Effect of muscle-fiber velocity recovery function on motor unit action potential properties in voluntary contractions

Measurements of muscle-fiber conduction velocity during voluntary contractions have been used in the diagnosis of neuromuscular diseases. However, the velocity of propagation of action potentials depends on the interspike interval of activation due to the velocity recovery function (VRF) of muscle fibers. The comparison of muscle-fiber conduction velocity estimates between individuals may thus be influenced by differences in motor unit discharge rate. This study investigates action potential properties of motor units of the sternocleidomastoid muscle during voluntary modulation of discharge rate with the purpose of assessing the effect of the VRF on motor unit properties in voluntary contractions. Nineteen healthy men trained to control a target motor unit with feedback of surface multichannel electromyographic (EMG) signals. The subjects performed three 30-s contractions of cervical flexion/rotation modulating the discharge rate of the target motor unit from 6.6 +/- 1.6 pps to 28.0 +/- 6.4 pps. Action potential conduction velocity was correlated to instantaneous discharge rate (R = 0.38 +/- 0.21). Action potential conduction velocity, peak-to-peak amplitude, and duration varied between minimum and maximum discharge rate (P < 0.01; percent change 12.3 +/- 5.0, -11.8 +/- 9.9, and -12.9 +/- 7.3). Thus, the properties of surface motor unit action potentials vary with modulation of discharge rate. This has implications for the use of conduction velocity values measured during voluntary contractions to differentiate patient populations from healthy individuals and for the development of normative data.     

Reference values of motor unit action potentials obtained with multi-MUAP analysis

We collected reference values of motor unit action potentials (MUAPs) from healthy deltoid, brachial biceps, first dorsal interosseous, lateral vastus, and anterior tibial muscles in 105 subjects between 15 and 86 years. The MUAPs were recorded with a concentric needle electrode and extracted with a decomposition method we call multi-MUAP analysis. The main goal is to identify and extract MUAPs. Also, the firing pattern of the motor units can be followed. No significant changes with age were found for duration, spike duration, thickness, amplitude, area, size index, or number of phases in all muscles studied. We did not find any influence of gender or height. We found higher amplitudes and shorter durations compared with previous studies. This may be due to a higher contraction level that can be used with a decomposition technique. No right-left side differences were found. The coefficient of variation of the parameters in repeated examinations was small, which implies a good reliability of the measurements. Interexaminer variability of four investigators was not greater than in repeated studies. © 1994 John Wiley & Sons, Inc.     

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.     

New approaches to motor unit potential analysis

The current state of motor unit potential (MUP) analysis is reviewed. New quantitative analysis methods are described and compared with traditional manual measurements. The emphasis in this review is on a new method, multi-MUP analysis and the detection of individual outliers. Multi-MUP analysis is based on decomposition EMG. The main advantage of these new methods is that they save time and allow immediate comparison between obtained results and reference values. The results are repeatable and independent of the investigator. This study was supported by a grant from Consiglio Nazionale delle Ricerche CNR (CI) and Grant 135 Swedish Medical Research Council (ES).     

Outliers,a way to detect abnormality in quantitative EMG

In visual analysis of motor unit potentials it is common to decide abnormality by a few motor unit potentials with definitely abnormal amplitude, duration, and shape. The aim of the present investigation was to define limits of normal values and to compare the diagnostic yield of assessing definitely abnormal values outliers, with conventional mean values of MUP parameters. MUPs were extracted and measured with a new decomposition method. Reference values were obtained for three commonly studied muscles. Patients with various types of neuropathies and myopathies were studied in the same way with measurement of outliers and mean values. It was found that outliers were as sensitive as mean values in neuropathies and better in myopathies. Often an increased number of outliers could already be detected after only a few MUPs had been obtained. It would not have been necessary to obtain all 20 MUPs in these patients. The conclusion is that the outlier method is as sensitive as mean values. Because the number of MUPs required may be reduced, the investigation takes a shorter time and is less painful for the patient. If the degree of abnormality is to be quantified, calculation of mean values is still necessary. The combination of outliers and mean values may be the optimal way to detect and express abnormality. © 1994 John Wiley & Sons, Inc.     

Fatigue-related changes in motor unit action potentials of adult cats.

The purpose of this study was to quantify the changes in motor-unit action potentials (MUAP) and force during a standard motor-unit fatigue test. MUAP waveforms were characterized by the measurement of amplitude, duration, area, and shape (as reflected in a coefficient of proportionality). Fatigue-resistant motor units exhibited small, but statistically significant, changes in MUAP amplitude and area during the fatigue test, whereas fatigable motor units displayed variable changes in MUAP amplitude, duration, and area. For all motor-unit types, the coefficient of proportionality did not change, and hence the change in MUAP area was proportional to the combined changes in amplitude and duration. The between- and within-train changes in MUAP were also distinct for the fatigue-resistant and fatigable motor units. Although several mechanisms could be responsible for the changes in the MUAP as the fatigue test proceeded, the dissociation of the time courses for MUAP and force indicated that these MUAP changes were not the principal reason for the decline in force under these conditions.     

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.     

Quantitative EMG in inflammatory myopathy

Fifty-four quantitative electromyographic (EMG) studies were made in 37 patients with inflammatory myopathy (IM) at different points in their clinical course and treatment. All studies were performed in the biceps brachii which varied in clinical strength. Motor unit action potentials (MUAPs) in 45 studies and EMG interference pattern (IP) in 48 studies were recorded using a concentric needle electrode. Macroelectromyographic (Macro-EMG) MUAPs were recorded from 10 patients in 14 studies. MUAP analysis revealed a myopathic pattern (decreased duration and/or area: amplitude ratio) in 69% of studies. IP analysis was more sensitive than MUAP analysis, demonstrating a myopathic pattern in 83% of studies. Macro-EMG MUAP amplitudes were reduced in two studies, minimally increased in one study and normal in the remainder; in 6 (40%) studies, fiber density was slightly increased. Thus, reinnervation does not seem to play an important role in motor unit remodeling in IM.     

Turns-amplitude analysis and motor unit potential analysis in myasthenia gravis

OBJECTIVES: The aims of this study were to investigate myopathic changes in myasthenia gravis (MG) by using turns-amplitude analysis (TAA) and quantitative motor unit potential duration analysis (MUPan), to correlate myopathic changes with severity and duration of the disease and the results of diagnostic tests including repetitive nerve stimulation test (RNS), single fiber electromyography (SFEMG), and anti-acetylcholine receptor antibody (AChR-ab), and to compare the sensitivities of these two methods in detecting myopathic changes in MG. MATERIALS AND METHODS: We studied both MUPan and TAA in 32 patients with MG. RESULTS: The MUPan study showed myopathic changes in 12 patients (37.5%); TAA revealed a myopathic pattern in 4 (12.5%) and a neurogenic pattern in 4 cases (12.5%). Two of the 4 patients with a myopathic change by TAA also had short-duration mean MUP on the MUPan. No statistically significant association was found between the myopathic changes either by MUPan or TAA, and the various clinical and laboratory features. CONCLUSION: We conclude that MUPan is a more sensitive method than TAA in showing myopathic changes in MG, and that TAA is of limited help in demonstrating them.     

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     

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.     

Comparison of three algorithms for multi-motor unit detection and waveform marking

Quantitative EMG (QEMG) techniques include automated motor unit action potential (MUAP) detection and marking of clinically useful waveform metrics. Different computer algorithms are available on modern EMG machines to perform these operations rapidly. However, the efficiency and accuracy of available algorithms are rarely directly compared. We have assessed three commercially available algorithms using both synthesized and biologic interference patterns and found differences among algorithms, some of which are clinically significant. Our results point out the importance of assessing for duplicate MUAPs (same waveform detected as two separate waveforms) and accuracy of markings used to determine MUAP metrics.     

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.     

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.     

Influence of recording site withing the muscle on motor unit potentials

The influence of the recording site on the motor unit potentials (MUPs) was investigated in the brachial biceps muscle of 8 healthy subjects. The MUPs were recorded with a concentric needle electrode and analyzed with a new decomposition EMG program we call multi-MUP analysis. MUPs had shorter durations and smaller amplitudes at superficial recording sites than at deeper sites in the muscle. This is mainly due to the cannula of the concentric electrode, which records a higher potential at superficial recording sites and partially cancels the recorded potentials from the tip in a differential recording. The MUPs had longer durations and higher amplitudes distally than in the middle of the muscle. The longer durations and spike durations are probably due to increased temporal dispersion at a greater distance from the endplate zone. We do not have an adequate explanation for the larger amplitudes distally in the biceps, they may be due to anatomical factors. To increase the diagnostic sensitivity of quantitative MUP analysis the recordings should be obtained from standardized recording sites. © 1995 John Wiley & Sons, Inc.     

Principal component analysis of the features of concentric needle EMG motor unit action potentials

Motor unit action potentials (MUAPs) were recorded from the biceps muscle of normal subjects and of patients with nerve or muscle diseases. Principal component analysis of the MUAP amplitude, area, area/amplitude ratio, duration, and the number of turns and phases produced three components that among them contained 90% of the variance of the data set. Thus the dimensionality of data was reduced from six to three. The first component reflected changes in the size of the MU, whereas the second reflected variations in the arrival time at the recording electrode of the action potentials of muscle fibers in the motor unit. The third factor reflected local loss of muscle fibers within the MU territory. Patterns of variations in the three components were different in patients with neuropathy and myopathy.     

Motor unit properties of biceps brachii in chronic stroke patients assessed with high‐density surface EMG

The aim of this study was to investigate motor unit (MU) characteristics of the biceps brachii in poststroke patients using high‐density surface electromyography (sEMG). Eighteen chronic hemiparetic stroke patients took part. The Fugl‐Meyer score for the upper extremity was assessed. Subjects performed an isometric step contraction consisting of force levels from 5%–50% maximal voluntary contraction while sEMG of the biceps brachii was recorded with a two‐dimensional 16‐channel electrode array. This was repeated for both sides. Motor unit action potentials (MUAPs) were extracted from the EMG signals, and their root‐mean‐square value (RMS_MUAP, reflecting MU size) and mean frequency of the power spectrum (FMEAN_MUAP, reflecting recruitment threshold) were calculated. FMEAN_MUAP was smaller on the affected than on the unaffected side, indicating an increased contribution of low‐threshold MUs, possibly related to degeneration of high‐threshold MUs. The ratio of RMS_MUAP on the affected side divided by that on the unaffected side correlated significantly with the Fugl‐Meyer score. This ratio may reflect the extent to which reinnervation has occurred on the affected side. © 2008 Wiley Periodicals, Inc. Muscle Nerve 2008     

Motor unit number index: Guidelines for recording signals and their analysis

Introduction: This study proposes guidelines for motor unit number index (MUNIX) recording and analysis. Methods: MUNIX was measured in control participants and in patients with amyotrophic lateral sclerosis. Changes in MUNIX values due to E1 electrode position, number of surface electromyography interference pattern (SIP) epochs, SIP epoch duration, force of contraction, and outlier data points were investigated. Results: MUNIX depends on optimized compound muscle action potential (CMAP) amplitude. Individual muscles showed variations when the number of epochs was low or when the SIP duration was short. Longer SIP duration allowed better recognition of artifacts. MUNIX results were affected by SIP values at all force levels but was more affected when SIP area was low. Discussion: We recommend changing the E1 electrode position to maximize CMAP amplitude. Twenty or more SIP signals of 500‐ms duration should be recorded by using force levels ranging from slight to maximum. Traces should be reviewed to identify and exclude signals with tremor or solitary spikes. Muscle Nerve 58 : 374–380, 2018