amplitude parameters and conduction velocity estimates of M-waves was tested on the vastus medialis muscle of 9 normal subjects. Isometric contractions sustained for 30 s were elicited by stimulation of the main muscle motor point and repeated on 5 different days. The initial value and two indicators of myoelectric manifestations of muscle fatigue were computed for each variable. The test-retest Pearson correlation coefficient, the paired Wilcoxon test, and the intraclass correlation coefficient (ICC) showed that parameters of spectral variables are more repeatable than those of amplitude variables and conduction velocity. The ICC ranged from 86.4% for the initial value of median frequency to 21.6% for the initial value of the average rectified value. Fatigue indices showed ICCs in the range of 20-64%. It is concluded that: (a) initial values and fatigue indices based on spectral variables are more repeatable than those based on amplitude variables; (b) the repeatability of conduction velocity estimates is not yet sufficient for clinical application; (c) M-wave shape, rather than amplitude or width, is a characteristic of individual muscles; and (d) electrode location is critical.

Adult↗
PubMed1997

Surface EMG signal processing during isometric contractions.

This paper provides an overview of techniques suitable for the estimation, interpretation and understanding of time variations that affect the surface electromyographic (EMG) signal during sustained voluntary or electrically elicited contractions. These variations concern amplitude variables, spectral variables and muscle fiber conduction velocity, are interdependent and are referred to as the 'fatigue plot'. The fatigue plot provides information suitable for the classification of muscle behavior. In addition, the information obtainable by means of linear electrode arrays is discussed, and applications of mathematical models for the interpretation of array signals are presented. The model approach provides additional ways for the classification of muscle behavior.

Journal Article↗
PubMed1996

Myoelectric and mechanical manifestations of muscle fatigue in voluntary contractions.

Endurance is a clinically relevant muscle parameter. It would be desirable to be able to estimate it without the need for a contraction sustained to exhaustion. The purpose of this work was to investigate the capability of the initial rate of spectral compression of the surface electromyographic (EMG) signal to predict mechanical endurance during sustained voluntary contractions of the human tibialis anterior muscle. Six healthy subjects performed voluntary isometric contractions of the tibialis anterior at 80, 70, 60, and 50% of the maximal voluntary contraction level. The contractions were sustained for 90, 120, 150, and 170 seconds, respectively. These intervals exceed the normal endurance time for this muscle and allow for a decrease of torque output. The slope of the median frequency, computed over the first 30 seconds of the contraction, was used to describe the initial spectral compression of the EMG signal. Significant correlations were found: 1) between contraction level and endurance time (p < 0.05 for each subject) and 2) between median (or mean) frequency slope and endurance time (p < 0.0001 for all subjects pooled together). The regression between median frequency slope and endurance time showed intersubject variations possibly related to the tibialis anterior muscle fiber type content. It is concluded that clinical use of the EMG spectral technique in assessing muscle fatigue may enable the clinician to estimate the endurance time without having the subject sustain a contraction until the point of contractile failure. This could be an advantage for some patient populations, such as the severely disabled, arthritic, or frail elderly, that might not be able to tolerate long duration contractions.

Adult↗
PubMed1995

Advances in processing of surface myoelectric signals: Part 1.

During sustained voluntary or electrically elicited muscle contractions the surface myoelectric signal is nonstationary and it undergoes progressive changes reflecting the modifications of the motor unit action potentials and their propagation velocity. In particular, during sustained electrical stimulation, the evoked signals show progressive amplitude, time scaling and shape modification. The quantitative evaluation of these changes is important for non-invasive muscle characterisation and may be performed in either the time or frequency domain using parametric and nonparametric spectral analysis as well as alternative methodologies. The paper introduces the detection techniques, reviews and compares the methods of spectral estimation based on FFT and autoregressive models, and discusses their applications and limitations in extracting information from the surface myoelectric signal with particular regard to myoelectric manifestations of localised muscle fatigue during sustained contractions.

Electric Stimulation↗
PubMed1995

Advances in processing of surface myoelectric signals: Part 2.

The paper focuses on the analysis of myoelectric evoked potentials and their progressive scaling, as well as morphological changes using orthogonal basis functions with essentially finite time support; and the compression of the information content using principal component analysis. The application of the Hermite-Rodriguez and the associated Hermite functions is discussed as a means to provide compact information about the shape of the M-wave or of the power spectral density function of either voluntary or electrically elicited myoelectric signals; a means to estimate scaling factors; and a means to describe and classify nonstationarities. The principal component analysis shows the possibility of a compression ratio of at least 10:1 in the storage of M-wave sequences. The paper also describes three methods for the estimation of delay between similar signals, and therefore for estimation of conduction velocity. They are based on normalised integrals. Fourier transform matching and matching in the time domain. In particular conditions they provide different results for the same pair of signals. The concept of delay and the performance of these methods are reviewed and discussed. The paper is not exhaustive. It has the main objective of making the reader aware of the wealth of methods available for nonstationary myoelectric signal analysis and conduction velocity estimation, and of the need to use them with knowledge of their respective advantages, disadvantages, peculiarities and limitations.

Electric Stimulation↗
PubMed1995

Estimation of shape characteristics of surface muscle signal spectra from time domain data.

Myoelectric manifestations of muscle fatigue have been described by monitoring the first-order moment (mean frequency) of the power spectral density function during voluntary or electrically elicited sustained contractions. Higher order central moments provide additional information about the width, skewness, and kurtosis of the spectrum and its shape changes, thereby providing a description of slow nonstationarities more accurate than that allowed by the mean frequency alone. In 1986, B. Saltzberg introduced a method of representing the moments of the power spectral density function of band limited signals, without computing the Fourier transform, as weighted sums of samples of the autocorrelation function. If we allow for oversampling of the signal (and therefore of its autocorrelation function), more efficient weighted sums can be found which give Saltzberg's formula as a limiting case. The faster rate of decay of the weights implies a faster convergence of the estimates and the need to compute fewer samples of the autocorrelation function. The algorithm is particularly suitable for: 1) analysis of evoked potentials (M-waves), because it does not need zero padding to increase resolution and operates on any number of samples, and 2) on-line implementation by dedicated microprocessors performing simultaneous spectral moment analysis on a number of parallel channels.

Electrophysiology↗
PubMed1994

Analogue and digital instruments for non-invasive estimation of muscle fibre conduction velocity.

Analogue and digital instruments for non-invasive on-line measurement of muscle fibre conduction velocity (CV) have been designed, built and compared using test signals and real myo-electric signals. Their inputs consist of two single-differential or double-differential myo-electric signals, obtained using a three- or four-contact surface electrode system. The analogue device computes CV by tracking the lag of the zero-crossing of the cross-correlation between the first signal and the derivative of the second. The digital device computes the peak of the cross-correlation function between the two signals by sampling them at 50 KHz for 20 ms (or longer, up to 320 ms), computing CV in about 30 ms (or longer, up to about 670 ms) and resuming sampling. Both devices allow estimation of CV during either voluntary or electrically elicited contractions and include a stimulation stage and a signal conditioner with artefact suppression features. Both devices provide analogue and numerical outputs and allow interfacing with analogue and digital instrumentation. They can be used in clinical or in research environments for easy and quick identification of appropriate electrode locations and/or for monitoring CV during sustained voluntary or electrically elicited contractions. The digital version is more versatile and requires no adjustments; it provides an estimate based on intermittent reading of the signals and is more sensitive to noise and momentary CV fluctuations.

Electric Conductivity↗
PubMed1994

Hermite expansions of compact support waveforms: applications to myoelectric signals.

Nonstationary signals with finite time support are frequently encountered in electrophysiology and other fields of biomedical research. It is often desirable to have a compact description of their shape and of their time evolution. For this purpose, Fourier analysis is not necessarily the best tool. The Hermite-Rodriguez and Associated Hermite basis functions are applied in this work. Both are based on the product of Hermite polynomials and Gaussian functions. Their general properties relevant to biomedical signal processing are reviewed. Preliminary applications are described concerning the analysis and description of: a) test signals such as a square pulse and a single cycle of a sinewave, b) electrically evoked myoelectric signals, and c) power spectra of either voluntary or evoked signals. It is shown that expansions with only five to ten terms provide an excellent description of the computer simulated and real signals. It is shown that these two families of Hermite functions are well suited for the analysis of nonstationary biological evoked potentials with compact time support. An application to the estimation of scaling factors of electrically evoked myoelectric signals is described. The Hermite functions show advantages with respect to the more traditional spectral analysis, especially in the case of signal truncation due to stimulation with interpulse intervals smaller than the duration of the evoked response. Finally, the Hermite approach is found to be suitable for classification of spectral shapes and compression of spectral information of either voluntary or evoked signals. The approach is very promising for neuromuscular diagnosis and assessment because of its capability for information compression and waveform classification.

Electrophysiology↗
PubMed1994

Electrically evoked myoelectric signals in back muscles: effect of side dominance.

This work had two goals, to study the effect of hand dominance on myoelectric signal variables and fatigue indexes in back muscles and to assess the repeatability of the estimates of such variables. Myoelectric manifestations of muscle fatigue were studied in the right and left longissimus dorsi muscles of five right-handed and five left-handed normal male subjects. Myoelectric signals (M waves), evoked by stimulation applied to a muscle's motor point, were detected with surface electrodes. Each test consisted of eliciting a tetanic contraction of 30 s duration with supramaximal stimulation at 25 Hz and was repeated five times on 5 different days for each subject. The mean and median frequencies of the resulting power spectra of the M waves were plotted vs. time, and fatigue indexes were obtained from the time course of these variables. Only two-thirds of the elicited contractions provided signals of sufficiently good quality to obtain reliable estimates of the mean and median frequencies. Criteria for acceptability are described. Analysis of variance and paired comparisons showed a statistically significant effect of side dominance on fatigue indexes in the right-handed subjects but not in the left-handed subjects. Normalized initial slope and other fatigue indexes based on spectral variables demonstrated myoelectric manifestations of fatigue that were greater on the dominant side. We surmise that the differences are related to the fiber type modifications associated with the unilateral usage of the upper limbs and the consequent activation of the nondominant side of the back.

Adult↗
PubMed1993

Effect of ankle joint position on electrically evoked surface myoelectric signals of the tibialis anterior muscle.

The relationship between surface myoelectric signal variables and ankle joint angle is studied in the tibialis anterior muscle of ten normal subjects during electrical stimulation of the main muscle motor point in isometric conditions. Rectangular current pulses of 0.1 ms width and 20Hz repetition rate were applied for 10s with a monopolar technique using a stimulation/detection system providing stimulation artifact suppression. Electrically evoked responses (M-waves) were detected with a four-bar electrode placed on the skin below the lowest motor point near the tendon end of the muscle. Average rectified value, root mean square value, mean and median spectral frequency, conduction velocity, and latency of the surface signal were computed for ankle angles of 15 degrees dorsal flexion (-15 degrees), 0 degrees, 15 degrees, 30 degrees and 45 degrees of plantar flexion. All variables were considerably affected by the joint angle. Spectral variables and latency showed a statistically significant decrement as angle increased from -15 degrees to 45 degrees whereas conduction velocity and amplitude variables had a less regular behavior. The evoked response showed a latency decrement and a change of shape (mostly a widening of the second phase), as ankle joint angle increased from -15 degrees to 45 degrees. In most subjects conduction velocity appeared to be over estimated at -15 degrees and 45 degrees, a fact attributed to the small distance between the electrodes and either the tendon or the innervation zone at these two angles.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Electrically evoked myoelectric signals.

Electrical stimulation of the nervous system is attracting increasing attention because of the possibilities it offers for physiological investigations, clinical diagnosis, muscle function assessment, noninvasive muscle characterization, and functional control of paralyzed extremities. Parameters of the myoelectric signal evoked by surface stimulation of a muscle motor point or by stimulation of a nerve trunk by means of implanted electrodes provide information about muscle performance and properties if the stimulation artifact is properly removed or suppressed. Comparison of these parameters with those obtained during voluntary contractions provides additional insight into muscle physiology. The relationships between myoelectric signal amplitude parameters, spectral parameters, and conduction velocity are discussed with special reference to muscle fatigue. This review focuses on a few methodological aspects concerning electrical stimulation of the peripheral nervous system, detection, and processing of the electrically evoked myoelectric signals in skeletal muscles. The state of the art of the following issues is discussed: (1) properties of voluntary and electrically evoked myoelectric signals; (2) techniques for evoking and detecting myoelectric signals; (3) techniques for suppression of stimulation artifacts; (4) effect of stimulation waveforms and electrode properties; (5) signal processing techniques for electrically evoked myoelectric signals; (6) physiological significance of myoelectric signal variables; (7) order of recruitment of motor units during electrical stimulation; (8) myoelectric manifestations of fatigue in electrically stimulated muscles; (9) assessment of crosstalk by electrical stimulation; and (10) applications in sport, rehabilitation, and geriatric medicine.

Animals↗
PubMed1992

Age related changes in surface myoelectric signals.

The initial values and the time course of muscle fiber conduction velocity and of surface myoelectric signal spectral variables were studied during voluntary or electrically elicited contractions of the tibialis anterior muscle of 15 healthy elderly human subjects. Age ranged from 65 to 84 years. Isometric voluntary contractions were performed at 20% MVC (Maximal Voluntary Contraction) and 80% MVC for 20 s. Tetanic electrical stimulation was then applied to the main muscle motor point for 20 s with surface electrodes. Two stimulation rates (20 Hz and 40 Hz) and two stimulation amplitudes were used to induce different degrees of fatigue. One stimulation amplitude was supramaximal, the second was adjusted to induce a response (M-wave) about 30% of the maximal. Results were compared with those reported in previous work on healthy adults (age range 18 to 43 years). The main findings of this work are: (a) when voluntary contraction level is increased from 20% MVC to 80% MVC conduction velocity and spectral variables increase; this increase is significantly smaller in elderly subjects, (b) during sustained contractions at 80% MVC the decrease of conduction velocity and spectral variables is significantly smaller in elderly subjects, (c) during contractions induced by supramaximal stimulation at 40 Hz the decrease of conduction velocity and spectral variables is not significantly different in the two age groups. It is concluded that points a and b reflect the age related decrease of number and size of fast twitch fibers indicated by histological data. Point c is discussed and possible explanations are suggested.

Adult↗
PubMed1990

Clinical engineering in Italy: the activity of the National Research Council.

The results of a five-year effort to rationalise the process of acquisition and management of medical instrumentation in Italy are reported and discussed. The research programme, sponsored by the National Research Council of Italy, was completed in 1988 and focused on (1) acquisition of technology by hospitals; (2) assessment of performance evaluation and preventive maintenance procedures for biomedical equipment; (3) cost analysis of high-technology health services; (4) analysis of clinical engineering activities in Italy. The present situation and guidelines for the future are outlined. In Italy there are no official positions for clinical engineers in public hospitals; however seven health-delivery institutions have established some activity in the field and a total of 15 engineers are involved in it. Despite the obvious advantage shown by the experience of these pilot institutions, the field has serious difficulties in developing due to the policy of the Ministry of Health and the consequent lack of positions. To reach the level of service provided in other European or North American countries, Italy would need at least 500 clinical engineers and 2500 biomedical equipment technicians. However, the lack of professional opportunities does not encourage training activities in the field.

Biomedical Engineering↗
PubMed1990

Inference of motor unit recruitment order in voluntary and electrically elicited contractions.

The relationship between surface myoelectric signal parameters and the level of voluntary or electrically elicited contractions was studied in 32 experiments on the tibialis anterior muscle of 22 healthy human subjects. Contractions were performed at 20 and 80% of the maximum voluntary contraction torque. Two levels of stimulation current were used, yielding, respectively, a maximum M wave and an M wave approximately 30% of the maximum. A four-bar electrode probe was used to detect single- and double-differential signals from which mean and median frequency of the power spectrum and average muscle fiber conduction velocity were estimated. Measurements obtained from voluntary contractions showed a positive correlation between contraction levels and both conduction velocity and spectral parameters. Conduction velocity increased by 21.2 +/- 10.9% when voluntary contraction level increased from 20 to 80% of the maximal value. Spectral parameters increased by similar amounts. Tetanic electrical stimulation was applied to a muscle motor point for 20 s via surface electrodes. Rectangular current pulses with 0.1-ms width and frequencies of 20, 25, 30, 35, and 40 Hz were used. Four types of behavior were observed with increasing stimulation level: 1) the two spectral parameters and conduction velocity both increased with stimulation in 15 experiments, 2) the two spectral parameters decreased and conduction velocity increased in 8 experiments, 3) the two spectral parameters and conduction velocity both decreased in 6 experiments, and 4) the two spectral parameters increased and conduction velocity decreased in 3 experiments. Conduction velocity increased with increasing stimulation current in 72% of the experiments, indicating a recruitment order similar to that of voluntary contractions, although it decreased in the other 28% of the cases, indicating a reverse order of recruitment. Contrary to what is observed in direct stimulation of nerves, motor units are not in general recruited in reverse order of size during electrical stimulation of a muscle motor point. This discrepancy may be the result of geometric factors or a lack of correlation between axonal branch diameter and the diameter of the parent motoneuron axon. Changes of conduction velocity and spectral parameters in opposite directions may be the result of the combined effect of the motor unit recruitment order and of the different tissue filtering function associated with the geometric location of the recruited motor units within the muscle.

Adult↗
PubMed1990

Myoelectric manifestations of fatigue in voluntary and electrically elicited contractions.

The time course of muscle fiber conduction velocity and surface myoelectric signal spectral (mean and median frequency of the power spectrum) and amplitude (average rectified and root-mean-square value) parameters was studied in 20 experiments on the tibialis anterior muscle of 10 healthy human subjects during sustained isometric voluntary or electrically elicited contractions. Voluntary contractions at 20% maximal voluntary contraction (MVC) and at 80% MVC with duration of 20 s were performed at the beginning of each experiment. Tetanic electrical stimulation was then applied to the main muscle motor point for 20 s with surface electrodes at five stimulation frequencies (20, 25, 30, 35, and 40 Hz). All subjects showed myoelectric manifestations of muscle fatigue consisting of negative trends of spectral variables and conduction velocity and positive trends of amplitude variables. The main findings of this work are 1) myoelectric signal variables obtained from electrically elicited contractions show fluctuations smaller than those observed in voluntary contractions, 2) spectral variables are more sensitive to fatigue than conduction velocity and the average rectified value is more sensitive to fatigue than the root-mean-square value, 3) conduction velocity is not the only physiological factor affecting spectral variables, and 4) contractions elicited at supramaximal stimulation and frequencies greater than 30 Hz demonstrate myoelectric manifestations of muscle fatigue greater than those observed at 80% MVC sustained for the same time.

Adolescent↗
PubMed1988

Surface myoelectric signal cross-talk among muscles of the leg.

Surface myoelectric signals were detected from the skin surface above the tibialis anterior muscle, the peroneus brevis muscle, the soleus muscle and the tibial bone during selective maximal electrical stimulation of the tibialis anterior muscle in 12 normal subjects. The double differential technique developed by Broman et al. (1985) was used to determine if the detected signal was due to volume conduction from the tibialis anterior fibers. The peak-to-peak (PP), average rectified (ARV) and root mean square (RMS) amplitudes of the M waves were computed for each detection location. The values detected on the tibial bone, on the peroneus and on the soleus muscles were normalized with respect to those detected on the tibialis anterior and ranged from 4.8% to 33.0% (PP), 4.7% to 36.0% (ARV), and 7.7% to 37.4% (RMS) for the tibial bone area; from 4.0% to 20.0% (PP), 3.5% to 10.0% (ARV), and 3.0% to 10.0% (RMS) for the peroneus brevis muscle area; and from 3.0% to 8.0% (PP), 3.4% to 9.1% (ARV), and 2.0% to 9.8% (RMS) for the soleus muscle area. Neither peak-to-peak values, average rectified values nor root mean square values appeared to be correlated with leg size. It is concluded that a surface myoelectric signal detected on the skin above a leg muscle and having a peak-to-peak amplitude of up to 16.6% of a signal detected above a neighboring muscle may be due to cross-talk rather than to activation of the muscle below the electrode.

Adult↗