Methods for estimating numbers of motor units in biceps-brachialis muscles and losses of motor units with aging

Maximal motor unit discharge rates in the quadriceps muscles of older weight lifters

Although the existence of "neural factors" is regularly cited as an important contributor to muscular strength, we have little specific knowledge regarding the existence of such neural factors or how they contribute to the expression of muscular force.

PURPOSE

The present investigation sought to assess maximal motor unit discharge rates in older, highly resistance-trained adults to determine whether maximal motor unit discharge rates might be one such neural contributor to maximal strength production.

METHODS

Subjects consisted of seven well-trained older weight lifters (ages 67-79 yr) and five untrained age-matched older adults. While subjects performed 50 and 100% maximal voluntary knee extensor contractions (MVC), recordings from groups of motor units were obtained from the rectus femoris muscle by using an indwelling electrode. Off-line analysis was performed to identify individual motor unit firing occurrences and to compute maximal motor unit discharge rates.

RESULTS

As expected, knee extension strength in the trained weight lifters (367.0 +/- 72.0 N) was significantly greater than that in the control subjects (299.9 +/- 35.9 N; P < 0.05). Motor unit discharge rates were similar in the two subject groups at the 50% MVC force level (P > 0.05), but maximal (100% MVC) motor unit discharge rate in the weight lifters (23.8 +/- 7.71 pps) was significantly greater than that in the age-matched controls (19.1 +/- 6.29 pps; P < 0.05).

CONCLUSION

Motor unit discharge rates may comprise an important neural factor contributing to maximal strength in older adults.

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.

Age-related changes in fastest and slowest conducting axons of thenar motor units

Single thenar motor unit F waves (FMUPs) were collected from 23 healthy volunteers (age range 21-91 years, mean 46 +/- 20 SD). In each subject, 10 distinct FMUPs were recorded, using surface stimulating and recording electrodes, and the conduction velocity (CV) of each motor unit was calculated. The distribution of CVs (overall range 42-66 m/s; individual FMUP CV dispersion range 6-27% of the maximal FMUP CV) was close to those previously reported whatever the technique used. With age, a progressive CV reduction was observed, and maximal FMUP CV was significantly correlated with age (r = -0.58, P < 0.01), whereas no statistically significant correlation was found between minimal FMUP CV and age (r = -0.27, ns). Individual FMUP CV dispersion presented a statistically significant decrease with age (r = -0.46, P < 0.05). Furthermore, thenar motor unit number (MUNE), estimated by the adapted multiple point stimulation method, decreased progressively with age and was statistically correlated with maximal FMUP CV (r = 0.59, P < 0.01), whereas there was no correlation with minimal FMUP CV (r = 0.34, ns). Thus, we propose that motor unit loss is progressive with age throughout life, affecting particularly the largest and fastest conducting motor units. Preferential involvement of these fibers could be responsible for the age-related changes in motor nerve CV.

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.

1998 ISEK Congress Keynote Lecture: Motor units: how many, how large, what kind? International Society of Electrophysiology and Kinesiology

There are now at least nine methods for motor unit number estimation (MUNE) in living human muscles. All methods are based on the comparison of an average single motor unit potential (or twitch) with the response of the whole muscle. Such estimations have been performed for proximal and distal muscles of the arm and leg in healthy subjects and in patients with various neuromuscular disorders. In healthy subjects there is a loss of motor units which is most evident in distal muscles and after the age of 60 years. Substantial losses of motor units have been measured in patients with ALS, post-polio symptoms, and diabetic peripheral neuropathies. In contrast, normal MUNEs have been found in approximately half of patients with persisting obstetric brachial palsies. The sizes of motor units show considerable variations within the same muscle and also between muscles; very large units are usually present in severe partial denervation. Although many motor unit properties are largely governed by motoneurons, some exhibit less plasticity in humans than in other mammals.

Age-associated changes in the innervation of muscle fibers and changes in the mechanical properties of motor units

In both humans and animals there is a progressive loss of muscle strength with age. Tests of handgrip and knee extension in men show that some decline in strength is evident by the age of 55 years and is pronounced by the age of 65, compared with the 25- to 35-year period when strength is at a maximum. A comparable age-related decline in peak force development has also been shown in hind-limb muscles of aged rats. Motoneurons and consequently motor units are lost with age, and this is apparent in man after the age of 60. Again, a comparable decline has been demonstrated in the motoneuron population of hindlimb muscles of rats aged 20-24 months. Loss of motoneurons in young adults (through either injury or disease) results in the remaining intact motoneurons sprouting to innervate the denervated fibers. This capacity for sprouting has been shown to be seriously impaired in the hindlimb muscles of aged rats. Furthermore, the well-established relationship between motor unit size and fatigability (smaller units tend to be more fatigue resistant) also tends to break down, with large units just as likely to be fatigable as fatigue resistant. The normally large, fatigable motor units also appear to be reduced in size in the aged muscles. The age-related loss of motoneurons and associated loss of muscle fibers accounts in part for the reduced functional capacity of muscle with age. The reason for the impairment of the aged motoneuron remains to be investigated, but it may relate to the integrity of the oxidative metabolic pathways within the cell, given that mitochondrial respiratory chain function is known to be reduced with age.

Longitudinal study of the contractile and electrical properties of single human thenar motor units

Serial motor unit number estimates have shed important light on the extent and rates of motoneuron losses in aging and amyotrophic lateral sclerosis. However, the estimates alone provide few clues to the health and functional status of surviving motor units. A reliable means for assessing the functional status of the surviving motor units would therefore by a welcome addition to our present tools for studying motor units. Examining the physiological properties of samples of motor units drawn at intervals during the course of a motoneuronal disease suffers from the important limitation that the samples may not be representative of one another. The latter problem could be circumvented by serially studying the same motor units. This study describes a noninvasive technique capable of longitudinally tracking the contractile and electrical properties of specific single thenar motor units in healthy subjects, in some instances over several years. The technique proved to be reasonably reliable and provided information on a wide range of contractile and electrical properties of motor units. Such an approach could serve as a potentially powerful and sensitive means of studying the life histories of single motor units in aging, diseases of the motoneuron, and in the latter instances, the responses of the motoneurons to treatment.

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.

Comparison of corticomotoneuronal EPSPs and macro-MUPs in amyotrophic lateral sclerosis

We correlated the size of the corticomotoneuronal excitatory postsynaptic potential (CM-EPSP) arising in a single spinal motor neuron with the function of the target motor unit as measured by conventional and macro EMG in early amyotrophic lateral sclerosis (ALS). Macro motor unit potentials (macro-MUPs) were recorded from a surface electrode after spike-triggered averaging in the extensor digitorum communis muscle. The size of the CM-EPSP projecting to the same motor unit was measured from changes in the firing probability of single motor units induced by transcranial magnetic stimulation using peristimulus time histograms. In controls, the amplitudes of CM-EPSPs and macro-MUPs correlated inversely, probably reflecting a lower input resistance of larger spinal motoneurons. In ALS the amplitude of macro-MUPs did not correlate with that of CM-EPSPs and one third of normal ALS motor units had a reduced or temporally dispersed CM-EPSP. The findings indicate primary dysfunction of the corticomotoneuronal projection system in ALS that is independent of functional changes of spinal motoneurons.

Diverse abnormalities of corticomotoneuronal projections in individual patients with amyotrophic lateral sclerosis

Using peristimulus time histograms (PSTHs), abnormalities of composite excitatory postsynaptic potentials (EPSPs) induced by transcranial magnetic stimulation were studied in multiple motor units from individuals with amyotrophic lateral sclerosis (ALS) and normal subjects. We studied 97 motor units in the extensor digitorum communis muscle of 22 patients with sporadic ALS and 47 motor units of 10 healthy control subjects. Four or five motor units were studied in each patient and normal subject. For each unit, macro motor unit potentials (Macro-MUPs) were simultaneously recorded from a surface electrode after spike-triggered averaging. The composite EPSPs in ALS showed a generally bi-directional deviation from the normal curve, with small EPSPs at one end, and larger amplitude EPSPs with a prolonged rise time at the other end. The variability of EPSPs from adjacent motor units in the same individual was significantly larger in ALS than in controls. In normal subjects there is a significant negative correlation between the amplitude of composite EPSPs and the Macro-MUPs. In ALS, the trend is reversed (positive) suggesting that the abnormalities of composite EPSPs are supraspinal in origin. A combination of partial attrition of the corticomotoneuronal core and hyper-excitability of surviving corticomotoneurons projecting to a given spinal motoneuron pool best explains the diversity of the composite EPSP in individuals with ALS.

Decomposition-enhanced spike-triggered averaging: contraction level effects

Decomposition-enhanced spike-triggered averaging (DE-STA) was applied to the vastus medialis muscle to examine size distributions of surface-detected motor-unit action potentials (S-MUAPs) at various force levels. Using DE-STA, 15-20 S-MUAPs were identified during 5%, 10%, 20%, and 30% of maximum voluntary contraction. Average S-MUAPs showed increase in peak to peak (and negative peak) amplitude with force (In microV): 5% = 37.9 +/- 6.1 (16.6 +/- 2.5), 10% = 44.0 +/- 4.0 (20.4 +/- 1.8), 20% = 80.7 +/- 9.3 (41.3 +/- 4.5), and 30% = 102.5 +/- 10.3 (53.6 +/- 5.0). Test-retest variability of peak to peak (and negative peak amplitude) between repeated trials was 0.10 (0.14), 0.14 (0.14), 0.17 (0.15), and 0.21 (0.20) at 5%, 10%, 20%, and 30% respectively. A relationship was found between the S-MUAP amplitude and force (r2 = 0.78, df = 90, F = 160, P < 0.001). Increase in average S-MUAP amplitude with force suggests that STA performed only at low levels of contraction may result in a biased sampling and small average S-MUAP amplitudes.

Age-related muscle stiffness: predominance of non-reflex factors

This study was aimed at assessing the contribution of reflex and non-reflex factors to the muscle tone of old female Wistar rats. The hind foot of a rat was flexed or extended at the ankle joint by 25 degrees over 250 ms. The resistance of the foot to passive movements (torque, mechanomyogram), as well as the reflex electromyographic activity in the gastrocnemius and tibialis anterior muscles, were recorded simultaneously. Moreover, the impact of the blockade of the reflex activity caused by the local anesthetic lignocaine (1-2 ml of a 2% solution, injected in the vicinity of the sciatic nerve) on the muscle tone was investigated. Additionally, old rats' hind leg muscle samples were analysed using fluorescent microscopy for the expression of fibronectin, which is an early marker of connective tissue formation. It has been shown that old rats are characterized by (i) a substantially increased resistance of flexor muscle stiffness (measured during extension) and unchanged resistance of extensors (measured during flexion), (ii) the loss of a major part of the reflex electromyographic activity and (iii) the increased content of fibronectin in muscles. Moreover, it has been shown that lignocaine, which completely blocked the electromyographic reflex activity in the gastrocnemius and tibialis anterior muscles in young animals, was unable to counteract the resistance of these muscles to passive movements in old rats. The present results suggest that the muscle stiffness seen in old rats is not due to a reflex response, but depends mainly on non-reflex factors--chiefly on a large overgrowth of non-elastic connective tissue replacing degenerated active muscle fibers.

Validity of electromyograms and tension as a means of motor unit number estimation

The purpose of this study was to validate three different techniques for obtaining motor unit number estimates of the rat medial gastrocnemius muscle. These consisted of two electromyographic techniques using unprocessed and digitally averaged unitary muscle action potentials, and one mechanical technique. We also injected subunit B of cholera toxin into this muscle and counted the number of spinal motor neurons labeled by the toxin. Our results revealed that a motor unit number estimate obtained by using the unprocessed unitary muscle action potential was statistically different from the actual number of motor neurons. The other two motor unit number estimates, however, were not statistically different from the actual motor neuron number. These two methods thus seem more appropriate than the first electromyographic method for obtaining an accurate motor unit number estimate.

Evidence for nervous system degeneration with advancing age

Human skeletal muscle undergoes major structural and functional changes with advancing age. A progressive degeneration of the nervous system is now considered a major factor underlying these alterations. This review will briefly describe the changes that occur in the human motor unit with increasing age and focus specifically on the changes that relate to a degeneration of the nervous system.

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.

Exercise for the older woman: choosing the right prescription

Many elderly women in industrially developed countries are at, or near to, functionally important strength related thresholds and so have either lost, or are in danger of losing, the ability to perform some important everyday tasks. The increased rate of healthcare expenditure due to loss of physical function is a major economic issue. Even though women make up most of the senior population, little current research on the impact of physical activity on strength and function in elderly people has included women. Elderly women typically have more barriers to participation in physical activity than do other groups and because of decreased participation, may possibly experience higher disability rates. Physical activity in old age may delay the progression of osteoporosis and is of paramount importance for maintaining the functional abilities needed to carry out daily tasks. Current research on exercise and the elderly population suggests that strength training may be the exercise mode of choice for maintenance of strength, physical function, bone integrity, and psychosocial health. This review summarises recent research on the impact of strength training on the fitness and health of elderly women and highlights considerations and potential barriers to physical activity that must be taken into account when planning exercise programmes for them.

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.

Age-related changes in the twitch contractile properties of human thenar motor units

The purpose of this study was to examine the effects of aging on the contractile and electrophysiological properties of human thenar motor units (MUs). Percutaneous electrical stimulation of single motor axons within the median nerve was used to isolate and examine the twitch tensions, contractile speeds, and surface-detected MU action potential (S-MUAP) sizes of 48 thenar MUs in 17 younger subjects (25-53 yr) and 44 thenar MUs in 9 older subjects (64-77 yr). A wide range of twitch tensions, contractile speeds, and S-MUAP sizes was observed in both age groups. However, older subjects had significantly larger MU twitch tensions and slower MU twitch contraction and half-relaxation times. These changes were accompanied by increased S-MUAP sizes. These findings suggest that the human thenar MU pool undergoes significant age-related increase in MU size and slowing of contractile speed. Such adaptation may help to overcome previously reported age-related losses of thenar MUs.

Influence of age on concentric isokinetic torque and passive extensibility variables of the calf muscles of women

The purpose of this study was to investigate the influence of age on concentric isokinetic torque (CIT) and passive extensibility (PE) variables of the calf muscles of healthy women. Ten younger women [31.9 (SD 6.1) years] and ten older women [71.1 (SD 6.6) years] were tested using a KIN-COM 500H dynamometer. The PE was tested by stretching the muscles from relaxed plantarflexion to the maximal dorsiflexion (DF) angle at 5 degrees.s-1 without raw electromyogram (EMG) activity exceeding 0.05 mV. The maximal CIT was tested from the maximal DF angle 60 degrees into plantarflexion at four randomly ordered velocities of 30,60, 120, and 180 degrees.s-1. Separate analysis of variance (ANOVA) tests showed that the standardized (% body mass) concentric peak and mean torques were lower for the older women for all isokinetic velocities (p < 0.001). The "angular delay" from the onset of concentric activation to peak torque was smaller for the older women at 120 and 180 degrees.s-1 (p < 0.05). Age showed negative relationships (Pearson r) with all standardized peak torques (p < or = 0.001) and mean torques (p < 0.001), and the "angular delay" at 120 and 180 degrees.s-1 (p < or = 0.05). Independent t-tests showed that the maximal DF angle and the change in the PE angle from an initial angle (defined at 10% of the maximal passive torque) to the maximal DF angle were less for the older women (p < 0.05). Age was negatively related to the maximal DF angle and the change in the PE angle (p < 0.01). The results suggest an age-related decrease in calf muscle CIT, muscle length and PE. The smaller "angular delay" for the older women at 120 and 180 degrees.s-1 indicates that CIT testing at rapid velocities can be used to examine age-related changes in calf muscle contractile properties in relation to rapid velocities of movement.

Quantitative motor unit potential analysis

A review of quantitative methods for electromyography is given. Background information about motor unit anatomy, physiology, and pathology is provided to explain some of the presented electrophysiological phenomena. Different aspects of quantitation, such as motor unit action potential parameters, automatic analysis methods, reference values, and findings in abnormal conditions, are discussed.

Motor unit estimates obtained using the new "MUESA" method

Motor unit number estimates were obtained from the extensor digitorum brevis and thenar muscles using a new method called MUESA. MUESA is distinguished from other estimation methods in the manner in which it deals with probabilistic motor unit activation, which is more commonly referred to as "alternation." Because of "alternation," incremental increases in the observed muscle potentials often cannot be interpreted in terms of the successive activation of single motor units. In the MUESA method, the nerve is subjected to a number of constant-intensity stimulus trains, and the resultant muscle response sequences are decomposed into their constituent motor unit action potentials. In general, if a stimulus train results in the probabilistic activation of n motor units, we can expect to see up to 2n different potentials, with each potential representing a unique combination of active and/or inactive motor units. If all 2n potentials are indeed observed, the decomposition of the observed potential sequence into its constituent motor unit action potentials is very straightforward. For the majority of the cases in which the number of observed potentials is not an integer power of 2, we have developed a novel decomposition method based on the analysis of the relative firing rates of the motor units.

Changes in motor unit estimates with aging

Elderly persons usually exhibit some degree of muscle atrophy, together with a reduction in voluntary strength, but there is still argument concerning the nature of the cellular events involved. This issue was reexamined by estimating the numbers and relative sizes of motor units in three limb muscles, using a fully automated system (Galea et al., 1993). In 79 healthy volunteers aged 20-98 years, estimations of motor unit numbers were performed on the thenar, biceps brachii, and extensor digitorum brevis muscles. Motor unit populations were noted to decrease significantly with age in the distal muscles but appeared to remain constant in the biceps. The excitable muscle fiber mass, as reflected in the peak-to-peak amplitude and area of the maximum M-wave, was diminished in all three muscles. Although the area of the average motor unit action potential was not significantly different between groups, the ratio of this potential to the M-wave area increased with increasing age. The results suggest that muscle deterioration in the elderly is due to a combination of changes in the muscle fibers and in their nerve supply and that the extent may differ between proximal and distal muscles.

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.

Associated movement as a sequel to thoracotomy: aberrant regeneration to the latissimus dorsi muscle

We describe two patients with unusual associated movements as a sequel to thoracotomy. The two patients developed involuntary twitchings of the latissimus dorsi, which were induced by contractions of the adjacent muscles, the serratus anterior muscle in patient 1 and the external intercostal muscle in patient 2, after lobectomy for lung cancer. Electrophysiological and radiological studies showed that aberrant regeneration from the adjacent nerves to the denervated latissimus dorsi could be responsible for the associated movements.

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.

Macro-EMG and motor unit recruitment threshold: differences between the young and the aged

The relationship between macro-EMG (electromyography) and motor unit recruitment threshold was studied in the first dorsal interosseous (FDI) muscle of normal young and aged subjects. During voluntary isometric contraction, motor unit action potentials (MUAP) were collected by a special quadrifilar electrode and decomposed to each MUAP train (MUAPT) using an EMG signal decomposition technique. Macro-EMG was obtained from the electrode shaft, then triggered and averaged for each MUAPT. A positive linear correlation was observed in both the young and aged subjects. However, the correlation coefficients were significantly lower in the aged individuals than in the young individuals.

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.

Knee extension strength and walking speed in relation to quadriceps muscle composition and training in elderly women

Knee extension strength, walking speed, quadriceps muscle mass and composition of the muscle compartment were studied in 66 to 85-year-old female athletes and controls. Maximal voluntary knee extension force, force/body mass, extension torque, torque/body mass and walking speed were higher for the athletes than the controls. A muscle index indicating intramuscular fat and connective tissue measured using ultrasonography was lower for the athletes than the controls. There were no differences between the study groups in knee extension force related either to cross-sectional area (CSA) or lean tissue area (CSAL) of the quadriceps. Within the subgroups, there was no significant correlation between knee extension torque and CSA or CSAL of the same muscle. In the athletes high knee extension torque/body mass was related to a low muscle index and high walking speed to a low relative proportion of fat in the muscle. The muscle index was lower the more kilometers trained during the preceding year. In the controls high knee extension torque/body mass and high walking speed were related to a low relative proportion of fat. Knee extension torque and walking speed were higher the more kilometers walked during the preceding year. The results indicate that elderly female athletes have superior muscle performance compared to their age-peers. Performance in a maximal isometric strength test in elderly women is not clearly related to muscle mass. However, to some extent it is related to the composition of the same muscle, especially the degree to which fat is infiltrated into the muscle.

Effects of ageing on the motor unit: a brief review

This review briefly summarizes the current state of knowledge regarding age related changes in skeletal muscle, followed by a more in-depth review of ageing effects on animal and human motor units (MUs). Ageing in humans is generally associated with reductions in muscle mass (atrophy), leading to reduced voluntary and electrically evoked contractile strength by the 7th decade for most muscle groups studied. As well, contraction and one-half relaxation times are typically prolonged in muscles of the elderly. Evidence from animal and human studies points toward age associated MU loss as the primary mechanism for muscle atrophy, and such losses may be greatest among the largest and fastest MUs. However, based on studies in animals and humans, it appears that at least some of the surviving MUs are able to partially compensate for MU losses, as indicated by an increase in the average MU size with age. The fact that muscles in the elderly have fewer, but on average larger and slower, MUs has important implications for motor control and function in this population.

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).

Ageing and human muscle: observations from Sweden

The purposes of this review are to summarize studies of cross-sections of autopsied whole muscles from previously physically healthy males and to focus on the cause of the ageing atrophy. The ageing atrophy begins around 25 years of age and thereafter accelerates. This is caused mainly by a loss of muscle fibres, and to a lesser extent by a reduction in fibre size, mostly of the proportion of the fibre area in the muscle cross-section occupied by type 2 (fast-twitch) fibres. In muscle from old subjects, there is a significant increase in the number of enclosed fibres, indicating an increased incidence of fibre type grouping, a loss of motor neurons in the spinal cord, and a reduction in the number of functioning motor units. These findings strongly suggest a combination of a progressive denervation process and an altered physical activity level as the two major mechanisms underlying the effects of normal ageing on human muscle. These changes have obvious implications for old individuals and their participation in physical activity and in sports, which must be accommodated in rehabilitation regimes or in training programmes.

Muscle stiffness and continuous electromyographic activity in old rats; an animal model for spasticity?

A mechanomyographic response of the hind foot to passive straightening and bending, as well as an electromyographic activity of the gastrocnemius and tibialis anterior muscles were recorded in old (35-44-month-old) and young female rats. In old rats, spontaneous, tonic electromyographic activity patterns were concurrently observed in both antagonistic muscles; they were low-amplitude, dense tonic activity and continuous, high-amplitude, sparse electromyographic activity. The tonic electromyographic activity was correlated with a decline in the strength and mass of muscles, as well as with motor disturbances, including paresis of the rigidly straightened backward hind legs, dragged behind by an animal. In muscles of old rats, morphological features of a chronic denervation atrophy were found. Baclofen (10 and 15 mg/kg, i.p.) diminished the spontaneous tonic electromyographic activity and potently decreased the whole body muscle tone, whereas Madopar (50 mg/kg of L-DOPA+12.5 mg/kg of benerazide) was ineffective. It is suggested that old rats in which the above-described pathologic alterations are observed might be a useful animal model in the search for basic etiopathological mechanisms of spasticity and similar disturbances found in humans.

AAEM case report #24: electrodiagnosis in posttraumatic syringomyelia

An adult male with C-7 quadriplegia developed neck pain. Axillary F central latencies were prolonged, and MRI showed a syrinx extending to C-1. After shunting, F latencies normalized. At subsequent follow-up, a rostral syrinx persisted by magnetic resonance imaging (MRI); motor evoked potential (MEP) latencies were prolonged but F latencies were normal. Later, the syrinx was less distended by MRI, MEPs normalized, and strength improved. We discuss the electrophysiologic methods available for diagnosing and monitoring posttraumatic syringomyelia.

Amyotrophic lateral sclerosis (ALS): a phylogenetic disease of the corticomotoneuron?

It is proposed that the primary cell involved in amyotrophic lateral sclerosis (ALS) is the corticomotoneuron. The spinal motoneuron becomes affected as a result of antegrade effects. This hypothesis does not negate most of the presently popular theories regarding the pathogenesis of ALS, but directs focus to one cell type--the corticomotoneuron. It takes cognizance of the complex, monosynaptic, corticomotoneuronal-spinomotoneuronal connections that have evolved in primates, and especially in man. It might explain the lack of any natural or thus far induced animal model which closely mimics the human disease. Threshold measurements to transcotical magnetic stimulation might be used to test the hypothesis. Replication of ALS in an animal is only likely to succeed in a nonhuman primate.

The numbers and relative sizes of motor units estimated by computer

A fully automated system is described for estimating the numbers and relative sizes of functioning motor units in proximal and distal muscles of the arm and leg. In this system, a computer controls the motor nerve stimulation, and analyzes the potentials evoked from the muscles; a subprogram searches for instances of "alternation." In 33 healthy volunteers, aged 21 to 56 years, the median-innervated thenar muscles of one hand were tested 2 to 3 times; the mean motor unit estimate was 228 +/- 93 SD. For similar numbers of biceps brachii, extensor digitorum brevis, and vastus medialis muscles, the respective mean values were 113 +/- 40, 131 +/- 45, and 229 +/- 108 units. The reproducibility of the method was such that the overall coefficient of variation, for the normalized results from the 121 muscles studied, was 22%. The reliability of the automated method was doubled if 3 estimates, rather than one, were performed on each muscle. Comparisons of the results obtained by automated and "manual" methods indicated that the computer-derived values tended to be lower by approximately 33%.