Publications
787 results found
Farina D, Negro F, 2007, Estimation of muscle fiber conduction velocity with a spectral multidip approach., IEEE Trans Biomed Eng, Vol: 54, Pages: 1583-1589, ISSN: 0018-9294
We propose a novel method for estimation of muscle fiber conduction velocity from surface electromyographic (EMG) signals. The method is based on the regression analysis between spatial and temporal frequencies of multiple dips introduced in the EMG power spectrum through the application of a set of spatial filters. This approach leads to a closed analytical expression of conduction velocity as a function of the auto- and cross-spectra of monopolar signals detected along the direction of muscle fibers. The performance of the algorithm was compared with respect to that of the classic single dip approach on simulated and experimental EMG signals. The standard deviation of conduction velocity estimates from simulated single motor unit action potentials was reduced from 1.51 m/s [10 dB signal-to-noise ratio (SNR)] and 1.06 m/s (20 dB SNR) with the single dip approach to 0.51 m/s (10 dB) and 0.23 m/s (20 dB) with the proposed method using 65 dips. When 200 active motor units were simulated in an interference EMG signal, standard deviation of conduction velocity decreased from 0.95 m/s (10 dB SNR) and 0.60 m/s (20 dB SNR) with a single dip to 0.21 m/s (10 dB) and 0.11 m/s (20 dB) with 65 dips. In experimental signals detected from the abductor pollicis brevis muscle, standard deviation of estimation decreased from (mean +/- SD over 5 subjects) 1.25 +/- 0.62 m/s with one dip to 0.10 +/- 0.03 m/s with 100 dips. The proposed method does not imply limitation in resolution of the estimated conduction velocity and does not require any iterative procedure for the estimate since it is based on a closed analytical formulation.
Serrao M, Arendt-Nielsen L, Ge H-Y, et al., 2007, Experimental muscle pain decreases the frequency threshold of electrically elicited muscle cramps., Exp Brain Res, Vol: 182, Pages: 301-308, ISSN: 0014-4819
This study in humans tested the hypothesis that nociceptive muscle afferent input facilitates the occurrence of muscle cramps. In 13 healthy adults, muscle cramps were experimentally induced in the foot by stimulating the tibialis posterior nerve at the ankle with 2-s bursts of stimuli separated by 30 s, with stimulation frequency increasing by 2-Hz increments from 10 Hz until the cramp appeared. The minimum stimulation frequency that induced the cramp was defined "cramp frequency threshold". In 2 days, elicitation of the cramp was performed in the two-feet with and without (baseline condition) injection of hypertonic (painful condition) or isotonic (control condition) saline into the deep midportion of the flexor hallucis brevis muscle, from where surface EMG signals were recorded. The cramp frequency threshold was lower for the painful condition with respect to its baseline (mean +/- SE, hypertonic saline: 25.7 +/- 2.1 Hz, corresponding baseline: 31.2 +/- 2.8 Hz; P < 0.01) while there was no difference between the threshold with isotonic injection with respect to baseline. EMG average rectified value and power spectral frequency were higher during the cramp than immediately before the stimulation that elicited the cramp (pre-cramp: 13.9 +/- 1.6 muV and 75.4 +/- 3.8 Hz, respectively; post-cramp: 19.9 +/- 3.2 muV and 101.6 +/- 6.0 Hz; P < 0.05). The results suggest that nociceptive muscle afferent activity induced by injection of hypertonic saline facilitates the generation of electrically elicited muscle cramps.
Carotti ESG, De Martin JC, Merletti R, et al., 2007, ACELP-based compression of multi-channel surface EMG signals, ISSN: 1520-6149
In this paper we extend a lossy compression technique for surface EMG signals, which is based on the Algebraic Code Excited Linear Prediction (ACELP) paradigm, to compress multi-channel surface EMG recordings by exploiting the correlation between the Line Spectral Frequencies (LSF). Experimental results show that the LSFs of the inner signals in a multi-channel recording can be efficiently represented with 13 bit/frame, versus the 38 bit/frame needed by independent ACELP coding of each signal, thus saving 66% of the bandwidth needed to transmit these coefficients while maintaining comparable performance in terms of the SNR, Average Rectified Value and Root Mean Square of the waveform, and mean and median frequencies of the power spectrum. © 2007 IEEE.
Maitrot A, Lucas MF, Doncarli C, et al., 2007, Erratum: Signal-dependent wavelets for electromyogram classification (Medical and Biological Engineering and Computing (2005) vol. 43 (4) (487-492) 10.1007/BF02344730), Medical and Biological Engineering and Computing, Vol: 45, ISSN: 0140-0118
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Campanini I, Merlo A, Degola P, et al., 2007, Effect of electrode location on EMG signal envelope in leg muscles during gait., J Electromyogr Kinesiol, Vol: 17, Pages: 515-526, ISSN: 1050-6411
The aim of the study was to assess the variability of EMG signal envelope with electrode location during gait. Surface EMG signals were recorded from 10 healthy subjects from the tibialis anterior (TA), peroneus longus (PL), gastrocnemius medialis (GM), gastrocnemius lateralis (GL), and soleus (SO) muscles. From TA, PL, GL and GM, signals were acquired using a two-dimensional grid of 4 x 3 electrodes (10 x 15 mm in size, as used in most gait laboratories) with 20-mm interelectrode distance in both directions. A similar grid of 3 x 3 electrodes was used for SO. EMG envelope was characterized by its peak value, area after normalization by the peak value, and time instant corresponding to the maximum. The maximum relative change in peak value with electrode location, expressed as a percentage of the peak value in the central location, was (mean+/-SD) 31+/-18% for TA, 29+/-13% for PL, 25+/-15% for GL, 14+/-8% for GM, and 26+/-14% for SO. The maximum relative change in area was 29+/-13% for TA, 73+/-40% for PL, 31+/-23% for GL, 35+/-20% for GM, 20+/-13% for SO, and in the position of maximum, computed as distance from the maximum position in the central channel, it was 5+/-10% of the gait cycle for TA, 26+/-16% for PL, 3+/-2% for GL, 3+/-1% for GM, 3+/-3% for SO. A crosstalk index, defined on the basis of the expected intervals of muscle activation for healthy subjects, indicated that estimated crosstalk was present between TA and PL, in an amount which depended on electrode location. It was concluded that the estimate of muscle activation intensity during gait from surface EMG is variable with location of the electrodes while timing of muscle activity is more robust to electrode displacement and can be reliably extracted in those cases in which crosstalk is limited. These results are valid for healthy subjects, where the level of muscular activity during gait is much lower than maximum.
Farina D, Ferguson RA, Macaluso A, et al., 2007, Correlation of average muscle fiber conduction velocity measured during cycling exercise with myosin heavy chain composition, lactate threshold, and VO2max., J Electromyogr Kinesiol, Vol: 17, Pages: 393-400, ISSN: 1050-6411
The aim of the study was to investigate the correlation between myosin heavy chain (MHC) composition, lactate threshold (LT), maximal oxygen uptake VO2max, and average muscle fiber conduction velocity (MFCV) measured from surface electromyographic (EMG) signals during cycling exercise. Ten healthy male subjects participated in the study. MHC isoforms were identified from a sample of the vastus lateralis muscle and characterized as type I, IIA, and IIX. At least three days after a measure of LT and VO2max, the subjects performed a 2-min cycling exercise at 90 revolutions per minute and power output corresponding to LT, during which surface EMG signals were recorded from the vastus lateralis muscle with an adhesive electrode array. MFCV and instantaneous mean power spectral frequency of the surface EMG were estimated at the maximal instantaneous knee angular speed. Output power corresponding to LT and VO2max were correlated with percentage of MHC I (R2=0.77; and 0.42, respectively; P<0.05). MFCV was positively correlated with percentage of MHC I, power corresponding to LT and to VO2max (R2=0.84; 0.74; 0.53, respectively; P<0.05). Instantaneous mean power spectral frequency was not correlated with any of these variables or with MFCV, thus questioning the use of surface EMG spectral analysis for indirect estimation of MFCV in dynamic contractions.
Kamavuako EN, Hennings K, Farina D, 2007, Velocity recovery function of the compound muscle action potential assessed with doublet and triplet stimulation., Muscle Nerve, Vol: 36, Pages: 190-196, ISSN: 0148-639X
Normative values of muscle fiber conduction velocity depend on the conditions in which conduction velocity is measured due to the velocity recovery function (VRF) of muscle fibers. In this study the VRF of the compound muscle action potential (CMAP) was assessed following doublet and triplet stimulation in order to investigate the effect of repetitive muscle activation on muscle fiber conduction velocity. The VRF from doublet and triplet activation showed a peak of 4.6%-15.0% and 6.4%-25.9%, respectively, which is not significantly different. The VRF of the CMAP with doublet stimulation had a plateau between 25-75 ms, similar to that reported for single muscle fibers, and changed as a consequence of previous activation. The VRFs with doublet and triplet stimulation were different for interstimulus intervals in the range of 12-250 ms, where the triplet resulted in a plateau of supernormal conduction velocity. The VRF of the triplet could be explained by linear summation of the effects from doublet stimulations only for small distances between the two conditioning stimuli. These results provide new information on the adaptation of membrane properties of muscle fibers to repetitive activation. Changes in CMAP properties due to repeated activation may influence the accuracy of techniques based on CMAP recordings, such as collision methods.
Hedayatpour N, Arendt-Nielsen L, Farina D, 2007, Motor unit conduction velocity during sustained contraction of the vastus medialis muscle., Exp Brain Res, Vol: 180, Pages: 509-516, ISSN: 0014-4819
The aim of the study was to analyze motor unit conduction velocity at varying force of the vastus medialis muscle during sustained contraction. Surface (8-electrode array) and intramuscular (two wire electrodes) EMG signals were recorded from the distal part of the dominant vastus medialis muscle of ten healthy male subjects. The subjects sat on a chair with the knee 90 degrees flexed and performed seven 180-s long contractions at forces in the range 2.5-30% of the maximal voluntary contraction force. For each force level, the discharge patterns of the newly recruited motor units with respect to the previous force level were identified from the intramuscular recordings and used as trigger for averaging the surface EMG signals. Motor unit conduction velocity was estimated from the averaged surface EMG. Average discharge rate at which motor units were analyzed was the same for each force level (mean +/- SD, 8.3 +/- 0.8 pulses per second). Motor unit conduction velocity at the beginning of the contraction and its rate of change over time increased with force (P < 0.05). Conduction velocity at the beginning of the contraction estimated from the interference surface EMG (4.44 +/- 0.66 m/s) and from single motor units (4.75 +/- 0.56 m/s) were positively correlated (R (2) = 0.46; P < 0.0001) but significantly different (P < 0.05). The results indicate that single motor unit conduction velocity and its rate of change during sustained contraction, assessed at a fixed discharge rate, depend on force level.
Falla D, Farina D, Graven-Nielsen T, 2007, Spatial dependency of trapezius muscle activity during repetitive shoulder flexion., J Electromyogr Kinesiol, Vol: 17, Pages: 299-306, ISSN: 1050-6411
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.
Farina D, do Nascimento OF, Lucas M-F, et al., 2007, Optimization of wavelets for classification of movement-related cortical potentials generated by variation of force-related parameters., J Neurosci Methods, Vol: 162, Pages: 357-363, ISSN: 0165-0270
The paper presents a novel pattern recognition approach for the classification of single-trial movement-related cortical potentials (MRCPs) generated by variations of force-related parameters during voluntary tasks. The feature space was built from the coefficients of a discrete dyadic wavelet transformation. Mother wavelet parameterization allowed the tuning of basis functions to project the signals. The mother wavelet was optimized to minimize the classification error estimated from the training set. Classification was performed with a support vector machine (SVM) approach with optimization of the width of a Gaussian kernel and of the regularization parameter. The efficacy of the optimization procedures was representatively shown on electroencephalographic recordings from two subjects who performed unilateral isometric plantar flexions at two target torques and two rates of torque development. The proposed classification method was tested on four pairs of classes corresponding to the change in only one of the two parameters of the task. Misclassification rate (test set) in the classification of 1-s EEG activity immediately before the onset of the tasks was reduced from 50.8+/-2.9% with worst wavelet and nearest representative classifier, to 40.2+/-7.3% with optimal wavelet and nearest representative classifier, and to 15.8+/-3.4% with optimal wavelet and SVM with optimization of the kernel and regularization parameter. The proposed pattern recognition method is promising for classification of MRCPs modulated by variations of force-related parameters.
Dewhurst S, Graven-Nielsen T, De Vito G, et al., 2007, Muscle temperature has a different effect on force fluctuations in young and older women., Clin Neurophysiol, Vol: 118, Pages: 762-769, ISSN: 1388-2457
OBJECTIVE: To investigate the effect of muscle temperature on force fluctuations during isometric contractions in young and older females. METHODS: Fifteen young and 11 older subjects performed 3x30-s long submaximal isometric ankle dorsi-flexions (5%, 10%, and 15% of the maximal force). Tibialis anterior muscle temperature was monitored with an intramuscular probe and manipulated to obtain a cold, control, and warm condition. The coefficient of variation (CofV) and the relative power in the frequency bands 0-3Hz (low), 4-6Hz (middle), and 8-12Hz (high) of the force signal were computed to characterise steadiness. Intramuscular EMG signals were recorded from the tibialis anterior muscle to assess motor unit discharge pattern. RESULTS: CofV was higher in the older than in the young subjects (P<0.001) in all conditions. In the older group only, CofV increased with cooling with respect to control temperature (P<0.001), whereas in the young group only, relative power of force fluctuations at high frequency decreased with cooling. Motor unit discharge rate and inter-pulse interval variability were not different between groups and across temperatures. CONCLUSIONS: The findings indicate a different effect of temperature on the ability to maintain constant force in young and older subjects. SIGNIFICANCE: These results highlight the risk of further impairment to the motor control of older individuals with varying temperature.
Falla D, Farina D, Graven-Nielsen T, 2007, Experimental muscle pain results in reorganization of coordination among trapezius muscle subdivisions during repetitive shoulder flexion., Exp Brain Res, Vol: 178, Pages: 385-393, ISSN: 0014-4819
The aim of the study was to examine the effect of experimental unilateral upper trapezius muscle pain on the relative activation of trapezius muscle subdivisions bilaterally during repetitive movement of the upper limb. Surface EMG signals were detected from nine healthy subjects from the upper, middle and lower divisions of trapezius during a repetitive bilateral shoulder flexion task. Measurements were performed before and after injection of 0.5 ml hypertonic (pain condition) and isotonic (control) saline into the upper division of the right trapezius muscle in two experimental sessions. On the painful side, upper trapezius showed decreased EMG amplitude (average rectified value, ARV) and lower trapezius increased ARV throughout the entire task following the injection of hypertonic saline (40.0 +/- 22.2 vs. 26.0 +/- 17.4 microV, and 12.5 +/- 7.6 vs. 25.6 +/- 14.8 microV, respectively, at the beginning of the contraction). On the side contralateral to pain, greater estimates of ARV were identified for the upper division of trapezius as the task progressed (37.4 +/- 20.2 vs. 52.7 +/- 28.4 microV, at the end of the contraction). Muscle fiber conduction velocity did not change with pain in all three divisions of the right trapezius muscle. The results suggest that local elicitation of nociceptive afferents in the upper division of the trapezius induces reorganization in the coordinated activity of the three subdivisions of the trapezius in repetitive dynamic tasks.
Mesin L, Damiano L, Farina D, 2007, Estimation of average muscle fiber conduction velocity from simulated surface EMG in pinnate muscles., J Neurosci Methods, Vol: 160, Pages: 327-334, ISSN: 0165-0270
The aim of this simulation study was to assess the bias in estimating muscle fiber conduction velocity (CV) from surface electromyographic (EMG) signals in muscles with one and two pinnation angles. The volume conductor was a layered medium simulating anisotropic muscle tissue and isotropic homogeneous subcutaneous tissue. The muscle tissue was homogeneous for one pinnation angle and inhomogeneous for bipinnate muscles (two fiber directions). Interference EMG signals were obtained by simulating recruitment thresholds and discharge patterns of a set of 100 and 200 motor units for the pinnate and bipinnate muscle, respectively (15 degrees pinnation angel in both cases). Without subcutaneous layer and muscle fibers with CV 4m/s, average CV estimates from the pinnate (bipinnate) muscle were 4.81+/-0.18 m/s (4.80+/-0.18 m/s) for bipolar, 4.71+/-0.19 m/s (4.71+/-0.12 m/s) for double differential, and 4.78+/-0.16 m/s (4.79+/-0.15m/s) for Laplacian recordings. When subcutaneous layer was added (thickness 1mm) in the same conditions, estimated CV values were 4.93+/-0.25 m/s (5.16+/-0.41 m/s), 4.70+/-0.21 m/s (4.83+/-0.33 m/s), and 4.89+/-0.21 m/s (4.99+/-0.39 m/s), for the three recording systems, respectively. The main factor biasing CV estimates was the propagation of action potentials in the two directions which influenced the recording due to the scatter of the projection of end-plate and tendon locations along the fiber direction, as a consequence of pinnation. The same problem arises in muscles with the line of innervation zone locations not perpendicular to fiber direction. These results indicate an important limitation in reliability of CV estimates from the interference EMG when the innervation zone and tendon locations are not distributed perpendicular to fiber direction.
Viljoen S, Hanekom T, Farina D, 2007, Effect of characteristics of dynamic muscle contraction on crosstalk in surface electromyography recordings, SAIEE Africa Research Journal, Vol: 98, Pages: 18-28, ISSN: 0038-2221
An investigation into the ability of different spatial filters to reduce the amount of crosstalk in a surface electromyography measurement was conducted. A simulation model was implemented to compare the performance of four spatial filters under dynamic muscle contractions. Two parameters of a dynamic muscle contraction, namely muscle shortening and varying contraction force, were evaluated separately. The normal double differential filter resulted in the best crosstalk rejection for varying contraction force simulations, while the double differential filter performed best when incorporating muscle shortening. It is furthermore suggested that crosstalk is influenced more by muscle shortening than by changes in the contraction force.
Carotti E, De Martin JC, Merletti R, et al., 2007, Compression of surface EMG signals with algebraic code excited linear prediction., Med Eng Phys, Vol: 29, Pages: 253-258, ISSN: 1350-4533
Despite the interest in long timescale recordings of surface electromyographic (EMG) signals, only a few studies have focused on EMG compression. In this paper we investigate a lossy coding technique for surface EMG signals that is based on the algebraic code excited linear prediction (ACELP) paradigm, widely used for speech signal coding. The algorithm was adapted to the EMG characteristics and tested on both simulated and experimental signals. The coding parameters selected led to a compression ratio of 87.3%. For simulated signals, the mean square error in signal reconstruction and the percentage error in average rectified value after compression were 11.2% and 4.90%, respectively. For experimental signals, they were 6.74% and 3.11%. The mean power spectral frequency and third-order power spectral moment were estimated with relative errors smaller than 1.23% and 8.50% for simulated signals, and 3.74% and 5.95% for experimental signals. It was concluded that the proposed coding scheme could be effectively used for high rate and low distortion compression of surface EMG signals. Moreover, the method is characterized by moderate complexity (approximately 20 million instructions/s) and an algorithmic delay smaller than 160 samples (approximately 160ms).
Keenan KG, Farina D, Meyer FG, et al., 2007, Sensitivity of the cross-correlation between simulated surface EMGs for two muscles to detect motor unit synchronization., J Appl Physiol (1985), Vol: 102, Pages: 1193-1201, ISSN: 8750-7587
The purpose of the study was to evaluate the use of cross-correlation analysis between simulated surface electromyograms (EMGs) of two muscles to quantify motor unit synchronization. The volume conductor simulated a cylindrical limb with two muscles and bone, fat, and skin tissues. Models of two motor neuron pools were used to simulate 120 s of surface EMG that were detected over both muscles. Short-term synchrony was established using a phenomenological model that aligned the discharge times of selected motor units within and across muscles to simulate physiological levels of motor unit synchrony. The correlation between pairs of surface EMGs was estimated as the maximum of the normalized cross-correlation function. After imposing four levels of motor unit synchrony across muscles, five parameters were varied concurrently in the two muscles to examine their influence on the correlation between the surface EMGs: 1) excitation level (5, 10, 15, and 50% of maximum); 2) muscle size (350 and 500 motor units); 3) fat thickness (1 and 4 mm); 4) skin conductivity (0.1 and 1 S/m); and 5) mean motor unit conduction velocity (2.5 and 4 m/s). Despite a constant and high level of motor unit synchronization among pairs of motor units across the two muscles, the cross-correlation index ranged from 0.08 to 0.56, with variation in the five parameters. For example, cross-correlation of EMGs from pairs of hand muscles, each having thin layers of subcutaneous fat and mean motor unit conduction velocities of 4 m/s, may be relatively insensitive to the level of synchronization across muscles. In contrast, cross-correlation of EMGs from pairs of leg muscles, with larger fat thickness, may exhibit a different sensitivity. These results indicate that cross correlation of the surface EMGs from two muscles provides a limited measure of the level of synchronization between motor units in the two muscles.
Falla D, Farina D, Dahl MK, et al., 2007, Muscle pain induces task-dependent changes in cervical agonist/antagonist activity., J Appl Physiol (1985), Vol: 102, Pages: 601-609, ISSN: 8750-7587
This study examined the effect of experimental neck muscle pain on the EMG-force relationship of cervical agonist and antagonist muscles. Surface EMG signals were detected from the sternomastoid, splenius capitis, and upper trapezius muscles bilaterally from 14 healthy subjects during cervical flexion and extension contractions of linearly increasing force from 0 to 60% of the maximum voluntary contraction (MVC). Measurements were performed before and after injection of 0.5 ml hypertonic and isotonic saline into either the sternomastoid or splenius capitis in two experimental sessions. EMG average rectified value (ARV) of the sternomastoid, splenius capitis, and upper trapezius muscles and the muscle fiber conduction velocity (CV) of the sternomastoid muscle were estimated at 5% MVC force increments. During cervical flexion with injection of hypertonic saline in sternomastoid, ARV of sternomastoid was lower on the side of pain in the force range 25-60% MVC (P < 0.05) and was associated with a bilateral reduction of splenius capitis and upper trapezius ARV (P < 0.01). During cervical extension, injection of hypertonic saline in splenius capitis resulted in lower estimates of splenius capitis ARV on the painful side from 45 to 60% MVC (P < 0.05), which was associated with a bilateral increase in upper trapezius ARV estimates from 50 to 60% MVC (P < 0.001). However, no significant change was identified for estimates of sternomastoid ARV. Experimentally induced neck muscle pain resulted in task-dependent changes in cervical agonist/antagonist activity without modifications in muscle fiber CV.
Hennings K, Kamavuako EN, Farina D, 2007, The recruitment order of electrically activated motor neurons investigated with a novel collision technique., Clin Neurophysiol, Vol: 118, Pages: 283-291, ISSN: 1388-2457
OBJECTIVE: The development of a novel collision technique for assessment of the activation order of electrically activated nerve fibers, which is an important question in functional electrical therapy or for interpretation of results of motor unit number estimates. METHODS: Compound muscle action potentials were recorded with the belly-tendon configuration from the abductor digiti minimi. A novel modified Hopf's collision technique was applied on ten healthy male subjects to determine the distributions of conduction velocities (DCV) of all ulnar nerve fibers and of the fibers activated by electrical stimuli eliciting 20%, 50%, and 80% of the maximal muscle response. RESULTS: The maximum nerve conduction velocity was (means+/-SE) 64.1+/-0.85m/s. The median conduction velocity of estimated DCV was 58.9+/-0.97m/s (stimulus at 20%), 58.0+/-0.98m/s (50%), 57.2+/-0.91m/s (80%), and 56.5+/-0.84m/s (whole nerve) (all different between each other, P<0.001). CONCLUSIONS: The proposed collision technique allows the assessment of nerve conduction velocity distributions at maximal and sub-maximal stimulation levels and provided evidence for an inverse activation order of nerve fibers with electrical stimulation. SIGNIFICANCE: The excessive fatigue seen with nerve electrical stimulation can be explained by a preferential activation of large diameter nerve fibers. The motor units first activated with electrical stimulation are likely not representative of the motor unit pool in the muscle, which poses limitations in the reliability of some of the proposed methods for motor unit counting.
Cescon C, Madeleine P, Graven-Nielsen T, et al., 2007, Two-dimensional spatial distribution of surface mechanomyographical response to single motor unit activity., J Neurosci Methods, Vol: 159, Pages: 19-25, ISSN: 0165-0270
In order to better understand the mechanisms of generation of mechanomyography (MMG) signals, the two-dimensional distribution of surface MMG produced by the activity of single motor units was analyzed by a novel two-dimensional recording method. Motor unit action potentials were identified from intramuscular electromyographic (EMG) signals and used to trigger the averaging of MMG signals detected over the tibialis anterior muscle of 11 volunteers with a grid of 5x3 accelerometers (20-mm inter-accelerometer distance). The intramuscular wires were inserted between the first and second accelerometer in the middle column of the grid, proximal to the innervation zone. The subjects performed three contractions with visual feedback of the intramuscular EMG signals. In each contraction, a new motor unit was recruited at the minimum stable discharge rate (mean+/-S.D., N = 11 subjects, 7.3+/-2.3 pulse/s), resulting in torque of 2.4+/-2.8% of the maximal voluntary contraction (MVC), 4.6+/-2.7% MVC, and 6.3+/-3.1% MVC (all different, P < 0.01). For 23 out of 33 detected motor units, it was possible to extract the motor unit surface acceleration map (MUAM). A negative MUAM peak (-2.7+/-2.2 mm/s2) was detected laterally and a positive MUAM peak (4.1+/-2.4 mm/s2) medially (P < 0.001). The time-to-peak was shorter in the medial part of the muscle (2.9+/-0.4 ms) than in the other locations (3.4+/-0.5 ms, P < 0.001). The double integrated signals (muscle displacement) indicated negative deflection in the lateral part and inflation close to the tibia bone. The maps of acceleration showed spatial dependency in single motor unit MMG activities. The technique provides a new insight into motor unit contractile properties.
Kamavuako EN, Farina D, 2007, Estimation of muscle fiber conduction velocity of doublet discharges, Biomedical Signal Processing and Control, Vol: 2, Pages: 331-338, ISSN: 1746-8094
A doublet is defined as two consecutive discharges of a motor unit occurring at short time interval between each other (e.g., <20 ms). In this paper, we propose a method for the estimation of muscle fiber conduction velocity (CV) from two partly overlapping action potentials generated by the same motor units. The method is based on the minimization of the mean square error between time-filtered versions of two surface EMG signals recorded along the direction of muscle fibers. The minimization is performed over the filter parameters that define the two propagation delays. The method was tested on simulated and experimental signals. Simulation results showed that the method is only in some cases superior to the simpler peak approach, due to limitations in the ideal model used for the algorithm development. However, application to experimental signals that mimic doublet motor unit discharges showed a substantial improvement in estimation quality of the new method with respect to the peak method. © 2007 Elsevier Ltd. All rights reserved.
Holobar A, Gazzoni M, Farina D, et al., 2007, Non-invasive estimation of the degree of motor unit synchronization in the biceps brachii muscle, Pages: 109-113, ISSN: 1680-0737
This paper addresses estimation of motor unit (MU) synchronization by means of surface electromyogram decomposition. Firstly, the so called convolution kernel compensation method for identifying the discharge patterns of individual motor units is briefly described. The method builds on independent component analysis framework and is, hence, highly robust to superimpositions of MU action potentials (AP). In this study, the method was tested with synthetic signals in presence of MU synchronization, when the MU discharges are not strictly independent anymore. The level of MU synchronization was measured by the cross-interval histograms of reconstructed MU discharge patterns. Two synchronization indices were computed: the average number of synchronized MU pairs per contraction and the average percentage of synchronized AP per synchronized MU pair. Results revealed that both indices vary substantially with the properties of reconstructed MUs, but can nevertheless be used as reliable estimators of MU synchronization. The method was then applied to experimental electromyographic signals, acquired during low force contractions of the dominant biceps brachii muscle. Up to 14 concurrently active MUs were identified. MU synchronization was observed in approx. 50% of MU pairs. It is thus concluded, that MU synchronization can be assessed by decomposing the surface electromyogram.
Madeleine P, Tuker K, Arendt-Nielsen L, et al., 2007, Heterogeneous mechanomyographic absolute activation of paraspinal muscles assessed by a two-dimensional array during short and sustained contractions., J Biomech, Vol: 40, Pages: 2663-2671, ISSN: 0021-9290
Spatial dependency of paraspinal muscle activity was assessed using a new two-dimensional MMG recording system. MMG signals were detected over the left and right paraspinal muscles of 10 volunteers using a grid of 12 accelerometers. During two separate trials subjects maintained a 20 degrees flexed position and held loads that ranged from 0 to 15 kg (in 2.5 kg increments) for 20s; and 7.5 kg for 6 min. Maps of absolute and normalised (with respect to initial values) average rectified value, mean power frequency, variance and skewness of the power spectral density were obtained from the two-dimensional MMG recordings. For both the short duration and sustained contractions, the MMG absolute average rectified value, mean power frequency, variance and skewness depended on accelerometer location (P<0.05), while, with the exception of the skewness (P<0.05), normalised values did not. These results demonstrate both inhomogeneous MMG absolute activity and homogeneous MMG normalised activity in paraspinal muscles for short duration and sustained contractions. Moreover, the effect of accelerometer location on spectral variables confirmed the limited validity of general relationships between MMG spectral changes and motor unit recruitment strategies. This study underlines the importance of using multiple recording sites when assessing back muscle activity.
Carotti ESG, De Martin JC, Merletti R, et al., 2006, Compression of surface EMG signals with algebraic code excited linear prediction, ISSN: 1520-6149
In this paper we investigate a lossy coding technique for surface EMG signals which is based on the Algebraic Code Excited Linear Prediction (ACELP) paradigm, widely used for speech signal coding. The algorithm was adapted to the EMG characteristics and tested on both simulated and experimental signals. A fixed compression ratio of 87.3% was chosen. On simulated signals, the mean square error in signal reconstruction and the percentage error in average rectified value after compression were 10.43 % and 5.52 %, respectively. On experimental signals, they were 6.74% and 3.11%. The mean power spectral frequency and third order power spectral moment were estimated with relative error smaller than 1.36% and 1.70%, respectively, for simulated signals, and 3.74% and 2.28% for experimental signals. It was concluded that the proposed coding scheme can be effectively used for high rate, low distortion and low-delay compression of surface EMG signals © 2006 IEEE.
Nielsen M, Kamavuako N, Andersen MM, et al., 2006, Biomedical signal compression with optimized wavelets, ISSN: 1520-6149
In this work, we propose a novel scheme of signal compression based on signal-dependent wavelets. To adapt the mother wavelet to the signal for the purpose of compression, it is necessary to define a family of wavelets that depend on a set of parameters and a quality criterion for wavelet selection (i.e., wavelet parameter optimization). We propose the use of orthogonal wavelets parameterized by their scaling filter, with optimization criterion based on the minimization of signal distortion rate given the desired compression rate. For coding the wavelet coefficients we adopted the embedded zerotree wavelet coding algorithm. Results on electromyographic signals show that optimization significantly improves performance. © 2006 IEEE.
Madeleine P, Cescon C, Farina D, 2006, Spatial and force dependency of mechanomyographic signal features., J Neurosci Methods, Vol: 158, Pages: 89-99, ISSN: 0165-0270
The aim was to investigate with a novel technique the spatial inhomogeneity in surface mechanomyographic (MMG) response to muscle contraction at varying force levels. MMG signals were detected over the dominant tibialis anterior muscle of 10 volunteers using a 5 x 3 grid of accelerometers. The subjects performed 3 s long isometric contractions at forces ranging from 0% to 100% of the maximal force (10% increment) in a randomised order. From the two-dimensional MMG recordings, maps of absolute and normalized temporal and spectral MMG descriptors were obtained. The centroid and entropy of these maps were computed to describe the spatial centre of activity and degree of homogeneity, respectively. MMG absolute amplitude did not depend on location over the muscle while normalized amplitude did and the centroid shifted with increasing force. Amplitude increased with force and its entropy decreased. Absolute and normalized spectral variables depended on location over the muscle and their centroid shifted with increasing force. In addition, the dependency of absolute and normalized spectral variables on force was affected by location. These results highlight limitations when using single-channel MMG features for the assessment of motor unit control strategies, due to a substantial effect of position on the relation between force and MMG characteristics.
Madeleine P, Leclerc F, Arendt-Nielsen L, et al., 2006, Experimental muscle pain changes the spatial distribution of upper trapezius muscle activity during sustained contraction., Clin Neurophysiol, Vol: 117, Pages: 2436-2445, ISSN: 1388-2457
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.
Mesin L, Tizzani F, Farina D, 2006, Estimation of motor unit conduction velocity from surface EMG recordings by signal-based selection of the spatial filters., IEEE Trans Biomed Eng, Vol: 53, Pages: 1963-1971, ISSN: 0018-9294
Muscle fiber conduction velocity (CV) can be estimated by the application of a pair of spatial filters to surface electromagnetic (EMG) signals and compensation of the spatial filter transfer function with equivalent temporal filters. This method integrates the selection of the spatial filters for signal detection to the estimation of CV. Using this approach, in this paper, we propose a novel technique for signal-based selection of the spatial filter pair that minimizes the effect of nonpropagating signal components (end-of-fiber effects) on CV estimates (optimal filters). The technique is applicable to signals with one propagating and one nonpropagating component, such as single motor unit action potentials. It is shown that the determination of the optimal filters also allows the identification of the propagating and nonpropagating signal components. The new method was applied to simulated and experimental EMG signals. Simulated signals were generated by a cylindrical, layered volume conductor model. Experimental signals were recorded from the abductor pollicis brevis with a linear array of 16 electrodes. In the simulations, the proposed approach provided CV estimates with lower bias due to nonpropagating signal components than previously proposed methods based on the entire signal waveform. In the experimental signals, the technique separated propagating and nonpropagating signal components with an average reconstruction error of 2.9 +/- 0.9% of the signal energy. The technique may find application in single motor unit studies for decreasing the variability and bias of CV estimates due to the presence and different weights of the nonpropagating components.
Castroflorio T, Icardi K, Becchino B, et al., 2006, Reproducibility of surface EMG variables in isometric sub-maximal contractions of jaw elevator muscles., J Electromyogr Kinesiol, Vol: 16, Pages: 498-505, ISSN: 1050-6411
The aims of this study were: (1) to develop and assess reproducibility of a new method for measuring masticatory force in the intercuspal position; (2) to test the reproducibility of surface EMG signal amplitude and spectral variables in constant force contractions of jaw elevator muscles and its dependency on inter-electrode distance. The study was performed on the masseter and temporalis anterior muscles of both sides of nine healthy volunteers. An intraoral compressive-force sensor was used to measure maximal voluntary contraction forces in the intercuspal position and to provide a visual feedback on sub-maximal forces to the subject. Three experimental sessions were performed in three days. In each session, three isometric contractions at 80% of the maximal force were sustained by the subjects for 30s. The intra-class correlation coefficient (ICC) of the maximal force measure was 71.9%. ICC of average rectified value and mean power spectral frequency of the EMG signal increased with inter-electrode distance, with values larger than 70% with 30 mm inter-electrode distance. It was concluded that surface EMG variables measured in isometric contractions of the jaw elevator muscles with the proposed force recording system show good reproducibility for clinical applications when a 30 mm inter-electrode distance is considered.
Falla D, Graven-Nielsen T, Farina D, 2006, 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., J Neurosci Methods, Vol: 156, Pages: 236-241, ISSN: 0165-0270
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.
Pozzo M, Alkner B, Norrbrand L, et al., 2006, Muscle-fiber conduction velocity during concentric and eccentric actions on a flywheel exercise device., Muscle Nerve, Vol: 34, Pages: 169-177, ISSN: 0148-639X
A gravity-independent flywheel exercise device (FWED) has been proven effective as a countermeasure to loss of strength and muscle atrophy induced by simulated microgravity. This study assessed muscle-fiber conduction velocity (CV) and surface EMG instantaneous mean power spectral frequency (iMNF) during brief bouts of fatiguing concentric (CON) and eccentric (ECC) exercise on a FWED in order to identify electromyographic (EMG) variables that can be used to provide objective indications of muscle status when exercising with a FWED. Multichannel surface EMG signals were recorded from vastus lateralis and medialis muscles of nine men during: (1) isometric, 60-s action at 50% of maximum voluntary action (MVC); (2) two isometric, linearly increasing force ramps (0-100% MVC); and (3) dynamic CON/ECC coupled actions on the FWED. Muscle-fiber CV and iMNF were computed over time during the three tasks. During ramps, CV, but not iMNF, increased with force (P < 0.001). Conduction velocity and iMNF decreased with the same normalized rate of change in constant-force actions. During CON/ECC actions, the normalized rate of change over time was larger for CV than iMNF (P < 0.05). These results suggest that, during fatiguing, dynamic, variable-force tasks, changes in CV cannot be indirectly inferred by EMG spectral analysis. This underlines the importance of measuring both CV and spectral variables for muscle assessment in dynamic tasks.
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