Accuracy assessment of CKC high-density surface EMG decomposition in biceps femoris muscle.

The aim of this study was to assess the accuracy of the convolution kernel compensation (CKC) method in decomposing high-density surface EMG (HDsEMG) signals from the pennate biceps femoris long-head muscle. Although the CKC method has already been thoroughly assessed in parallel-fibered muscles, there are several factors that could hinder its performance in pennate muscles. Namely, HDsEMG signals from pennate and parallel-fibered muscles differ considerably in terms of the number of detectable motor units (MUs) and the spatial distribution of the motor-unit action potentials (MUAPs). In this study, monopolar surface EMG signals were recorded from five normal subjects during low-force voluntary isometric contractions using a 92-channel electrode grid with 8 mm inter-electrode distances. Intramuscular EMG (iEMG) signals were recorded concurrently using monopolar needles. The HDsEMG and iEMG signals were independently decomposed into MUAP trains, and the iEMG results were verified using a rigorous a posteriori statistical analysis. HDsEMG decomposition identified from 2 to 30 MUAP trains per contraction. 3 ± 2 of these trains were also reliably detected by iEMG decomposition. The measured CKC decomposition accuracy of these common trains over a selected 10 s interval was 91.5 ± 5.8%. The other trains were not assessed. The significant factors that affected CKC decomposition accuracy were the number of HDsEMG channels that were free of technical artifact and the distinguishability of the MUAPs in the HDsEMG signal (P < 0.05). These results show that the CKC method reliably identifies at least a subset of MUAP trains in HDsEMG signals from low force contractions in pennate muscles.

Estimating motor-unit architectural properties by analyzing motor-unit action potential morphology.

OBJECTIVE: We investigated the architectural organization of groups of neighboring motor units (MUs) in normal brachial biceps and tibialis anterior muscles by analyzing morphological landmarks of their MU action potentials (MUAPs). METHODS: EMG signals containing multiple MUAPs were recorded using a monopolar needle electrode during moderate isometric voluntary contractions. The MUAPs were identified using computer-aided decomposition, and averaged. For each MUAP the onset, spike, terminal wave, and slow afterwave were identified; then the location of the MU's endplate and muscle/tendon junction were estimated from the latencies of the spike and terminal wave with respect to the MUAP onset. RESULTS: The analysis revealed a variety of architectural organizations, including single and multiple endplate zones, MU fractions, pennation, intramuscular aponeuroses, and centrally and non-centrally located endplates. CONCLUSIONS: This type of morphological analysis of the MUAP promises to be informative for studying normal MU properties as well as evaluating MU reorganization in disease.

A model of the muscle action potential for describing the leading edge, terminal wave, and slow afterwave.

The leading edge, terminal wave, and slow afterwave of the motor-unit action potential (MUAP) are produced by changes in the strength of electrical sources in the muscle fibers rather than by movement of sources. The latencies and shapes of these features are, therefore, determined primarily by the motor-unit (MU) architecture and the intracellular action potential (IAP), rather than by the volume-conduction characteristics of the limb. We present a simple model to explain these relationships. The MUAP is modeled as the convolution of a source function related to the IAP and a weighting function related to the MU architecture. The IAP waveform is modeled as the sum of a spike and a slow repolarization phase. The MU architecture is modeled by assuming that the individual fibers lie along a single equivalent axis but that their action potentials have dispersed initiation and termination times. The model is illustrated by simulating experimentally recorded MUAPs and compound muscle action potentials.

A model of the muscle-fiber intracellular action potential waveform, including the slow repolarization phase.

Recent studies have shown that the slow repolarization phase or "negative afterpotential" of the intracellular muscle-fiber action potential (IAP) plays an important role in determining the shape of the extracellularly recorded motor-unit action potential (MUAP). This paper presents a model of the IAP waveform as the sum of a spike and an afterpotential, both represented by simple analytical expressions. The model parameters that specify the sizes of the spike and afterpotential are shown to be proportional to the quadrupole and dipole moments of the transmembrane current distribution associated with the spike of the wave of excitation. The model provides a computationally efficient method for simulating the MUAP, and it can be reliably inverted to estimate the model parameters from empirical IAP and MUAP waveforms.

Satellite potentials of motor unit action potentials in normal muscles: a new hypothesis for their origin.

OBJECTIVE: A satellite potential is a late component of the motor unit action potential (MUAP) that occurs both in pathologic and normal muscle. We investigated the physiological mechanisms responsible for satellite potentials in normal muscle by relating the latencies of MUAP features to the timing of the underlying electrical events. METHODS: We analyzed 21 MUAPs with satellite potentials that had been recorded using a monopolar needle electrode from brachial biceps and tibialis anterior muscles in 10 normal subjects. We estimated the endplate-to-electrode, endplate-to-tendon, and satellite propagation times from the latencies, with respect to the MUAP onset, of the MUAP spike, terminal wave, and satellite. RESULTS: Satellite latencies ranged from 8.8 to 32 ms, too long to be explained by mechanisms involving regenerating axons or atrophic muscle fibers. The spike-to-satellite time intervals approximated either twice the spike-to-terminal-wave interval (17 MUAPs) or twice the terminal-wave latency (4 MUAPs). CONCLUSIONS: These results are consistent with the hypothesis that satellite potentials are due to retrograde propagation in a non-innervated muscle fiber that is connected with an innervated muscle fiber at one of the muscle/tendon junctions. Such a configuration could arise as a result of longitudinal muscle-fiber splitting.

The contribution of the interosseous muscles to the hypothenar compound muscle action potential.

The contributions of the various ulnar-innervated muscles of the hand to the hypothenar compound muscle action potential (CMAP) were estimated by directly stimulating individual muscles and by analyzing CMAP shape changes resulting from manipulations that changed individual muscle lengths. The results show that the first peak of the negative phase of the hypothenar CMAP comes from the hypothenar muscles, but that the second peak is due to a large volume-conducted potential from the interosseous muscles. The interosseous contribution affects both the amplitude and the area of the CMAP, and makes these parameters sensitive to changes in the configuration of the fingers and the temperature gradient in the hand. To reduce the interosseous contribution, a "balanced reference" consisting of two reference electrodes, one over each tendon, is proposed.

The physiological origin of the slow afterwave in muscle action potentials.

OBJECTIVE: Both intramuscularly-recorded motor unit action potentials (MUAPs) and surface recorded MUAPs and compound muscle action potentials (CMAPs) have slow afterwaves which can contribute as much as half their measured duration. This study tested the hypothesis that the slow afterwave has its physiological origin in the negative afterpotential of the muscle fiber intracellular action potential (IAP). METHODS: We investigated the slow afterwave in MUAPs and CMAPs from brachial biceps, tibialis anterior, first dorsal interosseous, thenar and hypothenar muscles in 15 normal subjects, and using computer simulations. RESULTS: The slow afterwaves did not match the time constant of the amplifier's high-pass filter, and so were not filtering artifacts. They lasted long after propagation had terminated at the muscle/tendon junction, and so were not due to the temporal or spatial dispersion of propagating single-fiber potentials. Their amplitude and polarity varied with the recording site as predicted by computer simulations that modeled the IAP as having a negative afterpotential. They also changed with double-pulse stimulation and decreasing temperature in ways consistent with the results of intracellular studies of the IAP negative afterpotential. CONCLUSIONS: The presented results support our hypothesis that the slow afterwave is a manifestation of the IAP negative afterpotential.

Anatomical and electrophysiological determinants of the human thenar compound muscle action potential.

Clinical interpretation of the compound muscle action potential (CMAP) requires a precise understanding of its underlying mechanisms. We recorded normal thenar CMAP5 and motor unit action potentials using different electrode configurations and different thumb positions. Computer simulations show that the CMAP has four parts: rising edge, negative phase, positive phase, and tail which correspond to four distinct stages of electrical activity in the muscle: initiation at the end-plate, propagation, termination at the muscle/tendon junctions, and slow repolarization. The shapes of volume-conducted signals recorded beyond the muscle are also explained by these four stages. Changes in CMAP shape associated with thumb abduction are due to changes in termination times resulting from changes in muscle-fiber lengths. These findings demonstrate that the negative and positive phases of the CMAP are due to different mechanisms, and that anatomical factors, particularly muscle-fiber lengths, play an important role in determining CMAP shape.

Effect of recording electrode position along a muscle fibre on surface potential power spectrum.

Extracellular action potentials produced by a muscle fibre of finite length were calculated for recordings at the skin surface. The sensitivity of power spectra to variations in propagation velocity (ν) and intracellular action potential (IAP) duration (T(in)) was studied theoretically. The magnitude and distribution of the spectral power of muscle fibre potentials depend on the electrode longitudinal position. The relative shifts of the spectra in dB induced by variation in ν or T(in) hardly depend on the longitudinal position of the electrode. A variation in ν affects only the power spectrum positive slope and the initial part of the high-frequency roll-off and a variation in T(in) affects only the remaining part of the high-frequency roll-off. The total spectral amplitude is practically non-sensitive to variations in the wavelength, b = ν.T(in). The total power is sensitive to variations in ν, T(in) as well as in b, and its relative changes depend on the electrode longitudinal position. The whole power spectrum is shifted along the frequency axis and mode (F(max)), median (F(med)) and mean (F(mean)) frequencies have practically equal percentage changes only when ν and T(in) vary jointly in such a way that the product ν.T(in) keeps unchanged.

Use of surface potential spectral characteristics for solving the inverse problem in electroneurography.

The changes in the power spectra of single-fibre extracellular action potentials (SFEAPs) generated in an infinite anisotropic frequency-dependent volume conductor, which occurred as a result of alterations in the propagation velocity v and duration T(in) of the intracellular action potential (IAP) were analytically determined. Effects of the temporal and spatial dispersions of almost synchronously activated fibres on the power spectrum of compound extracellular potentials (CEPs) were analysed for different shapes and sizes of the activated fibres' territory. It was found that, as a result of desynchronisation in the fibres' activation, dips existed in the CEP power spectra and that the frequencies of the dips depended on the degree of desynchronisation but did not depend on the velocity. It was shown that the hypothetical power spectrum of compound IAP was sensitive to the variations in the desynchronisation in the fibres' activation and in the risetime and duration of IAP even at a great fibre electrode distance typical for surface recordings.

Radial changes of extracellular potential amplitude and integral characteristics and the inverse problem in electroneurography.

The possibility of solving the inverse problem in electroneurography, i.e. of estimating the main parameters specifying the activated fibre's functional state, using the amplitude and integral characteristics of the surface potentials generated by infinite homogeneous fibres, has been analysed. An analytical expression has been found for the amplitude of the negative phase Anph of the single fibre extracellular action potential (SFEAP) as a function of the wavelength b, the fibre-electrode distance y and a scale factor Ao proportional to the intracellular action potential amplitude Vm, to the square of the fibre radius a and to the ratio of the axoplasm conductivity sigma a and volume conductor conductivity sigma e. For a large fibre-electrode distance, typical of surface recordings, an analytical expression of the integral of the negative phase Inph of the SFEAP as a function of Ao, b, y and the propagation velocity v was also found. Simple methods are proposed for estimating v, the location of the electrical centre of the activated fibres' territory and the product of the number of activated fibres N, duration T(in) of the intracellular action potential and of the factor Ao. The estimation errors due to the temporal and spatial dispersion of the activated fibres were analysed as a function of the fibre-electrode distance and the territory shape.

Influence of the fiber length on the power spectra of single muscle fiber extracellular potentials.

Influence of the fiber length on the power spectra of the single muscle fiber extracellular action potentials (SMFEAPs) as well as on the spectral changes under alterations in the propagation velocity (v) and intracellular action potential duration (Tin) was studied theoretically for a fiber-electrode distance typical of surface recordings. It was shown that the magnitude and distribution of the SMFEAP spectral power depend on the fiber length. The shorter the fiber, the wider the frequency region in which the total spectral power is distributed and the smaller the spectrum max amplitude. The fiber length affects the spectrum changes that are due to alterations in v or Tin, and, consequently, the spectrum characteristic frequencies--Fmax, Fmed and Fmean, as well as the total spectral amplitude (TSA) and total power (TP). Whereas for a relatively long fiber Fmax depends linearly on v and Fmed and Fmean increase with v increasing, for a short fiber the linearity of the relation between Fmax and v can be disturbed, and Fmed and Fmean can decrease with v increasing. Under the condition of a relatively short fiber, a change in Tin can cause changes not only in Fmed and Fmean, but in Fmax as well. It was shown, that as a result of the finite length of the muscle fibers as well as of the asymmetry of the end-plate location in relation to the fiber ends, dips can be observed in the spectrum even in a case of monopolar recording. Positions of the dips in the power spectra depend on propagation velocity.

Power spectra of single infinite fibre extracellular potentials recorded by a bipolar electrode.

The power spectra of bipolarly recorded extracellular action potentials (EAPs) generated by an infinite, homogeneous, excitable fibre in an infinite, resistive, isotropic and homogeneous volume conductor were theoretically analysed. The changes in the power spectrum of EAP, which occurred as a result of alterations in the propagation velocity v and duration Tin of the intracellular action potential IAP, were analytically determined for bipolar parallel and radial electrodes with a small interpole distance. It was found that the sensitivity of the spectral characteristics to alterations in v, Tin and/or the IAP asymmetry substantially depends on the fibre-electrode distance; information on the IAP fast changes, that seems to be lost in unipolar recording as a result of the filtering effect of the fibre-electrode distance, can be restored. The orientation of the recording electrode need not be taken into account when a qualitative analysis is carried out, but when a quantitative analysis has to be performed, then the electrode orientation has a significant influence. A method is suggested for determination of the fibres' orientation by means of the spectrum of EAPs recorded bipolarly. The selectivity of the bipolar electrodes is analysed.

Power spectra of extracellular potentials generated by an infinite, homogeneous excitable fibre.

The power spectra of the extracellular potentials (EPs) generated under activation of an infinite, homogeneous excitable fibre immersed into an infinite, resistive, isotropic and homogeneous volume conductor are theoretically analysed. The changes in the power spectrum related to the changes in the propagation velocity v, amplitudes Vm and duration Tin of the intracellular action potential (IAP) are analytically determined. It is found that in the ultra-low-frequency region the EP spectral power follows the course of alteration in the square of the modified Bessel function of the second kind and order zero multiplied by the fourth power of the frequency, and the Tin can be assessed by the deviation of the EP power spectrum from this function. It is shown why the sensitivity of the spectral characteristics depends substantially on the radial distance yo from the activated fibre to the point of observation; why the total spectral amplitude depends directly on the IAP wavelength but the total spectral power depends on the IAP wavelength as well as on its duration and propagation velocity; and why the EPs are not proportional to the IAP second spatial derivative even in close proximity to the fibre.

Integral characteristics of extracellular single fibre action potentials.

The changes in the integral of the extracellular action potentials (EAPs) generated by an infinite homogeneous fibre in an infinite homogeneous and isotropic volume conductor were studied at different radial distances (yo) from the fibre axis, depending on the propagation velocity (v), duration (Tin) and asymmetry of the intracellular action potential (IAP). The method used was a mathematical modelling. It was obtained that: the EAPs generated by an infinite fibre have no DC component, i.e. the integral value of the negative phase (INPh) is equal to that of the two positive phases (IPPh); the character of the integral dependences is different at different radial distances; INPh is related both to the EAP amplitude and to the temporal parameters of the EAP; at small radial distances INPh normalized in respect of the negative phase amplitude is in generally linear relationship with the Tin, it decreases when the IAP asymmetry increases (with the Tin being unchanged), and it is not practically affected by the changes in the yo (up to 0.2 mm) and by the changes in the v; at large yo the normalized INPh is almost inversely proportional to the v and is not practically affected by the changes in the Tin and in IAP asymmetry.

BACOMP--database of bioactive compounds for structure-activity relationship.

BACOMP database is presented for structure-activity relationship (SAR) investigations; it was realized on a BK-1300 general purpose microcomputer using the MICRO-SETOR network database management system. Some general considerations of database design are given and the models and facilities for a development of a microcomputer-based SAR oriented database are described. The database contains the following information for the bioactive compounds: chemical structures, biological activities, trade names, reference numbers and information sources. For computer representation of chemical structures SAR oriented language is used. The database software includes: system software, data capture and data editing software, information retrieval and data processing applications. The software development is done in FORTRAN IV and MACRO assembly language. The programs are written in a completely interactive mode. The information retrieval software includes 12 functions giving an information for the database as a whole and for a single compound as well. The data processing software includes 8 functions for finding common structural fragments among compounds with similar biological activity and for estimating a structural similarity between different compounds. The functions are selected from corresponding screen MENUs. The function realization results are framed as appropriate screen formats and the receipt of hardcopies is available. The database can be used to develop predictive methods in respect of the investigated biological activity.