Reliability of quantitative motor unit action potential parameters

We evaluated the test-retest reliability (reproducibility) of motor unit action potential (MUAP) parameters in multi-MUAP analysis over time. Reproducibility studies are not available for needle quantitative electromyography (QEMG) performed by the same examiner. Fourteen consecutive individuals (10 men and 4 women) had repeat QEMG at 3 hours after the first examination, and seven (5 men and 2 women) had a repeat QEMG after 4-10 days. The intraclass correlation coefficient (ICC) was 87-97% with same-day testing and 52-81% with different-day testing. Size index and firing rate were the most reproducible, suggesting use in follow-up multi-MUAP studies.     

A new approach to motor unit estimation with surface EMG triggered averaging technique

A new method for estimating the number of motor units using a surface EMG triggered averaging technique is described. This method provides an estimation of mean motor unit potential (MUP) amplitude at different leveles of contraction, which can be utilized to estimate the number of motor units in a given muscle. Motor unit count estimated in abductor pollicis brevis (APB) muscle of 11 normal healthy subjects ranged from 131 to 371 with a mean of 246 ± 68. In our preliminary study of patients with lower motor neuron lesions, there was a significant reduction in the number of motor units. We believe our new noninvasive method of motor unit counting is a relatively simple and reproducible physiological technique.© 1995 John Wiley &Sons, Inc.     

Determinants of motor unit action potential duration.

OBJECTIVE: Motor unit action potential (MUAP) recordings are modeled by means of a single muscle fiber simulation program, to define two key subcomponents comprising the complete physiologic MUAP duration. A number of defining properties of these subcomponents are further developed. METHODS: A single muscle fiber simulation program is utilized with various muscle fiber lengths and conduction velocities to generate near-field and far-field waveforms. RESULTS: Two key subcomponents to the total physiologic single muscle fiber and hence MUAP duration are identified. One, defined as the near-field component, is directly dependent upon muscle fiber hemi-length. The other, defined as the far-field component, is independent of fiber length, but matches the internal action potential in duration. Both the near-field and far-field components are inversely dependent upon intracellular action potential conduction velocity. Additionally, temporal dispersion among the individual fibers contributing to a MUAP must be included in the overall MUAP duration calculation. CONCLUSIONS: It is hoped that this approach to MUAP duration may allow a more complete appreciation of the components contributing to the MUAP, than permitted by the empirically derived values for MUAP duration presently under clinical use.     

Comparison of single-fiber and macro electrode recordings: Relationship to motor unit action potential duration

The factors contributing to the duration of a motor unit action potential (MUAP) are believed to be well known, with both manual measurements and computer simulations agreeing with respect to MUAP durations approaching 10 ms. In this investigation, it is clearly demonstrated that use of a wide-open amplifier bandpass combined with signal-to-noise ratio enhancement results in MUAP durations approaching 30 ms recorded with either a macro or single-fiber electrode. Why the clinically recorded MUAP duration differs significantly from these physiologic durations is discussed. A hypothesis is presented whereby the major contributing factor toward MUAP duration is the total time of action potential transmembrane current flow along the muscle fiber from end-plate zone to musculotendinous junction. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1381–1388, 1997     

Sliding window averaging in normal and pathological motor unit action potential trains

Objective

To evaluate the performance of a recently proposed motor unit action potential (MUAP) averaging method based on a sliding window, and compare it with relevant published methods in normal and pathological muscles.

On the selection of concentric needle electromyogram motor unit action potentials: Is the rise time criterion too restrictive?

Concentric needle electromyogram motor unit action potentials (MUAPs) were recorded from the biceps brachii muscle of normal subjects and in patients with neuromuscular diseases. Although the MUAPs had a crisp sound and appeared sharp, their rise time (RT), measured from the maximum negative peak to the preceding maximum positive peak before it, was often >500 μs. All MUAPs with a RT ≤500 μs were recorded from within the motor unit (MU) territory. MUAP recordings from outside the MU territory had a long RT but also low amplitude (<50 μV) and/or a characteristic initial negative deflection. In the remaining recordings from within the MU territory, MUAP duration remained relatively constant while MUAP amplitude and RT varied inversely with each other. These MUAPs may be useful in electrodiagnosis but discarded due to their longer RT. We feel that while the current RT criterion for MUAP selection ensures that the electrode tip is within the MU territory, it is also too restrictive. © 1996 John Wiley & Sons, Inc.     

Effects of intramuscular needle position on motor unit action potential metrics

It is unclear whether there are clinically significant differences in amplitude, duration, and numbers of turns and phases if an electromyographic (EMG) study is performed near to, or far from, the end-plate zone. The effects of temporal dispersion of arriving muscle-fiber action potentials on quantitative motor unit action potential (MUAP) metrics were assessed in simulated and biologic muscles. Two muscle simulation models were studied with electrode recording positions near the motor end-plate zone and 50-75 mm away. When the electrode was moved away from the end-plate zone, averages of 20 MUAPs significantly decreased in amplitude and area, and increased in numbers of turns and phases, but there was no significant change in duration. In biologic muscles (both normal and pathologic), similar changes in average metrics were observed, but to lesser degrees; few were statistically significant. Zones of innervation in biologic muscles are broadly distributed and, during routine electrode studies, distances between random electrode placements and end-plate zones are therefore relatively short, leading to clinically insignificant changes in quantitative MUAP metrics with distance from the end-plate zone. Thus, electrode position within a muscle is unlikely to affect clinical MUAP interpretation.     

Tracking motor unit action potentials in the tibialis anterior during fatigue

New surface electromyogram (SEMG) techniques offer the potential to advance knowledge of healthy and diseased motor units. Conduction velocity (CV) estimates, obtained from indwelling electrodes, may provide diagnostic information, but the standard method of CV estimation from SEMG may be of only limited value. We developed a motor unit (MU) tracking algorithm to extract motor unit conduction velocity (MUCV) and motor unit action potential (MUAP) amplitude estimates from SEMG. The technique is designed to provide a noninvasive means of accessing fatigue and recruitment behavior of individual MUs. We have applied this MU tracking algorithm to SEMG data recorded during isometric fatiguing contractions of the tibialis anterior (TA) muscle in nine healthy subjects, at 30%-40% maximum voluntary contraction (MVC). The results reveal that MUCVs and MUAP amplitudes of individual MUs can be estimated and tracked across time. Time-related changes in the MU population may also be monitored. Thus, the SEMG technique employed provides insight into the behavior of the underlying muscle at the MU level by noninvasive means.     

Impact of stimulus duration on motor unit thresholds and alternation in compound muscle action potential scans

ObjectiveTo investigate the impact of stimulus duration on motor unit (MU) thresholds and alternation within compound muscle action potential (CMAP) scans.MethodsThe stimulus duration (0.1, 0.2, 0.6, and 1.0 ms) in thenar CMAP scans and individual MUs of 14 healthy subjects was systematically varied. We quantified variability of individual MU’s thresholds by relative spread (RS), MU thresholds by stimulus currents required to elicit target CMAPs of 5% (S5), 50% (S50) and 95% (S95) of the maximum CMAP, and relative range (RR) by 100*[S95-S5]/S50. We further assessed the strength-duration time constant (SDTC). Experimental observations were subsequently simulated to quantify alternation.ResultsRS, unaffected by stimulus duration, was 1.65% averaged over all recordings. RR increased for longer stimulus duration (11.4% per ms, p < 0.001). SDTC shortened with higher target CMAPs (0.007 ms per 10% CMAP, p < 0.001). Experiments and simulations supported that this may underlie the increased RR. A short compared to long stimulus duration recruited relative more MUs at S50 (more alternation) than at the tails (less alternation).ConclusionsThe stimulus duration significantly affects MU threshold distribution and alternation within CMAP scans.SignificanceStimulation settings can be further optimized and their standardization is preferred when using CMAP scans for monitoring neuromuscular diseases.     

Size does matter: The influence of motor unit potential size on statistical motor unit number estimates in healthy subjects

Objective

The statistical method of motor unit number estimation (MUNE) assumes that all motor unit potentials (MUPs) have the same size. The present study aims to evaluate the consequences of this assumption as well as its implications for the validity of statistical MUNEs.

Methods

We performed statistical and multiple point stimulation (MPS) MUNE with an array of 120 electrodes on the thenar muscles of 15 healthy subjects. These recordings allow isolation and quantification of the effect of non-uniform MUP size on MUNE, because the differences in submaximal CMAP size (and, hence, in MUNE) between electrodes are due almost entirely to differences in (summed) MUP size.

Results

We found no correlation between statistical and MPS MUNEs. Statistical MUNEs proved very sensitive to small variations in the “bandwidth” (variance) of the response series; MUNEs from electrodes only 8 mm apart could deviate by as much as 60%. This variation in bandwidth resulted from spatial (and, hence, size) differences between the contributing MUPs.

Conclusions

Statistical MUNEs are very sensitive to violation of the uniform MUP-size assumption, to an extent that blurs any correlation with MPS MUNE in healthy subjects.

Significance

Statistical MUNE cannot be used to detect mild to moderate motor unit losses.     

Effects of evoked pain on the electromyogram and compound muscle action potential of the brachial biceps muscle

Muscle pain is often accompanied by a feeling of muscle fatigue and weakness. We examined the effect of experimental muscle pain on the electromyogram (EMG) during maximal voluntary contraction (MVC), and on the compound muscle action potential (CMAP) of the brachial biceps muscle. Twenty-one healthy subjects were injected intramuscularly with 0.1 ml/30 microg of the vanilloid receptor agonist (capsaicin) or 0.2 ml of 5% hypertonic saline. A Teflon-coated cannulated EMG needle was used to record the EMG interference pattern (IP) at MVC. The CMAP of the brachial biceps muscle was obtained by stimulation of the musculocutaneous nerve at the axilla using surface electrodes. Amplitude, mean frequency of the power spectrum, and turns/s of the interference pattern were reduced after pain induced by capsaicin or hypertonic saline. Latency, amplitude, and area-under-curve of the CMAP did not change after injection of either substance. Acute stimulation of muscle nociceptors thus produced a fatiguelike change in the interference pattern during MVC, possibly due to a decrease in motoneuron firing rate and increased muscle fatigability.     

Evaluation of motor unit potential wavelet analysis in the electrodiagnosis of neuromuscular disorders

Introduction: Electrophysiological studies of human motor units can use various electromyographic techniques. Together with the development of new techniques for analysis and processing of bioelectric signals, motor unit action potential (MUAP) wavelet analysis represents an important change in the development of electromyographic techniques. Methods: The proposed approach involves isolating single MUAPs, computing their scalograms, taking the maximum values of the scalograms in 5 selected scales, and averaging across MUAPs to give a single five‐dimensional feature vector per muscle. After Support Vector Machine analysis, the feature vector is reduced to a single decision parameter that allows the subject to be assigned to 1 of 3 groups: myogenic, healthy, or neurogenic. The software is available as freeware. Results: MUAP wavelet analysis yielded consistent results for the diagnostic index and muscle classification, with only 7 incorrect classifications out of a total of 1,015 samples. Conclusions: This proposed approach provides a sensitive and reliable method for evaluating and characterizing MUAPs. Muscle Nerve 46: 63–69, 2012     

Multipoint incremental motor unit number estimation versus amyotrophic lateral sclerosis functional rating scale and the medical research council sum score as an outcome measure in amyotrophic lateral sclerosis

Introduction:

Monitoring the disease progression in amyotrophic lateral sclerosis (ALS) is a challenge due to different rates of progression between patients. Besides clinical methods to monitor disease progression, such as the ALS functional rating scale (ALSFRS) and the medical research council (MRC) sum score, quantitative methods like motor unit number estimation (MUNE) are of interest.

Objective:

The objective of the present study is to evaluate the rate of progression in ALS using multipoint incremental MUNE and to compare MUNE, ALSFRS and MRC sum score at baseline and at 6 months for progression of the disease.

Materials and Methods:

Multipoint incremental MUNE using median nerve, ALS-FRS and MRC sum score was carried out in 29 ALS patients at baseline and then at 6 months.

Results:

Of the 29 ALS patients studied, the mean MUNE at baseline was 21.80 (standard deviation [SD]: 19.46, range 4-73), 15.9 in the spinal onset group (SD: 14.60) and 30.16 (SD: 22.89) in the bulbar onset group. Spinal onset patients had 74.02% of baseline MUNE value while bulbar onset patients had only 24.74% baseline value MUNE at 6 months follow-up (Unpaired t-test, P = 0.001). ALSFRS and MRC sum score showed statistically significant decline (P < 0.001) at 6 months follow-up. MUNE had the highest sensitivity for progression of the disease when compared to the ALS FRS and MRC sum score.

Conclusion:

Multipoint incremental MUNE is a valuable tool for outcome measure in ALS and other diseases characterized by motor unit loss. The rate of decline of multipoint incremental MUNE is more sensitive than that of MRC sum score and ALSFRS-R, when expressed as the percentage change from baseline.     

Effect of maximal voluntary contraction on the amplitude of the compound muscle action potential: Implications for the interpolated twitch technique

The compound muscle action potential (M_MAX) during a maximal voluntary contraction (MVC) may be measured to determine if the motor nerve has been supramaximally stimulated during the interpolated twitch technique (ITT). Ten males performed isometric knee extension MVCs. M_MAX for the vastus medialis was recorded during MVC and rest. To examine the effect of stimulating electrode movement, the M_MAX of the thenar group and antidromic sensory nerve action potentials (SNAPs) to the third digit were recorded in a separate experiment. M_MAX during MVC was reduced by 18% (P < 0.0001) and 43% (p < 0.0001) for the quadriceps and thenar group, respectively. The SNAP amplitude was not different between rest and MVC (P = 0.18). Reduction of M_MAX during MVC suggests that some motor axons are refractory and unable to respond to a superimposed maximal stimulus. These results have implications for the sensitivity of the interpolated twitch technique. Muscle Nerve, 2010     

Determination of the motor unit behavior of lumbar erector spinae muscles through surface EMG decomposition technology in healthy female subjects

Introduction: The aims of this study were to determine the motor unit behavior of the erector spinae muscles and to assess whether differences exist between the dominant/nondominant sides of the back muscles. Methods: Nine healthy women, aged 21.7 years (SD = 0.7), performed a back extension test. Surface electromyographic decomposition data were collected from both sides of the erector spinae and decomposed into individual motor unit action potential trains. The mean firing rate for each motor unit was calculated, and a regression analysis was performed against the corresponding recruitment thresholds. Results: The mean firing rate ranged from 15.9 to 23.9 pps and 15.8 to 20.6 pps on the dominant and nondominant sides, respectively. However, the early motor unit potentials of the nondominant lumbar erector spinae muscles were recruited at a lower firing rate. Conclusions: This technique may further our understanding of individuals with back pain and other underlying neuromuscular diseases. Muscle Nerve 55 : 28–34, 2017     

Noise reduction based on ICA decomposition and wavelet transform for the extraction of motor unit action potentials

We have studied methods for noise reduction of myoelectric signals and for extraction of motor unit action potentials from these signals. Effective MUAP peak detection is the first important step in EMG decomposition. We first combined independent component analysis and wavelet filtering to remove power line interference, and then applied a wavelet filtering method and threshold estimation calculated using wavelet transform to suppress background noise and Gaussian white noise. The technique was applied to single-channel, short-period real myoelectric signals from normal subjects and to artificially generated EMG recordings. In contrast to existing methods based on amplitude single-threshold filtering of the original myoelectric signal or a conventional digitally filtered signal, our technique is fast and robust. Moreover, the proposed algorithm is substantially automatic. The performance has been evaluated with a set of synthetic and experimentally recorded myoelectric signals. The basic tool for testing was power spectrum density (PSD) estimation by the Welch method, which allowed us to analyze the PSD of nonstationary signals.     

Utility of the distal compound muscle action potential duration for diagnosis of demyelinating neuropathies

To assess the significance of distal compound muscle action potential (CMAP) duration for diagnosis of demyelinating neuropathies, electrophysiologic data were reviewed from 471 subjects, including 145 normal controls, 60 patients with chronic inflammatory demyelinating polyneuropathy (CIDP), 205 with other neuropathy, and 61 with amyotrophic lateral sclerosis (ALS). The duration of distally evoked CMAP was measured in the median, ulnar, tibial, and peroneal nerves. Optimal cut-off values were calculated with receiver-operating characteristic (ROC) curves. In comparison of normal controls and CIDP patients, ROC analyses showed the sufficient area under the curves (82-93%). When the cut-off values in the detection of demyelination were determined as the point with 98% specificity vs. normal on the ROC curves (median, 6.6 ms; ulnar, 6.7 ms; peroneal, 7.6 ms; tibial, 8.8 ms), the sensitivity was 77% for CIDP, with a specificity of 90% vs. ALS and 95% vs. diabetic neuropathy. The distal CMAP duration is a useful index for the detection of distal demyelination. We suggest the above cut-off values for each nerve as one of the electrodiagnostic criteria for demyelinating neuropathies, preferentially affecting the distal nerve terminals, such as CIDP.     

The distal sensory nerve action potential as a diagnostic tool for the differentiation of lesions in dorsal roots and peripheral nerves

Summary On the basis of the conception that, in preganglionic lesions, peripheral sensory nerve fibers should remain intact, the question arises whether evaluation of distal sensory nerve action potentials can be helpful in differentiating between cervical dorsal root and peripheral nerve lesions. Amplitudes of sensory nerve action potentials (SNAP) and corresponding distal sensory conduction velocities (SCV) of the median and ulnar nerves were measured in 194 patients with distal sensory impairment and in 20 control subjects. In dorsal root lesions attributable to degenerative changes of cervical spines and/or discs, SNAP and SCV were within the normal range. In peripheral nerve lesions, located in the brachial plexus, or at the elbow or wrist, diminished SNAP amplitudes and/or slowing of SCV were found. It is concluded that in patients with sensory deficit in the hands recordings of SNAP make a differentiation between lesions of dorsal roots and peripheral nerves possible.

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Zusammenfassung Geht man davon aus, daß bei präganglionären Läsionen die peripheren sensiblen Nervenfasern intakt bleiben, so ergibt sich die Frage, ob Registrierungen distaler sensibler Nervenaktionspotentiale eine Differenzierung zwischen Läsionen zervikaler Hinterwurzeln und Läsionen peripherer Nerven ermöglichen. Bei 194 Patienten mit objektivierbaren distalen Sensibilitätsstörungen und bei 20 Kontrollpersonen wurden die Amplituden des distalen sensiblen Nervenaktionspotentials (SNAP) des Nervus medianus und des Nervus ulnaris sowie die entsprechenden distalen sensiblen Nervenleitgeschwindigkeiten gemessen. Im Falle von Hinterwurzelschädigungen, die durch degenerative Veränderungen der zervikalen Wirbelkörper und/oder Bandscheiben verursacht wurden, lagen die Amplituden des SNAP und die sensiblen Nervenleitgeschwindigkeiten im Normbereich. Bei Läsionen peripherer Nerven im Plexusbereich, am Ellenbogen oder am Handgelenk hingegen wurde eine Amplitudenreduktion des SNAP und/oder eine Verringerung der sensiblen Nervenleitgeschwindigkeit gefunden. Die Ergebnisse sprechen dafür, daß bei Sensibilitätsstörungen im Bereich der Hände die Auswertung der sensiblen Nervenaktionspotentiale und der entsprechenden Nervenleitgeschwindigkeiten eine Differenzierung zwischen Wurzelläsion und Läsion peripherer Nerven ermöglicht.

     

Quantitative studies of lower motor neuron degeneration in amyotrophic lateral sclerosis: Evidence for exponential decay of motor unit numbers and greatest rate of loss at the site of onset

Objective

To use our Bayesian method of motor unit number estimation (MUNE) to evaluate lower motor neuron degeneration in ALS.

Methods

In subjects with ALS we performed serial MUNE studies. We examined the repeatability of the test and then determined whether the loss of MUs was fitted by an exponential or Weibull distribution.

Results

The decline in motor unit (MU) numbers was well-fitted by an exponential decay curve. We calculated the half life of MUs in the abductor digiti minimi (ADM), abductor pollicis brevis (APB) and/or extensor digitorum brevis (EDB) muscles. The mean half life of the MUs of ADM muscle was greater than those of the APB or EDB muscles. The half-life of MUs was less in the ADM muscle of subjects with upper limb than in those with lower limb onset.

Conclusions

The rate of loss of lower motor neurons in ALS is exponential, the motor units of the APB decay more quickly than those of the ADM muscle and the rate of loss of motor units is greater at the site of onset of disease.

Significance

This shows that the Bayesian MUNE method is useful in following the course and exploring the clinical features of ALS.