Invited review: motor unit estimation: methods, results, and present status

Number of Edb motor units estimated using an adapted multiple point stimulation method: normal values and longitudinal studies in ALS and peripheral neuropathies

OBJECTIVE

To validate the adapted multiple point stimulation (AMPS) method to estimate the number of motor units (MUNE) from the extensor digitorum brevis (Edb) muscle.

METHODS

Twenty controls (10 young and 10 old) were examined on both sides and 10 patients with amyotrophic lateral sclerosis (ALS) and 5 with acute peripheral neuropathy (PN) were examined longitudinally on one side during a series of repeated electrophysiological sessions.

RESULTS

In the controls, the median MUNE and size of the motor unit action potentials (S-MUAPs) were found to be age-related (411 and 70 microVms in the young group; 164 and 142 microVms in the old group; P < 0.01), with a coefficient of variation of MUNE values of 27% and 20%, respectively. In the ALS group, the median MUNE value at diagnosis was 31 (P < 0.05 vs. controls), and during a mean follow-up period of 11.5 months a continuous decrease in the MUNE value was seen, together with an initial increase, followed by a later decrease in 4 cases, in S-MUAP size. In the PN group, the MUNE value was initially similar to that in controls, but then decreased, accompanied by an increase in S-MUAP size, and then showed a progressive increase, together with a decrease in S-MUAP size.

CONCLUSIONS

AMPS, a MUNE method developed in the upper extremity, also appears to be a useful procedure for quantifying changes in the MUNE value in the Edb muscle without specific software in order to study age-related changes or changes in patients with ALS or PN.

Motor unit number estimation: new techniques and new uses

MUNE is a unique neurophysiologic tool because it can quantitatively estimate the number of motor neurons innervating a muscle or group of muscles. All other neurophysiologic techniques are influenced by collateral reinnervation and provide only a qualitative estimate of motor unit loss. Further, the S-MUPs obtained with MUNE provide quantitative information about the whole motor unit. Other routine neurophysiologic techniques provide information restricted to a portion of the motor unit. These unique features of MUNE have been applied to neurogenic disorders to yield a better understanding of disease processes. Various modifications are being developed that will provide more data and ease of use. It is anticipated that the availability of MUNE on EMG machines will grow and it use will expand from a research tool to a routine neurophysiologic test.

Assessment of axonal loss in Charcot–Marie–Tooth neuropathies

Sensory loss and weakness in Charcot-Marie-Tooth (CMT) neuropathy is due to axonal loss. However, the pattern and degree of axonal loss cannot be accurately determined from routine electrodiagnostic or strength testing due to collateral reinnervation. We sought to quantify axonal loss in two upper extremity muscles in CMT1A and CMT2 subjects using the electrophysiologic endpoint measure of motor unit number estimation (MUNE). Hypothenar and biceps-brachialis muscle groups were studied in 9 CMT1A, 9 CMT2, and 10 control subjects. The spike-triggered averaging (STA) technique was used to collect surface motor unit potentials for MUNE calculations, and a needle electrode was used to collect corresponding intramuscular data. Maximal voluntary hypothenar and handgrip strength was measured quantitatively, while biceps-brachialis strength was measured qualitatively. Compared to normal subjects, CMT1A and CMT2 subjects had significantly lower MUNE values in hypothenar muscles. Biceps-brachialis MUNE values were reduced in CMT2 but not in CMT1A subjects. In support of proximal axonal loss in CMT2 subjects, surface motor unit and intramuscular potential amplitudes were higher in biceps-brachialis muscles compared to controls. Correlations between quantitative strength and MUNE were significant for hypothenar but not for grip muscle groups. Axonal loss is demonstrated in distal muscles in CMT1A and CMT2 supporting a length-dependent axonopathy. Despite clinical findings of normal or near-normal strength and small reductions in compound muscle action potential (CMAP) amplitude, MUNE values were significantly lower in CMT2 subjects in proximal muscles, consistent with more diffuse denervation. These data indicate that subclinical axonal loss is present that cannot be appreciated using clinical examination or routine electrodiagnostic techniques.

Invited review: Aging and sarcopenia

Aging is associated with progressive loss of neuromuscular function that often leads to progressive disability and loss of independence. The term sarcopenia is now commonly used to describe the loss of skeletal muscle mass and strength that occurs in concert with biological aging. By the seventh and eighth decade of life, maximal voluntary contractile strength is decreased, on average, by 20-40% for both men and women in proximal and distal muscles. Although age-associated decreases in strength per unit muscle mass, or muscle quality, may play a role, the majority of strength loss can be accounted for by decreased muscle mass. Multiple factors lead to the development of sarcopenia and the associated impact on function. Loss of skeletal muscle fibers secondary to decreased numbers of motoneurons appears to be a major contributing influence, but other factors, including decreased physical activity, altered hormonal status, decreased total caloric and protein intake, inflammatory mediators, and factors leading to altered protein synthesis, must also be considered. The prevalence of sarcopenia, which may be as high as 30% for those >/=60 yr, will increase as the percentage of the very old continues to grow in our populations. The link between sarcopenia and disability among elderly men and women highlights the need for continued research into the development of the most effective interventions to prevent or at least partially reverse sarcopenia, including the role of resistance exercise and other novel pharmacological and nutritional interventions.

Motor Unit Estimate Number in the Anterior Tibial Muscle: Normative Data versus Findings in Critically Ill Patients in Intensive Care Units

OBJECTIVES

To determine the number of motor units (MUNEs) in the anterior tibial muscle of normal subjects for comparison with those of severely paretic or paralytic muscles of critically ill patients in intensive care units.

RESULTS

The mean MUNE for 24 normal subjects (194 +/- 5; mean +/- standard deviation) was similar to that of the 22 patients with critical illness (184 +/- 10). However, both the mean amplitude of the evoked compound muscle action potential (CMAP) and of the single motor unit action potential (S-MUAP) among patients were approximately one third of those in normal subjects.

CONCLUSION

Critically ill patients in this study demonstrated normal MUNEs with reduced CMAP and S-MUAP amplitudes in the setting of severe clinical weakness, suggestive of predominantly myopathic injury. MUNE may provide a valuable tool for distinguishing between neuropathy and myopathy in critically ill patients.

Motor unit number estimation in infants and children with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a disease of lower motor neurons. Motor unit number estimation (MUNE) is an electrophysiologic method to estimate the number of motor neurons innervating a muscle group. We applied the multiple point stimulation technique to the ulnar nerve--hypothenar muscle group to study lower motor neuron loss in 14 SMA subjects, including those presymptomatic, and varying from newborn through 45 years of age. Preliminary data support the value of MUNE to help understand the time course of motor neuron loss in SMA.

Motor unit number estimates in idiopathic Parkinson's disease

We previously reported changes in motor unit morphology in patients with Parkinson's disease (PD) using subjective and computerized quantitative electromyography. Now, we present data on motor unit number estimates (MUNE) to address the hypothesis of motor neuron dropout in PD. Twenty patients with PD and 20 age-matched control subjects were screened by clinical criteria and nerve conduction studies to exclude those with neuropathy. Motor unit number estimates in the extensor digitorum brevis and hypothenar group were assessed by three different MUNE techniques. The MUNE technique types included (1) the statistical method developed by Daube, (2) a threshold method, and (3) an F-wave method. The overall multivariate comparison for the six MUNE measurements was significantly lower for the patients than the controls (P=0.02). The only significant difference in the individual measures was found in the threshold MUNE method of the hypothenar group (P<0.05). These results are consistent with those of our previous work, and both support the hypothesis that mild motor neuron dropout occurs in idiopathic PD. However, MUNE methods characteristically have large standard deviations which make it difficult to detect small changes. Progress in decreasing the variance of MUNEs will facilitate their use in detecting small motor unit number changes in neurodegenerative disease.

Depletion and sizes of motor units in spinal muscular atrophy

Motor unit number estimation (MUNE) was applied to the biceps brachii muscles of 13 young patients (age 5--24 years) with spinal muscular atrophy (SMA) and the results compared with those of healthy control subjects matched for age and gender. In the SMA patients, all motor unit (MU) estimates fell below the control range, and there was good correspondence between the values for the two arms in the same subject. No correlation could be found between the MUNEs and the severity of the weakness. This unexpected result was attributed to the presence of small and normal-sized MUs in the muscles of patients, in addition to MUs that appeared to be considerably enlarged. The threefold mean increase in MU potential size was insufficient to compensate for the MU loss. In addition, the study confirmed that there are, on average, approximately 130 MUs in the healthy biceps brachii muscle.

Electrophysiologic endpoint measures in a multicenter ALS drug trial

We report the analysis of a battery of secondary electrophysiologic measurements to assess the progression of amyotrophic lateral sclerosis (ALS) in a two center, six month, double-blind, three arm trial comparing branched chain amino acids to L-threonine with pyridoxal 5-phosphate to placebo. The endpoint measurements were chosen to separately assess the effects of lower motor neuron loss and collateral reinnervation. For tests of inter-center reliability, we found no differences that could not be readily explained by variations in electrophysiologic testing techniques. Since the drug study was negative for the primary endpoint measure (muscle strength), we combined data from both centers and the three treatment arms. For measures of progression, all measures changed in the expected direction during the 6 months of the trial. We conclude that a battery of electrophysiologic measures can be used in a multicenter ALS drug trial to provide information on changes in lower motor neuron numbers and the effects of collateral reinnervation.

Quantification of upper motor neuron loss in amyotrophic lateral sclerosis

OBJECTIVE

To quantitatively estimate upper motor neuron (UMN) loss in ALS.

METHODS

We used the recently developed triple stimulation technique (TST) to study corticospinal conduction to 86 abductor digiti minimi muscles of 48 ALS patients. This method employs a collision technique to estimate the proportion of motor units activated by a transcranial magnetic stimulus. At the same time, it yields an estimate of lower motor neuron (LMN) integrity.

RESULTS

The TST disclosed and quantified central conduction failures attributable to UMN loss in 38 sides of 24 patients (subclinical in 15 sides), whereas conventional motor evoked potentials detected abnormalities in only 18 sides of 12 patients (subclinical in two sides). The increased sensitivity of the TST to detect UMN dysfunction was particularly observed in early cases. Increased central motor conduction times (CMCT) occurred exclusively in sides with conduction failure. In sides with clinical UMN syndromes, the TST response size (but not the CMCT) correlated with the muscle weakness. In sides with clinical LMN syndromes, the size of the peripherally evoked compound muscle action potentials correlated with the muscle weakness.

CONCLUSION

The TST is a sensitive method to detect UMN dysfunction in ALS. It allows a quantitative estimate of the UMN loss, which is related to the functional deficit. Therefore, the TST has a considerable impact on diagnostic certainty in many patients. It will be suited to follow the disease progression and therapeutic trials.

Physiologic decrease of single thenar motor units in the F-response in stroke patients

OBJECTIVE

To investigate the left-right difference and the reproducibility by the F-wave motor unit number estimation and to compare the motor unit number between the hemiplegic and unaffected side in stroke patients.

SETTING

A referral center and institutional practice providing outpatient care.

SUBJECTS

Seven healthy volunteers and 15 consecutive stroke patients.

DESIGN

Diagnostic statistical test and correlational study.

METHOD

Submaximal stimuli were used to evoke a sample of surface motor unit action potentials (S-MUAPs) in the F-waves that are entirely representative of the relative numbers of detected S-MUAPs of different sizes. The average S-MUAP amplitude was calculated from a selected population of F-wave responses for each abductor pollicis brevis (APB) muscle. The motor unit number was calculated by dividing the maximum M-potential negative peak amplitude by the average S-MUAP negative peak amplitude.

RESULT

There was no statistical difference between motor unit numbers on either side and between test and retest in this motor unit number estimation method among normal subjects. The motor unit number on the hemiplegic side was significantly lower than on the unaffected side (p < .05, Mann-Whitney test) among stroke patients.

CONCLUSION

The motor unit could decrease in the hemiplegic side after a moderate-to-severe hemiplegic stroke and this decrement might be due to the transsynaptic degeneration secondary to an upper motor neuron lesion.

Motor unit number estimate-based rates of progression of ALS predict patient survival

We have examined, as predictors of survival in patients with amyotrophic lateral sclerosis (ALS), linear estimates of rates of disease progression (LEP), based on motor unit number estimates (MUNE). Motor unit number estimates of thenar, hypothenar, and extensor digitorum brevis muscles (according to the manual method of McComas), isometric grip and foot dorsiflexion (FD) strength, and forced vital capacity (FVC) were available in 34 patients. Linear estimates of rates of disease progression were derived. Probability of survival was calculated using the Kaplan-Meier method. Motor unit number estimates, LEP based on MUNE, and demographic characteristics were tested as risk factors within the Cox Proportional Hazards Model, using regression techniques. Individually, all MUNE-based LEP were highly significant (P < 0.00005); bulbar onset attained modest significance (P = 0.044). Secondary analysis showed MUNE-based LEP were more significant than regionally concordant function-based LEP. Linear estimates of rates of disease progression based on MUNE may thus predict survival of patients with ALS better than LEP based on function.

Role of motor unit number estimate electromyography in experimental canine laryngeal reinnervation

Laryngeal electromyography has been used clinically to differentiate neuromuscular pathology from other causes of vocal fold immobility such as arytenoid dislocation, tumor invasion, or cricoarytenoid joint fixation. Electromyography has also been used to predict the prognosis for nerve recovery in laryngeal paralysis. Existing electromyographic techniques either record activity with voluntary motion or study nerve conduction. In this study a new technique, motor unit number estimation, a commercially available quantitative method of electromyographic analysis, is used to study the progress of recovery of vocal fold function after recurrent laryngeal nerve injury. Four dogs underwent transection and immediate reanastomosis of selected branches of the adductor and abductor branches of the recurrent laryngeal nerve on 1 side; the opposite side served as a control. Baseline electromyographic and videolaryngoscopic studies were performed. These measures were then repeated in a longitudinal fashion every 6 weeks after denervation. The motor unit number estimation technique indicated a return of motor unit numbers with time, along with estimates of their size. This was consistent with the expected progress of laryngeal reinnervation. These data and their predictive value for nerve recovery will be discussed.

The size index as a motor unit identifier in electromyography examined by numerical calculation

A computer simulation was performed to investigate the size index as a motor unit identifier in electromyography. The size index calculated from the amplitude and area of the simulated motor unit action potential (MUP) was plotted against the distance between the needle electrode and current source to show how the index changes as a function of the distance. The index of the MUP also was plotted against the number of muscle fibers belonging to a single motor unit, the size of the motor unit territory, and the diameter of the muscle fibers in order to establish the major determinants of the index. The index was relatively constant for the distance less than 2 mm between the needle electrode and closest edge of the current source. It changed logarithmically with the number of muscle fibers and with the diameter of the fibers.

Ongoing trials in motor neurone disease

Motor neurone disease (MND) is a group of progressive neurodegenerative disorders that cause disability from weakness and lead to death from respiratory failure. The pathophysiology of the several forms of MND is unknown, but recent advances have led to clinical trials of therapeutic agents based on an improved understanding of the pathologic processes. The design of clinical trials in MND is challenging, because an effective drug cannot restore strength, but rather slow the rate of progression. Measurement of progression poses difficulties, and an optimum end-point measure has not been determined. This article will include the clinical features of MND, present the leading hypotheses about causes as they relate to drug therapy, discuss factors to consider in selecting informative end-point measures, and will review past and current drug trials.

Denervation and reinnervation in congenital brachial palsy

Motor unit number estimation (MUNE) was shown to be useful in assessing the neurophysiological status of 18 subjects with congenital brachial palsy. This was especially so since conventional M-wave measurements may give misleading impressions as to the extent of motor axon regeneration. In most subjects the involvement of sensory nerve fibers indicated that the traumatic lesions included postganglionic segments of the fibers, with or without preganglionic damage. In a minority the lesions were purely preganglionic. Digital sensory nerve involvement was more in a mediolateral direction, consistent with greater damage to the uppermost elements in the brachial plexus. In 5 individuals, MUNE and sensory testing showed that there had been trauma to the supposedly unaffected arm. Discrepancies between sensory and motor results suggested that reinnervation of the biceps brachii muscle was greater than that of the intrinsic muscles of the hand. In one subject examined serially, reinnervation of the hand muscles was detected by 10 months and continued in the hypothenar muscles for the next 6 years.

Expression of acetylcholine receptor mRNAs in atrophying and nonatrophying skeletal muscles of old rats

We examined the age-related association in skeletal muscle between atrophy and expression of mRNAs encoding both the gamma-subunit of the nicotinic acetylcholine receptor (AChR), and myogenin, a transcription factor that upregulates expression of the gamma-subunit promoter. Gastrocnemius and biceps brachii muscles were collected from young (2-mo-old), adult (18-mo-old), and old (31-mo-old) Fischer 344/Brown Norway F1 generation cross male rats. In the gastrocnemius muscles of old vs. young and adult rats, lower muscle mass was accompanied by significantly elevated AChR gamma-subunit and myogenin mRNA levels. In contrast, the biceps brachii muscle exhibited neither atrophy nor as drastic a change in AChR gamma-subunit and myogenin mRNA levels with age. Expression of the AChR epsilon-subunit mRNA did not change with age in either gastrocnemius or biceps brachii muscles. Thus changes in skeletal muscle AChR gamma-subunit and myogenin mRNA levels may be more related to atrophy than to chronological age in old rats.

Neurophysiologic evaluation of lower motor neuron damage in tetraplegia

We quantitatively investigated the extent of damage to motor neurons in tetraplegic subjects. Numbers of motor units in the patients were significantly lower for thenar, wrist extensor, and biceps brachii as compared to controls. Reduction in counts occurred even when M-response amplitudes were normal. Standard electromyography suggested a surprising frequency of lower motor neuron dysfunction below the level of injury. These results confirm previous reports and add data on motor units in the biceps brachii.

Evident trans-synaptic degeneration of motor neurons after stroke: a study of neuromuscular jitter by axonal microstimulation

Neuronal degradation accompanied by axonal degeneration has been known to occur in lower motor neurons following a stroke. In the present study, the functional integrity of neuromuscular transmission was assessed, utilizing a sensitive electrodiagnostic method consisting of stimulated single-fiber electromyography (SFEMG), along with axonal microstimulation, in paralytic muscles of stroke patients. Neuromuscular jitter was measured in the hemiplegic side extensor digitorum communis (EDC) as well as in anterior tibial (AT) muscles for 28 stroke patients and also for 13 age-matched controls. The disease duration, i.e. from the onset of stroke until the stimulated SFEMG examination, extended from 2 months to 8 years. Mean jitters obtained in EDC and AT muscles of stroke patients were found to be significantly greater than those in normal controls. Mean jitters obtained in severely weak muscles of stroke patients were greater than those in moderately weak muscles. Positive correlations were noted between the increased jitter and the disease duration from the onset of stroke until the time of the stimulated SFEMG test. These findings demonstrate a dysfunction of neuromuscular transmission in the paralytic muscles of stroke patients and suggests that trans-synaptic degeneration of motor neurons may occur in stroke. Furthermore, the neuronal degradation in stroke was positively correlated with the course duration of the disease.

Electrodiagnostic studies in clinical trials for motor neuron disease

Motor neuron disease (MND) is a group of neurodegenerative disorders characterized by death of upper motor neurons (UMN) and lower motor neurons (LMN). Clinical study of UMN loss is limited, but electrodiagnostic studies can be used readily to assess the functional state of LMN. Electrodiagnostic studies are regularly used in making the diagnosis of amyotrophic lateral sclerosis (ALS) and other forms of MND. They can also be used to follow disease progression and serve as end point measures in drug trials. In this role, electrodiagnostic studies can provide specific information not readily available from measurement of muscle strength or clinical functional scales. This article emphasizes the primary pathologic and secondary physiologic changes that take place after LMN loss and how they can be assessed by electrodiagnostic studies. It reviews the uses of routine electrodiagnostic studies for the diagnosis of MND but focuses on how special electrodiagnostic studies can be used as end point measures in drug trials.

Motor unit number estimation by spatial-temporal summation of single motor unit potentials

Current techniques for motor unit number estimation (MUNE) rely on the amplitude of the compound muscle action potential (CMAP) evoked by supramaximal stimulation and mean amplitude of single motor unit potentials (SMUPs). The phase cancellation during summation is not considered. We developed a technique to address this issue. Slow and fast types of motor unit potentials were collected from 5 normal subjects from their abductor pollicis brevis muscles by low-level voluntary contractions, and near-threshold nerve stimulation, respectively. Two of each type of SMUPs were used as templates for reconstructing the best fitted CMAP using a feed-forward neural network. The total number of SMUPs simulated from the four templates during the reconstruction served as MUNE. The mean MUNE was 222 +/- 98. The technique is simple and noninvasive, and may be applied in the future for MUNE in patients.

Clinical and prognostic features in unilateral femoral neuropathies

We have examined the clinical features of patients with femoral neuropathy and the factors that influence the prognosis. Of 80 consecutive patients referred for neurophysiological evaluations of proximal lower limb weakness, 32 fulfilled strict inclusion criteria and had adequate information, including estimates of axon loss (AxL) by stimulation of the bilateral femoral nerve. In 31, the Kaplan-Meier method was used to describe the time course of the outcome, while logistic regression was employed to determine the contributing factors. Excellent, satisfactory, and poor outcomes were seen in 10 (31%), 11 (34%), and 10 (31%) patients, respectively. Logistic regression analysis of seven factors demonstrated that the estimate of AxL was the only significant variable. The best prognostic factor was an estimate of AxL < or = 50%, with all patients fulfilling this criterion showing improvement with 1 year; fewer than half the patients with AxL > 50% should be expected to improve. This study clearly shows that, irrespective of the cause of femoral neuropathy, functional improvement is seen in 2 out of 3 patients within 2 years and that the estimate of AxL is the only factor influencing prognosis.

A longitudinal study comparing thenar motor unit number estimates to other quantitative tests in patients with amyotrophic lateral sclerosis

The following data were obtained on 21 amyotrophic lateral sclerosis (ALS) patients, aged 36-76 years (mean: 58 years), at baseline and months 4, 8, and 12: thenar motor unit number estimate (MUNE) using multiple point stimulation, mean thenar surface-recorded motor unit action potential negative-peak area, thenar compound muscle action potential amplitude, isometric hand grip strength, total Medical Research Council (MRC) manual muscle testing score, Appel ALS rating scale, and forced vital capacity (FVC). The absolute mean rate of change per month was significantly greater (P < 0.01) for MUNE values than for MRC and FVC values in the 21 ALS patients. In a subset of patients (n = 6) with slowly progressive disease, the absolute mean rate of change per month was significantly greater (P < 0.01) for MUNE values than for all other test values. In addition, MUNE values were the most sensitive index for documenting changes in disease progression over time.

Motor unit number estimation: sample size considerations

A computer model of the motor unit number estimation procedure was developed to evaluate the sampling error associated with estimates of the number of motor units in muscles. Two different distributions were used to model the motor unit amplitude distribution and were chosen in such a manner that they qualitatively matched the distributions observed under both normal and neurogenic conditions. As expected, the results indicated that estimation error decreases as a function of sample size. However, the relationship between these two variables was nonlinear in the sense that successive increases in sample size lead to progressively smaller decreases in estimation error. The results also indicated that the shape of the motor unit amplitude distribution plays an important role. Specifically, estimates obtained using the distribution modeling normal muscle were generally higher than the actual number of motor units in the muscle, which was not the case for the distribution modeling neurogenic muscle. In addition, the neurogenic distribution was associated with much smaller estimation error, suggesting that motor unit number estimation is well suited to the analysis of neurogenic disease processes.

Progressive motor neuron impairment in an animal model of familial amyotrophic lateral sclerosis

Mutations of Cu,Zn superoxide dismutase cause an autosomal dominant form of familial amyotrophic lateral sclerosis. An animal model of the disease has been produced by expressing mutant human SOD1 in transgenic mice (G93A). In order to quantify the dysfunction of the motor unit in transgenic mice, electromyographic recordings were performed during the course of the disease. The first alterations in neuromuscular function appeared between P63 and P90. The deficits became even more striking after P100; compound muscle action potentials in the hindlimb decreased by 80% of initial value. Spontaneous fibrillation potentials were measured in more than 50% of transgenic mice. The number of motor units in the gastrocnemius muscle was progressively reduced over time, down to 18% of the control value at P130. Moreover, distal motor latencies increased after P120. These data suggest that the initial dysfunctions of motor unit are related to a severe motor axonal degeneration, which is followed at later periods by myelin alteration.

A simulation study of the electromyographic volley at initiation of rapid isometric contractions in the first dorsal interosseous muscle

Monopolar surface electromyograms (EMGs) of rapid isometric abduction of the first dorsal interosseous muscle (FDI) were initiated from an EMG volley that was characterized by a negative potential lasting over several tens of milliseconds. An EMG model was developed to study how the EMG volley was generated. EMGs were defined as the linear summations of surface-recorded action potential trains originating from single motor units (MUs). All action potential trains had the same discharge pattern but different recruitment thresholds, depending on the potential amplitude. Real action potentials in single MUs in FDI were recorded with a monopolar surface electrode, one of which was used as a prototype wave in simulation. The model predicted an initial negative potential comparable to that of the EMG volley observed in rapid contractions of FDI. Results from our simulation studies suggest that the EMG volley is caused by at least two independent factors: (1) the negative phase of the action potential is greater in area than the positive one, in which the effect is enhanced by the high discharge rate of many MUs; (2) many MUs are recruited within a short time in an orderly fashion starting from those with small action potentials to those with large ones.

Relationships between motor-unit number estimates and isometric strength in distal muscles in ALS/MND

Measurement of progression in amyotrophic lateral sclerosis/motor neuron disease (ALS/MND) is useful for charting the natural history and assessing efficacy in drug trials. Common measures are maximal voluntary isometric contraction (MVIC) which mainly focuses on-proximal muscles, and electrophysiologic measures of the compound muscle action potential (CMAP) and motor-unit number estimation (MUNE) which focus on distal muscles. We have undertaken a study to compare the relationships between MVIC, CMAP and MUNE recorded in intrinsic hand muscles of 10 subjects with ALS/MND. Grip and pinch (between the first and fifth digits) MVIC were recorded. The CMAP and MUNE (determined by the multipoint stimulation technique) were recorded from thenar and hypothenar muscle groups. To facilitate comparisons between strength and electrophysiologic measures, the MUNE values and the CMAP values from the thenar and hypothenar muscle groups were each summed. Test-retest correlations were high for all measures (r = 0.75-0.99). Pinch and grip MVIC were highly correlated (r = 0.83). However, MVIC measures showed weaker correlations with summed MUNE values (pinch r = 0.56, grip r = 0.64) and summed CMAP values (pinch r = 0.58 and grip r = 0.65). The weak correlations between MVIC and electrophysiologic measures are due to two factors. First, grip and pinch MVIC are correlated through co-activation of agonist muscle groups, for we recorded strong concomitant muscle activity from forearm flexor muscles during pinch. In contrast, CMAP and MUNE reflect measurement from isolated muscle groups in the hand. Second, MVIC and the CMAP are affected by collateral reinnervation and only indirectly assess motor-unit loss, while MUNE is uninfluenced by reinnervation and directly addresses the degree of motor-unit loss. These factors determine the information available from endpoint measures. We offer guidelines for choosing useful measures based upon the goals of a given study.

Surface potentials generated by synchronous activation of different fractions of the motor pool

Surface electromyogram potentials were recorded from the abductor pollicis brevis, abductor digiti minimi, and extensor digitorum brevis muscles in response to finely graded nerve stimuli. Successive potentials were subtracted to obtain intermediate potentials 2%, 5%, 10%, 20%, and 50% of the maximal compound muscle action potential (mCMAP). The average latency of the onset and negative and positive peaks, and the average duration of the negative phase and whole potential were similar for all degrees of fractionation, although smaller fractionation was associated with increasing variability. An initial positivity occurred with some of the smaller fractions of the CMAP, particularly those with the lowest stimulus threshold. Submaximal CMAPs closely resembled the mCMAP once their amplitude was greater than 5-10% of the mCMAP. Our results support the common practice of expressing H-reflex amplitudes as a percentage of the M wave and may partly explain why reflexes "scale" in response to tonic activation.

Thenar motor unit number estimates using the multiple point stimulation technique: reproducibility studies in ALS patients and normal subjects

Thenar motor unit number estimate (MUNE) reproducibility was assessed in 20 patients with amyotrophic lateral sclerosis (ALS) and 16 normal subjects using the multiple point stimulation (MPS) technique. The MUNE was calculated by dividing the thenar compound muscle action potential negative-peak (n-p) area by the mean n-p area of 10 lowest threshold, all-or-nothing, surface-recorded motor unit action potentials. Two trials (test-retest) were performed by the same examiner either on separate days or on the same day with new electrode placements. The mean test MUNE was 43.4 (SD: 35.9, range: 6-145) for ALS patients and 219.4 (SD: 80.8, range: 122-368) for normal subjects. Test-retest MUNE differences were not significant for ALS patients or normal subjects. The test-retest correlation coefficient (r) was 0.99 for ALS patients and 0.85 for normal subjects. The mean difference between test-retest values was 10% for ALS patients and 17% for normal subjects. Test-retest reproducibility of the thenar MUNE using the MPS technique is high in both ALS patients and normal subjects. The reliability of the MPS technique in estimating motor unit numbers may make it a useful outcome measure in following the course of patients with progressive lower motor neuron disease, especially those enrolled in experimental drug trials.

A new approach to motor unit estimation with surface EMG triggered averaging technique

A new method for estimating the number of motor units using a surface EMG triggered averaging technique is described. This method provides an estimation of mean motor unit potential (MUP) amplitude at different levels of contraction, which can be utilized to estimate the number of motor units in a given muscle. Motor unit count estimated in abductor pollicis brevis (APB) muscle of 11 normal healthy subjects ranged from 131 to 371 with a mean of 246 +/- 68. In our preliminary study of patients with lower motor neuron lesions, there was a significant reduction in the number of motor units. We believe our new noninvasive method of motor unit counting is a relatively simple and reproducible physiological technique.

Sources of error in the spike-triggered averaging method of motor unit number estimation (MUNE)

Motor unit number estimation (MUNE) is an electrophysical technique to estimate the number of motor units innervating a muscle or muscle group. MUNE may be useful as a measure of progression of lower motor neuron loss in amyotrophic lateral sclerosis (ALS). Several methods of MUNE have been developed. The spike-triggered averaging method can be readily performed on EMG machines with signal averaging capabilities and is suitable for estimating the number of motor neurons innervating proximal muscles. We have used MUNE as a measure of disease state in a drug efficacy trial for ALS. From our experience with this method we have identified sources of error which can affect MUNE accuracy. We have investigated these sources and report their effect on MUNE.

Number and relative size of thenar motor units estimated by an adapted multiple point stimulation method

An adapted multiple point stimulation (AMPS) method is described for estimating the number and relative size of thenar motor units. With this method, the median nerve was stimulated at various sites from the wrist to the elbow. To avoid alternation, only two or three clearly identifiable surface-recorded motor unit action potentials (S-MUAPs) were recruited at each point by incremental stimulation. A total of 10 S-MUAPs, elicited from four to five distinct stimulation points, was used to calculate the average S-MUAP size. By dividing the maximum M-potential size by that value, a motor unit number estimate (MUNE) was derived. In 59 healthy volunteers, from 19 to 87 years old, the mean average S-MUAP size was 87 +/- 27.6 microV.ms and the mean MUNE was 278 +/- 113 motor units. When performed repeatedly, the results were reproducible. The number of motor units declined exponentially with age while average S-MUAP sizes increased only moderately. To assess the validity of the AMPS method, its results were correlated with those obtained using the F-response technique. The correlation coefficient was 0.83 (P < 0.001).

Motor unit estimation: anxieties and achievements

The history of motor unit number estimation (MUNE) is given, together with brief descriptions of the various methods presently available. A small muscle of the hand contains about 100 motor units and greater numbers are found in larger muscles; beyond 60 years the numbers begin to decline. In ALS approximately half the motor units cease to function within 6 months of the involvement of the motoneuron pool, while in adult spinal muscular atrophy further loss may not occur over several years. The reduction in MUNE values in myotonic dystrophy remains an enigma, but even more curious are the losses and subsequent recoveries occasionally observed in hyperthyroidism and chronic renal failure; possibly, nontransmitting ("silent") synapses are involved. MUNE may also be used to study CNS problems such as hemiplegia and congenital brachial palsy. The availability of more powerful computers for EMG should lead to advances in MUNE.

Recording characteristics of the surface EMG electrodes

Routine motor nerve conduction studies are conducted using surface EMG electrodes. Most techniques of estimating the number of motor units (MUs) are based on surface EMG recordings. Therefore, it is important to assess the uptake area of these electrodes. We recorded surface EMG motor unit action potentials (SMUAPs) from the biceps muscle of normal subjects. The SMUAP amplitude fell from 42 microV for the superficially located MUs (i.e., within 10 mm of skin surface) to 11 microV for the deep MUs (i.e., more than 20 mm from the skin surface). We infer that the pickup radius of the surface electrode is less than 20 mm. The implications of the limited uptake area of the surface electrodes to the analysis of compound muscle action potentials, estimation of the number of MUs, and the surface EMG recordings are discussed.

Motor unit number estimates based on the automated analysis of F-responses

An automated technique for estimating the number of motor units based on single motor unit action potentials in the F-response is described. The average surface detected motor unit action potential (S-MUAP) was calculated from the datapoint-by-datapoint average of a sample of S-MUAPs automatically selected from a population of F-responses. The technique was applied to the thenar muscles of young (n = 18, aged 31 +/- 11 years) and older (n = 15, aged 68 +/- 3) subjects. Motor unit number estimates based on the automated selection of S-MUAPs from the F-responses compared well with those derived using a computer-assisted manual method for selecting S-MUAPs from the F-response (automated 245 +/- 105 vs. manual 241 +/- 100, r = 0.93) and were similar to estimates obtained using multiple point stimulation (219 +/- 77). The advantages of the automated technique for collecting S-MUAPs from the F-response include the ready tolerance of the technique by subjects, the minimal amount of operator interaction required, and the additional information relating to the conduction velocities and latencies of single motor axons.

The control of human motor units

1. The motor unit, consisting of a single motor neuron and the skeletal muscle fibres that it innervates, is the final output pathway of the motor system. 2. Much is now known about the way that human motor neurons are recruited and controlled during voluntary and reflex movements. This review briefly summarizes some of the recent experimental data that has contributed to our present understanding. The review is largely limited to data obtained in human experiments. While much of what we know about the organization of the nervous system has come from studies of the anatomy and physiology of experimental animals, there are some questions that cannot be addressed in reduced animal preparations. The development of new techniques has made it possible to investigate the human nervous system at a level of detail that has not hitherto been possible.

Determinants of mean motor unit size: impact on estimates of motor unit number

The purpose of this study was to compare two fundamentally different methods of deriving the average surface-detected motor unit action potential (S-MUAP) size from which to calculate a motor unit number estimate (MUNE), namely: (1) the simple arithmetic average of S-MUAP parameter values; and (2) a computer-derived datapoint by datapoint average waveform which takes account of differences in S-MUAP shapes and durations. Multiple point stimulation was used to collect representative samples of between 11 and 20 S-MUAPs (mean 15 +/- 2 SD) from the median-innervated thenar muscles of 20 healthy control subjects between 20 and 76 years of age (mean 48 +/- 19 SD). The average S-MUAP size based on peak-to-peak amplitude, negative peak amplitude, and negative peak area measurements was calculated using the two different methods. The mean S-MUAP sizes based on the average waveform were significantly lower in all cases than those based on the simple average of S-MUAP parameter values. Differences tended to be greatest for MUNEs based on peak-to-peak amplitude (35%), less for negative peak amplitude (20%), and least for negative peak area (16%).

Motor unit number estimation, isometric strength, and electromyographic measures in amyotrophic lateral sclerosis

Pathologic progression in amyotrophic lateral sclerosis (ALS) results from motor neuron death, while the clinical expression also reflects the compensatory effects of collateral reinnervation consequent to lower motor neuron loss. In a cross-sectional study of ALS subjects, we made comparisons between motor unit number estimation (MUNE) values and several measures reflecting collateral reinnervation, including isometric strength, compound muscle action potential (CMAP) amplitude, surface motor unit action potential (S-MUAP) amplitude, fiber density (FD), macro-EMG potential amplitude, turns-to-amplitude (T/A) ratio, and amplitude and recruitment pattern of low threshold voluntary motor units in elbow flexor muscles. Before comparisons were made, test-retest reproducibility of these measures was assessed in ALS subjects, and is highest for isometric strength, and lower but similar for EMG measures. When the effects of multiple comparisons are considered, borderline significant correlations are found between MUNE values and isometric strength. Neither MUNE values nor isometric strength are significantly correlated with macro-EMG amplitude, FD, T/A ratio, or amplitude and recruitment rate of low threshold voluntary motor units. There are significant correlations of CMAP and S-MUAP with MUNE values, but these are statistical artifacts with no independent interpretation. We conclude that collateral reinnervation prevents isometric strength and EMG measures from accurately reflecting lower motor neuron death in ALS. MUNE measurements are better suited to provide insight into the true natural history of the disease process and may be clinically useful to follow progression and response in drug trials.

Motor unit estimation: reproducibility of the spike-triggered averaging technique in normal and ALS subjects

Reproducibility of the spike-triggered averaging technique of motor unit estimation (MUE) was assessed in biceps-brachialis muscle in 10 normal subjects and 15 subjects with amyotrophic lateral sclerosis (ALS). MUE was calculated by dividing the compound muscle action potential by the mean amplitude of 15 surface motor unit potentials (S-MUPs) of low recruitment threshold. Averaged MUE values in normal subjects were higher than in ALS subjects, with few values overlapping. Differences between test and retest MUE values were not significant for either subject group. The relative differences between test-retest values were 45.3% for normal subjects and 32.6% for ALS subjects. Correlation coefficients between test and retest values were low (r = 0.07) for normal subjects when influential outlying points were removed, and higher (r = 0.65) for ALS subjects when individuals with MUE values within the normal range were removed. The higher correlation of test-retest MUE values in ALS subjects compared to normal subjects may be due to a greater probability of resampling among the smaller number of motor units in ALS subjects. In summary, the reproducibility and technical aspects of the spike-triggered averaging technique are similar to those reported for other MUE techniques.

The estimated numbers and relative sizes of thenar motor units as selected by multiple point stimulation in young and older adults

Multiple point stimulation (MPS) is described as a method of estimating the numbers of motor units in the median innervated thenar muscles of young and older control subjects. Stimulation at multiple sites along the course of the median nerve was employed to collect a sample of the lowest threshold, all-or-nothing surface-recorded motor unit action potentials (S-MUAPs). The average, negative peak area, and peak-to-peak amplitude of the sample of S-MUAPs was determined and divided into the corresponding value for the maximal compound muscle action potential to derive the motor unit estimate (MUE). In 37 trials from 17 younger subjects (20-40 years), the mean MUE was 288 +/- 95 SD based on negative peak area and, in 33 trials from 20 older subjects, mean values were 139 +/- 68. In 23 young and older subjects, MPS was performed on at least two occasions and the MUEs were found to be highly correlated (r = 0.88).