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Investigation of the neuromuscular system during involuntary muscle contractions - Methodological issues and clinical applications
No full textIn the last ten years the techniques for non invasive investigation of the neuromuscular system made consider¬able progress. Recently they have been associated to multichannel surface electromyography (EMG) detected with linear or two-dimensional elec¬trode arrays. This approach consists in measuring electrical muscle activ¬ity, generated by the most superficial motor units (MUs), with multiple (more than two) closely spaced elec¬trodes overlying either a restricted area of the skin or the entire surface of the muscle. Although the behavior of the MUs is classically studied with intramuscular EMG, these recently developed techniques allow the analy¬sis of EMG recorded in multiple loca¬tions over the skin surface. The analy¬sis of MUs from the surface EMG is useful when the insertion of needles is not desirable or not possi¬ble. Moreover, multichannel surface EMG allows the measure of MU properties which are difficult to assess with inva¬sive technology (e.g., muscle fiber conduction velocity, location of inner¬vation zones, global and single motor unit manifestations of fatigue) and increases the number of detectable motor units with respect to selective intramuscular recordings. The application of surface EMG to the study of electrically elicited con¬tractions allows a study of the com¬pound muscle action potentials (CMAPs) or massed action potentials (M-waves) which represent the syn¬chronous summation of the MUAPs that add asynchronously during a vol¬untary contraction. M-wave analysis represents an important investigative tool in several different areas of neuro¬physiological research. Evoking the maximum M-wave by supramaxi¬mal stimulation of either a peripheral nerve or the muscle motor point is the electrical equivalent of the recruitment of all motor units within the selected motoneuron pool and allows to inves¬tigate the properties of these motor units. Moreover, progressive incre¬ments of stimulation intensity elicit progressively greater M-waves. The difference between M-waves elicited using two different stimulation levels allows the analysis of the MUs whose activation thresholds are in between the two stimulation levels. This proc¬ess allows the characterization of small groups of MUs and therefore the order of recruitment and MU charac¬terization during electrical stimula¬tion. The use of incremental M-wave technique together with bi-dimen-sional detection systems is an inter¬esting application to study the spatial localization of MU territories within pinnate muscles. Electrical stimulation (combined with surface EMG) provides also in¬teresting experimental paradigms to study muscular properties during fa¬tiguing contractions, because it gives to the experimenter the control of MU firing frequency and recruitment and, if selectively applied, eliminates the problem of cross-talk from nearby muscles. In fact, selective electrical stimulation of a nerve branch or of the motor point of a muscle allows, in first approximation, to "disconnect" the investigated muscle from the central nervous system and to activate only one (or a portion of one) muscle at a time at a controlled frequency and with a motor unit pool that is more likely to be stable. During fatiguing muscle contractions, M-waves (de¬tected with linear or two-dimensional electrode arrays) change shape be¬cause of the presence of motor units with different conduction velocities and fatigue profiles. With respect to the analysis of myoelectric fatigue during voluntary contractions, electri¬cally elicited contractions provide es¬timates of the changes in surface EMG signal variables that have lower esti¬mation errors. The issue of fatigue during electrically elicited contrac¬tions is of paramount importance in functional electrical stimulation tech¬niques for external control of para¬lyzed extremities. Furthermore, the application of bursts of electrical pulses to a periph¬eral nerve (or to the muscle motor point) may induce, in certain muscles, involuntary muscle activity that per¬sists for some time after the interrup¬tion of the stimulation. Such activity is referred to as "fascicula¬tion" and "cramp", which are defined, respectively, as "the random, sponta¬neous twitching of a group of muscle fibers belonging to a single motor unit" and "an involuntary, painful muscle contraction associated with electrical activity". Even if cramps are a common complaint encountered by both neurologists and primary care physicians, their pathophysiology still remains poorly understood, mainly due to the unpredictable occurrence and the relative inaccessibility to experi¬mental investigation. However, sur¬face EMG signals detected during cramps induced by means of electrical stimulation of the muscle, in con¬trolled conditions of fibre length and orientation, provide interesting infor¬mation about cramp pathophysiology. This PhD thesis contributes to this research field through the analysis and interpretation of multichannel surface EMG signals during electrically elic¬ited contractions and during different types of involuntary muscle contrac¬tions, such as fasciculations and cramps.The main objectives of the thesis were to: (1) establish the influence of selected parameters of the electrical stimu¬lation (current amplitude, pulse shape, injected charge) on the de¬gree of MU activation during transcutaneous stimulation; (2) assess the differences in the myoelectric fatigue profiles during electrical stimulation among differ¬ent muscles and subject popula¬tions; (3) evaluate whether the electrical stimulation of the muscle motor point can trigger a cramp in differ¬ent foot and leg muscles, as it was already observed in foot muscles following the electrical stimulation of the nerve trunk; (4) study surface EMG signals during cramp contractions; (5) examine whether different bursts of electrical stimulation, applied to the muscle motor point, trigger dif¬ferent cramps of the muscle under study; (6) provide novel insights into cramp pathophysiology from detection and analysis of surface and intra¬muscular EMG signals during electrically elicited cramps. Part 1. NMES: Methodological issues This section focuses on methodological aspects concerning neuromuscular electrical stimulation (NMES). Chapters I and II include a brief introduction to the stimulation techniques, to the spinal involvement in electrically elicited muscle contractions, and to the acquisition of EMG signals during electrically elicited contractions (study of M-waves and incremental M-waves). Further, Chapter II reports a study on the incremental M waves detected from the medial gastrocnemius muscle. It has been preliminarly demonstrated that the use of incremental M-waves acquired with high density detection systems allows the spatial localization of MU territories in a pinnate muscle. This could have an important application in the study of the compartmentalization of muscles involved in posture, such as the gastrocnemii. Chapter III describes an experimental study on the effect of the stimulation parameters (stimulation waveform, amplitude, and injected charge) on the degree of MU activation in electrically-elicited contractions of the biceps brachii. The main outcome of this work was that the degree of MU activation was a function of the injected charge and not of the stimulation waveform. Moreover, MUs tended to be recruited in order of increasing conduction velocity with increasing charge of transcutaneous stimulation (regardless of the pulse waveform). This study concerned the biceps brachii, a muscle which is not usually electrically stimulated other than for M-wave studies. Other muscles, particularly the thigh muscles, are either of greater interest in sport sciences and rehabilitation medicine. Whether MUs are recruited in an orderly or random manner during motor point electrical stimulation of these muscles was one of the topics of the study reported in Chapter IV. The objectives of the study described in Chapter IV were to investigate, in three muscles of the thigh, the MU recruitment order in stimulated contractions and the electrically-induced manifestations of muscle fatigue. This study, performed on two groups of healthy subjects, was also aimed to test the possibility of detecting differences among muscles and subject groups in the response to the transcutaneous stimulation. Results of this study confirmed the observations of the previous study (Chapter III) on the recruitment order: MUs tended to be activated from low to high conduction velocities with increasing current of the transcutaneous stimulation. Moreover, EMG variables during electrically-evoked fatigue showed different properties in different muscles and subject groups. Overall, the results of the above-described methodological studies contributed to the design of clinical protocols for neuromuscular function assessment (sarcolemmal excitability and fatigability) in clinical relevant conditions. These studies are described in the second part of this thesis. Part 2. NMES: clinical applications Chapter V describes two experimental protocols aimed to investigate the neuromuscular effects of both glucocorticoid administration in healthy subjects and chronic endogenous hypercortisolism in pathological subjects (affected by Cushing's disease). Results from the protocol performed on healthy subjects showed that muscle fiber conduction velocity and myoelectric manifestations of fatigue significantly decreased after the short-term administration of a synthetic glucocorticoid (dexamethasone), in doses well within the range used clinically. Results from the protocol conducted on pathological subjects showed that muscle fiber conduction slowing is a sensitive marker of steroid myopathy which is suitable to be used in combination with standard electrodiagnostic tests for identifying early signs of myopathy. Part 3. NMES induced muscle cramps The third section is focused on the study of involuntary muscle contractions. Chapter VI reports the description of a method, developed within this project, for inducing cramps in an intrinsic foot muscle, the abductor hallucis, by means of electrical stimulation of the main muscle motor point. This method was reliable and suitable to be used concomitantly with multichannel surface EMG. The test of this method in different foot and leg muscles is the topic of the second part of Chapter VI. We found that the neurostimulation method was effective in eliciting cramps in four muscles of the lower limb bilaterally, and that differences exist between the cramp elicitability profiles of the different foot and leg muscles. The analysis of surface EMG signals during cramp contractions as well as the study of the effect of different stimulation frequencies on the elicited muscle cramp is reported in Chapter VII. The aim of this study was to provide new insight into cramp pathophysiology (generation and self-sustaining mechanisms) through the analysis of EMG signals. Results showed that the choice of the frequency of the stimulation burst affects the temporal and spectral properties of EMG in electrically-elicited cramps. However, these findings did not provide insight into the exact physiological mechanism(s) underlying cramp generation since they could fit both the hypothesis of peripheral origin and that of spinal origin of cramps. Hence, the differences between cramps elicited with different stimulation frequencies were further investigated by means of a joint analysis of surface and intramuscular EMG signals (Chapter VIII). The analysis of the discharge behaviour of single MUs during cramp (discharge rate and variability, and coherence between discharge rate oscillations of different MUs) indicated that cramp development and self-sustaining mechanisms involve spinal pathways, although the origin may be peripheral. Future studies will be aimed to confirm recent preliminary evidences supporting the hypothesis of cramps being initialized at the peripheral level but subsequently supported by spinal reflex loops. HDsEMG and decomposition of cramp signals into the constituent trains of motor unit action potential represent a promising approach to investigate the behavior of individual MUs during cramp contraction
Integration of HD-sEMG and Ultrasounds for the Assessment of Muscle Function
Full text linkElectromyograms (EMGs) and ultrasound (US) images provide complementary information on muscle function. The integration of these two techniques may provide key insights into the electromechanical properties of skeletal muscles as well as into physiological adaptations due to ageing, pathologies, injuries or rehabilitation. From a technological point of view, the simultaneous acquisition of US and surface EMG from the same muscle region is challenging as the two detection systems may interfere with each other. This paper will describe these methodological issues and how they have been addressed to ensure high quality detection of both surface EMGs and US images. Finally, an overview on current and possible, future applications will be provided
Elicitability of muscle cramps in different leg and foot muscles
No full textTo explore the efficacy of muscle motor point stimulation in eliciting muscle cramps, 11 subjects underwent eight sessions of electrical stimulation of the following muscles bilaterally: abductor hallucis flexor hallucis brevis, and both heads of the gastrocnemius muscles. Bursts of 150 square wave stimuli (duration: 152 micros; current intensity: 30% supramaximal) were applied. The stimulation frequency was increased from 4 pulses per second (pps) at increments of 2 pps until a cramp was induced. The number of cramps that could be elicited was smaller in flexor hallucis brevis than in abductor hallucis (16 vs. 22 out of 22 trials each; P < 0.05) and in the lateral gastrocnemius than in the medial gastrocnemius (5 vs. 20 out of 22 trials each; P < 0.0001). We show that leg and foot muscles have different cramp susceptibility, and the intermuscle variability in the elicitability profile for electrically induced cramps supports the use of the proposed method for cramp research
Device for the joint acquisition of at least one surface electromyographic signal and one echographic image of a same portion of a muscle of a living being, in particular for non-invasive applications, and related manufacturing method
No full textThe present invention relates to a device (1) for the joint acquisition of at least one surface electromyographic signal and one echographic image of a same portion of a muscle of a living being, in particular for non-invasive applications. The invention is characterized in that said device (1) comprises a first layer (10) consisting of cross-linked polymers, said first layer (10): - comprising at least one cavity (11) adapted to house at least one first portion (12A) of an electrode (12) for acquiring said at least one surface electromyographic signal; - being transparent to ultrasounds, so as to allow obtaining an acoustic impedance adaptation such that said joint acquisition of said at least one surface electromyographic signal and of said at least one echographic image of a same muscle portion can be obtained
A new method for the reduction of power line interference from multichannel bioelectric recordings
No full textFiltered virtual reference: A new method for the reduction of power line interference with minimal distortion of monopolar surface EMG
No full textGoal: This study tests and validates a new method to remove power line interference from monopolar EMGs detected by multichannel systems: the filtered virtual reference (FVR). FVR is an adaptation of the virtual reference (VR) method, which consists in referencing signals detected by each electrode in a grid to their spatial average. Signals may however be distorted with the VR approach, in particular when the skin region where the detection system is positioned does not cover the entire muscle. Methods: Simulated and experimental EMGs were used to compare the performance of FVR and VR in terms of interference reduction and distortion of monopolar signals referred to a remote reference. Results: Simulated data revealed the monopolar EMG signals processed with FVR were significantly less distorted than those filtered by VR. These results were similarly observed for experimental signals. Moreover, FVR method outperformed VR in removing power line interference when it was distributed unevenly across the signals of the grid. Conclusion: Key results demonstrated that FVR improves the VR method as it reduces interference while preserving the information content of monopolar signals. Significance: Although the actual distribution of motor unit action potential is represented in monopolar EMGs, collecting high quality monopolar signals is challenging. This study presents a possible solution to this issue; FVR provides undistorted monopolar signals with negligible interference and is insensitive to muscle architecture. It is therefore relevant for EMG applications benefiting from a clean monopolar detection (e.g., decomposition, control of prosthetic devices, motor unit number estimation
Surface EMG and muscle fatigue: multi-channel approaches to the study of myoelectric manifestations of muscle fatigue
Full text linkIn a broad view, fatigue is used to indicate a degree of weariness. On a muscular level, fatigue posits the reduced capacity of muscle fibres to produce force, even in the presence of motor neuron excitation via either spinal mechanisms or electric pulses applied externally. Prior to decreased force, when sustaining physically demanding tasks, alterations in the muscle electrical properties take place. These alterations, termed myoelectric manifestation of fatigue, can be assessed non-invasively with a pair of surface electrodes positioned appropriately on the target muscle; traditional approach. A relatively more recent approach consists of the use of multiple electrodes. This multi-channel approach provides access to a set of physiologically relevant variables on the global muscle level or on the level of single motor units, opening new fronts for the study of muscle fatigue; it allows for: (i) a more precise quantification of the propagation velocity, a physiological variable of marked interest to the study of fatigue; (ii) the assessment of regional, myoelectric manifestations of fatigue; (iii) the analysis of single motor units, with the possibility to obtain information about motor unit control and fibre membrane changes. This review provides a methodological account on the multi-channel approach for the study of myoelectric manifestation of fatigue and on the experimental conditions to which it applies, as well as examples of their current applications
A Modular, Smart, and Wearable System for High Density sEMG Detection
Full text linkThe use of linear or bi-dimensional electrode arrays for surface EMG detection (HD-sEMG) is gaining attention as it increases the amount and reliability of information extracted from surface EMG. However, the complexity of the setup and the encumbrance of HD-sEMG hardware currently limits its use in dynamic conditions. The aim of this work was to develop a miniaturized, wireless, and modular HD-sEMG acquisition system for applications requiring high portability and robustness to movement artifacts
Myoelectric fatigue profiles during electrically-elicited contractions of vastus lateralis, vastus medialis obliquus, and vastus medialis longus muscles
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