Biomedical subjects

Roberto Merletti

Publications and source records attributed to Roberto Merletti.

13 recordsLinked to original sources

PubMed2003

Effect of side dominance on myoelectric manifestations of muscle fatigue in the human upper trapezius muscle.

The purpose of this study was to investigate whether differences in the peripheral and control properties of the neuromuscular system due to long-term preferential use, related to side dominance, affect postural muscles, such as the upper trapezius. Therefore, fatigability properties of the upper trapezius muscles of the dominant and non-dominant side were assessed. Surface EMG signals were detected from the upper trapezius muscles of both sides of nine right- and five left-handed subjects with adhesive linear electrode arrays consisting of eight contact points. Static constant force contractions with the arms 90 degrees abducted were performed by the subjects while holding hand loads of 0 kg, 0.5 kg, and 1 kg. Surface EMG spectral and amplitude variables were computed from the recorded signals. EMG spectral variable rate of change (indicating fatigue) showed a statistically significant difference between the two sides, with the dominant side less fatigable than the non-dominant one. The observed differences held for both the right- and left-handed subject group. A possible explanation for the results is that long preferential use of one side with respect to the other leads to changes in muscle fiber membrane and control properties, in agreement with previous results on limb muscles.

Adult↗

PubMed2003

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.

Electromyography↗

PubMed2003

Assessment of low back muscle fatigue by surface EMG signal analysis: methodological aspects.

This paper focuses on methodological issues related to surface electromyographic (EMG) signal detection from the low back muscles. In particular, we analysed (1) the characteristics (in terms of propagating components) of the signals detected from these muscles; (2) the effect of electrode location on the variables extracted from surface EMG; (3) the effect of the inter-electrode distance (IED) on the same variables; (4) the possibility of assessing fatigue during high and very low force level contractions. To address these issues, we detected single differential surface EMG signals by arrays of eight electrodes from six locations on the two sides of the spine, at the levels of the first (L1), the second (L2), and the fifth (L5) lumbar vertebra. In total, 42 surface EMG channels were acquired at the same time during both high and low force, short and long duration contractions. The main results were: (1) signal quality is poor with predominance of non-travelling components; (2) as a consequence of point (1), in the majority of the cases it is not possible to reliably estimate muscle fiber conduction velocity; (3) despite the poor signal quality, it was possible to distinguish the fatigue properties of the investigated muscles and the fatigability at different contraction levels; (4) IED affects the sensitivity of surface EMG variables to electrode location and large IEDs are suggested when spectral and amplitude analysis is performed; (5) the sensitivity of surface EMG variables to changes in electrode location is on average larger than for other muscles with less complex architecture; (6) IED influences amplitude initial values and slopes, and spectral variable initial values; (7) normalized slopes for both amplitude and spectral variables are not affected by IED and, thus, are suggested for fatigue analysis at different postures or during movement, when IED may change in different conditions (in case of separated electrodes); (8) the surface EMG technique at the global level of amplitude and spectral analysis cannot be used to characterize fatigue properties of low back muscles during very low level, long duration contractions since in these cases the non-stable MU pool has a major influence on the EMG variables. These considerations clarify issues only partially investigated in past studies. The limitations indicated above are important and should be carefully discussed when presenting surface EMG results as a means for low back muscle assessment in clinical practice.

Adult↗

PubMed2003

A fast and reliable technique for muscle activity detection from surface EMG signals.

The estimation of on-off timing of human skeletal muscles during movement is an important issue in surface electromyography (EMG) signal processing with relevant clinical applications. In this paper, a novel approach to address this issue is proposed. The method is based on the identification of single motor unit action potentials from the surface EMG signal with the use of the continuous wavelet transform. A manifestation variable is computed as the maximum of the outputs of a bank of matched filters at different scales. A threshold is applied to the manifestation variable to detect EMG activity. A model, based on the physical structure of the muscle, is used to test the proposed technique on synthetic signals with known features. The resultant bias of the onset estimate is lower than 40 ms and the standard deviation lower than 30 ms in case of additive colored Gaussian noise with signal-to-noise ratio as low as 2 dB. Comparison with previously developed methods was performed, and representative applications to experimental signals are presented. The method is designed for a complete real-time implementation and, thus, may be applied in clinical routine activity.

Action Potentials↗

PubMed2003

Selectivity of spatial filters for surface EMG detection from the tibialis anterior muscle.

Many spatial filters have been proposed for surface electromyographic (EMG) signal detection. Although theoretical and modeling predictions on spatial selectivity are available, there are no extensive experimental validations of these techniques based on single motor unit (MU) activity detection. The aim of this study was to compare spatial selectivity of one- and two-dimensional (1-D and 2-D) spatial filters for EMG signal detection. Intramuscular and surface EMG signals were recorded from the tibialis anterior muscle of ten subjects. The simultaneous use of intramuscular wire and surface recordings (with the spike triggered averaging technique) allowed investigation of the activity of single MUs at the skin surface. The surface EMG signals were recorded with a grid of point electrodes (3 x 3 electrodes) and a ring electrode system at 15 locations over the muscle, with the wires detecting signals from the same intramuscular location. For most subjects, it was possible to classify, from the intramuscular recordings, the activity of the same MUs for all the contractions. The surface EMG signals were averaged with the intramuscularly detected MU action potentials as triggers. In this way, eight spatial filters--longitudinal and transversal, single and double differential (LSD, TSD, LDD, TDD), Laplacian (NDD), inverse binomial filter of the second order (IB2), inverse rectangle filter (IR), and differential ring system (C1)--could be compared on the basis of their spatial selectivity. The distance from the source (transversal with respect to the muscle fiber orientation) after which the surface detected potential did not exceed +/- 5% of the maximal peak-to-peak amplitude (detection distance) was statistically smaller for the 2-D systems and TDD than for the other filters. The MU action potential duration was significantly shorter with LDD and with the 2-D systems than with the other filters. The 2-D filters investigated (including C1) showed very similar performance and were, thus, considered equivalent from the point of view of spatial selectivity.

Action Potentials↗

PubMed2002

Repeatability of surface EMG variables in the sternocleidomastoid and anterior scalene muscles.

In this study we examined the repeatability and reliability of the surface electromyographic (sEMG) signal mean frequency (MNF), average rectified value (ARV) and conduction velocity (CV) measured for the sternocleidomastoid (SCM) and the anterior scalene (AS) muscles in nine healthy volunteers during 15-s isometric cervical flexion contractions at 50% of the maximal voluntary contraction level over 3 non-consecutive days. Repeatability and reliability estimates were obtained for the initial values and rates of change of each sEMG variable by using both the Intraclass Correlation Coefficient (ICC) and the normalised standard error of the mean (nSEM). Results from SCM indicated good levels of repeatability for the initial value and slope of ARV (ICC>65%). For the AS, high levels of repeatability were identified for the initial value of MNF (ICC>70%) and the slope of ARV (ICC>75%). Values of nSEM in the range 2.8-7.2% were obtained for the initial values of MNF and CV for both SCM and AS, indicating clinically acceptable measurement precision. The low value obtained for the nSEM of the initial value of MNF for the AS, in combination with the high ICC, indicates that of all of the variables examined, this variable could offer the best normative index to distinguish between subjects with and without neck pain, and represents the sEMG variable of choice for future evaluation purposes.

Adult↗

PubMed2002

Upper trapezius muscle mechanomyographic and electromyographic activity in humans during low force fatiguing and non-fatiguing contractions.

The purposes of this study were firstly to compare and investigate localised fatigue in the upper trapezius muscle in various arm positions as assessed by mechanomyographic (MMG) and surface electromyographic (EMG) signals and secondly to study the effects of different normalisation methods on MMG and EMG signals during non-fatiguing and fatiguing low level isometric contractions. The MMG, EMG and rate of perceived exertion were recorded from 11 subjects in five arm positions (0 degrees abduction and 0 degrees flexion, 45 degrees and 90 degrees flexion, 45 degrees and 90 degrees abduction) with different bilateral arm loads during 3 s for non-fatiguing (0-0.5-1 kg hand-load) and 3 min for fatiguing contractions (1 kg hand-load). The root mean square (RMS), average rectified value (ARV), mean power frequency (MNF), and median power frequency (MDF) of the MMG and EMG signals were computed and normalised with respect to the initial values obtained in the current arm position or in the reference position (0 degrees abduction and 0 degrees flexion) corresponding to the normal postural activity of the trapezius muscle. For fatiguing contractions, differences in magnitude of the increase in the RMS or ARV and decrease in the MNF or MDF were observed for EMG and MMG. The MMG amplitude and spectral changes followed the subjective sensation of fatigue and were not correlated to their EMG counterparts, suggesting that they may reflect different phenomena. For non-fatiguing contractions, normalisation to the current arm position entailed the loss of dynamic amplitude changes suggesting that a single reference contraction in the middle part of the range of movement is enough for proper normalisation of EMG and MMG signals. For fatiguing contractions, normalisation of the EMG and MMG to some extent can lead to a misleading interpretation. Assessment of the upper trapezius muscle by means of MMG may be valuable in ergonomics.

Adult↗

PubMed2002

Assessment of single motor unit conduction velocity during sustained contractions of the tibialis anterior muscle with advanced spike triggered averaging.

This paper describes an improved spike triggered averaging technique for the assessment of control properties and conduction velocity (CV) of single motor units (MUs) of the tibialis anterior muscle during voluntary muscle contractions. The method is based on the detection of multi-channel surface EMG signals (with linear electrode arrays) and intramuscularly recorded single MU action potentials (MUAPs). Intramuscular electrodes were inserted in the muscle taking into account the MU structural properties (innervation zone, tendon locations, length of the fibers), assessed by the linear array surface EMG detection technique. An algorithm for intramuscular EMG signal decomposition is used to identify single MUAP trains. The MUAPs detected by the intramuscular EMG decomposition algorithm were used to trigger and average the multi-channel EMG signals. CV of single averaged surface MUAPs was estimated by the use of advanced signal processing methods based on multi-channel recordings which allow to consistently reduce the variance of CV estimates compared with traditional two channel delay estimators. The number of averaged potentials can thus be limited, resulting in high temporal resolution CV estimates. The developed technique was tested on recordings from the tibialis anterior muscle in 11 volunteers during fatigue. It was shown that the method allows the assessment of single MU CV changes (fatigue) as small as 0.1 m/s with less than 2 s data epochs. The method allows reliable assessment of firing rate and conduction properties of single MUs with applications for the investigation of central and peripheral fatigue mechanisms.

Action Potentials↗

PubMed2002

Standardising surface electromyogram recordings for assessment of activity and fatigue in the human upper trapezius muscle.

The objectives of this work were to determine optimal surface electromyogram (EMG) electrode locations, and inter-electrode distance (IED), when assessing activity and fatigue in the human upper trapezius muscle. Surface EMG signals were recorded from the upper trapezius muscle of 11 healthy male subjects using a linear array of 16 surface electrodes. Five arm positions were investigated (arms at the side of the body, 45 degrees and 90 degrees flexion, 45 degrees and 90 degrees abduction). Fatiguing (1 kg hand load held for 3 min) and non-fatiguing (no load, 0.5 kg and 1 kg hand load held for 3 s) contractions were made. The variabilities of the average rectified value, root mean square, mean and median power spectral frequency and slope over time of these parameters as functions of electrode location and IED (from 5 mm over a range of 35 mm in steps of 5 mm) were quantitatively evaluated. A criterion for selecting the optimal electrode position was applied. This criterion indicated an optimal location measured from the acromion (38% of the distance from the lateral edge of acromion to the spine of the seventh cervical vertebra) which was statistically the same for all the EMG descriptors, arm positions and IED investigated. Finally, it was found that both EMG variables and indexes of muscle fatigue depended on IED which should thus be properly standardised. On the basis of the sensitivity of the EMG descriptors to electrode location and cross-talk reduction, an IED of 20 mm is suggested when a global analysis of activity in the upper trapezius muscle is made using a single pair of electrodes. This study emphasises that a surface EMG analysis of the upper trapezius muscle, following a proper placement of the electrodes and selection of IED, can give reliable indications of muscle activity and fatigue. Data on the myoelectric manifestations of muscle fatigue of the upper trapezius muscle are provided for the optimal electrode location.

Adult↗

PubMed2002

Effect of age on muscle functions investigated with surface electromyography.

The purpose of this study was to investigate changes in the surface electromyographic (EMG) signal as a means of defining age-related central and peripheral mechanisms affecting muscle fatigue. Spectral and temporal variables of the surface EMG signal were studied during voluntary isometric contractions of the dominant biceps brachii muscle in a group of 8 healthy elderly men (age range 67-86 years) and a group of 10 healthy young men (age range 23-34 years). The maximal torque developed and the rate of decrease (slope) of spectral variables and conduction velocity (CV) were statistically higher in the young subjects than in the elderly subjects. Motor unit (MU) CV distribution was also estimated from the surface EMG signal and no statistical difference was observed in its variance between the two groups. These results confirm previous findings from the tibialis anterior muscle. Thus, changes in fiber type distribution and decrease in MU firing rate with aging may be factors determining the decrease in maximal voluntary contraction torque and in myoelectric manifestations of muscle fatigue.

Action Potentials↗

PubMed2002

Surface EMG crosstalk between knee extensor muscles: experimental and model results.

Surface electromyographic (EMG) crosstalk between vastus lateralis, vastus medialis, and rectus femoris muscles was evaluated by selective electrical stimulation of one muscle and recording from the stimulated and another muscle with linear surface arrays of eight electrodes. The ratio between the amplitude of the signals recorded over nonstimulated and stimulated muscles and their correlation coefficient were used as indices to quantify crosstalk. Single-differential and double-differential detection systems were used with interelectrode distances in the range 10-40 mm. The multichannel EMG signals clearly showed that crosstalk is largely due to nonpropagating potentials that correspond in time to the end of the propagation of the action potentials generated by the stimulated muscle. The crosstalk signal increased with increasing interelectrode distance and was statistically higher for single- than for double-differential recordings. The correlation-based indices of crosstalk were poorly correlated with the amplitude-based indices. Moreover, the characteristic spectral frequencies of the signals detected over the nonstimulated muscles were statistically higher than those from the stimulated muscles. A mathematical model of signal generation was used to explain the experimental findings. This study clarifies many controversial findings of past investigations and creates the basis for crosstalk interpretation, simulation, and reduction.

Action Potentials↗

PubMed2002

Influence of anatomical, physical, and detection-system parameters on surface EMG.

Many previous studies were focused on the influence of anatomical, physical, and detection-system parameters on recorded surface EMG signals. Most of them were conducted by simulations. Previous EMG models have been limited by simplifications which did not allow simulation of several aspects of the EMG generation and detection systems. We recently proposed a model for fast and accurate simulation of the surface EMG. It characterizes the volume conductor as a non-homogeneous and anisotropic medium, and allows simulation of EMG signals generated by finite-length fibers without approximation of the current-density source. The influence of thickness of the subcutaneous tissue layers, fiber inclination, fiber depth, electrode size and shape, spatial filter transfer function, interelectrode distance, length of the fibers on surface, single-fiber action-potential amplitude, frequency content, and estimated conduction velocity are investigated in this paper. Implications of the results on electrode positioning procedures, spatial filter design, and EMG signal interpretation are discussed.

Action Potentials↗

PubMed2002

Motor unit recruitment strategies investigated by surface EMG variables.

During isometric contractions of increasing strength, motor units (MUs) are recruited by the central nervous system in an orderly manner starting with the smallest, with muscle fibers that usually show the lowest conduction velocity (CV). Theory predicts that the higher the velocity of propagation of the action potential, the higher the power at high frequencies of the detected surface signal. These considerations suggest that the power spectral density of the surface detected electromyogram (EMG) signal may give indications about the MU recruitment process. The purpose of this paper is to investigate the potential and limitations of spectral analysis of the surface EMG signal as a technique for the investigation of muscle force control. The study is based on a simulation approach and on an experimental investigation of the properties of surface EMG signals detected from the biceps brachii during isometric linearly increasing torque contractions. Both simulation and experimental data indicate that volume conductor properties play an important role as confounding factors that may mask any relation between EMG spectral variables and estimated CV as a size principle parameter during ramp contractions. The correlation between spectral variables and CV is thus significantly lower when the MU pool is not stable than during constant-torque isometric contractions. Our results do not support the establishment of a general relationship between spectral EMG variables and torque or recruitment strategy.

Adult↗