Surface detected potentials of normal and reinnervated motor units: a simulation study for muscles consisted of short fibres

We aimed to check whether the characteristics used up to now in macro EMG to distinguish between normal and reinnervated motor unit potentials (MUPs), were suitable for surface detected MUPs. MUPs produced by normal and reinnervated MUs were simulated with a fast and precise convolution model. An increased number of fibres in the MU territory enhanced the amplitude, area and RMS of the MUP proportionally irrespective of the MU-electrode distance. An increased scatter of the end-plates and greater desynchronization in the fibres' activation decreased the MUP amplitude and affected the temporal characteristics of the MUP (duration of the negative phase and its area to amplitude ratio). The effects were more pronounced at shorter distances. At larger distances, the effect of the MU-electrode distance on temporal and amplitude characteristics of MUPs was much stronger than that of the parameters changed with reinnervation. We conclude that reinnervated MUs consisting of short fibres can not be distinguished from the normal ones by means of characteristics of MUP used in macro EMG. To discriminate reinnervated MUs non-invasively, the MUP amplitude should be normalized in respect of the MU-electrode distance or other MUP characteristics (independent of MU-electrode distance and sensitive to reinnervation) should be used.     

Comparison of amplitudes of surface macro motor unit potentials recorded from various muscle sites

Peak-to-peak amplitudes and total areas of surface macro motor unit potentials (S-MMUPs) were measured in 19 healthy volunteers. While participants maintained minimal isometric muscle contraction of the left biceps brachii, motor unit potentials (MUPs) were recorded from a needle and surface electrodes. The largest MUP recorded by the needle electrode was designated the trigger source. Electrical activities from the surface electrodes, which emerged synchronously with the trigger-potential, were averaged by the spike-triggered averaging (STA) technique. When the surface electrodes were placed over the muscle belly at a right angle to the muscle fibers, the S-MMUP amplitude and area decreased gradually with the distance of the electrodes from the point of insertion of the needle electrode. In contrast, when the surface electrodes were arranged parallel to the muscle fibers, the S-MMUP amplitude and area did not always decrease. In addition, negative peak positions in individual S-MMUPs showed a time delay along the muscle fibers. The placement and size of the surface electrodes, as well as the depth of the needle electrode, must be carefully considered when MUPs are analyzed by the STA technique. Muscle fiber conduction velocity (MFCV) is measurable by the STA technique combined with surface electrodes.     

The presence of unknown layer of skin and fat is an obstacle to a correct estimation of the motor unit size from surface detected potentials

To overcome problems with a strong distance-dependence of the motor unit potentials (MUPs), different methods to estimate the MU location and size have been proposed. Distance-independence of the exponent of the power function, that describes the MUP distance decline, and homogeneity of the volume conductor, are assumed in all methods. Some of them consider the exponent value as unique, irrespective of persons, muscles and their functional state. One method estimates the current exponent value. We evaluate this method by computer simulation of MUPs in infinite and semi-infinite volume conductor. Our results show that although the first assumption is not fulfilled, it does not affect considerably the estimate of the MU location and size obtained for infinite or semi-infinite homogeneous volume conductor. The errors of the MU location can be insignificant even in inhomogeneous volume conductor with a layer of lower conductivity (skin and fat) between the muscle tissue and electrode. The accurate location of the MU electrical axis is, however, not a sufficient condition for a correct MU size estimation that depends considerably on actual parameters of the layer. Thus, the surface EMG could hardly be considered as non-invasive alternative to macro EMG for detection of the enlarged MUs.     

平山病的临床与神经电生理学特点分析

目的：分析平山病的临床与神经电生理学特点,以提高对平山病的诊断水平。方法：对21例确诊为平山病的患者进行运动和感觉传导速度、肌电图（EMG）、F波及交感皮肤反应（SSR）等神经电生理学检查。结果：对126条神经进行检测,神经传导速度（NCV）测定总异常率为16．7％（21／126）,主要表现为远端运动潜伏期（DML）延长及复合肌肉动作电位（CMAP）波幅下降,感觉神经传导速度均正常。F波平均最小潜伏期为（28．70±3．10）ms,平均出现率为47．70％,总异常率为76．2％。患者患侧上肢及对侧上肢远端肌（不包括肱桡肌）EMG呈神经原性改变者分别为1000及90．5％,主要异常表现为出现自发电位运动单位电位（MUP）时限延长、波幅增宽、多相波增多及募集相等异常。上肢远端肌EMG异常率为91．5％,明显高于上肢近端肌的8．0％及下肢肌的3．5％。结论：平山病可出现神经电生理上的亚临床改变,神经电生理检查对其有较高的诊断价值。     

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.     

A morphological analysis of the macro motor unit potential

The technique of macro EMG is used to investigate the motor unit architecture in a number of pathological conditions. Amplitude and area are the most commonly used criteria, but these parameters alone are not sufficient to assess the complexity of the macro MUP morphology. In an attempt to examine the morphology of the macro MUP in more detail, additional measures were investigated including, (i) average power, (ii) duration, and (iii) number of phases. Macro MUP duration was defined as the time parameter that contains a particular fraction (90%) of the total power of the potential. The above mentioned parameters were evaluated for normal subjects and for patients suffering with motor neuron disease (MND), spinal muscular atrophy (SMA), and Becker's muscular dystrophy (BMD). It is shown that high amplitude and average power macro MUPs give shorter macro MUP duration than macro MUPs with normal amplitude. In contrast, in low amplitude macro MUPs there is a tendency towards a higher duration measure, as compared with the duration of the normal amplitude macro MUPs. Also, t–test results for the duration measure gave a significant difference between the NOR–MND, and no significant difference between the NOR–BMD and NOR–SMA groups at P<0.05. Significant difference between the NOR and the three disease groups investigated was obtained for the parameters log amplitude, log area, and log average power. The number of phases was not significantly different between the NOR and the rest of the groups. In conclusion, the average power and duration parameters can possibly be used as additional discriminators to detect abnormalities of the macro motor unit potential in both needle and surface EMG but further investigation is necessary.     

Effect of electrode dimensions on motor unit potentials

Different effects of longitudinal and transversal electrode dimensions on nerve or muscle single fibre action potentials detected monopolarly, were reported in the literature. The results were contradictory. We studied motor unit potentials (MUPs) detected at a large distance (typical of surface recording) on the basis of a mathematical model without source simplification. The MUPs were calculated as a single convolution of the first temporal derivative of a realistic intracellular action potential and MU impulse response. The spatial averaging of the MUPs by rectangular plate electrodes was performed through analytical integration of the MU impulse response over the electrode area. The effects of longitudinal dimension of the electrode were stronger than those of a transversal one. The effects were distance dependent. The longitudinal dimension of the electrode influenced the main phases (that reflected the excitation origin and propagation) more than the terminal phases (that reflected the excitation extinction at the muscle fibers' ends). This was due to differences in the character of the potential fields (quadrupole or dipole) during generation of individual MUP phases. It was shown that the relative weight of the individual MUP phases could be stressed or suppressed by a proper choice of electrode dimensions, position and orientation.     

Effects of muscle fibre shortening on the characteristics of surface motor unit potentials

Traditionally, studies dealing with muscle shortening have concentrated on assessing its impact on conduction velocity, and to this end, electrodes have been located between the end-plate and tendon regions. Possible morphologic changes in surface motor unit potentials (MUPs) as a result of muscle shortening have not, as yet, been evaluated or characterized. Using a convolutional MUP model, we investigated the effects of muscle shortening on the shape, amplitude, and duration characteristics of MUPs for different electrode positions relative to the fibre–tendon junction and for different depths of the MU in the muscle (MU-to-electrode distance). It was found that the effects of muscle shortening on MUP morphology depended not only on whether the electrodes were between the end-plate and the tendon junction or beyond the tendon junction, but also on the specific distance to this junction. When the electrodes lie between the end-plate and tendon junction, it was found that (1) the muscle shortening effect is not important for superficial MUs, (2) the sensitivity of MUP amplitude to muscle shortening increases with MU-to-electrode distance, and (3) the amplitude of the MUP negative phase is not affected by muscle shortening. This study provides a basis for the interpretation of the changes in MUP characteristics in experiments where both physiological and geometrical aspects of the muscle are varied.     

Area-to-amplitude ratio of the terminal phase of the belly-tendon detected motor unit potentials could be used to recognize reinnervated motor units

Besides the increased number of fibres, the reinnervated motor units (MUs) are characterised by an increased scattering the end-plates, greater desynchronization in the fibres' activation, greater dispersion in the diameters of the MU fibres and thus in propagation velocities along them. As a result, desynchronization in the moments, at which the excitation waves reach the fibres' ends, increases in reinnervated MUs. The possibility to recognize reinnervated MUs in short (hand) muscles on the basis of changes in duration of the terminal (second) phase of the belly-tendon detected motor unit potentials (MUPs) was examined by numerical experiments. A convolution model that took into account the finite fibre length, was used to calculate MUPs for distances typical of surface detection. It was shown that the ratio between the area of the terminal phase and its amplitude, as a measure of duration of the terminal phase, was sensitive to desynchronisation of the waves of excitation. The ratio was independent of the distance from the MU axis and of the volume conductor properties. Basing on the results obtained, we can conclude that the ratio reflects main functional compensations in reinnervated MUs and could be used for discrimination between reinnervated and normal MUs.     

Electromyography of the infrahyoid muscles: pathological and normal findings

In recent years, the infrahyoid muscles (IHM) have been used by plastic reconstructive surgeons as a neurovascular muscle flap in the neck and mouth region. METHODS: A preoperative electromyographic examination (EMG) of the IHM was performed in 10 patients, of whom 9 suffered from tongue cancer, in order to detect neurogenic lesions caused by possible metastases or lymph nodes. The results were compared to those of 10 healthy controls. RESULTS: The EMG at rest showed no pathological spontaneous activity in any patient. During light voluntary innervation, the motor unit potentials (MUPs) were normal in controls and in patients with normal sonographic images, computertomographic scans, and histologic findings after surgery in the neck region. When metastatic lymph nodes were found on one side of the neck, the number of polyphasic MUPs in the IHM of that side was increased in some cases (n = 6), and normal in others (n = 5). Traumatic or radiogenic lesions clearly resulted in pathological EMG findings (n = 6). A maximal innervation of the IHM was achieved during head bending and jaw opening, there was no activation of the IHM with tongue movements and vice versa. CONCLUSIONS: If the presence of lymph node pathology was demonstrated using imaging techniques, a resulting lesion of the ansa cervicalis can functionally be demonstrated by EMG. In patients without lymph node metastases and without concurrent other lesions in the cervical region, EMG of the IHM seems to give no further clinical information. A clear postoperative functional differentiation of the transplanted IHM and the indigenous tongue muscles is possible.     

EMG dynamics in polymyositis and dermatomyositis in adults

In order to analyze the EMG dynamics in acute and chronic polymyositis 44 patients were examined. Thirty-four were seen in the acute stage, 28 in the chronic stage and 18 serially. Investigations included quantitative electromyography using the Polish minicomputer "ANOPS 105" connected to a DISA electromyograph. Additionally fiber density was analyzed by single fiber electromyography in the chronic stage only. The acute stage findings confirmed the observations of earlier authors with the classical expression of excessive spontaneous activity, polyphasic potentials of short duration and low amplitude. In the chronic stage, motor unit potentials with increased duration and amplitude and with late components of the type seen in satellite potentials were noted. This was compared with the increased fiber density found at this stage. Additionally, in some muscles in the chronic stage, motor unit potentials were seen with increased duration, but also a reduction in the mean amplitude of motor unit potentials counted by the automatic analysis method. The decreased amplitude of the motor unit potentials in the chronic polymyositis may be the result of the smaller size of regenerating muscle fibers.     

肌电图在诊断突眼性甲状腺肿合并肌病中的作用

目的：探讨肌电图在评估突眼性甲状腺肿合并肌病患者的病程、病情、治疗效果中的作用。方法：对44例突眼性甲状腺肿患者检测正中神经和胫后神经共88条神经感觉伟导速度（SCV）、运动末端潜伏期（ML）及波幅;F波潜伏期及出现率;并观察三角肌、肌四头肌共88块肌肉的肌电图。结果：88条神经中有16条SCV减慢或波降低;12条ML延长或波幅降低。4条F波潜伏期延长,4条F波出现率降低。检测88块肌肉,有5块骨肉出现自发电位,58块肌肉出现短时限多相运动单位电位,49块肌肉募集反应呈病理干扰相。结论：肌电衅对突眼性甲状腺肿合并肌病有客观评价病情和疗效的作用。     

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.     

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.     

Oculopharyngeal muscular dystrophy. Description of a case with involvement of the central nervous system

A sporadic case of oculopharyngeal muscular dystrophy occurred in a 74-year-old woman is described. High levels of IgA and IgG in the serum, and esophageal smooth muscle involvement are shown. Electromyography of several limb muscles displayed myopathic pattern with giant polyphasic motor unit potentials, suggesting superimposed denervation. The histological examination of peroneus brevis muscle biopsy specimen showed myopathic changes with dystrophic features, associated with neurogenic changes, including atrophic angulated fibers, small-group atrophy and type-grouping: concomitant involvement of spinal motor neuron pathway is hypotized, normal values of motor and sensory nerve conduction velocities excluding associated polineuropathy. Furthermore, Somatosensory Evoked Potentials recording revealed bilaterally increased Central Conduction Time. Referring to other similar cases previously reported in the literature, the significance of neurogenic involvement in oculopharyngeal muscular dystrophy is discussed.     

Evaluation of motor unit firing rates by standard concentric needle electromyography.

Motor unit firing rates at slight voluntary contraction were studied by standard concentric needle electromyography. Employing digital signal analysis techniques firing rates of motor units (MU) could be evaluated as long as four or less different MUs were activated in the vicinity of the concentric needle electrode. The extension of the recording area is defined by the recording properties of the electrode and the upper limit of rise-time for all MUPs being evaluated. Distant MUs, generating volume conducted potentials with rise-times greater than 0.8 ms, were excluded. In biceps muscles of 15 healthy controls the firing rate of the MU activated first was evaluated at that moment, when a second MU was recruited and was found to be 12.1 +/- 2.1 Hz (mean +/- S.D., n = 40). The firing rate of the fastest MU out of 2, 3, or 4 simultaneously active MUs was 10.7 +/- 2.5 Hz, 10.9 +/- 2.5 Hz and 10.6 +/- 2.4 Hz respectively. Hence at low innervation level there is no increase of firing rate with rising number of activated MUs. The upper normal limit of MU firing rate (3 sigma interval) is calculated as 17 Hz, irrespective whether 1, 2, 3 or 4 MUs are active within the recording area. Fifteen patients with partially denervated biceps muscles were investigated. Maximal firing rates were increased in 10 patients, all showing moderate or severe paresis (grade 1-3). In 10 patients suffering from myopathies firing rates always were normal. The presented data may serve as an additional criterion in evaluating MU firing rates during standard clinical EMG.     

Peculiarities of extracellular potentials produced by deep muscles. Part 2: motor unit potentials

The potential fields generated by single fibres far from the sources are non-propagating. This suggests that there should be differences in the features of surface motor unit (MU) potentials (MUPs) detected from deep and superficial muscles. We explored the features using a simulation approach. We have shown that the non-propagating character and similar shapes among surface MUPs recorded over a wide area above deep muscles with monopolar or longitudinal single differential (LSD) electrodes are natural. Contrary to close distances, at large radial distances single differentiation did not emphasize MUP main phase relative weight. The position of the end plate area could be estimated with LSD detections only for MUs with long (123 mm) almost symmetric fibres. With short fibres, the LSD main phase was masked by the outlined terminal phases. This could be misleading in MUP analysis since the terminal phases reflect standing sources. The highly asymmetric fibres could yield peculiar MUP shapes resembling MUPs of two distinct MUs. We have shown that the relative weight of terminal phases at large fibre-electrode distance decreases under abnormal peripheral conditions. However, the changes in membrane depolarization due to fatigue or pathology could be assessed non-invasively also from deep muscles.     

Impact of obstructive apnea syndrome on upper airway respiratory muscles

This article reviews studies of upper airway muscles in humans, with emphasis on muscle fiber structural and electrophysiological changes observed in patients with obstructive sleep apnea syndrome (OSAS). The concept of OSAS as a progressive disease is discussed and also possible causes. These include local nervous lesions in the upper airway, both motor and sensory. Previous muscle biopsy studies have given evidence for motor neuron lesions such as, e.g., the phenomenon of type grouping in histological sections. New data obtained with concentric needle EMG recordings from the palatopharyngeus muscles are also presented. In 10/12 OSAS patients there were typical findings indicating motor neuropathy (reduced EMG activity at maximal voluntary effort, long and polyphasic motor-unit potentials and, in two cases, spontaneous denervation activity), whereas such findings were only present in 3/15 patients with habitual snoring. This supports the hypothesis that progression from habitual snoring to the clinical disease of OSAS could be attributed to peripheral neurogenic lesions.     

Collateral nerve sprouting and twitch forces of single motor units in conditions with partial denervation in man

Single motor units (MUs) were studied in the first dorsal interosseus muscle of 7 patients with slight partial denervation and in 10 controls. MU action potentials were recorded using the macro-EMG technique and their size was taken to assess collateral nerve sprouting. Simultaneously, the muscle force was monitored to determine the voluntary recruitment thresholds of the MUs and spike-triggered-averaging was applied to measure their twitch forces. At comparable force recruitment thresholds, both macro-EMG potentials and twitch forces were increased in the patients. We conclude, that collateral nerve sprouting increases MU force and can compensate for MU loss.     

Muscle fiber conduction velocity studied by the multi-channel surface EMG.

Muscle fiber conduction velocity in single motor unit (MU) during voluntary contraction was measured by using the multi-channel surface EMG. The subjects examined were 28 normal controls (5-40 years old), four patients with myopathy and three patients with neuropathy. The tibialis anterior and the biceps brachii muscles were investigated at weak and moderate contraction levels. In normal muscles, the mean muscle fiber conduction velocity increased with muscle force (P less than 0.01). In three of four cases with myopathy, the conduction velocities were reduced compared to normal subjects (P less than 0.01), and there was little or no correlation between the conduction velocity and muscle force in myopathy. The conduction velocities were within the normal range in the muscles of patients with neuropathy. However, high-amplitude MUAPs with fast conduction velocities were detected during weak voluntary contraction in one case with neuropathy.