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Biomedical subjects

K C McGill

Publications and source records attributed to K C McGill.

At least 19 recordsLinked to original sources
PubMed2004

Surface electromyogram signal modelling.

The paper reviews the fundamental components of stochastic and motor-unit-based models of the surface electromyogram (SEMG). Stochastic models used in ergonomics and kinesiology consider the SEMG to be a stochastic process whose amplitude is related to the level of muscle activation and whose power spectral density reflects muscle conduction velocity. Motor-unit-based models for describing the spatio-temporal distribution of individual motor-unit action potentials throughout the limb are quite robust, making it possible to extract precise information about motor-unit architecture from SEMG signals recorded by multi-electrode arrays. Motor-unit-based models have not yet been proven as successful, however, for extracting information about recruitment and firing rates throughout the full range of contraction. The relationship between SEMG and force during natural dynamic movements is much too complex to model in terms of single motor units.

Action Potentials↗
PubMed2001

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.

Action Potentials↗
PubMed2001

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.

Action Potentials↗
PubMed2001

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.

Action Potentials↗
PubMed1999

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.

Action Potentials↗
PubMed1999

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.

Action Potentials↗
PubMed1998

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.

Action Potentials↗
PubMed1996

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.

Action Potentials↗
PubMed1995

Action potentials of curved nerves in finite limbs.

Previous simulations of volume-conducted nerve-fiber action-potentials have modeled the limb as semi-infinite or circularly cylindrical, and the fibers as straight lines parallel to the limb surface. The geometry of actual nerves and limbs, however, can be considerably more complicated. This paper presents a general method for computing the potentials of fibers with arbitrary paths in arbitrary finite limbs. It involves computing the propagating point-source response (PPSR), which is the potential arising from a single point source (dipole or tripole) travelling along the fiber. The PPSR can be applied to fibers of different conduction velocities by simple dilation or compression. The method is illustrated for oblique and spiralling nerve fibers. Potentials from oblique fibers are shown to be different for orthodromic and antidromic propagation. Such results show that the straight-line models are not always adequate for nerves with anatomical amounts of curvature.

Action Potentials↗
PubMed1993

The nerve gap dilemma: a comparison of nerves repaired end to end under tension with nerve grafts in a primate model.

The objective of this study was to compare, in a clinically relevant primate model, axon regeneration after epineurial repair under tension (15 mm gap) with interfascicular nerve grafts with the use of either standard microsuture techniques or a new interfascicular nerve graft technique termed fascicular tubulization that uses a hypoantigenic collagen membrane formed into a tube to approximate nerve ends. Electrophysiologic analysis demonstrated that the percentage of proximal axons that conducted across the repair site was greater in those nerves repaired under tension with epineurially placed sutures than in either of the tensionless repairs involving interfascicular graft techniques. The mean diameters of the regenerated axons repaired under tension with epineurial sutures were greater than those of the nerves repaired with interfascicular grafts, although the difference was not statistically significant. Interfascicular nerve grafting with tubulization using the current collagen tube resulted in regeneration equal to the sutured interfascicular nerve grafts. For modest defects (perhaps up to 3 to 4 cm in the adult), it seems advantageous to accept the modest tension associated with an epineurial repair rather than to use an autograft (or artificial graft) to achieve a tension-free repair.

Anastomosis, Surgical↗
PubMed1991

A comparison of turns analysis and motor unit analysis in electromyography.

We compared the results of turns analysis and motor unit analysis on 4056 electromyographic interference patterns (IPs) from normal subjects and patients with neuromuscular disorders. The motor unit analysis involved decomposing the IPs into their component motor unit action potentials (MUAPs) using automatic decomposition electromyography (ADEMG). We checked the accuracy of the decompositions by attempting to reconstruct some of the IPs from their identified MUAPs using computer simulations. The simulations revealed that ADEMG typically identified more than 60% (but not all) of the MUAPs in a given IP. Both turns and MUAP properties showed regular and related changes with force, age, muscle, and recording electrode type. The number of turns in each IP was highly correlated with the number of active MUAPs (r = 0.65), the mean MUAP firing rate (r = 0.72), the mean number of turns per MUAP (r = 0.34), and the product of these 3 properties (r = 0.83). The mean amplitude change per turn was highly correlated with the mean MUAP amplitude (r = 0.82), but also depended on the number of turns per MUAP. Due to the lack of a one-to-one relationship between the turns analysis properties and the MUAP properties, the turns analysis properties by themselves did not provide sufficient information to infer unambiguous physiological information about motor unit morphology or firing behavior.

Action Potentials↗
PubMed1991

A comparison of suture and tubulization nerve repair techniques in a primate.

This study compared standard methods of nerve repair, epineurial or perineurial sutures with a technique termed fascicular tubulization using a biodegradable polyglycolic acid tube in a nonhuman primate model. Electrophysiologic analysis demonstrated that the percentage of proximal axons that conducted across the repair site did not significantly differ among the three techniques while epineurial suture repairs were associated with significantly longer conduction delays across the repair site compared with the other two techniques. Even though fascicular tubulization using the current polyglycolic acid tube resulted in regeneration equal to the currently perceived best suture repair technique, associated technical problems with the current tube design indicate that this fascicular tubulization technique cannot, at present, be considered as an alternative to present clinically used nerve suture techniques.

Animals↗
PubMed1990

Triphasic behavioral response of motor units to submaximal fatiguing exercise.

We have measured the firing rate and amplitude of 4551 motor unit action potentials (MUAPs) recorded with concentric needle electrodes from the brachial biceps muscles of 10 healthy young adults before, during, and after 45 minutes of intermittent isometric exercise at 20% of maximum voluntary contraction (MVC), using an automatic method for decomposition of electromyographic activity (ADEMG). During and after exercise, MUAPs derived from contractions of 30% MVC showed progressive increase in mean firing rate (P less than or equal to .01) and amplitude (P less than or equal to .05). The firing rate increase preceded the rise in mean amplitude, and was evident prior to the development of fatigue, defined as reduction of MVC. Analysis of individual potentials revealed that the increase in firing rate and in amplitude reflected different MUAP subpopulations. A short-term (less than 1 minute) reduction in MUAP firing rates (P less than or equal to .05) was also observed at the onset of each test contraction. These findings suggest that motor units exhibit a triphasic behavioral response to prolonged submaximal exercise: (1) short-term decline and stabilization of onset firing rates, followed by (2) gradual and progressive increase in firing rates and firing variability, and then by (3) recruitment of additional (larger) motor units. The (2) and (3) components presumably compensate for loss of force-generating capacity in the exercising muscle, and give rise jointly to the well-known increase in total surface EMG which accompanies muscle fatigue.

Action Potentials↗
PubMed1989

Motor unit firing rates and firing rate variability in the detection of neuromuscular disorders.

We have used automatic decomposition electromyography (ADEMG) to study 41 muscles in 29 patients with well-defined peripheral and central motor disorders. In motor neuron diseases motor unit action potentials (MUAPs) showed increased amplitudes, firing rates and firing variability. Relatively large MUAPs sometimes were not identified by the computer program if they lacked sufficient high-frequency signal content, or were too variable in shape. In myopathies the MUAPs showed reduced amplitudes, durations and turns, and sometimes dramatic increases in firing rates. Also, the mean number of MUAPs per recording site was often increased, indicating excessive recruitment. In polymyositis (the best studied myopathy) the nature and magnitude of the MUAP shape and firing abnormalities were usually similar at different levels of contractile force, suggesting that motor units are affected without regard to recruitment order. In upper motor neuron paresis (multiple sclerosis), the shape properties of the MUAPs were normal, but mean firing rates were reduced, and firing variability increased. These findings confirm many of the traditional criteria for distinguishing neurogenic from myopathic disease electrophysiologically at the level of the individual MUAP. In addition, they demonstrate the potential diagnostic sensitivity of MUAP firing rate measurements for detecting neuromuscular dysfunction, and for differentiating between some cases of central and peripheral paresis, but not for distinguishing peripheral neurogenic from myopathic weakness, since firing rates tend to increase in both. Increased firing rate variability may be a marker of central or peripheral neurogenic weakness.

Action Potentials↗
PubMed1988

Age effects on properties of motor unit action potentials: ADEMG analysis.

We have measured the configurational and firing properties of 13,206 motor unit action potentials (MUAPs) from the brachial biceps, brachial triceps, and anterior tibial muscles in 10 young (20-40 years), 10 middle-aged (40-60 years), and 10 elderly (60-80 years) normal individuals, using an automatic method for decomposition of the electromyographic (EMG) interference pattern (ADEMG). Recording were made during stable isometric contractions at threshold, 10%, and 30% of maximum voluntary contraction using standard concentric needle electrodes. At supra-threshold forces, an average of 5.9 simultaneously active MUAPs were identified at each recording site. Mean amplitudes, durations, and numbers of turns all increased linearly with age in both low-threshold and high-threshold MUAPs (p less than 0.01), suggesting an ongoing process of progressive denervation and compensatory reinnervation. Mean MUAP firing rates decreased with age (p = 0.01) when force was measured proportionately, but not when measured absolutely. In a subgroup of 12 age-matched gender pairs, men had larger mean MUAP amplitudes, rise rates, and numbers of turns (p less than 0.05), probably reflecting larger muscle fiber diameters. These findings amplify previous observations from traditional analysis of lowest-threshold single MUAPs; establish a base of normative adult data for ADEMG; and further validate the clinical applicability of rapid, automatic EMG decomposition.

Action Potentials↗
PubMed1988

AAEE minimonograph #29: automatic quantitative electromyography.

The present status of different computerized methods of automatic quantitative electromyography are reviewed. Interference pattern methods-turns analysis, spectral analysis-are efficient, but the results usually cannot be directly related to the physiological properties of the motor units. Integration analysis does not currently have a major role in diagnostic electromyography. Traditional measurement of single motor unit action potentials during weak contraction can be facilitated and made more objective with computer assistance, but only the lowest-threshold motor units in the muscle are amenable to study. A new class of methodologies under development permit the decomposition of interference patterns into their constituent motor unit action potentials for measurement of configurational and behavioral properties. Patient data from these various methods can be statistically compared with normative data bases available on-line in computerized electromyographs. Both quantitative and quantitative electromyography have applications in the neuromuscular electrodiagnostic examination.

Electromyography↗