The surface interference EMG signal provides some information on the neural drive to muscles. However, the association between neural drive to muscle and muscle activation has long been debated with ...controversial indications due to the unavailability of motor unit population data. In this study, we clarify the potential and limitations of interference EMG analysis to infer motor unit recruitment strategies with an experimental investigation of several concurrently active motor units and of the associated features of the surface EMG. For this purpose, we recorded high-density surface EMG signals during linearly increasing force contractions of the tibialis anterior muscle, up to 70% of maximal force. The recruitment threshold (RT), conduction velocity (MUCV), median frequency (MDF
), and amplitude (RMS
) of action potentials of 587 motor units from 13 individuals were assessed and associated with features of the interference EMG. MUCV was positively associated with RT (
= 0.64 ± 0.14), whereas MDF
and RMS
showed a weaker relation with RT (
= 0.11 ± 0.11 and 0.39 ± 0.24, respectively). Moreover, the changes in average conduction velocity estimated from the interference EMG predicted well the changes in MUCV (
= 0.71), with a strong association to ankle dorsiflexion force (
= 0.81 ± 0.12). Conversely, both the average EMG MDF and RMS were poorly associated with motor unit recruitment. These results clarify the limitations of EMG spectral and amplitude analysis in inferring the neural strategies of muscle control and indicate that, conversely, the average conduction velocity could provide relevant information on these strategies.
The surface EMG provides information on the neural drive to muscles. However, the associations between EMG features and neural drive have been long debated due to unavailability of motor unit population data. Here, by using novel highly accurate decomposition of the EMG, we related motor unit population behavior to a wide range of voluntary forces. The results fully clarify the potential and limitation of the surface EMG to provide estimates of the neural drive to muscles.
Key points
Previous studies have indicated that several weeks of strength training is sufficient to elicit significant adaptations in the neural drive sent to the muscles.
There are few data, ...however, on the changes elicited by strength training in the recruitment and rate coding of motor units during voluntary contractions. We show for the first time that the discharge characteristics of motor units in the tibialis anterior muscle tracked across the intervention are changed by 4 weeks of strength training with isometric voluntary contractions.
The specific adaptations included significant increases in motor unit discharge rate, decreases in the recruitment‐threshold force of motor units and a similar input–output gain of the motor neurons.
The findings suggest that the adaptations in motor unit function may be attributable to changes in synaptic input to the motor neuron pool or to adaptations in intrinsic motor neuron properties.
The strength of a muscle typically begins to increase after only a few sessions of strength training. This increase is usually attributed to changes in the neural drive to muscle as a result of adaptations at the cortical or spinal level. We investigated the change in the discharge characteristics of large populations of longitudinally tracked motor units in tibialis anterior before and after 4 weeks of strength training the ankle‐dorsiflexor muscles with isometric contractions. The adaptations exhibited by 14 individuals were compared with 14 control subjects. High‐density electromyogram grids with 128 electrodes recorded the myoelectric activity during isometric ramp contractions to the target forces of 35%, 50% and 70% of maximal voluntary force. The motor unit recruitment and derecruitment thresholds, discharge rate, interspike intervals and estimates of synaptic inputs to motor neurons were assessed. The normalized recruitment‐threshold forces of the motor units were decreased after strength training (P < 0.05). Moreover, discharge rate increased by 3.3 ± 2.5 pps (average across subjects and motor units) during the plateau phase of the submaximal isometric contractions (P < 0.001). Discharge rates at recruitment and derecruitment were not modified by training (P < 0.05). The association between force and motor unit discharge rate during the ramp‐phase of the contractions was also not altered by training (P < 0.05). These results demonstrate for the first time that the increase in muscle force after 4 weeks of strength training is the result of an increase in motor neuron output from the spinal cord to the muscle.
Key points
Previous studies have indicated that several weeks of strength training is sufficient to elicit significant adaptations in the neural drive sent to the muscles.
There are few data, however, on the changes elicited by strength training in the recruitment and rate coding of motor units during voluntary contractions. We show for the first time that the discharge characteristics of motor units in the tibialis anterior muscle tracked across the intervention are changed by 4 weeks of strength training with isometric voluntary contractions.
The specific adaptations included significant increases in motor unit discharge rate, decreases in the recruitment‐threshold force of motor units and a similar input–output gain of the motor neurons.
The findings suggest that the adaptations in motor unit function may be attributable to changes in synaptic input to the motor neuron pool or to adaptations in intrinsic motor neuron properties.
Key points
Despite the non‐linear property of individual motor neurons, the pool of motor neurons linearizes the relation between their common synaptic input and the neural drive to the muscle, i.e. ...the ensemble of axonal action potentials reaching the muscle from the spinal cord.
In the frequency bandwidth relevant for force generation, the motor neuron pool attenuates the input signals sent independently to each motor neuron and transfers only the common signal components with a pure scaling.
The effective neural drive to the muscle tends to exactly replicate, without phase distortion, the common synaptic input to motor neurons for increasing number of active motor neurons.
The classic definition and functional meaning of motor unit synchronization are discussed in relation to the role of common input in determining the neural drive to muscle.
We analysed the transformation of synaptic input to the pool of motor neurons into the neural drive to the muscle. The aim was to explain the relations between common oscillatory signals sent to motor neurons and the effective component of the neural signal sent to muscles as output of the spinal cord circuitries. The approach is based on theoretical derivations, computer simulations, and experiments. It is shown theoretically that for frequencies smaller than the average discharge rates of the motor neurons, the pool of motor neurons determines a pure amplification of the frequency components common to all motor neurons, so that the common input is transmitted almost undistorted and the non‐common components are strongly attenuated. The effective neural drive to the muscle thus mirrors the common synaptic input to motor neurons. The simulations with three models of motor neuron confirmed the theoretical results by showing that the coherence function between common input components and the neural drive to the muscle tends to 1 when increasing the number of active motor neurons. This result, which was relatively insensitive to the type of model used, was also supported experimentally by observing that, in the low‐pass signal bandwidth, the peak in coherence between groups of motor units of the abductor digiti minimi muscle of five healthy subjects tended to 1 when increasing the number of motor units. These results have implications for our understanding of the neural control of muscles as well as for methods used for estimating the strength of common input to populations of motor neurons.
Research on pattern recognition for myoelectric control has usually focused on a small number of electromyography (EMG) channels because of better clinical acceptability and low computational load ...with respect to multi-channel EMG. However, recently, high density (HD) EMG technology has substantially improved, also in practical usability, and can thus be applied in myocontrol. HD EMG provides several closely spaced recordings in multiple locations over the skin surface. This study considered the use of HD EMG for controlling upper limb prostheses, based on pattern recognition. In general, robustness and reliability of classical pattern recognition systems are influenced by electrode shift in dons and doff, and by the presence of malfunctioning channels. The aim of this study is to propose a new approach to attenuate these issues. The HD EMG grid of electrodes is an ensemble of sensors that records data spatially correlated. The experimental variogram, which is a measure of the degree of spatial correlation, was used as feature for classification, contrary to previous approaches that are based on temporal or frequency features. The classification based on the variogram was tested on seven able-bodied subjects and one subject with amputation, for the classification of nine and seven classes, respectively. The performance of the proposed approach was comparable with the classic methods based on time-domain and autoregressive features (average classification accuracy over all methods ~ 95% for nine classes). However, the new spatial features demonstrated lower sensitivity to electrode shift (±1 cm) with respect to the classic features (p<;0.05). When even just one channel was noisy, the classification accuracy dropped by ~ 10% for all methods. However, the new method could be applied without any retraining to a subset of high-quality channels whereas the classic methods require retraining when some channels are omitted. In conclusion, the new spatial feature space proposed in this study improved the robustness to electrode number and shift in myocontrol with respect to previous approaches.
A
bstract
We investigate the topological properties of
N
f
= 2 + 1 QCD with physical quark masses, at temperatures around 500 MeV. With the aim of obtaining a reliable sampling of topological modes ...in a regime where the fluctuations of the topological charge
Q
are very rare, we adopt a multicanonical approach, adding a bias potential to the action which enhances the probability of suppressed topological sectors. This method permits to gain up to three orders magnitude in computational power in the explored temperature regime. Results at different lattice spacings and physical spatial volumes reveal no significant finite size effects and the presence, instead, of large finite cut-off effects, with the topological susceptibility which decreases by 3-4 orders of magnitude while moving from
a
≃ 0
.
06 fm towards the continuum limit. The continuum extrapolation is in agreeement with previous lattice determinations with smaller uncertainties but obtained based on ansatzes justified by several theoretical assumptions. The parameter
b
2
, related to the fourth order coefficient in the Taylor expansion of the free energy density
f
(
θ
), has instead a smooth continuum extrapolation which is in agreement with the dilute instanton gas approximation (DIGA); moreover, a direct measurement of the relative weights of the different topological sectors gives an even stronger support to the validity of DIGA.
We investigate the static QQ¯ potential at zero and finite temperature in the presence of a constant and uniform external magnetic field B→, for several values of the lattice spacing and for ...different orientations with respect to B→. As a byproduct, we provide continuum limit extrapolated results for the string tension, the Coulomb coupling and the Sommer parameter at T=0 and B=0. We confirm the presence in the continuum of a B-induced anisotropy, regarding essentially the string tension, for which it is of the order of 15% at |e|B∼1 GeV2 and would suggest, if extrapolated to larger fields, a vanishing string tension along the magnetic field for |e|B≳4 GeV2. The angular dependence for |e|B≲1 GeV2 can be nicely parametrized by the first allowed term in an angular Fourier expansion, corresponding to a quadrupole deformation. Finally, for T≠0, the main effect of the magnetic field is a general suppression of the string tension, leading to a early loss of the confining properties: this happens even before the appearance of inverse magnetic catalysis in the chiral condensate, supporting the idea that the influence of the magnetic field on the confining properties is the leading effect originating the decrease of Tc as a function of B.
Non‐technical summary
Since the human central nervous system controls muscle contraction through inputs to spinal motoneurons, oscillations recorded on the primary motor cortex during voluntary ...movements are correlated with the electrical activity produced on the surface of the muscles. We show through theoretical derivations and experimental recordings that cortical input is transmitted partly in a linear way to the population of motoneurons. The results demonstrate the effective spread of the cortical projections to the motoneuron pool to allow an efficient control of the muscle force output.
Oscillations in the primary motor cortex are transmitted through the corticospinal tract to the motoneuron pool. This transmission has been previously investigated using coherence analysis between concurrent recordings of EEG and surface EMG signals. In this study we used a mathematical derivation and motor unit recordings in vivo to investigate the origin of linear transmission of cortical input to the motoneuron spike trains (neural drive to muscle). The theoretical derivation showed that a common input spread to a relatively small number of motoneurons is partly transmitted in a linear way, overcoming the interference signal generated by the non‐linearity of the individual motoneurons. We further calculated the corticomuscular coherence between experimental EEG signals and the cumulative spike trains of motor units in the abductor digiti minimi muscle of seven healthy men. The experimental results indicated that, on average, only four to five motor units were sufficient to reach the same level of coherence as estimated from the surface EMG. The results demonstrate that linearity in the transmission of the cortical input to motoneurons through the corticospinal tract is achieved because (i) this input is largely common to all motoneurons, and (ii) its frequency content requires only a small fraction of active motoneurons to be accurately sampled. In this way, the central nervous system can directly transmit oscillations to the control signals to muscles for practically the entire range of functionally relevant forces.
The hepatitis B virus (HBV) and the hepatitis C virus (HCV) affect approximately 400–500 million individuals worldwide. Both infections are characterised by a significant morbidity and mortality: ...chronic hepatitis B and C may evolve towards the development of cirrhosis and primary hepatocellular carcinoma. During the last two decades, several new antivirals have been developed that are active against HBV and HCV, allowing sustained cure rates in a significant proportion of patients. All these drugs have side effects, which may represent a major barrier to achieve cure in many patients in need. I will review the most common adverse events reported during the therapy of chronic hepatitis B and C, with some recommendations for proper management.
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