Spasticity,weakness, force variability,and sustained spontaneous motor unit discharges of resting spastic–paretic biceps brachii muscles in chronic stroke

Introduction: The purpose of our study was to examine relations among spasticity, weakness, force variability, and sustained spontaneous motor unit discharges in spastic–paretic biceps brachii muscles in chronic stroke. Methods: Ten chronic stroke subjects produced submaximal isometric elbow flexion force on impaired and non‐impaired sides. Intramuscular EMG (iEMG) was recorded from biceps and triceps brachii muscles. Results: We observed sustained spontaneous motor unit discharges in resting biceps on iEMG. Spontaneous discharges increased after voluntary activation only on the impaired side. The impaired side had greater matching errors and greater fluctuations in isometric force. Spontaneous discharges were not related functionally to spasticity, force variability, or weakness. However, greater strength on the impaired side correlated with less force variability. Conclusion: Weakness rather than spasticity is a main factor interfering with voluntary force control in paretic–spastic biceps brachii muscles in chronic stroke. Muscle Nerve, 2013     

Electromyographs of the flexor digitorum profundus muscle are useful for the diagnosis of inclusion body myositis

Introduction: The frequent observation of high‐amplitude and long‐duration motor unit potentials (MUPs) in inclusion body myositis (IBM) is problematic, because it may lead to a misdiagnosis of amyotrophic lateral sclerosis (ALS). Objective: To document the diagnostic utility of EMG from the flexor digitorum profundus (FDP) muscle for IBM. Methods: Quantitative analyses of MUP parameters were performed in the FDP and biceps brachii (BB) muscles from 7 biopsy‐confirmed IBM patients. Results: In the FDP muscle, all MUP parameters were significantly decreased in IBM patients, which indicated the predominance of low‐amplitude and short‐duration MUPs in this muscle. In the BB muscle, most parameters were increased, suggesting the frequent contamination of high‐amplitude and long‐duration MUPs. Conclusions: Low‐amplitude MUPs in the FDP muscle indicate the presence of an advanced myopathy in this muscle that was extremely weak for all subjects. Examining the FDP muscle would reduce the chance of misdiagnosing IBM as ALS. Muscle Nerve 46: 181–186, 2012     

Anconeus motor unit number estimates using decomposition‐based quantitative electromyography

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

The influence of prolonged vibration on motor unit behavior

Introduction: The purpose of this study was to determine the effects of vibration (VIB) on motor unit (MU) behavior of the vastus lateralis (VL) muscle during a 40% maximal voluntary contraction (MVC). Methods: Eleven healthy (age 21.3 ± 2.6 years) individuals participated in the study. Surface electromyography (EMG) data were recorded from the VL during isometric trapezoidal muscle contractions at 40% MVC. Firing events of single MUs and EMG amplitude were reported for the first, middle, and final seconds of a 12‐second steady force segment at 40% MVC. VIB was applied at 55 Hz to the patellar tendon for 15 minutes before and continued throughout the remainder of testing (VIB) or remained off (CON). Results: There were significant increases in MU firing rates during VIB in comparison to CON and no differences in EMG amplitude between VIB and CON. Conclusion: The VIB‐mediated reduction in muscle spindle function altered MU behavior at 40% MVC. Muscle Nerve 55 : 500–507, 2017     

Electrophysiological studies in a mouse model of Schwartz–Jampel syndrome demonstrate muscle fiber hyperactivity of peripheral nerve origin

Schwartz–Jampel syndrome (SJS) is an autosomal‐recessive condition characterized by muscle stiffness and chondrodysplasia. It is due to loss‐of‐function hypomorphic mutations in the HSPG2 gene that encodes for perlecan, a proteoglycan secreted into the basement membrane. The origin of muscle stiffness in SJS is debated. To resolve this issue, we performed an electrophysiological investigation of an SJS mouse model with a missense mutation in the HSPG2 gene. Compound muscle action potential amplitudes, distal motor latencies, repetitive nerve stimulation tests, and sensory nerve conduction velocities of SJS mice were normal. On electromyography (EMG), neuromyotonic discharges, that is, bursts of motor unit action potentials firing at high rates (120–300 HZ ), were constantly observed in SJS mice in all muscles, except in the diaphragm. Neuromyotonic discharges were not influenced by general anesthesia and disappeared with curare administration. They persisted after complete motor nerve section, terminating only with Wallerian degeneration. These results demonstrate that perlecan deficiency in SJS provokes a neuromyotonic syndrome. The findings further suggest a distal axonal localization of the generator of neuromyotonic discharges. SJS should now be considered as an inherited disorder with peripheral nerve hyperexcitability. Muscle Nerve, 2009     

Motor unit control strategies of endurance‐ versus resistance‐trained individuals

Introduction: We examined motor unit (MU) control properties of resistance‐trained (RT) and endurance‐trained (ET) individuals. Methods: Five RT (age 25 ± 4 years) and 5 ET (age 19 ± 1 years) subjects participated. Surface electromyography (EMG) data were recorded from the vastus lateralis during isometric trapezoid muscle actions at 40% and 70% of maximal voluntary contraction. Decomposition and wavelet transform techniques were applied to the EMG signals to extract the firing events of single MUs and EMG intensity across the frequency spectrum. Results: There were significant differences between RT and ET for the mean MU firing rate and derecruitment threshold versus recruitment threshold relationships and EMG intensity at various wavelet bands during the linearly increasing, steady force, and linearly decreasing segments of the trapezoid contraction. Conclusions: MU behavior is altered as a function of training status and is likely the result of differences in the physical properties of the MU. Muscle Nerve 52 : 832–843, 2015     

Motor unit size estimation of enlarged motor units with surface electromyography

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

Caffeine improves neuromuscular function during maximal dynamic exercise

Introduction: In this study we tested the hypothesis that caffeine supplementation improves neuromuscular function, which has both nutritional and clinical relevance. Methods: Fourteen male subjects (mean ± SD: 23.8 ± 2.8 years) volunteered in a double‐blind, repeated‐measures study with placebo (PLA) or caffeine (CAFF) (6 mg kg^?1). Maximal voluntary isometric contractions (MVCs), evoked maximal twitch, and maximal isokinetic contractions during elbow flexion were assessed. Mechanical and electromyographic (EMG) signals from the biceps brachii muscle were recorded, and muscle fiber conduction velocity (CV) was calculated to evaluate changes in the muscle force–velocity relationship and muscle fiber recruitment. Results: The torque–angular velocity curve was enhanced after CAFF supplementation. This was supported by a concomitant increase of CV values (8.7% higher in CAFF). Conclusions: Caffeine improves muscle performance during short‐duration maximal dynamic contractions. The concomitant improvement of mean fiber CV supports the hypothesis of an effect of caffeine on motor unit recruitment. Muscle Nerve, 2011.     

Macro EMG follow-up study in post-poliomyelitis patients

We investigated the muscle strength and motor unit (MU) territory of five patients with postpolio syndrome (PPS), six stable patients with prior poliomyelitis, and five healthy volunteers. The MU territory was assessed by measuring amplitudes of motor unit potentials (MUPs) recorded by the macro EMG technique. The investigations were repeated after 11–20 months (mean 15.6). The macro MUP amplitudes in both patient groups were markedly increased (P = 0.02). However, no statistical difference was found between the two groups in the initial amplitude values. Macro MUP amplitudes obtained on repeated examinations did not differ significantly from the initial macro MUP amplitudes in any of the three groups. In three individual PPS patients, a decline in muscle strength on the follow-up study was documented, providing the diagnosis of post-poliomyelitis muscular atrophy (PPMA). The three PPMA patients had the highest initial macro MUPs. Two of them showed a decrease in macro MUP amplitudes on follow-up. These findings suggest that a later breakdown of oversized MUs may play a role in the pathogenesis of PPMA.     

Feature selection for motor unit potential train characterization

Introduction: Ten new features of motor unit potential (MUP) morphology and stability are proposed. These new features, along with 8 traditional features, are grouped into 5 aspects: size, shape, global complexity, local complexity, and stability. Methods: We used sequential forward and backward search strategies to select subsets of these 18 features to discriminate accurately between muscles whose MUPs are predominantly neurogenic, myopathic, or normal. Results and Conclusions: Results based on 8102 motor unit potential trains (MUPTs) extracted from 4 different limb muscles (n = 336 total muscles) demonstrate the usefulness of these newly introduced features and support an aspect‐based grouping of MUPT features. Muscle Nerve 49 : 680–690, 2014     

Adult-onset lower extremity weakness caused by venous malformation detected by magnetic resonance imaging

Introduction: Symptomatic venous malformation (VM) of muscle in adults is rare and usually presents in childhood or adolescence as the individual is growing. We describe an atypical presentation of a malformation affecting the gastrocnemius muscle asymmetrically with onset in adulthood, which created a diagnostic challenge. Electromyography (EMG) and muscle biopsy did not fit clinically and MRI of the gastrocnemius led to the diagnosis. Methods: The setting for the patient studied was a neuromuscular outpatient clinic. Results: EMG showed decreased insertional activity and motor unit potential recruitment in the right gastrocnemius muscle. Muscle biopsy showed mild neurogenic changes. MRI demonstrated VM in the contralateral gastrocnemius muscle. Conclusions: This case represents a rare cause of atrophic weakness in adults, but muscle MRI should be considered when other tests are equivocal. Muscle Nerve 46:129–133, 2012     

Syringomyelia: Electrophysiologic aspects

The clinical and EMG findings in 44 patients with syringomyelia who were seen at the Mayo Clinic between 1976 and 1985 are presented. In 10 of the patients, somatosensory evoked potentials (SEPs) of the upper and lower extremities were obtained. All 44 patients had radiographic or surgical evidence of a cervical syrinx. The most common abnormality on nerve conduction studies was a reduced hypothenar compound muscle action potential amplitude (23 patients). Abnormal findings on needle electromyography were present in 33 patients and included sparse fibrillation potentials, reduced motor unit potential (MUP) recruitment, and chronic neurogenic MUP changes in muscles innervated by the C-5, T-1 roots, with the most pronounced changes in small hand muscles. Ulnar and median nerve SEPs were usually normal in the presence of a dissociated sensory loss and were usually abnormal when all sensory modalities were impaired. Abnormalities of tibial nerve SEPs were frequent and were related to impaired proprioceptive sensation in the lower extremities.     

Neurotonia: impulse-induced repetitive discharges in motor nerves in peripheral neuropathy

Delayed relaxation of muscle following voluntary contraction was an unusual feature of a mild chronic sesorimotor peripheral neuropathy in an adult. This abnormality resulted from rapid repetitive discharges in motor nerves occurring only in response to passing impulses; there was no spontaneous nerve discharge. Voluntary contraction of affected muscles generated involuntary high-frequency discharges of motor unit potentials, which persisted briefly after attempted relaxation. Nerve blocks localized independent zones of hyperexcitability in distal and proximal sections of nerve from which such repetitive discharges could be triggered. Intravenous administration of phenytoin diminished the discharge. Examination of intramuscular nerve bundles revealed loss of myelinated nerve fibers with numberous sprouting and remyelinating axons. These findings, along with muscle biopsy changes of neurogenic atrophy and type grouping, strongly favor an axonal neuropathy. An explanation for the repetitive nerve discharge is slow waning of heightened excitability of a motor nerve after passage of an impulse.     

EMG power spectrum, turns-amplitude analysis and motor unit potential duration in neuromuscular disorders

The diagnostic value of power spectrum analysis of the needle EMG pattern at a force of 30% of maximum was compared to that of turns-amplitude analysis and to that of manual measurements of motor unit potential (MUP) duration in the brachial biceps muscle of 20 patients with myopathy and 11 patients with neurogenic disorders. In myopathy the power spectrum analysis had the same diagnostic value as the turns-amplitude analysis and MUP duration measurements and the 3 methods supplemented each other. In patients with neurogenic disorders the diagnostic value of the power spectrum analysis as well as that of the turns-amplitude analysis were lesser than that of MUP duration measurement. In diseased muscles the amount of high frequencies increased with increasing ratio of turns to mean amplitude while there was no relation between the power spectrum and the MUP changes. The results suggest that the power spectrum analysis of EMG can be used as a diagnostic tool in patients with neuromuscular disorders.     

Neuromuscular junction disorders mimicking myopathy

Introduction: Small‐amplitude, short‐duration motor unit action potentials are non‐specific findings seen in myopathies and neuromuscular junction (NMJ) disorders. NMJ studies (repetitive nerve stimulation and single‐fiber electromyography) can determine if such findings are related to NMJ abnormalities but are not considered routinely in atypical cases. Methods: Medical records of 338 patients with confirmed NMJ disorders were reviewed to identify cases with a clinical or electrodiagnostic impression of myopathy during initial evaluation. A history of muscle biopsy with findings that did not support a myopathic process was required for inclusion. Results: Four patients met the inclusion criteria. NMJ studies were abnormal in all cases. One patient had elevated acetylcholine receptor antibodies. Three patients were antibody negative: 2 demonstrated immunotherapy responsiveness, and 1 had a Rapsyn mutation. Conclusions: NMJ disorders may mimic myopathies, and NMJ studies should be performed to clarify so‐called “myopathic” electromyographic findings to avoid unnecessary testing and delayed diagnosis. Muscle Nerve 50 : 854–856, 2014     

多发性肌炎患者神经电生理特点分析

目的增进对多发性肌炎患者肌电图特点的认识,提高其检查的阳性率。方法对91例多发性肌炎患者进行肌电图(EMG)、神经传导速度测定。结果肌电图异常率为87.9%,肌源性损害者占79.1%,神经源性损害者占8.8%。其中插入电位延长、自发电位的阳性率分别为6%和52%,肱二头肌出现率较高,外展拇短肌出现率最低(p<0.05);运动单位电位(MUP)时限缩短的阳性率为71%,胫前肌出现率最高,外展拇短肌最低(p<0.05);MUP波幅降低的阳性率较低,仅为7%;多相波增多的阳性率为29%,胫前肌出现率最高,股四头肌最低(p<0.05);重收缩时波形异常的阳性率为26%,以股四头肌出现率最高;重收缩时峰值电压降低的阳性率为31%,胫前肌出现率最高,外展拇短肌最低(均p<0.05)。5例患者EMG呈神经源性损害,1例感觉神经传导速度减慢,2例运动神经传导速度减慢。肌电图正常组、肌源性损害组及神经源性损害组患者的病程、年龄无明显差异。结论 EMG对多发性肌炎诊断的阳性率为87.9%,其中以MUP时限缩短出现率最高为71%,其次为自发电位为52%。EMG异常最多见于肱二头肌、股四头肌和胫前肌。     

Simulation studies on the motor unit potentials with satellite components in amyotrophic lateral sclerosis and spinal muscle atrophy

Introduction: We compared motor unit potentials (MUPs) with satellite components recorded in two anterior horn disorders: amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA, types II and III). Methods: We analyzed MUPs recorded from biceps brachii muscle, including 209 associated with ALS (12 patients) and 127 with SMA (5 patients). Simulations were applied to determine the origin of satellites in these processes. Results: MUP parameters differ in ALS and SMA. Simulations indicate that the satellite potential in ALS often originated from a single fiber, whereas in SMA it originated from a group of fibers of smaller diameters than the surrounding ones. Conclusions: These results suggest that, except for neurogenic factors, the variability of muscle fiber diameters also leads to the formation of MUPs with satellites. This variability seems to be responsible for the differences in the shape of the main and satellite MUP components in ALS and SMA. Muscle Nerve, 2012     

An atypical case of SCN9A mutation presenting with global motor delay and a severe pain disorder

Introduction: Erythromelalgia due to heterozygous gain‐of‐function SCN9A mutations usually presents as a pure sensory–autonomic disorder characterized by recurrent episodes of burning pain and redness of the extremities. Methods: We describe a patient with an unusual phenotypic presentation of gross motor delay, childhood‐onset erythromelalgia, extreme visceral pain episodes, hypesthesia, and self‐mutilation. The investigation of the patient's motor delay included various biochemical analyses, a comparative genomic hybridization array (CGH), electromyogram (EMG), and muscle biopsy. Once erythromelalgia was suspected clinically, the SCN9A gene was sequenced. Results: The EMG, CGH, and biochemical tests were negative. The biopsy showed an axonal neuropathy and neurogenic atrophy. Sequencing of SCN9A revealed a heterozygous missense mutation in exon 7; p.I234T. Conclusions: This is a case of global motor delay and erythromelalgia associated with SCN9A. The motor delay may be attributed to the extreme pain episodes or to a developmental perturbation of proprioceptive inputs. Muscle Nerve 49 : 134–138, 2014     

Prior heat stress effects fatigue recovery of the elbow flexor muscles

Introduction: Long‐lasting alterations in hormones, neurotransmitters, and stress proteins after hyperthermia may be responsible for the impairment in motor performance during muscle fatigue. Methods: Subjects (n = 25) performed a maximal intermittent fatigue task of elbow flexion after sitting in either 73° or 26°C to examine the effects of prior heat stress on fatigue mechanisms. Results: The heat stress increased the tympanic and rectal temperatures by 2.3° and 0.82°C, respectively, but there was full recovery prior to the fatigue task. Although prior heat stress had no effects on fatigue‐related changes in volitional torque, electromyographic (EMG) activity, torque relaxation rate, motor evoked potential (MEP) size, and silent period (SP) duration, prior heat stress acutely increased the pre‐fatigue relaxation rate and chronically prevented long‐duration fatigue (P < 0.05). Conclusions: These findings indicate that prior passive heat stress alone does not alter voluntary activation during fatigue, but prior heat stress and exercise produce longer‐term protection against long‐duration fatigue. Muscle Nerve 44: 115–125, 2011