Inter-operator agreement in decomposition of motor unit firings from high-density surface EMG.

High-density surface EMG can be used to obtain a spatially selective representation of several motor unit action potentials. Recently, a decomposition of the signal into the underlying motor neuron firing patterns has been described. The reliability of the algorithm has not yet been tested. Eleven healthy subjects participated. High-density surface EMG was recorded from the vastus lateralis muscle during an isometric knee extension. Two independent operators analyzed the signals. After operator-supervised cluster analysis of spikes, motor unit action potential templates were constructed and an automatic template matching was performed. The decomposition was adjusted by hand. Agreement between operators was calculated for the number of coincident firings. Bland-Altman plots of peak-to-peak amplitude were constructed and limits of agreement were calculated. For completely decomposed motor unit action potential trains the between-operator agreement of firing events was very high. The peak-to-peak amplitude of monopolar motor unit action potentials was 115microV (SD 74microV). The agreement was within 3microV and independent of amplitude. With partial decomposition agreement within 26microV was achieved. For bipolarly derived motor unit action potentials the peak-to-peak amplitude was 54microV (SD 49microV), the agreement was within 3microV. Only for recordings obtained from a force level below 5% of the maximum voluntary contraction full decomposition was possible. It was concluded that when full decomposition is achieved, two independent operators are likely to arrive at nearly identical firing patterns.     

Some Models of Neuronal Variability

The pattern of nerve action potentials produced by unit permeability changes (quantal inputs) occurring at random is considered analytically and by computer simulation methods. The important parameters of a quantal input are size and duration. Varying both the mean and the probability density function of these parameters has calculable effects on the distribution of interspike intervals. Particular attention is paid to the relation between the mean rate of excitatory inputs and the mean frequency of nerve action potentials (input-output curve) and the relation between the coefficient of variation for the interval distribution and the mean interval (variability curve). In the absence of action potentials one can determine the parameters of the voltage distribution including the autocorrelation function and the power spectrum. These parameters can sometimes be used to approximate the variability of interspike intervals as a function of the threshold voltage. Different neuronal models are considered including one containing the Hodgkin-Huxley membrane equations. The negative feedback inherent in the Hodgkin-Huxley equations tends to produce a small negative serial correlation between successive intervals. The results are discussed in relation to the interpretation of experimental results.     

Detection of motor unit action potentials with surface electrodes: influence of electrode size and spacing

A model of the motor unit action potential was developed to investigate the amplitude and frequency spectrum contributions of motor units, located at various depths within muscle, to the surface detected electromyographic (EMG) signal. A dipole representation of the transmembrane current in a three-dimensional muscle volume was used to estimate detected individual muscle fiber action potentials. The effects of anisotropic muscle conductance, innervation zone location, propagation velocity, fiber length, electrode area, and electrode configuration were included in the fiber action potential model. A motor unit action potential was assumed to be the sum of the individual muscle fiber action potentials. A computational procedure, based on the notion of isopotential layers, was developed which substantially reduced the calculation time required to estimate motor unit action potentials. The simulations indicated that: 1) only those motor units with muscle fibers located within 10–12 mm of the electrodes would contribute significant signal energy to the surface EMG, 2) variation in surface area of electrodes has little effect on the detection depth of motor unit action potentials, 3) increased interelectrode spacing moderately increases detection depth, and 4) the frequency content of action potentials decreases steeply with increased electrode-motor unit territory distance.     

The methods of surface EMG analysis

The use of surface emg as a tool for quantification is described. First, the specific advantages of the surface emg are discussed. Techniques for analysis of the emg signal which estimate and detect the action potentials of the individual motor units and estimate some global properties of muscle activity are reviewed. A survey of data on relations between the properties of motor unit action potential, the properties of motor unit activity and the results of signal processing are given.     

Properties of synaptic noise in tonically active human motoneurons

The objective of these experiments was to determine the amount of synaptic noise on the cell membrane at various intervals after an action potential in a motoneuron firing at a specified frequency. Sources of noise such as variations in the level of voluntary drive were minimized by selecting only segments of the spike train in which the unit was running within prescribed frequency limits. The level of the membrane potential of the motoneuron during these intervals was determined using two test “pulses” (compound Ia excitatory postsynaptic potentials) of known amplitude. This enabled the probability of the membrane potential falling within a voltage “window” of known size at known times after the preceding spike to be determined. The probability density histograms showed that the fluctuations of membrane potential about a target interspike trajectory (i.e., the membrane noise) increased with time after the preceding spike. These fluctuations in the membrane potential can be accounted for by a one-dimensional “random walk” model of membrane noise. This model explains the salient features of the interval histograms, such as positive skewness at low target frequencies. A quantitative test of the model demonstrated its applicability to the motor pools of tibialis and masseter.     

Differences between properties of male and female motor units in the rat medial gastrocnemius muscle.

Differences between motor units in hindlimb locomotor muscles of male and female Wistar rats were studied. The contractile and action potential properties of various types of motor units as well as proportions of these units in the medial gastrocnemius muscle were analyzed. Experiments were based on functional isolation and electrical stimulation of axons of single motor units. Composition of motor units was different for male and female subjects, with higher number of the fast fatigable and lower number of slow type units in male animals. The contraction and the half-relaxation times were significantly longer in male motor units, what might be due to differences in muscle size. Slower contraction of male motor units likely corresponds to lower firing rates of their motoneurons. On the other hand, no significant differences between sexes were observed with respect to force parameters of motor units (the twitch and the maximum tetanus forces), except the fast resistant units (higher force values in male muscles). The mass of the muscle was approximately 1.5 time bigger in male rats. However, the mean ratio of motor unit tetanus force to the muscle mass was almost twice smaller in this group, what indirectly suggests that muscles of male rats are composed of higher number of motor units. Finally, female muscles appeared to have higher fatigue resistance as the effect of higher proportion of resistant units (slow and fast resistant) and higher values of the fatigue index in respective motor unit types. The motor unit action potentials in female rats had slightly lower amplitudes and shorter time parameters although this difference was significant only for fast resistant units.     

The effects of motor unit synchronization on the power spectrum of the electromyogram

A realistic model for two synchronized motor unit action potential trains (MUAPT) is presented in which the variability of the time difference between corresponding action potentials (hereafter denoted by delay) is taken into account. Specifically, this delay is modeled as a continuous random variable that may assume both positive and negative values.Expressions are derived for the auto- and cross-power spectra of two such trains using their relations with the auto- and cross-correlation functions, respectively, with which they form Fourier transform pairs.The results show that the auto- and the cross-power spectra of two such synchronized MUAPTs differ from the auto- and the cross-spectra of two independent MUAPTs. The contribution of the statistics of the interpulse intervals to one of the autopower spectra is smaller and the cross-power spectra no longer reduce to a Dirac -function at the origin but are now determined by the other auto-power spectrum and by the Fourier transform of the density function associated with the time difference between corresponding action potentials. As a consequence of this change in the cross-power spectra synchronization leads to an absolute increase of power at low frequencies and to a relative decrease of power at high frequencies.The results are then generalized to electromyograms (EMG) composed of more than just two MUAPTs and illustrated with simulated power spectra with which the theory shows excellent agreement.     

A study of the motor unit action potential by means of computer simulation

In order to study the motor unit action potential a computer simulation model was developed. It is based on the superposition of single muscle fibre potentials of the fibres belonging to the motor unit. The parameters which characterize each fibre (spatial position, diameter, and a dispersion of arrival time of the potential at the electrode) are chosen from statistical distributions which can be derived from anatomical and physiological data. The electrode type, position and dimensions can be specified. Simulated motor unit action potentials are presented in the time and frequency domain. The simulation results refer to (1) the influence of the electrode position and dimensions with respect to the motor unit territory, (2) the meaning of this model for the study of pathological phenomena, (3) the variability of some parameters characterizing the motor unit, (4) the selectivity of uni- and bipolar electrodes and finally (5) the influence of the geometrical situation of the motor end-plates within the muscle, on the shape of motor unit action potentials.     

Spectral analysis of the surface electromyogram as a tool for studying rate modulation: A comparison between theory,simulation, and experiment

Theoretical work suggests that if the interpulse intervals (IPIs) of motor unit action potential trains (MUAPTs) are independently and normally distributed, then spectral analysis of the electromyogram could be a useful tool for studying rate modulation by virtue of the presence of a peak in the power spectrum at the average firing frequency of all active motor units. It is shown in this paper that IPIs need not be normally distributed, specifically that the results are very much the same if the IPIs are distributed according to a Gamma probability density function (PDF). Simulation of the electromyogram based on this theory proved the applicability of the method. Experimental results obtained for the masseter, biceps brachii and first dorsal interosseus (FDI) muscles, however, were in disagreement with both theory and simulation except for the biceps muscle at force levels up to 20% of the maximal force and for the masseter and FDI muscles in 1 out of 5 subjects. This indicates that the models for MUAPTs hitherto used might not be generally correct. Apart from this discrepancy, our results reveal differences between masseter and FDI muscles on the one hand and the biceps brachii on the other, which indicate that motor unit synchronisation is much more pronounced in the latter muscle.     

Spatial analysis of motor unit potentials of the rat medial gastrocnemius

The spatial analysis of the potentials of single motor units of the rat medial gastrocnemius muscle evoked by stimulation of the fibres of split ventral roots was carried out with a bipolar electrode moving in the direction perpendicular to the longitudinal axis of the muscle fibres. During this movement of the electrode a variability was observed in the time of the biphasic potential from its maximum to minimum, and in the peak-to-peak amplitude of these potentials. The potentials recorded outside the territory of the motor unit had a lower amplitude in relation to the potentials from the territory of the unit. This made localization of the motor unit on the cross-section of the muscle possible. Differences in the duration of the potential from maximal to minimal amplitude (maximum-minimum amplitude time--M-MAT) of each investigated motor unit from successive recording sites reflected the number of fibres contributing to the action potential and the distance of the recording surface of the electrode from the zone of the motor end-plates of this motor unit. The greatest diameter of the territory of the observed motor units reached 2.5 mm.     

Some properties of the point processes generated by integral density modulation of renewal processes

Integral density modulation of point processes is defined, and the properties of the modulated point processes are described. When a homogeneous renewal process is modulated by a step random signal, the mathematical expressions are derived of the probability density functions of the sums of r-successive inter point intervals, the intensity functions and the first order correlation coefficient of intervals. These quantities are calculated and illustrated for several parameter values. Modulated point sequences are generated by computer simulation method. The interval histograms and the serial correlation coefficients of counts and of intervals of the sequences are obtained. The results are compared with the theoretical results on the point processes modulated by the step random signal.Abbreviations IDM Integral density modulation - SRS Step random signal - PP Point process - PS Point sequence - PDF Probability density function - CPDF Conditional probability density function - JCPDF Joint conditional probability density function - SED Special Earlangian distribution - SCC Serial correlation coefficients - SIC Serial intensity coefficients     

The mode of spontaneous transmitter release at the insect neuromuscular junction.

The time intervals between miniature excitatory postsynaptic potentials and the counts of them in the cockroach, Periplaneta americana, were analyzed, using a computer program to test for properties of a Poisson process. The miniature potentials occurred basically in random manner at this neuromuscular junction. Although the distribution of the potentials did not fit the criteria for a Poisson process when the muscle fiber exhibited the short burst of high-frequency discharges, it was suggested that the primary process of such a distribution is Poisson, which is occasionally contaminated by the burst phase of the release rates.     

海岱地区大汶口文化时期人口死亡年龄的统计分布特征

人口死亡年龄是揭示一个族群健康状况和社会经济条件的重要指标。本文根据海岱地区大汶口文化时期九个墓地人骨遗存的发掘报告,运用定量统计的方法检验了人口死亡年龄分布特征。发现该区大汶口文化时期人口的死亡年龄分布近似服从正态分布。最后探讨了造成人口低死亡年龄的可能原因,并给出了这一概率分布的数学意义以及在史前人口学中的应用前景。     

In vivo magnetic and electric recordings from nerve bundles and single motor units in mammalian skeletal muscle. Correlations with muscle force

Recent advances in the technology of recording magnetic fields associated with electric current flow in biological tissues have provided a means of examining action currents that is more direct and possibly more accurate than conventional electrical recording. Magnetic recordings are relatively insensitive to muscle movement, and, because the recording probes are not directly connected to the tissue, distortions of the data due to changes in the electrochemical interface between the probes and the tissue are eliminated. In vivo magnetic recordings of action currents of rat common peroneal nerve and extensor digitorum longus (EDL) muscle were obtained by a new magnetic probe and amplifier system that operates within the physiological temperature range. The magnetically recorded waveforms were compared with those obtained simultaneously by conventional, extracellular recording techniques. We used the amplitude of EDL twitch force (an index of stimulus strength) generated in response to graded stimulation of the common peroneal nerve to enable us to compare the amplitudes of magnetically recorded nerve and muscle compound action currents (NCACs and MCACs, respectively) with the amplitudes of electrically recorded nerve compound action potentials (NCAPs). High, positive correlations to stimulus strength were found for NCACs (r = 0.998), MCACs (r = 0.974), and NCAPs (r = 0.998). We also computed the correlations of EDL single motor unit twitch force with magnetically recorded single motor unit compound action currents (SMUCACs) and electrically recorded single motor unit compound action potentials (SMUCAPs) obtained with both a ring electrode and a straight wire serving as a point electrode. Only the SMUCACs had a relatively strong positive correlation (r = 0.768) with EDL twitch force. Correlations for ring and wire electrode-recorded SMUCAPs were 0.565 and -0.366, respectively. This study adds a relatively direct examination of action currents to the characterization of the normal biophysical properties of peripheral nerve, muscle, and muscle single motor units.     

Statistical Analysis of Motor Unit Firing Patterns in a Human Skeletal Muscle

A statistical analysis of the firing pattern of single motor units in the human brachial biceps muscle is presented. Single motor unit spike trains are recorded and analyzed. The statistical treatment of these spike trains is as stochastic point processes, the theory of which is briefly discussed. Evidence is presented that motor unit spike trains may be modelled by a renewal process with an underlying gaussian probability density. Statistical independence of successive interspike intervals is shown using scatter diagrams; the hypothesis of a gaussian distribution is accepted at the 99th percentile confidence limit, chi-square test, in 90% of the units tested. A functional relationship between the mean and standard deviation is shown and discussed; its implications in obtaining sample size are presented in an appendix.The results of higher order analysis in the form of autocorrelograms and grouped interval histograms are presented. Grouped interval histograms are discussed in the context of motor unit data, and used to confirm the hypothesis that a stable probability density function does not represent a good model of the data at this level of analysis.     

A new surface electromyography analysis method to determine spread of muscle fiber conduction velocities.

Muscle fiber conduction velocity (MFCV) estimation from surface signals is widely used to study muscle function, e.g., in neuromuscular disease and in fatigue studies. However, most analysis methods do not yield information about the velocity distribution of the various motor unit action potentials. We have developed a new method-the interpeak latency method (IPL)-to calculate both the mean MFCV and the spread of conduction velocities in vivo, from bipolar surface electromyogram (sEMG) during isometric contractions. sEMG was analyzed in the biceps brachii muscle in 15 young male volunteers. The motor unit action potential peaks are automatically detected with a computer program. Associated peaks are used to calculate a mean MFCV and the SD. The SD is taken as a measure of the MFCV spread. The main finding is that the IPL method can derive a measure of MFCV spread at different contraction levels. In conclusion, the IPL method provides accurate values for the MFCV and additionally gives information about the scatter of conduction velocities.     

Conduction velocity of low-threshold motor units during ischemic contractions performed with surface EMG feedback.

The aim of this study was to analyze the effect of ischemia on low-threshold motor unit conduction velocity. Nine subjects were trained to isolate the activity of a single motor unit (target motor unit) in the abductor pollicis brevis muscle with feedback on surface EMG signals recorded with a 16-electrode linear array. After training, the subjects activated the target motor unit at approximately 8 pulses per second (pps) for five 3-min-long contractions. During the third and fourth contractions, a cuff inflated at 180 mmHg around the forearm induced ischemia of the hand. The exerted force (mean +/- SE, 4.6 +/- 2.1% of the maximal voluntary contraction force), discharge rate (8.6 +/- 0.4 pps), interpulse interval variability (34.8 +/- 2.5%), and peak-to-peak amplitude of the target motor unit action potentials (176.6 +/- 18.2 microV) were not different among the five contractions. Conduction velocity, mean power spectral frequency, and action potential duration were the same in the beginning of the five contractions (2.8 +/- 0.2 m/s, 195.2 +/- 10.5 Hz, and 5.4 +/- 0.3 ms, respectively) and changed over the 3 min of sustained activation only during the fourth contraction. Conduction velocity and mean power spectral frequency decreased (10.05 +/- 1.8% and 8.50 +/- 2.18% during the 3 min, respectively) and action potential duration increased (8.2 +/- 4.6% in the 3 min) during the fourth contraction. In conclusion, 1) subjects were able to isolate the activity of a single motor unit with surface EMG visual feedback in ischemic conditions maintained for 16 min, and 2) the activation-induced decrease in single motor unit conduction velocity was significantly larger with ischemia than with normal circulation, probably due to the alteration of mechanisms of ion exchange across the fiber membrane.     

Statistical analysis of mediator release from the cyto-neural junction of the posterior canal of the labyrinth isolated from the frog]

The transmitter release mechanism was investigated at the cyto-neural junction of the frog labyrinth posterior canal. Low frequency (less than 100/s) non overlapping EPSPs were intracellularly recorded both at rest and during inhibitory mechanical stimulation of the canal (2-8 deg/s2). Recordings were obtained: in control solution; in the presence of increased external Ca2+ (9 mM); in Ca-free EGTA (5 mM) solution and during electrical activation at 50 Hz of the posterior canal inhibitory efferent system. Individual synaptic potentials were digitized and their peak amplitudes, their time integrals as well as the time intervals between them were evaluated. The time intervals proved to be exponentially distributed, suggesting a random EPSP occurrence. The analytical reconstruction of the EPSP waveform indicated that a gamma- function fitted reasonably well both the single and averaged events. As regards the averaged event, despite the scatter in the values of the gamma-function exponential factor (range 1.1-2.2), in the EPSP time-to-peak (0.6-1.2 ms) and peak amplitude (0.9-2.7 mV) displayed by the units, no significant differences were observed in the same fibre between control and test conditions. Moreover, the event peak amplitude distribution represented by cumulative plots or amplitude histograms was fitted by a lognormal function. The distributions obtained for the same unit in control solution proved to be not significantly different from those successively obtained under test conditions. The unimodal and continuous EPSP distributions, together with the unvarying characteristics of the single events, strongly suggest that the observed potentials are true mEPSPs due to the release of single quanta of transmitter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Use of surface EMG for studying motor unit recruitment during isometric linear force ramp.

The aim of this work was to demonstrate the rank order of motor unit (MU) recruitment by surface EMG based on a Laplacian detection technique and to document the MU features at their recruitment threshold. Surface EMG signals were recorded on the biceps brachii of 10 healthy subjects during linear force ramps. When achievable, the signals were decomposed into MU action potential (MUAP) trains. MU inter-pulse interval (IPI), conduction velocity (MUCV) and amplitude were estimated on the first 12 MUAPs of each detectable train in order to characterize the MU features at their firing onset. A strong correlation was found between MU recruitment threshold and IPI, MUCV, and amplitude, showing that the size principle can be demonstrated by a fully non-invasive EMG technique. However, signal decomposition was not possible on seven subjects due to the effects of the volume conductor when the skinfold thickness was too large. When requirements for an optimal detection of MUAP trains are met, surface EMG may be used to improve our understanding of MU activity.