Characteristics of motor unit potentials in human skeletal muscle during the denervation-reinnervation process

Traditionally measured parameters of motor unit potentials and parameters of the fast (spike) component of motor units were compared in the common extensor of fingers of 24 patients with chronic progressive diseases of motoneurons or their axons and 12 healthy subjects. The earliest signs of motor unit disorders are shown to be increased in the amplitude, duration, and number of turns in the spike (main) component of potentials. A corrrelation was found to exist between increases in muscle fiber density and in the number of turns. Analysis of the main component parameters indicates that both concentration of muscle fibers in the central zone of motor units and desynchronization of their activity occur in initial phases of the denervation-reinnervation process. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 122, No. 8, pp. 131–134, August, 1996     

The motor unit potential distribution over the skin surface and its use in estimating the motor unit location

The amplitude of a surface electromyogram is dependent on the number of active motor units, their size and the relative position of the recording electrode. It is not possible to interpret the surface electromyogram quantitatively without disentangling these different aspects. In this study the decline of different components of the motor unit potential with increasing radial distance from the motor unit is quantified. Fifty-two motor units in the biceps brachii muscle were studied using 36-channel surface electromyography combined with intramuscular scanning electromyography. Scanning electromyography was used to locate precisely the motor unit. The dependence of the surface motor unit potential magnitude on the radial distance between the motor unit and the recording electrodes can be described fairly well by an inverse power function. The steepness of this function depends on the chosen motor unit potential parameter and the interelectrode distance, but also varies between motor units. The change of the negative peak amplitude of the motor unit potential over the skin surface can be used to give a fairly accurate estimate of the location of the motor unit under the skin surface. We found that for all practical purposes the depth of a motor unit in the biceps brachii muscle can be estimated as 20% of the distance over the skin surface where motor unit potentials can be recorded with higher amplitudes than 50% of the maximal amplitude recorded at the skin surface caused by activity of the same motor unit.     

Motor unit remodelling in Duchenne muscular dystrophy. Electrophysiological assessment.

Conventional EMG, motor and sensory conduction velocities, averaging analysis of MUPs, SFEMG, and muscle fiber conduction velocity in situ were performed in 14 boys with Duchenne muscular dystrophy (DD) aged 5 to 11 years. MUPs parameters study showed a striking increment of long duration MUPs followed by satellites and increase of polyphasic potentials of variable duration. The main findings in SFEMG examination were increment in fiber density of the motor unit, large MISI and presence of complex potentials of long duration in all patients. Muscle fiber conduction velocity in situ was significantly slower than in controls, with significant decrease in minimum conduction and increased variability (large SD) in propagation velocity values. Low conduction velocity of muscle fibers, long duration of polyphasics and MUPs followed by satellites, and large MISI were significantly related. These findings support the hypotheses which have suggested that the motor unit remodelling in DD is mainly myogenic. The abnormalities in muscle fiber conduction velocity in situ reflect an increased diameter variation of muscle fibers consistent with splitting fibers, small groups of regenerating and necrotic fibers, and fiber diameter variation found in histological studies. Thus, increased variability in fiber diameter may be the cause of complex and long duration MUPs in DD.     

Motor unit changes in inflammatory myopathy and progressive muscular dystrophy

The aim of present study was to analyse the motor unit (MU) changes in progressive muscle dystrophy (PMD) and in inflammatory myopathy (IM) and to evaluate eventual neurogenic factors in MU reorganisation. The material consisted of 20 patients with (PMD), 20 patients with (IM) and 20 healthy age-matched volunteers. The shape of concentric needle motor unit potentials (cn MUPs), including their duration, amplitude, area, size index and number of phases, the interference pattern and the amplitude and area of macro MUPs were evaluated. The cn emg data satisfied the classical criteria for myopathy in all examined patients, at least in one of the tested muscles. A decreased amplitude and/or area of macro MUPs, compatible with myopathy, were observed in 32 of the 40 patients. In some cases of chronic IM and PDM the long duration polyphasic potentials were recorded. The size index (SI) value of long polyphasic MUPs was usually decreased or normal. This feature indicated that desynchronisation of "myopathic" MUPs results from a reduced number of muscle fibers and their degeneration and regeneration. The results indicated no difference in MU reorganization between PMD and IM and no evidence of neurogenic factors in MU changes.     

Elektromyographische Untersuchung bei Myopathien

Needle electromyography is the most helpful clinical test in a situation where physical examination and serum creatine kinase activity point towards a myopathy. In typical patients, a decrease of mean motor action potential amplitude and duration and an increased number of polyphasic potentials along with the early recruitment of an inappropriately large number of motor units are the hallmark of myopathies. Myotonic discharges indicate the specific group of myotonic disorders with or without clinical myotonia. Inflammatory myopathies are often characterized by spontaneous activity (fibrillation potentials and positive sharp waves) uncommon in chronic degenerative myopathies. Quantitative myography is best performed according to the Buchthal analysis but may be replaced or favourably supplemented by the outlier analysis. Sophisticated automated analysis programs have not proved superior in diagnosing myopathies.     

Laryngeal electromyography in normal Brazilian population

Quantitative analysis of normal values of motor unit action potentials duration and amplitude of muscles tireoaritenoideus (TA), cricotireoideus (CT), cricoaritenoideus lateralis (CAL), and cricoaritenoideus posterioris (CAP) was performed in 14 adult normal Brazilian volunteers. The recordings were obtained by percutaneously inserted concentric needle electrode. Different motor unit action potentials were manually selected in each muscle for quantitative computerized analysis of duration and amplitude. The mean values for duration and amplitude were respectively 3.8 ms and 413 microV for TA, 4.9 ms and 585 microV for CT 4.1 ms and 388 microV for CAL and 4.5 ms and 475 microV in CAP. There were no similar reports of normal values of motor unit action potentials in Brazilian subjects.     

Motor unit potential contribution to surface electromyography

The background of the bioelectric activity of muscle recorded from the surface of the skin (surface electromyography) in terms of the representation of single motor units of the underlying muscle(s) is not very well documented or understood. An insight into the composition of an electromyogram is essential for the proper interpretation of one of the most widely applied electrophysiological techniques. In the present paper, a study of the contribution of single motor unit potentials to the surface electromyogram is presented. To this end, the decline of different components of the motor unit potential with depth of the motor unit is quantified. Experimentally, the action potentials from motor units at several positions in the muscle were recorded by 30 skin surface electrodes. Simultaneous use of scanning electromyography provided information about the actual position and size of the motor unit. Observed linear log–log relationships between motor unit potential magnitudes and distance indicated the usefulness of a power function to describe the motor unit potential's dependence on recording distance. It is shown that different specific surface motor unit potential characteristics fall off differently with depth. The magnitude–distance relationship is shown to be dependent on the recording configuration (unipolar vs. bipolar recording, including the inter-electrode distance) and the chosen motor unit potential parameter (negative peak amplitude, positive peak amplitude and area).     

神经电生理检测对肌萎缩侧索硬化症患者的早期诊断价值

目的：探讨肌萎缩侧索硬化症(ALS)与脊髓型颈椎病(CSM)的神经电生理改变的差异。方法：对60例ALS与CSM患者进行常规肌电图(EMG)、神经电图(ENoG)检测。EMG检测包括三肢体肌、胸锁乳突肌、胸段棘旁肌等肌肉，观察静息状态时自发电位，测定运动单位电位(MUP)的时限、波幅，大力收缩时的募集相。测定运动、感觉神经的传导速度(MCV，SCV)及动作电位的末端潜伏期(ML)、波幅(Amp)。结果：ALS与CSM中自发电位的异常率分别为：胸锁乳突肌39％和17．5％，胸段棘旁肌分别为93．3％和3．3％，肢体肌分别为88．1％和75％，ALS患者MUP时限、波幅的增高不同于CSM患者，差异均有显著意义。结论：胸段棘旁肌EMG阳性率高、简便易行，为早期鉴别诊断ALS与CSM提供了更敏感的依据。同时表明电生理检测可作为ALS可疑或ALS与CSM两病并存患者的随访观察及判定病情和预后的客观指标。     

Changes in motor unit action potentials during the fatigue test.

The studies were performed on medial gastrocnemius motor units of the rat. Changes in amplitude, duration and latency of motor unit action potentials were monitored in the course of fatigue test in the three types of motor unites: FF (fast twitch, fatigable), FR (fast twitch, resistant), and S (slow twitch, resistant) motor unit. In a majority of the motor units the amplitude of action potential decreased. The most pronounced decrease took place in FF units, less pronounced one in FR units, while in S type motor units the decrease was insignificant. However, in a part of investigated motor units the amplitude of their action potential transiently increased at the beginning of the fatigue test and, then decreased or was maintained at the same level till the end of the fatigue test. In a few motor units, particularly the slow ones, no changes in action potential amplitude were observed. In the studied material, a decrease in action potential amplitude was on the average less pronounced than a decrease in tetanus tension in the course of the fatigue test. In a majority of studied motor units the duration of action potential was becoming prolonged, which was most pronounced in FF type motor units and least pronounced in S type ones. In few slow motor units the duration showed no alterations. Changes in action potential duration were much more marked than changes in their amplitude. The changes in action potential in fast motor units were less pronounced than changes in their tetanus tension, while in the slow motor units they resembled each other in intensity. Comparison of motor unit action potential changes with changes in their tetanic tension in the course of fatigue test demonstrated no clear-cut relationship between the two phenomena.     

Fiber density in congenital muscle fiber type disproportion. I. Congenital myopathies

In 15 cases with congenital muscle fiber type disproportion, concentric needle EMG (CNEMG) and single fiber EMG (SFEMG) with fiber density (FD) estimation was performed. The aim of the work was to establish the nature of congenital muscle fiber type disproportion. In 11 cases CNEMG revealed interference pattern on maximal effort and low, polyphasic motor unit action potentials (MUAPs) of short duration indicating a myopathic lesion. In four other cases electromyographic changes were less evident. The FD values were slightly increased in 9 cases. This finding could be explained by pronounced muscle fiber splitting with subsequent ephaptic transmission or even secondary denervation and reinnervation. The most important conclusion from our pilot study is the confirmation of the fact that reinnervation with excessive sprouting is not the only mechanism responsible for muscle fiber type disproportion.     

Single fiber electromyography. A method for the evaluation of motor axonopathy during toxicity studies in dogs

In man, single fiber electromyography is used as a very sensitive indicator for the assessment of functional changes in motor nerves. The purpose of the present study was to adapt the above testing procedure to allow repeated investigations of dogs used in subchronic toxicity tests.Experiments were performed on anesthetized purebred beagle dogs. Action potentials from single muscle fibers in response to electrical stimulation of motor nerves were recorded with Medelec microelectrodes, amplified with a Medelec system and monitored on a Tektronix oscilloscope. Repeated clectrical stimulation with pulses of 0.03 msec and 1 p.p.s. produced characteristic action potentials of single muscle fibers consisting of a positive, followed by a negative, deflection having a duration of from 500 to 800 sec altogether. Successive potentials occurred with a variable latency (the jitter) ranging from±5 to 15 sec.Quantitative evaluation of the jitter will allow the clinical evaluation of motor axonopathies in dogs.     

Optimal placement of bipolar surface EMG electrodes in the face based on single motor unit analysis

Locations of surface electromyography (sEMG) electrodes in the face are usually chosen on a macro‐anatomical basis. In this study we describe optimal placement of bipolar electrodes based on a novel method and present results for lower facial muscles. We performed high‐density sEMG recordings in 13 healthy participants. Raw sEMG signals were decomposed into motor unit action potentials (MUAPs). We positioned virtual electrode pairs in the interpolated monopolar MUAPs at different positions along muscle fiber direction and calculated the bipolar potentials. Electrode sites were determined where maximal bipolar amplitude was achieved and were validated. Objective guidelines for sEMG electrode placement improve the signal‐to‐noise ratio and may contribute to reduce cross talk, which is particularly important in the face. The method may be regarded as an important basis for improving the validity and reproducibility of sEMG in complex muscle areas.     

The influence of contraction amplitude and firing history on spike-triggered averaged trapezius motor unit potentials

The spike-triggered averaged (STA) technique was used to examine trapezius motor unit potentials and their dependence on contraction amplitude and firing history. Individual motor unit firings were identified by a fine-wire intramuscular electrode, while STA-derived potentials were extracted from the simultaneously recorded surface electromyographic (SEMG) signal. Amplitude-controlled contractions and contractions with typing tasks and mental stress were carried out. STA potentials were mostly derived from 20 s intervals of firing. Motor unit synchrony was estimated by peristimulus time histograms (PSTHs). An association between SEMG amplitude and STA-derived motor unit potentials was found: motor unit area showed a four-fold increase when SEMG amplitude increased from 1.5 to 10.5% of the root mean square-detected SEMG signal at maximal voluntary contraction (%EMG_max). Low- and higher threshold motor unit potentials, all with recruitment thresholds <10% EMG_max, had similar area at the same contraction amplitude. A significant increase in the STA-derived potentials was observed after 3 min of constant-amplitude contractions; however, this difference was reduced after 10 min and no longer present after 30 min of contraction. Motor unit synchrony accounted for, on average, 2.8% additional firings within 2 ms of the triggering motor unit. We conclude that the increase in STA-derived potentials with contraction amplitude is, to a major extent, due to motor unit synchrony, limiting the applicability of this method in postural muscles presenting wide motor unit potentials. The similar area of motor units at same SEMG amplitude may indicate that trapezius motor units recruited below 10% EMG_max are of similar size and thus not organized according to the Henneman size principle.     

Influence of motor unit properties on the size of the simulated evoked surface EMG potential

The purpose of the study was to quantify the influence of selected motor unit properties on the simulated amplitude and area of evoked muscle potentials detected at the skin surface. The study was restricted to a motor unit population simulating a hand muscle whose potentials were recorded on the skin over the muscle. Peak-to-peak amplitude and area of the evoked potential were calculated from the summed motor unit potentials and compared across conditions that simulated variation in different motor unit properties. The simulations involved varying the number of activated motor units, muscle fiber conduction velocities, axonal conduction velocities, neuronal activation times, the shape of the intracellular action potential, and recording configurations commonly used over hand muscles. The results obtained for the default condition simulated in this study indicated that ~7% of the motor unit potentials were responsible for 50% of the size of the evoked potential. Variation in the amplitude and area of the evoked muscle potential was directly related to the number of active motor units only when the stimulus activated motor units randomly, and not when activation was based on a parameter such as motor unit size. Independent adjustments in motor unit properties had variable effects on the size of the evoked muscle potential, including when the stimulus activated only a subpopulation of motor units. These results provide reference information that can be used to assist in the interpretation of experimentally observed changes in the size of evoked muscle potentials.     

Electromyographic pattern in Duchenne and Becker muscular dystrophy. Part I: Electromyographic pattern in subsequent stages of muscle lesion in Duchenne muscular dystrophy

BACKGROUND: The study was aimed to evaluate the EMG pattern in myopathy, in the muscles with early, slight abnormalities and in the muscles severely affected, and to analyze the sequence of abnormalities appearing with the progress of the impairment. MATERIAL AND METHOD: Fifty one boys with Duchenne muscular dystrophy were studied. Ninety nine proximal muscles (biceps brachii and rectus femoris) were examined. According to clinical criteria (defective force, atrophy) the muscles were assigned to the group of slight changes (group AB: 50 muscles) and to the group of severe abnormalities (group CD: 49 muscles). An own method of Functional-QEMG was applied in the CNEMG examination. RESULTS AND CONCLUSIONS: The sensitivity of the method applied allowed the detection of myogenic changes even at the subclinical, oligosymptomatic (group AB) stage of impairment. The earliest EMG abnormality detected was an increased percentage of the polyphasic potentials, even if other MUAPs parameters remained normal. However at this stage already a decrease appeared of amplitude, area and MUAPs duration, along with the decrease of IP amplitude and amplitude size. An increase of IP density and a slight increase of number of stable-shaped potentials (simple, but more often polyphasic) was also seen which we believed to reflect the compensatory process. At the further stage, when compensating mechanisms were no longer possible, a dramatic decrease appeared of the IP amplitude, amplitude size and density and, sometimes, also a decrease in the number of stable-shaped potentials. The structural changes such as decrease of amplitude, area and MUAPs duration also progressed. These findings reflect in our study the sequence of EMG abnormalities in the successive stages of myogenic lesion.     

A simulation model of the surface EMG signal for analysis of muscle activity during the gait cycle

This work describes a model able to synthetize the surface EMG (electromyography) signal acquired from tibialis anterior and gastrocnemious medialis muscles during walking of asymptomatic adult subjects. The model assumes a muscle structure where the volume conductor is represented by multiple layers of anisotropic media. This model originates from analysis of the single fiber action potential characterized by the conduction velocity. The surface EMG of voluntary contraction is calculated by gathering motor unit action potentials estimated by the summation of all activities of muscle fibers assumed to have a uniformly parallel distribution. The parameters related to the gait cycle, such as onset and cessation timings of muscle activation, amplitude of muscle contraction, periods and sequences of motor units' recruitment, are included in the model presented. In addition, the relative positions of the electrodes during gait can also be specified in order to adapt the simulation to the different acquisition settings.     

Twitch-obtaining intramuscular stimulation (TOIMS) in acute partial radial nerve palsy.

Twitch Obtaining Intramuscular Stimulation (TOIMS) is useful in the management of chronic nerve-related pain. The best understanding of the mechanism of action of TOIMS can occur on treating acute nerve-related symptoms. An opportunity to use TOIMS treatments in an acute, partial left radial palsy within 24 hours of onset did occur. Following treatment to the affected triceps and brachioradialis muscles, there was an immediate improvement in the amplitude, area and conduction velocity of the left radial motor and sensory nerves at the lower arm level. There was also improvement in numbness and all symptoms abolished after four treatments. Serial multiple motor unit action potential (multi-MUAP) analysis performed in the triceps and extensor communis (EDC) showed signs of motor unit enlargement. The triceps MUAPs showed an increase in duration and size index (area/amplitude) by the 3rd month. Both parameters stabilized at 18 months. Phases increased at the 6th month only. In EDC, the size index increased progressively by the 3rd month. The duration increased at the 6th month with stabilization by the 18th month. Phases and turns increased on day 3 examination only. EDC showed reduction in the firing rate with time. By relaxing the muscles through the effects of intramuscular exercise and also by improving local ischemia, TOIMS averted prolonged conduction abnormalities and probably a more serious axonal injury.     

Myopathy in acromegaly. A case study

The muscle findings in a patient with acromegaly of 20 years duration are presented. The patient demonstrated mild muscular weakness and atrophy. Standard needle electromyography was normal and single fiber electromyography showed normal motor unit densities. Muscle biopsy revealed normal size type-I muscle fibers, but three populations of type-II fibers - hypertrophied, atrophied, and normal-sized fibers. Hypertrophy of type-II fibers may be the direct result of the excess growth hormone, while the atrophic fibers may result from the disturbance of other endocrine function.     

自动分解肌电图运动单位动作电位各参数的分析和临床意义

目的：探讨运动单位动作电位（ＭＵＡＰ）各参数评价神经肌肉疾病的临床意义。方法：采用肌电图仪,以常规同心圆针电极记录,对７１例健康成人和４３例神经肌肉疾病患者的肱二头肌或／和胫前肌进行自动分解肌电图（ＡＤＥＭＧ）检测,共收集ＭＵＡＰ８３５６个,测定ＭＵＡＰ的５个参数：波幅、时限、转折数、发放率、棘波间隔变异系数（ＣＩＶ）。结果：１５例肌源性疾病患者表现为每一记录部位的ＭＵＡＰ数目增加;肱二头肌ＭＵＡＰ的波幅、时限、转折数减低、发放率增加,ＣＩＶ无改变;胫前肌ＭＵＡＰ的各参数无显著性改变。２８例神经源性疾病患者表现为每一记录部位的ＭＵＡＰ数目减少;ＭＵＡＰ的波幅、转折数及ＣＩＶ增加,时限及发放率无显著性增加。结论：ＡＤＥＭＧ检测的ＭＵＡＰ各参数对于评价神经肌肉疾病有重要的临床意义。     

"Pseudo-conduction block" in a patient with vasculitic neuropathy.

A 63-year-old man presented with progressive asymmetric weakness and numbness in his hands of 2 weeks duration. Nerve conduction studies showed low amplitude motor evoked potentials of both median nerves. The right ulnar, left tibial and peroneal nerves had normal potentials on distal stimulation with markedly decreased amplitudes proximally, suggestive of "conduction block". Three weeks later, amplitudes were decreased throughout. The patient was diagnosed with vasculitis. The acute ischemic injury presumably resulted in axonal damage between the distal and proximal stimulation sites, with subsequent Wallerian degeneration.