Decomposition-based quantitative electromyography: methods and initial normative data in five muscles

Quantitative electromyographic (EMG) techniques provide clinically useful information to aid in the diagnosis and follow the course or response to treatment of diseases affecting the motor system. The purpose of this study was to describe a decomposition-based quantitative electromyography method (DQEMG) designed to obtain clinically applicable information relating to motor unit potential (MUP) size and configuration, and motor unit (MU) firing characteristics. Additionally, preliminary normative data were obtained from the deltoid, biceps brachii, first dorsal interosseous, vastus medialis, and tibialis anterior muscles of 13 control subjects. DQEMG was capable of efficiently and accurately extracting MUP data from complex interference patterns during mild to moderate contractions. MUP amplitude, surface-detected MUP (S-MUP) amplitude, MUP duration, number of phases, and MU firing frequencies varied significantly across muscles. The mean parameter values for the individual muscles studied were similar to previous reports based on other quantitative methods. The main advantages of this method are the speed of data acquisition and processing, the ability to obtain MUPs from MUs with low and higher recruitment thresholds, and the ability to obtain both S-MUP or macro-MUP data as well as MU firing rate information.     

Motor unit number estimates and quantitative motor unit analysis in healthy subjects and patients with amyotrophic lateral sclerosis

Limitations associated with global measures of function in patients with amyotrophic lateral sclerosis (ALS) and the qualitative nature of needle electromyography have stimulated the development of alternate means of monitoring disease severity and progression in ALS. Thus, the objective of this study was to examine the ability of one these techniques, decomposition-based quantitative electromyography (DQEMG), to obtain electrophysiological data, including motor unit number estimates (MUNEs), from a group of patients with ALS. The first dorsal interosseous and biceps brachii muscles were studied in 10 healthy subjects and 9 patients with ALS. Following the acquisition of a maximum M wave, needle- and surface-detected EMGs were collected simultaneously during 30-second contractions performed at 10% of the maximum voluntary contraction force to obtain motor unit potential (MUP) trains. DQEMG was then used to extract the surface-detected MUP associated with each MUP train, the mean size of which was divided into the maximum M wave to obtain a MUNE. The results suggest that quantitative electrophysiological data obtained using DQEMG are representative of the pathophysiological changes in the lower motor system in ALS patients, supporting its use in studies documenting the natural history and progression of the disease.     

The motor unit and quantitative electromyography

Electromyography (EMG) assesses the anatomic motor unit (A-MU), but knowledge of its anatomy, physiology, and changes with pathology is limited. The electrophysiological motor unit (E-MU) and its motor unit potential (E-MUP) represents a fraction of the A-MU. Routine EMG assesses a limited number of E-MUP waveform characteristics (metrics) and their magnitudes qualitatively scaled in a nonlinear manner. Another approach is quantitative EMG (QEMG), whereby 20⁺ E-MUPs are extracted and both basic and derived metrics obtained and values expressed quantitatively. In diseased muscle, many E-MUP metrics may be normal, which complicates diagnostic interpretation. In QEMG, E-MUP metrics can be clustered and statistical analyses performed to assign probabilities that E-MUPs (and the muscle) are normal, neuropathic, or myopathic. In this article we review what is known about the A-MU, the restricted E-MU, E-MUP metrics, and what QEMG offers currently and in the future.     

Effect of small motor unit potentials on the motor unit number estimate

Small surface motor unit potentials (S-MUPs) may have a negative influence on the variability of the motor unit number estimate (MUNE). According to published consensus criteria S-MUPs with a negative peak amplitude smaller than 10 muV should be omitted. The effect of omitting small S-MUPs on the MUNE was evaluated using a simulation model. The model incorporated a healthy and amyotrophic lateral sclerosis (ALS) distribution formed with real S-MUPs. Using a random drawing process the MUNE was calculated with and without small S-MUPs. In the healthy population 27% of all S-MUPs were small. MUNE determined without these S-MUPs was marginally less variable. However, MUNE values dropped about 24% at a sample size of 20. In ALS, only 12% of the total population of 130 S-MUPs were small. MUNE dropped about 12% without the small S-MUPs. By omitting small S-MUPs the differences between the healthy and ALS distributions become smaller. Therefore, incorporating small S-MUPs in the estimate is suggested.     

Motor unit size estimation of enlarged motor units with surface electromyography

Surface EMG is hardly used to estimate motor unit (MU) characteristics, while its non-invasiveness is less stressful for patients and allows multi-electrode recordings to investigate different sites of the muscle and MU. The present study compares motor unit potentials (MUPs) obtained with surface EMG and macro EMG during voluntary contraction of the biceps brachii muscle of patients with enlarged MUs caused by prior poliomyelitis. Averaged surface MUPs were obtained by means of needle EMG (SMUP1) and surface EMG (SMUP2) triggering. The MUPs area and peak amplitudes correlated well when comparing the macro MUP and SMUP1 of the same MUs. When MU populations of different patients were compared, the SMUP1s and SMUP2s were equally sensitive to pathology as macro MUPs. In this, the late non-propagating positive wave (only present in unipolar recordings) is more robust than the triphasic propagating wave. Therefore, surface EMG can be used for detecting enlarged MUs. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:878–886, 1998.     

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.     

Optimizing acquisition time in quantitative electromyography

Quantitative electromyography (QEMG) relies on a number of discharges of the same motor unit action potential (MUAP) from a train to create an averaged MUAP considered to be representative of a true potential. The train of potentials may be affected by changes in position of the electrode relative to contributing muscle fibers of the motor unit due to operator or subject movement. The effect of changes in electrode position, along with consideration for patient comfort, prompted this study to determine the shortest duration of recording time necessary for sufficient data acquisition for QEMG studies. We determined that 10 seconds of moderate muscle activity is the most reasonable acquisition duration to isolate up to 6 MUAPs at a given electrode site and minimize the effects of movement artifact in the signal. Muscle Nerve, 2009     

Determinants of mean motor unit size: Impact on estimates of motor unit number

The purpose of this study was to compare two fundamentally different methods of deriving the average surface-detected motor unit action potential (S-MUAP) size from which to calculate a motor unit number estimate (MUNE), namely: (1) the simple arithmetic average of S-MUAP parameter values; and (2) a computer-derived datapoint by datapoint average waveform which takes account of differences in S-MUAP shapes and durations. Multiple point stimulation was used to collect representative samples of between 11 and 20 S-MUAPs (mean 15 ± 2 SD) from the median-innervated thenar muscles of 20 healthy control subjects between 20 and 76 years of age (mean 48 ± 19 SD). The average S-MUAP size based on peak-to-peak amplitude, negative peak amplitude, and negative peak area measurements was calculated using the two different methods. The mean S-MUAP sizes based on the average waveform were significantly lower in all cases than those based on the simple average of S-MUAP parameter values. Differences tended to be greatest for MUNEs based on peak-to-peak amplitude (35%), less for negative peak amplitude (20%), and least for negative peak area (16%). © 1993 John Wiley & Sons, Inc.     

Motor unit number estimation by spatial-temporal summation of single motor unit potentials

Current techniques for motor unit number estimation (MUNE) rely on the amplitude of the compound muscle action potential (CMAP) evoked by supramaximal stimulation and mean amplitude of single motor unit potentials (SMUPs). The phase cancellation during summation is not considered. We developed a technique to address this issue. Slow and fast types of motor unit potentials were collected from 5 normal subjects from their abductor pollicis brevis muscles by low-level voluntary contractions, and near-threshold nerve stimulation, respectively. Two of each type of SMUPs were used as templates for reconstructing the best fitted CMAP using a feed-forward neural network. The total number of SMUPs simulated from the four templates during the reconstruction served as MUNE. The mean MUNE was 222 ± 98. The technique is simple and noninvasive, and may be applied in the future for MUNE in patients. © 1997 John Wiley & Sons, Inc. Muscle Nerve, 20, 461–468, 1997     

Shape irregularity of motor unit potentials in some neuromuscular disorders

The aim of this study was to test whether analysis of the irregularity of the motor unit potential waveform may supplement conventional evaluation. We have found that the irregularity is not a characteristic feature of potentials either in neurogenic disorders or in myopathy. We have found, however, that within myo- and neuropathic disorders, the irregularities differ between slow (such as Becker muscle dystrophy and chronic spinal muscle atrophy) and fast progressing processes (such as amyotrophic lateral sclerosis and Duchenne muscle dystrophy). These differences depend on the different number of phases and turns contributing to wave formation. In slowly progressing processes, very irregular potentials are more often polyphasic, whereas in acute processes they may be polyturn or polyphasic. The results suggest that it is the irregularity of the potential that provides new information, not available so far, on the activity of the pathological process. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21: 1181–1187, 1998.     

Simulation of myopathic motor unit action potentials

Normal motor units (MUs) were simulated and their architecture altered to simulate the changes produced by myopathy. The concentric needle electromyographic recordings of motor unit action potentials (MUAPs) from the MUs were then also simulated. These simulated MUAPs showed features that are seen in myopathy: normal amplitude and slightly reduced area, MUAPs with simple waveform and reduced duration, and complex MUAPs with normal or increased duration. The MUAP waveforms were complex because of increased variability of fiber diameter and not because of loss of muscle fibers. The MUAP duration increased when the variability of fiber diameter increased. Finally, MUAPs similar to those seen in neurogenic diseases were produced from MUs in which the only abnormality was increased variability of fiber diameter.     

Long-duration polyphasic motor unit potentials in myopathies: a quantitative study with pathological correlation

In most myopathies, the duration of motor unit action potentials (MUAPs) is shorter than normal. However, polyphasic MUAPs of duration longer than 20% of the control mean, (long-duration polyphasic potentials, or LDPPs) may be seen. We therefore analyzed the incidence and meaning of LDPPs using quantitative MUAP analysis in 41 patients with different myopathies. The mean duration of all potentials was reduced in only 64% of patients because LDPPs increased the mean. When only simple potentials were considered, however, the mean duration was decreased in 95% of patients. This observation confirms the need to exclude LDPPs when calculating the mean duration of MUAPs for diagnosis. We found LDPPs most often in chronic polymyositis and in one patient with Becker Muscular Dystrophy. LDPPs are attributed to desynchronization of single-fiber potentials within the MUAP and may be due to slow conduction in regenerating muscle fibers.     

Effect of changing data collection parameters on statistical motor unit number estimates

The effect of number of samples and selection of data for analysis on the calculation of surface motor unit potential (SMUP) size in the statistical method of motor unit number estimates (MUNE) was determined in 10 normal subjects and 10 with amyotrophic lateral sclerosis (ALS). We recorded 500 sequential compound muscle action potentials (CMAPs) at three different stable stimulus intensities (10-50% of maximal CMAP). Estimated mean SMUP sizes were calculated using Poisson statistical assumptions from the variance of 500 sequential CMAP obtained at each stimulus intensity. The results with the 500 data points were compared with smaller subsets from the same data set. The results using a range of 50-80% of the 500 data points were compared with the full 500. The effect of restricting analysis to data between 5-20% of the CMAP and to standard deviation limits was also assessed. No differences in mean SMUP size were found with stimulus intensity or use of different ranges of data. Consistency was improved with a greater sample number. Data within 5% of CMAP size gave both increased consistency and reduced mean SMUP size in many subjects, but excluded valid responses present at that stimulus intensity. These changes were more prominent in ALS patients in whom the presence of isolated SMUP responses was a striking difference from normal subjects. Noise, spurious data, and large SMUP limited the Poisson assumptions. When these factors are considered, consistent statistical MUNE can be calculated from a continuous sequence of data points. A 2 to 2.5 SD or 10% window are reasonable methods of limiting data for analysis.     

Effects of temperature on motor unit action potentials during isometric contraction

The effect of temperature on motor unit action potential (MUAP) configuration and recruitment was studied using automatic decomposition electromyography (ADEMG) recordings from a concentric needle electrode placed in the first dorsal interosseous (FDI) muscle of 10 normal adult subjects during isometric contraction. Focally cooling the FDI resulted in prolonged MUAP duration (P < 0.001, ANOVA), a finding congruent with those of Buchthal. Focal ulnar cooling at the elbow resulted in the increased MUAP frequency. In contrast to previous studies, there were no significant differences in amplitude or turns. Greater understanding of normal motor unit electrophysiology is necessary to improve diagnostic accuracy of EMG testing © 1995 John Wiley & Sons, Inc.     

运动单位电位多项参数正常值的分析

目的探讨定量肌电图正常运动单位电位(MUP)4项参数的特征、相互关系、正常值及其临床神经生理意义。方法采用棘波触发平均技术,在轻度用力肌肉收缩下记录正常4条肌肉,胫骨前肌118例、股四头肌内侧头71例、手部第一骨间背侧肌63例、三角肌80例。测定MUP的4项参数:波幅、时限、面积、面积／波幅。结果在MUP所有参数中,胫骨前肌、第一骨间背侧肌、三角肌的面积与年龄相关性最强(F=24．591,19．788,16．839,P <0．01)。在互相独立的参数中,波幅与面积的相关性最为显著(r=0．635,0．780,0．663,P<0．01),除了第一骨间背侧肌外,这两种参数从不同角度反映了MUP的特征。面积为波幅依赖性参数。面积／波幅和波幅为互相独立的参数。面积／波幅与时限具有很强的相关性(r=0．489,0．372,0．781,0．476,P<0．01),但与波幅不具有相关性(P> 0．05)。波幅与时限不具有相关性(P>0．05)。结论新参数面积和面积／波幅可能在神经系统疾病诊断中有助于弥补传统肌电图的波幅和时限参数的不足。     

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.     

Intra- and inter-rater reliability of motor unit number estimation and quantitative motor unit analysis in the upper trapezius

Objective

To collect normative data and assess the intra- and inter-rater reliability of decomposition-enhanced spike-triggered averaging (DE-STA) motor unit number estimation (MUNE) and quantitative MU analysis obtained using decomposition-based quantitative electromyography (DQEMG) in the upper trapezius (UT).

Methods

In 10 control subjects, the experimental protocol was performed twice by the same examiner, and once by a second examiner.

Results

Mean MUNE values were 339 ± 121 (rater 1a), 320 ± 131 (rater 1b), and 262 ± 115 (rater 2) MUs. Intra- and inter-rater reliability was good for maximum CMAP (ICC = 0.77 and 0.79, respectively) and moderate for MUNE (ICC = 0.69 and 0.73, respectively), with poor inter-rater reliability for mean S-MUP (ICC = 0.42). Significant differences between rater 1a and 2 were found for mean S-MUP (p = 0.014) and MUNE (p = 0.002), and moderate to good levels of reliability found for quantitative needle-detected MUP parameters.

Conclusions

Various components of the protocol may have contributed to mean S-MUP variability, and may require particular attention in a large, proximal muscle like the UT.

Significance

This study has established preliminary data using DQEMG in a novel muscle which may be relevant to study in patients with ALS.     

Anconeus motor unit number estimates using decomposition‐based quantitative electromyography

Introduction: Motor unit number estimates (MUNEs) provide important information in health, aging, and disease, and can be determined using decomposition‐enhanced spike‐triggered averaging (DE‐STA). Discrimination of surface‐detected motor unit potentials (S‐MUPs) has been limited to contractile forces of ∽30% maximum voluntary contraction (MVC), which is insufficient to recruit a representative sample of the entire MU pool in most muscles. Unique features of the anconeus may permit MUNEs at high muscle activation levels. Methods: In 10 men (25 ± 3 years), anconeus MUNEs were performed using DE‐STA at 10%, 30%, and 50% root‐mean‐square of MVC (RMS_MVC). Results: The mean compound muscle action potential of the anconeus was ∽6 mV, and average S‐MUP amplitudes were ∽100 μV, 145 μV, and 235 μV at 10%, 30%, and 50% RMS_MVC, resulting in low average MUNEs of 58, 38, and 25, respectively. Conclusions: Elbow extensor force–EMG relationships suggest full recruitment of the anconeus MU pool at 50% RMS_MVC, thus providing a representative sample for MUNE. Muscle Nerve 50 : 52–59, 2014     

Standardization of anal sphincter electromyography: utility of motor unit potential parameters.

Advanced electromyography systems offer quantitative analysis of a number of motor unit potential (MUP) parameters. However, only limited data are available on the diagnostic usefulness of these parameters. In the present study, we compared the sensitivities of MUP parameters in revealing "neuropathic" changes in the external anal sphincter (EAS) muscles in 56 patients examined 5-240 months after damage to the cauda equina or conus medullaris. Using multi-MUP analysis, 20 MUPs were obtained from patients' EAS muscles. Their MUP parameters were compared with normative data from 64 controls. The diagnostic sensitivities of mean values/"outliers" of MUP parameters for detecting neuropathic EAS muscles were calculated (area 25%/30%; number of turns 18%/29%; size index 13%/24%; thickness 18%/18%; amplitude 17%/17%; spike duration 20%/9%; duration 15%/12%; number of phases 15%/11%; and their combination 51%/52%). Altogether, the cumulative sensitivity of multi-MUP analysis using both mean values and "outliers" was 62%. The combination of MUP parameters improves the diagnostic yield of MUP analysis, but the influence on specificity remains unknown.     

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.