[Show abstract][Hide abstract] ABSTRACT: Purpose:
The purpose of this study was to investigate how much the distance between stimulation electrodes affects the knee extension torque in tetanic, electrically elicited contractions.
Current pulses of progressively larger amplitude, from 0 mA to maximally tolerated intensities, were delivered at 20 pps to the vastus medialis, rectus femoris and vastus lateralis muscles of ten, healthy male subjects. Four inter-electrode distances were tested: 32.5 % (L1), 45.0 % (L2), 57.5 % (L3) and 70 % (L4) of the distance between the patella apex and the anterior superior iliac spine. The maximal knee extension torque and the current leading to the maximal torque were measured and compared between electrode configurations.
The maximal current tolerated by each participant ranged from 60 to 100 mA and did not depend on the inter-electrode distance. The maximal knee extension torque elicited did not differ between L3 and L4 (P = 0.15) but, for both conditions, knee torque was significantly greater than for L1 and L2 (P < 0.024). On average, the extension torque elicited for L3 and L4 was two to three times greater than that obtained for L1 and L2. The current leading to maximal torque was not as sensitive to inter-electrode distance. Except for L1 current intensity did not change with electrode configuration (P > 0.16).
Key results presented here revealed that for a given stimulation intensity, knee extension torque increased dramatically with the distance between electrodes. The distance between electrodes seems therefore to critically affect knee torque, with potential implication for optimising exercise protocols based on electrical stimulation.
Full-text · Article · Nov 2015 · Arbeitsphysiologie
[Show abstract][Hide abstract] ABSTRACT: Goal: 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).
No preview · Article · Oct 2015 · IEEE Transactions on Biomedical Engineering
[Show abstract][Hide abstract] ABSTRACT: Cortex-muscle coherence (CMC) reflects coupling between magnetoencephalography (MEG) and surface electromyography (sEMG), being strongest during isometric contraction, but absent, for unknown reasons, in some individuals. We used a novel non-magnetic high-density sEMG (HD-sEMG) electrode grid (36 mm x 12 mm; 60 electrodes separated by 3 mm) to study effects of sEMG recording site, electrode derivation and rectification on the strength of CMC. Monopolar sEMG from right thenar and 306-channel whole-scalp MEG were recorded from 14 subjects during 4-min isometric thumb abduction. CMC was computed for 60 monopolar, 55 bipolar, and 32 Laplacian HD-sEMG derivations, and two derivations were computed to mimic 'macroscopic' monopolar and bipolar sEMG (electrode diameter 9 mm; inter-electrode distance 21 mm). With unrectified sEMG, 12 subjects showed statistically significant CMC in 91-95% of the HD-sEMG channels, with maximum coherence at ~25 Hz. CMC was about a fifth stronger for monopolar than bipolar and Laplacian derivations. Monopolar derivations resulted in most uniform CMC distributions across the thenar and in most tight cortical source clusters in the left rolandic hand area. CMC was 19-27% stronger for HD-sEMG than for 'macroscopic' monopolar or bipolar derivations. EMG rectification reduced the CMC peak by a quarter, resulted in a more uniformly distributed CMC across the thenar, and provided more tightly clustered cortical sources than unrectifed sEMGs. Moreover, it revealed CMC at ~12 Hz. We conclude that HD-sEMG, especially with monopolar derivation, can facilitate detection of CMC and that the individual muscle anatomy cannot explain the high inter-individual CMC variability.
No preview · Article · Sep 2015 · Journal of Neurophysiology
[Show abstract][Hide abstract] ABSTRACT: Muscle architectural changes, induced by changes in joint position, can influence the amplitude distribution of surface electromyograms (EMGs). By combining grids of electrodes, ultra sound (US) imaging and three-dimensional (3D) kinematics, the relative contribution of different anatomical factors (e.g., pinnation angle, muscle thickness) on EMG features may be quantified. The aim of this paper was to explore the potentiality of this technique in the analysis of how much muscle architectural changes, induced by changes in joint position, can influence the EMG amplitude distribution. While provided with EMG visual-feedback, a participant was asked to recruit a single motor unit of the right tibialis anterior (TA) muscle at two different ankle positions. Surface EMGs, kinematic data and US images were acquired. The spatial distribution of the amplitude of motor unit action potentials was assessed from EMGs. Ankle angles were obtained from 3D kinematics. TA width and thickness variations between the two ankle positions were obtained through the segmentation of US images reconstructed in 3D space. When compared with ankle at plantar flexion, ankle at neutral position resulted in greater TA width and thickness, as well as, in more widely distributed EMG amplitude. These results suggest TA architecture may markedly affect the amplitude distribution of surface EMGs.
No preview · Article · Apr 2015 · Journal of Mechanics in Medicine and Biology
[Show abstract][Hide abstract] ABSTRACT: The integration of grids of electrodes, US imaging and 3D kinematic can allow investigating how muscle architectural changes influence surface EMGs: this innovative approach can provide novel spatio-temporal information regarding electromechanical function of muscle, relevant both for improving basic knowledge and for clinical applications. The aims of the present study were i) to verify whether movements of the US probe lead to artefacts in surface EMGs detected with a grid of electrodes transparent to US; ii) to analyse how much muscle architectural changes, induced by changes in joint positions, can influence the amplitude distribution of surface EMGs. A young healthy participant performed contractions of the right tibialis anterior (TA) at two different ankle positions: A) neutral and B) full plantarflexion. While provided with EMG visual feedback, the participant was asked to recruit a single motor unit: contractions lasted ~50 s. Surface EMGs, kinematic data and ultrasound images were acquired. The spatial distribution of the root mean square amplitude of motor unit action potentials was assessed from collected EMGs. Ankle angles were obtained from 3D kinematics. Muscle volume was obtained through the segmentation of US images reconstructed in 3D space: width and thickness variations between the two positions were measured. Movements of the US probe over the grid of electrodes did not result in artefacts in the surface EMGs. Results indicate a marked effect of TA architecture on the amplitude distribution of action potentials of a single TA motor unit. When moving the foot from plantarflexion to neutral position, TA width increased of 11.6%. Interestingly, a corresponding increase in the RMS spatial distribution was observed for the ankle neutral position. Presuming the same motor unit was recruited for both ankle positions, the changes in EMG amplitude reported here were predominantly due to TA architectural changes.
No preview · Article · Jan 2015 · IFMBE proceedings
[Show abstract][Hide abstract] ABSTRACT: In this study we investigated whether the spatial distribution of surface electromyographic (EMG) amplitude can be used to describe the activation of muscle portions with different biomechanical actions. Ten healthy subjects performed isometric contractions aimed to selectively activate a number of forearm muscles or muscle subportions. Monopolar electromyographic signals were collected with an electrode grid of 128 electrodes placed on the proximal, dorsal portion of the forearm. The monopolar EMG amplitude [root mean square (RMS) value] distribution was calculated for each contraction, and high-amplitude channels were identified through an automatic procedure; the position of the EMG source was estimated with the barycenter of these channels. Each of the contractions tested was associated to a specific EMG amplitude distribution, whose location in space was consistent with the expected anatomical position of the main agonist muscle (or subportion). The position of each source was significantly different from the others in at least one direction (ANOVA; transversally to the forearm: P < 0.01, F = 125.92; longitudinally: P < 0.01, F = 35.83). With such an approach, we could distinguish the spatial position of EMG distributions related to the activation of contiguous muscles [e.g., extensor carpi ulnaris (ECU) and extensor digitorum communis (EDC)], different heads of the same muscle (i.e., extensor carpi radialis (ECR) brevis and longus) and different functional compartments (i.e., EDC, middle, and ring fingers). These findings are discussed in terms of how forces along a given direction can be produced by recruiting population of motor units clustered not only in specific muscles, but also in muscle sub-portions. In addition, this study supports the use of high-density EMG systems to characterize the activation of muscle subportions with different biomechanical actions.
Full-text · Article · Dec 2013 · Frontiers in Physiology
[Show abstract][Hide abstract] ABSTRACT: Ankle movements in the frontal plane are less prominent though not less relevant than movements in the plantar or dorsal flexion direction. Walking on uneven terrains and standing on narrow stances are examples of circumstances likely imposing marked demands on the ankle medio-lateral stabilization. Following our previous evidence associating lateral bodily sways in quiet standing to activation of the medial gastrocnemius (MG) muscle, in this study we ask: how large is the MG contribution to ankle torque in the frontal plane? By arranging stimulation electrodes in a selective configuration, current pulses were applied primarily to the MG nerve branch of ten subjects. The contribution of populations of MG motor units of progressively smaller recruitment threshold to ankle torque was evaluated by increasing the stimulation amplitude by fixed amounts. From smallest intensities (12-32mA) leading to the firstly observable MG twitches in force-plate recordings, current pulses reached intensities (56-90mA) below which twitches in other muscles could not be observed from the skin. Key results showed a substantial MG torque contribution tending to rotate upward the foot medial aspect (ankle inversion). Nerve stimulation further revealed a linear relationship between the peak torque of ankle plantar flexion and inversion, across participants (Pearson R>.81, p<.01). Specifically, regardless of the current intensity applied, the peak torque of ankle inversion amounted to about 13% of plantar flexion peak torque. Physiologically, these results provide experimental evidence that MG activation may contribute to stabilize the body in the frontal plane, especially under situations of challenged stability.
Full-text · Article · Aug 2013 · Human movement science
[Show abstract][Hide abstract] ABSTRACT: Application of two dimensional surface electrode arrays can provide a means of mapping motor unit action potentials on the skin surface above a muscle. The resulting muscle tissue displacement can be quantified, in a single plane, using ultrasound imaging (US). Currently however, it is not possible to simultaneously map spatio-temporal propagation of activation and resulting tissue strain. In this manuscript we developed and tested a material that will enable concurrent measurement of 2D surface EMGs with US images. Specific protocols were designed to test the compatibility of this new electrode material both with EMG recording and with US analysis. Key results indicate that, for this new electrode material: i) the electrode-skin impedance is similar to that of arrays of electrodes reported in literature; ii) the reflection of ultrasound at the electrode-skin interface is negligible; iii) the likelihood of observing missing contacts, short-circuits and artefacts in EMGs is not affected by the US probe; iv) movement of tissues sampled by US can be tracked accurately. We therefore conclude this approach will facilitate multi-modal imaging of muscle to provide new spatio-temporal information regarding electromechanical function of muscle. This is relevant to basic physiology-biomechanics of active and passive force transmission through and between muscle, of motor unit spatio-temporal activity patterns, of their variation with architecture and task related function, and of their adaptation with ageing, training-exercise-disuse, neurological disease and injury.
Full-text · Article · Aug 2013 · Journal of Applied Physiology
[Show abstract][Hide abstract] ABSTRACT: The electromyogram (EMG) is a compound signal comprising the electrical activity of the motor units (MU) activated asynchronously during voluntary muscle contractions. This chapter provides a basic overview of the technologies for the detection and conditioning of surface EMG (sEMG) signals. The first section focuses on electrode technology and the electrode-skin interface. The electrical model of the interface and the effect of electrode characteristics (e.g., size, constituent material) on the features of the detected signal are discussed. The second section describes the most common solutions for the design of the front-end amplifier, signal filtering, and analog-to-digital (A/D) conversion. The third section describes and discusses the use of different configurations of electrodes for the detection of sEMG signals. The concepts of spatial filtering and spatial sampling (mono- and bi-dimensional) are introduced, and the effect of the detection system parameters on the sEMG characteristics is discussed.
[Show abstract][Hide abstract] ABSTRACT: The aims were to investigate the plasticity of the myosin heavy chain (MHC) phenotype following neuromuscular electrical stimulation (NMES) and to assess the correlation between MHC isoform distribution and muscle fibre conduction velocity (MFCV).14 men were subjected to 24 sessions of quadriceps NMES. Needle biopsies were taken from the dominant vastus lateralis and neuromuscular tests were performed on the dominant thigh before and after training. NMES significantly increased the quadriceps maximal force by 14.4±19.7% (P=0.02), vastus lateralis thickness by 10.7±8.6% (P=0.01), vastus lateralis MFCV by 11.1±3.5% (P<0.001), vastus medialis MFCV by 8.4±1.8% (P<0.001). The whole spectrum of possible MHC isoform adaptations to training was observed: fast-to-slow transition (4 subjects), bi-directional transformation from MHC-1 and MHC-2X isoforms toward MHC-2A isoform (7 subjects), shift toward MHC-2X (2 subjects), no MHC distribution change (1 subject). No significant correlation was observed between MHC-2 relative content and vastus lateralis MFCV (pre-training: R2=0.04, P=0.46; post-training: R2=0.02, P=0.67). NMES elicited distinct adaptations in the MHC composition and increased force, muscle thickness, and MFCV. The MHC isoform distribution did not correlate with MFCV, thus implying that the proportion of different fibre types cannot be estimated from this electrophysiological variable.
No preview · Article · Jan 2013 · International Journal of Sports Medicine
[Show abstract][Hide abstract] ABSTRACT: Motor unit behavior differs between contraction types at submaximal contraction levels, however is challenging to study during maximal voluntary contractions (MVCs). With multi-channel surface electromyography (sEMG), mean physiological characteristics of the active motor units can be extracted. Two 8-electrode sEMG arrays were attached on biceps brachii muscle (one on each head) to examine behavior of sEMG variables during isometric, eccentric and concentric MVCs of elbow flexors in 36volunteers. On average, isometric (364±88N) and eccentric (353±74N) MVCs were higher than concentric (290±73N) MVC (p<0.001). Mean muscle fiber conduction velocity (CV) was highest during eccentric MVC (4.42±0.49m/s) than concentric (4.25±0.49m/s, p<0.01) and isometric (4.14±0.45m/s, p<0.001) MVCs. Furthermore, eccentric MVC showed lower sEMG amplitude at the largest elbow joint angles (120-170°) and higher CV at the smallest (70-150°) elbow joint angles (p<0.05-0.001) than concentric MVC. The differences in CV and sEMG amplitude between the MVCs suggest that the control strategy of motor units differs between the contraction types during MVCs, and is dependent on the muscle length between the dynamic MVCs.
Full-text · Article · Nov 2012 · Journal of electromyography and kinesiology: official journal of the International Society of Electrophysiological Kinesiology
[Show abstract][Hide abstract] ABSTRACT: The aims of this study were to examine if surface EMG signals can be detected from the quadriceps femoris muscle of severely obese patients and to investigate if differences exist in quadriceps force and myoelectric manifestations of fatigue between obese patients and lean controls. Fourteen severely obese patients (body mass index, BMI, mean±SD: 44.9±6.3kg/m(2)) and fourteen healthy controls (BMI: 23.7±2.5kg/m(2)) were studied. The vastus medialis and lateralis of the dominant thigh were concurrently investigated during voluntary isometric contractions (10-s long at submaximal and maximal intensities and intermittent submaximal contractions until exhaustion) and sustained (120-s long) electrically elicited contractions. We found that the detection of surface EMG signals from the quadriceps is feasible also in severely obese subjects presenting increased thickness of the subcutaneous fat tissue. In addition, we confirmed and extended previous findings showing that the volume conductor properties determine the amplitude and spectral features of the detected surface EMG signals: the lower the subcutaneous tissue thickness, the higher the amplitude and mean frequency estimates. Further, we found no differences in the mechanical and myoelectric manifestations of fatigue during intermittent voluntary and sustained electrically elicited contractions between obese patients and lean controls.
No preview · Article · Oct 2012 · Journal of electromyography and kinesiology: official journal of the International Society of Electrophysiological Kinesiology
[Show abstract][Hide abstract] ABSTRACT: MINETTO, M. A., A. HOLOBAR, A. BOTTER, and D. FARINA. Origin and development of muscle cramps. Exerc. Sport Sci. Rev., Vol. 41, No. 1, pp. 3-10, 2013. Cramps are sudden, involuntary, painful muscle contractions. Their pathophysiology remains poorly understood. One hypothesis is that cramps result from changes in motor neuron excitability (central origin). Another hypothesis is that they result from spontaneous discharges of the motor nerves (peripheral origin). The central origin hypothesis has been supported by recent experimental findings, whose implications for understanding cramp contractions are discussed.
No preview · Article · Oct 2012 · Exercise and sport sciences reviews
[Show abstract][Hide abstract] ABSTRACT: Alterations in surface electromyographic (sEMG) signals of the abductor pollicis brevis muscle were evaluated in 24 non-manual workers and 40 manual workers (25 asymptomatic and 15 reporting CTS symptoms).
The initial value (IV) and the normalized rate of change (NRC) of average rectified value (ARV), mean frequency of the power spectrum (MNF), and muscle fiber conduction velocity (MFCV) were calculated during contractions at 20% and 50% of maximal voluntary contraction (MVC). Neuromuscular efficiency (NME) and kurtosis of the sEMG amplitude distribution were estimated.
With respect to controls, manual workers showed higher NME, lower ARV IV, and reduced myoelectric manifestations of fatigue (lower MNF NRC for both contraction levels, and lower MFCV NRC at 50% MVC). Kurtosis at 20% MVC showed higher values in symptomatic manual workers than in the other two groups.
Kurtosis seems to be a promising parameter for use in monitoring individuals who develop CTS.
[Show abstract][Hide abstract] ABSTRACT: We analysed the cramp threshold (i.e. the minimum frequency of electrical stimulation capable of inducing a cramp) and the behaviour of individual motor units during cramps electrically elicited in the absence (intact condition) and presence (blocked condition) of a peripheral nerve block in eight healthy subjects. The cramp threshold was significantly greater in the blocked than in the intact condition (18 ± 3 Hz vs. 13 ± 3 Hz; P = 0.01). Cramp duration and peak EMG amplitude in the intact condition (55.6 ± 19.2 s and 47.5 ± 24.8 μV, respectively) were significantly greater compared to the blocked condition (2.6 ± 1.3 s and 13.9 ± 8.8 μV; P < 0.01). All motor units identified in the blocked condition (n = 38) had a shorter interval of activity and a greater discharge rate compared to the intact condition (n = 37) (respectively, 1.1 ± 1.0 s vs. 29.5 ± 21.8 s, P < 0.0001; 25.7 ± 11.6 pulses s(-1) vs. 20.0 ± 5.9 pulses s(-1); P < 0.05). The motor unit activity detected during the blocked condition corresponded to spontaneous discharges of the motor nerves, while in the intact condition the motor unit discharge patterns presented the typical characteristics of motor neuron discharges. These results indicate a spinal involvement at the origin of cramps and during their development.
Full-text · Article · Dec 2011 · The Journal of Physiology
[Show abstract][Hide abstract] ABSTRACT: The Particle Swarm Optimization (PSO) algorithm is applied to the problem of "load sharing" among muscles acting on the same joint for the purpose of estimating their individual mechanical contribution based on their EMG and on the total torque. Compared to the previously tested Interior-Reflective Newton Algorithm (IRNA), PSO is more computationally demanding. The mean square error between the experimental and reconstructed torque is similar for the two algorithms. However, IRNA requires multiple initializations and tighter constraints found by trial-and-errors for the input variables to find a suitable optimum which is not the case for PSO whose initialization is random.
Full-text · Article · Aug 2011 · Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference
[Show abstract][Hide abstract] ABSTRACT: The aim of the study was to investigate the uniformity of the muscle motor point location for lower limb muscles in healthy subjects. Fifty-three subjects of both genders (age range: 18-50 years) were recruited. The muscle motor points were identified for the following ten muscles of the lower limb (dominant side): vastus medialis, rectus femoris, and vastus lateralis of the quadriceps femoris, biceps femoris, semitendinosus, and semimembranosus of the hamstring muscles, tibialis anterior, peroneus longus, lateral and medial gastrocnemius. The muscle motor point was identified by scanning the skin surface with a stimulation pen electrode and corresponded to the location of the skin area above the muscle in which an electrical pulse evoked a muscle twitch with the least injected current. For each investigated muscle, 0.15 ms square pulses were delivered through the pen electrode at low current amplitude (<10 mA) and frequency (2 Hz). 16 motor points were identified in the 10 investigated muscles of almost all subjects: 3 motor points for the vastus lateralis, 2 motor points for rectus femoris, vastus medialis, biceps femoris, and tibialis anterior, 1 motor point for the remaining muscles. An important inter-individual variability was observed for the position of the following 4 out of 16 motor points: vastus lateralis (proximal), biceps femoris (short head), semimembranosus, and medial gastrocnemius. Possible implications for electrical stimulation procedures and electrode positioning different from those commonly applied for thigh and leg muscles are discussed.
No preview · Article · Jul 2011 · Arbeitsphysiologie
[Show abstract][Hide abstract] ABSTRACT: Glucocorticoids are known to decrease protein synthesis and conduction velocity of muscle fibers. However, the degree of impairment of muscle protein synthesis and conduction slowing in patients with Cushing's disease remains poorly characterized. Our objective was to investigate whether and to what extent chronic endogenous hypercortisolism could decrease the circulating levels of muscle proteins and modify myoelectric indexes of sarcolemmal excitability and fatigability.
A total of ten patients with Cushing's disease and 30 healthy controls matched for age, sex, and body mass index were compared.
Blood sampling and electrophysiological tests on vastus lateralis, vastus medialis, and tibialis anterior muscles were performed.
Serum creatine kinase (CK) and plasma myoglobin were significantly lower in patients with respect to controls (P<0.001 and P<0.05 respectively): the mean relative difference between patients and controls was 48.9% for CK and 21.4% for myoglobin. Muscle fiber conduction velocity (MFCV) and myoelectric manifestations of fatigue were significantly decreased in all muscles of the patients with respect to controls. The mean relative difference in MFCV between patients and controls was 26.0% for vastus lateralis, 22.9% for vastus medialis, and 11.6% for tibialis anterior. These differences contrasted with the paucity of signs suggestive of myopathy that were obtained by needle electromyography in the patients.
Slowing of muscle fiber conduction and decreased levels of circulating muscle proteins are sensitive markers of impaired muscle function, which are suitable for use in combination with clinical assessment and standard electrodiagnostic tests for accurate identification and follow-up of myopathic patients.
Preview · Article · Mar 2011 · European Journal of Endocrinology
[Show abstract][Hide abstract] ABSTRACT: Electrical stimulation of innervated muscles has been investigated for many decades with alternations of high and low clinical interest in the fields of rehabilitation medicine and sports sciences. Early work demonstrated that afferent fibers have lower thresholds and are usually activated first (therefore eliciting an H-reflex). In the case of nerve trunk stimulation, the order of recruitment is mostly conditioned by the axonal dimension and excitability threshold. In the case of muscle motor point stimulation, the spatial distribution of nerve branches plays a predominant role. Sustained stimulation produces a progressive increase of force that is often maintained in subsequent voluntary activation by stroke patients. This observation suggested a facilitation mechanism at the spinal and/or supraspinal level. Such facilitation has been observed in healthy subjects as well, and may explain the generation of cramps elicited during stimulation and sustained for dozens of seconds after the stimulation has been interrupted. The most recent interpretations of facilitation resulting from peripheral stimulation focused on presynaptic (potentiation of neurotransmitter release from afferent fibers) or postsynaptic (generation of "persistent inward currents" in spinal motor neurons or interneurons) mechanisms. The renewed attention to these phenomena is once more increasing the interest toward electrical stimulation of the neuromuscular system. This is an opportunity for a structured investigation of the field aimed to resolving elements of confusion and controversy that still plague this area of electrophysiology.
No preview · Article · Mar 2011 · Artificial Organs