Properties Of Motor Units Research Articles

Motor unit remodelling in multifocal motor neuropathy: The importance of axonal loss

ObjectiveTo estimate the degree of axonal loss in patients diagnosed with multifocal motor neuropathy (MMN) using a novel assessment of motor unit numbers and size. MethodsAutomated motor unit number estimation using a compound muscle action potential (CMAP) scan was undertaken in median nerves with conduction block. Results were compared with 30 age-matched healthy controls. ResultsCompared with healthy controls, MMN patients had fewer motor units (MMN: 33±11vs HC: 93±36 [mean±SD]; p<0.0001) and larger ‘size of the largest unit’ (MMN: 1.2±0.5mVvs HC: 0.4±0.1mV; p<0.0001), despite having normal distal CMAP amplitudes (MMN: 7.6±1.8mVvs HC: 8.7±2.5mV; p=0.24). ConclusionsMMN is associated with marked axonal loss which may be masked by striking re-innervation resulting in preservation of distal CMAP amplitudes. SignificanceAssessment of motor unit properties should be incorporated into assessment of disease progression in MMN, given that nerve conduction studies are insensitive to motor unit remodelling.

A motor unit-based model of muscle fatigue.

Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-world applications. Moreover, the model provided insight into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approaches.

Electrophysiological and neuromuscular stability of persons with chronic inflammatory demyelinating polyneuropathy

We assessed motor unit (MU) properties and neuromuscular stability in the tibialis anterior (TA) of chronic inflammatory demyelinating polyneuropathy (CIDP) patients using decomposition-based quantitative electromyography. Dorsiflexion strength was assessed, and surface and concentric needle electromyography were sampled from the TA. Estimates of MU numbers were derived using decomposition-based quantitative electromyography and spike-triggered averaging. Neuromuscular transmission stability was assessed from concentric needle-detected MU potentials. CIDP patients had 43% lower compound muscle action potential amplitude than controls, and despite near-maximum voluntary activation, were 37% weaker. CIDP had 27% fewer functioning MUs in the TA, and had 90% and 44% higher jiggle and jitter values, respectively compared with controls. CIDP had lower strength and compound muscle action potential values, moderately fewer numbers of MUs, and significant neuromuscular instability compared with controls. Thus, in addition to muscle atrophy, voluntary weakness is also due to limitations of peripheral neural transmission consistent with demyelination. Muscle Nerve 56: 413-420, 2017.

Tracking motor units longitudinally across experimental sessions with high-density surface electromyography.

Key points Classic motor unit (MU) recording and analysis methods do not allow the same MUs to be tracked across different experimental sessions, and therefore, there is limited experimental evidence on the adjustments in MU properties following training or during the progression of neuromuscular disorders.We propose a new processing method to track the same MUs across experimental sessions (separated by weeks) by using high‐density surface electromyography.The application of the proposed method in two experiments showed that individual MUs can be identified reliably in measurements separated by weeks and that changes in properties of the tracked MUs across experimental sessions can be identified with high sensitivity.These results indicate that the behaviour and properties of the same MUs can be monitored across multiple testing sessions.The proposed method opens new possibilities in the understanding of adjustments in motor unit properties due to training interventions or the progression of pathologies. A new method is proposed for tracking individual motor units (MUs) across multiple experimental sessions on different days. The technique is based on a novel decomposition approach for high‐density surface electromyography and was tested with two experimental studies for reliability and sensitivity. Experiment I (reliability): ten participants performed isometric knee extensions at 10, 30, 50 and 70% of their maximum voluntary contraction (MVC) force in three sessions, each separated by 1 week. Experiment II (sensitivity): seven participants performed 2 weeks of endurance training (cycling) and were tested pre–post intervention during isometric knee extensions at 10 and 30% MVC. The reliability (Experiment I) and sensitivity (Experiment II) of the measured MU properties were compared for the MUs tracked across sessions, with respect to all MUs identified in each session. In Experiment I, on average 38.3% and 40.1% of the identified MUs could be tracked across two sessions (1 and 2 weeks apart), for the vastus medialis and vastus lateralis, respectively. Moreover, the properties of the tracked MUs were more reliable across sessions than those of the full set of identified MUs (intra‐class correlation coefficients ranged between 0.63—0.99 and 0.39–0.95, respectively). In Experiment II, ∼40% of the MUs could be tracked before and after the training intervention and training‐induced changes in MU conduction velocity had an effect size of 2.1 (tracked MUs) and 1.5 (group of all identified motor units). These results show the possibility of monitoring MU properties longitudinally to document the effect of interventions or the progression of neuromuscular disorders.

Increases in human motoneuron excitability after cervical spinal cord injury depend on the level of injury.

Unitary F waves were common in paralyzed thenar muscles of people who had a chronic spinal cord injury (SCI) at the C4 level compared with uninjured people, but F waves did not occur in people that had SCI at the C5 or C6 level. These results highlight that intrinsic motoneuron excitability depends, in part, on how close the motoneurons are to the site of the spinal injury, which could alter the generation and strength of voluntary and involuntary muscle contractions.

Contractile properties of motor units and expression of myosin heavy chain isoforms in rat fast-type muscle after volitional weight-lifting training.

Dynamic resistance training increases the force and speed of muscle contraction, but little is known about modifications to the contractile properties of the main physiological types of motor units (MUs) that contribute to these muscle adaptations. Although the contractile profile of MU muscle fibers is tightly coupled to myosin heavy chain (MyHC) protein expression, it is not well understood if MyHC transition is a prerequisite for modifications to the contractile characteristics of MUs. In this study, we examined MU contractile properties, the mRNA expression of MyHC, parvalbumin, and sarcoendoplasmic reticulum Ca2+ pump isoforms, as well as the MyHC protein content after 5 wk of volitional progressive weight-lifting training in the medial gastrocnemius muscle in rats. The training had no effect on MyHC profiling or Ca2+-handling protein gene expression. Maximum force increased in slow (by 49%) and fast (by 21%) MUs. Within fast MUs, the maximum force increased in most fatigue-resistant and intermediate but not most fatigable MUs. Twitch contraction time was shortened in slow and fast fatigue-resistant MUs. Twitch half-relaxation was shortened in fast most fatigue-resistant and intermediate MUs. The force-frequency curve shifted rightward in fast fatigue-resistant MUs. Fast fatigable MUs fatigued less within the initial 15 s while fast fatigue-resistant units increased the ability to potentiate the force within the first minute of the standard fatigue test. In conclusion, at the early stage of resistance training, modifications to the contractile characteristics of MUs appear in the absence of MyHC transition and the upregulation of Ca2+-handling genes.

The aging neuromuscular system and motor performance.

Age-related changes in the basic functional unit of the neuromuscular system, the motor unit, and its neural inputs have a profound effect on motor function, especially among the expanding number of old (older than ∼60 yr) and very old (older than ∼80 yr) adults. This review presents evidence that age-related changes in motor unit morphology and properties lead to impaired motor performance that includes 1) reduced maximal strength and power, slower contractile velocity, and increased fatigability; and 2) increased variability during and between motor tasks, including decreased force steadiness and increased variability of contraction velocity and torque over repeat contractions. The age-related increase in variability of motor performance with aging appears to involve reduced and more variable synaptic inputs that drive motor neuron activation, fewer and larger motor units, less stable neuromuscular junctions, lower and more variable motor unit action potential discharge rates, and smaller and slower skeletal muscle fibers that coexpress different myosin heavy chain isoforms in the muscle of older adults. Physical activity may modify motor unit properties and function in old men and women, although the effects on variability of motor performance are largely unknown. Many studies are of cross-sectional design, so there is a tremendous opportunity to perform high-impact and longitudinal studies along the continuum of aging that determine 1) the influence and cause of the increased variability with aging on functional performance tasks, and 2) whether lifestyle factors such as physical exercise can minimize this age-related variability in motor performance in the rapidly expanding numbers of very old adults.

Chronic Inflammatory Demyelinating Polyneuropathy: Muscle Atrophy Associated with Denervation and Neuromuscular Transmission Instability

IntroductionChronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired neuropathy of immunological origin. Increased understanding of the pathophysiology of CIDP is of specific interest due to the treatable nature of the disease. It is characterized, in most cases, by symmetrical weakness in proximal and distal muscles that progresses for greater than 2 months. Evidence indicates that disease‐related post‐synaptic damage and muscle fiber atrophy lead to neuromuscular junction remodeling and impaired neuromuscular transmission (Deschenes et al. 2011). The integrity and stability of neuromuscular transmission can be examined using decomposition‐based quantitative electromyography (DQEMG), which can assess the degree of variability in the shape of consecutively detected motor unit potentials (MUPs) termed ‘jiggle’. In this study we compared voluntary and stimulated contractile function, electrophysiological motor unit (MU) properties and muscle volume quantities in a group of CIDP patients (n=11) with a group of age and sex‐matched healthy controls (n=11).MethodsIn an isometric dynamometer, dorsiflexion strength with twitch interpolation to assess voluntary activation, was recorded. Surface and concentric needle electromyographic (EMG) signals were collected from the tibialis anterior (TA). Compound motor action potentials (CMAPs) for the TA were obtained by supra‐maximal stimulation of the fibular nerve. To estimate the number of MUs in the TA the decomposition enhanced spike‐triggered averaging technique was applied. This required subjects to contract at 25% of their maximal voluntary strength for a series of 30s contractions until at least 20 suitable MU trains were acquired. From the intramuscular needle‐detected signals of single MUs, neuromuscular transmission stability was assessed through calculation of MUP ‘jiggle’. Magnetic resonance imaging (MRI) was used to calculate TA muscle volume.ResultsDespite near maximum voluntary activation (~98%), CIDP patients were ~50% weaker and had ~60% lower CMAP amplitudes than controls. The estimated number of functioning MUs in the TA was ~30% fewer in CIDP and with ~68% higher ‘jiggle’ values indicative of greater neuromuscular transmission instability. MRI results indicated that CIDP patients had ~40% less muscle volume in the TA compared to controls. From these preliminary results, those with CIDP have less strength, but are highly activated. They have substantially lower CMAP values and fewer numbers of MUs compared with controls. Neuromuscular stability is also considerably lower in CIDP.ConclusionsVoluntary muscle weakness appears multifactorial and is due to limitations of peripheral neuromuscular transmission, muscle atrophy and moderate MU loss. MRI data indicate atrophy of the TA muscle in CIDP subjects who exhibit approximately half the muscle mass of controls. These measures that detect MU loss, neuromuscular transmission instability and muscle atrophy could be useful for objective assessment of disease severity leading to improved ability to monitor disease progression and treatment outcomes. Furthermore the use of DQEMG could be incorporated into larger studies to help identify CIDP patients in the early stages of neuromuscular degeneration allowing for earlier and perhaps more effective intervention.Support or Funding InformationSupported by NSERC.

Action potential amplitude as a noninvasive indicator of motor unit-specific hypertrophy.

Skeletal muscle fibers hypertrophy in response to strength training, with type II fibers generally demonstrating the greatest plasticity in regards to cross-sectional area (CSA). However, assessing fiber type-specific CSA in humans requires invasive muscle biopsies. With advancements in the decomposition of surface electromyographic (sEMG) signals recorded using multichannel electrode arrays, the firing properties of individual motor units (MUs) can now be detected noninvasively. Since action potential amplitude (APSIZE) has a documented relationship with muscle fiber size, as well as with its parent MU's recruitment threshold (RT) force, our purpose was to examine if MU APSIZE, as a function of its RT (i.e., the size principle), could potentially be used as a longitudinal indicator of MU-specific hypertrophy. By decomposing the sEMG signals from the vastus lateralis muscle of 10 subjects during maximal voluntary knee extensions, we noninvasively assessed the relationship between MU APSIZE and RT before and immediately after an 8-wk strength training intervention. In addition to significant increases in muscle size and strength (P < 0.02), our data show that training elicited an increase in MU APSIZE of high-threshold MUs. Additionally, a large portion of the variance (83.6%) in the change in each individual's relationship between MU APSIZE and RT was explained by training-induced changes in whole muscle CSA (obtained via ultrasonography). Our findings suggest that the noninvasive, electrophysiological assessment of longitudinal changes to MU APSIZE appears to reflect hypertrophy specific to MUs across the RT continuum.

18. Dysfunctional properties of single motor units in ALS

Background Abnormalities in the excitability of lower motor neurons in amyotrophic lateral sclerosis have been extensively examined using threshold tracking techniques. Several studies have found evidence for a down-regulation of K+, and an up-regulation of Na+ conductances. The results as a whole however have been somewhat mixed. Threshold-tracking studies test the excitability of a fixed fraction of the maximal compound action potential, and for most disease processes this accurately reflects the excitability of all axons of the same modality within a nerve. However, in ALS such a relationship is unreliable. The slow inexorable process of degeneration of individual motor units is likely to change their excitability, with hyperexcitability and reinnervation believed to impact on their viability. In ALS the disease process itself presents the opportunity to study single motor units identifiable in the recruitment curve, and in particular the large units which result from reinnervation of muscle fibres from motor units that have undergone degeneration. Such measurements provide an unequivocal snapshot of the excitability of an individual motor unit. Methods Measurements of the excitability of single motor units in the intrinsic muscles of the split hand were undertaken in 7 subjects, and mathematical modelling was performed to examine the nature of changes in individual motor units. Results As expected, the ALS single motor unit recordings were more varied in nature when compared to normal single motor units from a previous study. Significant differences were observed with a greater rheobasic current and lower resting I/V slope, relative refractory period and ‘fanned out’ threshold electrotonus in the ALS single motor units compared to the normal controls. Conclusions The excitability of individual motor units in ALS is heterogeneous in nature, with axonal abnormalities correlated with disease progression. The underlying biophysical mechanisms of these abnormal motor units were best modelled by an increase in the Na+/K+-ATPase pump current, and a near halving of slow K+ conductances. The relative proportions of changes to the Na+/K+-ATPase pump and slow K+ conductances are likely to vary between individual motor units, and with disease progression.

Estimating motor unit numbers from a CMAP scan.

Compound muscle action potential (CMAP) scans are detailed stimulus-response curves which provide information about motor unit properties in neuromuscular disorders. This study assessed a method of automatic motor unit number estimation (MUNE) from 5-min CMAP scans. A preliminary model, derived from the variance and slope of the scan, is refined to fit the CMAP scan more closely. The method was tested by application to 60 simulated scans, generated from between 5 and 160 motor unit potentials. The fitting procedure took an average of 1.5 min on a standard personal computer. Small unit numbers (5-20) were on average correctly estimated, but large unit numbers (>40) were slightly underestimated. Overall, the absolute MUNE error averaged 6.9%. This new MUNE method takes all excitable motor units into account and provides realistic estimates of unit numbers over the range 5 to 160. Validation as a clinical tool awaits further study. Muscle Nerve 53: 889-896, 2016.

Force regulation and electrical properties of motor units in overloaded muscle.

The influence of long-term muscle overload on force regulation and electrical properties of motor units (MUs) was investigated in rats. Compensatory overload of the medial gastrocnemius was induced by tenotomy of its synergists. Electrophysiological experiments were performed on functionally isolated MUs 3 months after the surgery. Force-frequency curves for overloaded MUs were shifted rightward compared with control, thus MUs developed the same relative tetanic forces at higher frequencies. Higher force increase was achieved in response to an increase in stimulation frequency in overloaded fast MUs compared with control. The optimal tetanic contraction, characterized by the highest force-time area per pulse, was evoked at higher stimulation frequencies for all overloaded MUs except FF. Only minor adaptive changes in MU action potentials occurred. Compensatory muscle overload leads to substantial modifications in MU force development mechanisms, which are MU-type-specific and influence whole muscle force regulation.

Motor unit loss is accompanied by decreased peak muscle power in the lower limb of older adults

This study investigated the relationship between motor unit (MU) properties and the isometric strength and power of two lower limb muscles in healthy young and older adults. Twelve older adults (6 men, mean age, 77±5years) and twelve young adults (6 men, mean age, 24±3years) were studied. MU properties of the tibialis anterior (TA) and vastus medialis (VM) muscles were determined electrophysiologically using decomposition-enhanced spike-triggered averaging (DE-STA). Motor unit number estimates (MUNEs) of the TA were significantly reduced (p<0.05) in older adults (102±76) compared to young adults (234±109), primarily as a result of significantly larger surface-detected motor unit potentials (S-MUPs) in older adults (63±29μV) compared to young adults (27±14μV). Although VM S-MUP values were larger in older adults (60±31μV) compared to young (48±42μV), the difference was not significant. Maximal isometric strength was significantly larger in both the TA and knee extensors of young adults (TA: 0.56Nm/kg, KE: 2.2Nm/kg) compared to old (TA: 0.4Nm/kg, KE: 1.3Nm/kg). Similar reductions in peak muscle power were observed between young (TA: 33W, KE: 357W) and old adults (TA: 26W, KE: 224W). The greatest deficit between young and old subjects in peak power output occurred at 20% MVC for the TA and 40% MVC for the knee extensors. Results from this study indicate that there are changes in MU properties with age, and that this effect may be greater in the more distal TA muscle. Further, this study demonstrates that muscle power may be a sensitive marker of changes in neuromuscular function with aging.

Inappropriate interpretation of surface EMG signals and muscle fiber characteristics impedes understanding of the control of neuromuscular function

as the final common pathway from the nervous system to muscle, the motor unit transmits an activation signal generated by the nervous system to engage the contractile proteins and produce the muscle forces needed for reflex responses, automatic behaviors, and voluntary actions ([11][1]). The net

Non-uniform recruitment along human rectus femoris muscle during transcutaneous electrical nerve stimulation.

To test the hypothesis that motor units with different axonal excitability levels are localized in specific portions of the rectus femoris (RF) muscle using transcutaneous electrical nerve stimulation. M-waves were elicited by transcutaneous electrical nerve stimulation and detected from 24 sites along longitudinal line of the muscle. The stimulation was applied to the femoral nerve, and the current level was gradually increased. The central locus activation, which is calculated from the spatial distribution of M-waves, appeared at the proximal regions at low stimulation level and then moved to the middle site of the muscle with an increase in the stimulation level. The results reveal that groups of motor units activated at different stimulation levels are located in different positions in the proximal-distal muscle direction. Our results suggest that motor unit properties in proximal and other regions are not uniform within the RF muscle.

Influence of common synaptic input to motor neurons on the neural drive to muscle in essential tremor.

Tremor in essential tremor (ET) is generated by pathological oscillations at 4-12 Hz, likely originating at cerebello-thalamo-cortical pathways. However, the way in which tremor is represented in the output of the spinal cord circuitries is largely unknown because of the difficulties in identifying the behavior of individual motor units from tremulous muscles. By using novel methods for the decomposition of multichannel surface EMG, we provide a systematic analysis of the discharge properties of motor units in nine ET patients, with concurrent recordings of EEG activity. This analysis allowed us to infer the contribution of common synaptic inputs to motor neurons in ET. Motor unit short-term synchronization was significantly greater in ET patients than in healthy subjects. Furthermore, the strong association between the degree of synchronization and the peak in coherence between motor unit spike trains at the tremor frequency indicated that the high synchronization levels were generated mainly by common synaptic inputs specifically at the tremor frequency. The coherence between EEG and motor unit spike trains demonstrated the presence of common cortical input to the motor neurons at the tremor frequency. Nonetheless, the strength of this input was uncorrelated to the net common synaptic input at the tremor frequency, suggesting a contribution of spinal afferents or secondary supraspinal pathways in projecting common input at the tremor frequency. These results provide the first systematic analysis of the neural drive to the muscle in ET and elucidate some of its characteristics that determine pathological tremulous muscle activity.

Changes in motor unit properties in SOD1 (G93A) rats

In amyotrophic lateral sclerosis (ALS), progressive death of motor neurons results in denervation and reinnervation of muscles. It is not clear how ALS affects the properties of motor units (MUs). Properties of single MUs in the medial gastrocnemius (MG) muscle of rats bearing the human mutated superoxide dismutase gene type 1 (SOD1) were determined at 3 stages: asymptomatic (ALS I); early symptomatic (ALS II); and terminal (ALS III). In ALS II, higher proportions of FF (fast fatigable) and S (slow) MUs were observed, whereas in ALS III higher percentages of S and lower percentages of FF MUs were noted compared with controls. S motor neurons reinnervated fast muscle fibers, and those MUs gained some properties of fast MUs, including lower fatigue resistance, greater force generation, and higher action potential amplitudes. Changes in MU properties of SOD1 rats have progressive and multidirectional character and speed depending on the MU type.

Single motor unit firing rate after stroke is higher on the less-affected side during stable low-level voluntary contractions.

Muscle weakness is the most common outcome after stroke and a leading cause of adult-acquired motor disability. Single motor unit properties provide insight into the mechanisms of post-stroke motor impairment. Motor units on the more-affected side are reported to have lower peak firing rates, reduced discharge variability and a more compressed dynamic range than healthy subjects. The activity of 169 motor units was discriminated from surface electromyography in 28 stroke patients during sustained voluntary contractions 10% of maximal and compared to 110 units recorded in 16 healthy subjects. Motor units were recorded in three series: ankle dorsiflexion, wrist flexion and elbow flexion. Mean firing rates after stroke were significantly lower on the more-affected than the less-affected side (p < 0.001) with no differences between dominant and non-dominant sides for healthy subjects. When data were combined, firing rates on the less-affected side were significantly higher than those either on the more-affected side or healthy subjects (p < 0.001). Motor unit mean firing rate was higher in the upper-limb than the lower-limb (p < 0.05). The coefficient of variation of motor unit discharge rate was lower for motor units after stroke compared to controls for wrist flexion (p < 0.05) but not ankle dorsiflexion. However the dynamic range of motor units was compressed only for motor units on the more-affected side during wrist flexion. Our results show that the pathological change in motor unit firing rate occurs on the less-affected side after stroke and not the more-affected side as previously reported, and suggest that motor unit behavior recorded in a single muscle after stroke cannot be generalized to muscles acting on other joints even within the same limb. These data emphasize that the less-affected side does not provide a valid control for physiological studies on the more-affected side after stroke and that both sides should be compared to data from age- and sex-matched healthy subjects.

Limitations of the Spike-Triggered Averaging for Estimating Motor Unit Twitch Force: A Theoretical Analysis

Contractile properties of human motor units provide information on the force capacity and fatigability of muscles. The spike-triggered averaging technique (STA) is a conventional method used to estimate the twitch waveform of single motor units in vivo by averaging the joint force signal. Several limitations of this technique have been previously discussed in an empirical way, using simulated and experimental data. In this study, we provide a theoretical analysis of this technique in the frequency domain and describe its intrinsic limitations. By analyzing the analytical expression of STA, first we show that a certain degree of correlation between the motor unit activities prevents an accurate estimation of the twitch force, even from relatively long recordings. Second, we show that the quality of the twitch estimates by STA is highly related to the relative variability of the inter-spike intervals of motor unit action potentials. Interestingly, if this variability is extremely high, correct estimates could be obtained even for high discharge rates. However, for physiological inter-spike interval variability and discharge rate, the technique performs with relatively low estimation accuracy and high estimation variance. Finally, we show that the selection of the triggers that are most distant from the previous and next, which is often suggested, is not an effective way for improving STA estimates and in some cases can even be detrimental. These results show the intrinsic limitations of the STA technique and provide a theoretical framework for the design of new methods for the measurement of motor unit force twitch.

Characterization of the motor units of the external anal sphincter in pregnant women with multichannel surface EMG

Locating the innervation zones (IZs) of the external anal sphincter (EAS) is helpful to obstetricians to identify areas particularly vulnerable to episiotomy in pregnant women. The aim was to investigate the motor unit (MU) properties of the EAS during voluntary contractions. Electromyographic signals were detected, from 478 pregnant women, by means of an intra-anal cylindrical probe carrying a circumferential array of 16 electrodes. The signals were decomposed into the constituent MU action potential trains and 5,947 templates were extracted and analyzed in order to identify the IZ position. MUs innervated at one end are concentrated in the dorsal portion of the sphincter, while MUs innervated in the middle are distributed symmetrically in the left and right portions of the EAS. The angular propagation velocity was estimated for each MU resulting in 260 ± 45 rad/s, corresponding to 1.8 m/s on the probe surface and to about 4 m/s at a radial depth of 10 mm from the probe surface. A novel method for identification and classification of MUs of the EAS is proposed and applied to a large-scale study. It is possible to distinguish MUs of the EAS in a minimally invasive way and identify their IZs. This information should be used to plan episiotomies and minimize risks of EAS denervation.