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

Toward the validation of a new method (MUNIX) for motor unit number assessment

INTRODUCTION

This prospectively designed study analyzed the correlation of a new, non-invasive neurophysiological method (Motor Unit Number Index - MUNIX) with two established Motor Unit Number Estimation (MUNE) methods.

METHODS

MUNIX and incremental stimulation MUNE (IS-MUNE) were done in the abductor digiti minimi muscle (ADM), while MUNIX and spike-triggered averaging MUNE (STA-MUNE) were tested in the trapezius muscle. Twenty healthy subjects and 17 patients with amyotrophic lateral sclerosis (ALS) were examined.

RESULTS

MUNIX and MUNE values correlated significantly (ADM: n=108; Spearman-Rho; r=0.88; p<0.01; trapezius muscle: n=49; Spearman-Rho; r=0.46; p<0.01).

DISCUSSION

MUNIX indeed reflects the number of motor units in a muscle, and may sensibly be recorded from the trapezius muscle. With MUNIX being both much more patient friendly and much more rapid to assess than MUNE, the results support the use of MUNIX when motor unit number assessment is desired.

Age-related motor unit remodeling in the Tibialis Anterior

Limited studies exist on the use of surface electromyogram (EMG) signal features to detect age-related motor unit remodeling in the Tibialis Anterior. Motor unit remodeling leads to declined muscle strength and force steadiness during submaximal contractions which are factors for risk of falls in the elderly. This study investigated the remodeling phenomena in the Tibialis Anterior using sample entropy and higher order statistics. Eighteen young (26.1 ± 2.9 years) and twelve elderly (68.7 ± 9.0 years) participants performed isometric dorsiflexion of the ankle at 20% maximal voluntary contraction (MVC) and their Tibialis Anterior (TA) EMG was recorded. Sample entropy, Gaussianity and Linearity Test statistics were calculated from the recorded EMG for each MVC. Shapiro-Wilk test was used to determine normality, and either a two-tail student t-test or Wilcoxon rank sum test was performed to determine significant difference in the EMG features between the young and old cohorts. Results show age-related motor unit remodeling to be depicted by decreased sample entropy (p <; 0.1), increased non-Gaussianity (p <; 0.05) and lesser degree of linearity in the elderly. This is due to the increased sparsity of the MUAPs as a result of the denervation-reinnervation process, and the decrease in total number of motor units.

Age-related neuromuscular changes affecting human vastus lateralis

KEY POINTS

Skeletal muscle size and strength decline in older age. The vastus lateralis, a large thigh muscle, undergoes extensive neuromuscular remodelling in healthy ageing, as characterized by a loss of motor neurons, enlargement of surviving motor units and instability of neuromuscular junction transmission. The loss of motor axons and changes to motor unit potential transmission precede a clinically-relevant loss of muscle mass and function.

ABSTRACT

The anterior thigh muscles are particularly susceptible to muscle loss and weakness during ageing, although how this is associated with changes to neuromuscular structure and function in terms of motor unit (MU) number, size and MU potential (MUP) stability remains unclear. Intramuscular (I.M.) and surface electromyographic signals were recorded from the vastus lateralis (VL) during voluntary contractions held at 25% maximal knee extensor strength in 22 young (mean ± SD, 25.3 ± 4.8 years) and 20 physically active older men (71.4 ± 6.2 years). MUP size, firing rates, phases, turns and near fibre (NF) jiggle were determined and MU number estimates (MUNEs) were made by comparing average surface MUP with maximal electrically-evoked compound muscle action potentials. Quadriceps cross-sectional area was measured by magnetic resonance imaging. In total, 379 individual MUs were sampled in younger men and 346 in older men. Compared to the MU in younger participants, those in older participants had 8% lower firing rates and larger MUP size (+25%), as well as increased complexity, as indicated by phases (+13%), turns (+20%) and NF jiggle (+11%) (all P < 0.0005). The MUNE values (derived from the area of muscle in range of the surface-electrode) in older participants were ∼70% of those in the young (P < 0.05). Taking into consideration the 30% smaller cross-sectional area of the VL, the total number of MUs in the older muscles was between 50% and 60% lower compared to in young muscles (P < 0.0005). A large portion of the VL MU pool is lost in older men and those recruited during moderate intensity contractions were enlarged and less stable. These MU changes were evident before clinically relevant changes to muscle function were apparent; nevertheless, the changes in MU number and size are probably a prelude to future movement problems.

Age-dependent motor unit remodelling in human limb muscles

Voluntary control of skeletal muscle enables humans to interact with and manipulate the environment. Lower muscle mass, weakness and poor coordination are common complaints in older age and reduce physical capabilities. Attention has focused on ways of maintaining muscle size and strength by exercise, diet or hormone replacement. Without appropriate neural innervation, however, muscle cannot function. Emerging evidence points to a neural basis of muscle loss. Motor unit number estimates indicate that by age around 71 years, healthy older people have around 40 % fewer motor units. The surviving low- and moderate-threshold motor units recruited for moderate intensity contractions are enlarged by around 50 % and show increased fibre density, presumably due to collateral reinnervation of denervated fibres. Motor unit potentials show increased complexity and the stability of neuromuscular junction transmissions is decreased. The available evidence is limited by a lack of longitudinal studies, relatively small sample sizes, a tendency to examine the small peripheral muscles and relatively few investigations into the consequences of motor unit remodelling for muscle size and control of movements in older age. Loss of motor neurons and remodelling of surviving motor units constitutes the major change in ageing muscles and probably contributes to muscle loss and functional impairments. The deterioration and remodelling of motor units likely imposes constraints on the way in which the central nervous system controls movements.

Redox regulation of muscle adaptations to contractile activity and aging

Superoxide and nitric oxide are generated by skeletal muscle, and these species are increased by contractile activity. Mitochondria have long been assumed to play the primary role in generation of superoxide in muscle, but recent studies indicate that, during contractile activity, membrane-localized NADPH oxidase(s) rapidly generate(s) superoxide that plays a role in redox signaling. This process is important in upregulation of rapid and specific cytoprotective responses that aid maintenance of cell viability following contractile activity, but the overall extent to which redox signaling contributes to regulation of muscle metabolism and homeostasis following contractile activity is currently unclear, as is identification of key redox-sensitive protein targets involved in these processes. Reactive oxygen and nitrogen species have also been implicated in the loss of muscle mass and function that occurs with aging, although recent work has questioned whether oxidative damage plays a key role in these processes. A failure of redox signaling occurs in muscle during aging and may contribute to the age-related loss of muscle fibers. Whether such changes in redox signaling reflect primary age-related changes or are secondary to the fundamental mechanisms is unclear. For instance, denervated muscle fibers within muscles from aged rodents or humans appear to generate large amounts of mitochondrial hydrogen peroxide that could influence adjacent innervated fibers. Thus, in this instance, a "secondary" source of reactive oxygen species may be potentially generated as a result of a primary age-related pathology (loss of neurons), but, nevertheless, may contribute to loss of muscle mass and function during aging.

Effect of number of motor units and muscle fibre type on surface electromyogram

Reduction in number of motor units (nMU) and fast fibre ratio (FFR) is associated with disease or atrophy when this is rapid. There is a need to study the effect of nMU and FFR to analyse the association with ageing and disease. This study has developed a mathematical model to investigate the relationship between nMU and FFR on surface electromyogram (sEMG) of the biceps muscles. The model has been validated by comparing the simulation outcomes with experiments comparing the sEMG of physically active younger and older cohort. The results show that there is statistically significant difference between the two groups, and the simulation studies closely model the experimental results. This model can be applied to identify the cause of muscle weakness among the elderly due to factors such as muscle dystrophy or preferential loss of type F muscle fibres.

Motor unit loss is accompanied by decreased peak muscle power in the lower limb of older adults

This study investigated the relationship between motor unit (MU) properties and the isometric strength and power of two lower limb muscles in healthy young and older adults. Twelve older adults (6 men, mean age, 77 ± 5 years) and twelve young adults (6 men, mean age, 24 ± 3 years) were studied. MU properties of the tibialis anterior (TA) and vastus medialis (VM) muscles were determined electrophysiologically using decomposition-enhanced spike-triggered averaging (DE-STA). Motor unit number estimates (MUNEs) of the TA were significantly reduced (p<0.05) in older adults (102 ± 76) compared to young adults (234 ± 109), primarily as a result of significantly larger surface-detected motor unit potentials (S-MUPs) in older adults (63 ± 29 μV) compared to young adults (27 ± 14 μV). Although VM S-MUP values were larger in older adults (60 ± 31 μV) compared to young (48 ± 42 μV), the difference was not significant. Maximal isometric strength was significantly larger in both the TA and knee extensors of young adults (TA: 0.56 Nm/kg, KE: 2.2 Nm/kg) compared to old (TA: 0.4 Nm/kg, KE: 1.3 Nm/kg). Similar reductions in peak muscle power were observed between young (TA: 33 W, KE: 35 7 W) and old adults (TA: 26 W, KE: 224 W). The greatest deficit between young and old subjects in peak power output occurred at 20% MVC for the TA and 40% MVC for the knee extensors. Results from this study indicate that there are changes in MU properties with age, and that this effect may be greater in the more distal TA muscle. Further, this study demonstrates that muscle power may be a sensitive marker of changes in neuromuscular function with aging.

Motor unit number in a small facial muscle, dilator naris

A loss of functioning motor units underlies many neuromuscular disorders. The facial nerve innervates the muscles of facial expression, including nasal muscles, which also play an important role in the regulation of airflow resistance. It is difficult to accurately assess motor unit number in the facial muscles, because the muscles are difficult to activate in isolation. Here, we apply the manual McComas method to estimate the number of motor units in a nasal dilator muscle. EMG of the dilator naris was recorded during graded stimulation of the zygomatic branch of the facial nerve in 26 subjects (12 males and 14 females), aged 20-41 years. Each subject was studied twice, on separate days, to estimate method reproducibility. As a check on our use of the McComas method, we also estimated motor unit number in the first dorsal interosseus muscle (FDI) of six subjects, as the muscle is also small and has been studied with the McComas method. Reproducibility was evaluated with a rigorous statistical approach, the Bland-Altman procedure. We estimate that dilator naris is composed of 75 ± 15.6 (SD) motor units, compared to 144 ± 35.5 in FDI. The coefficient of variation for test-retest reproducibility of dilator naris motor unit estimates was 29.6 %, similar to separate-day reproducibility reported for other muscles. Recording and stimulation were done with surface electrodes, and the recordings were of high quality and reproducible. This simple technique could be applied clinically to track motor neuron loss and to monitor facial nerve integrity.

Innervation zones of fasciculating motor units: observations by a linear electrode array

This study examines the innervation zone (IZ) in the biceps brachii muscle in healthy subjects and those with amyotrophic lateral sclerosis (ALS) using a 20-channel linear electromyogram (EMG) electrode array. Raster plots of individual waveform potentials were studied to estimate the motor unit IZ. While this work mainly focused on fasciculation potentials (FPs), a limited number of motor unit potentials (MUPs) from voluntary activity of 12 healthy and seven ALS subjects were also examined. Abnormal propagation of MUPs and scattered IZs were observed in fasciculating units, compared with voluntarily activated MUPs in healthy and ALS subjects. These findings can be related to muscle fiber reinnervation following motor neuron degeneration in ALS and the different origin sites of FPs compared with voluntary MUPs.

An examination of motor unit number index in adults with cerebral palsy

Spinal motor neuron loss may be a factor contributing to weakness in central disorders. The aim of this study was to assess whether motor unit numbers are reduced in the hand musculature of adults with cerebral palsy (CP) using the motor unit number index (MUNIX) technique. In this prospective, case-control study, 10 adults with CP were matched with healthy controls. MUNIX was computed using area and power of voluntary surface hypothenar electromyographic (EMG) signals and the compound muscle action potential (CMAP) recorded with ulnar nerve stimulation. The motor unit size index (MUSIX) was calculated based on maximum CMAP amplitude and MUNIX value. Gross Motor Function Classification Scale (GMFCS) and Manual Abilities Classification Scale (MACS) levels were rated for CP subjects. MUNIX was significantly lower for CP participants (Mean 167.8 vs. 214.4, p=.022). MUNIX values did not correlate with GMFCS or MACS. MUSIX values were higher, though not significantly, for CP subjects (p=.11). MUSIX increased with increasing MACS levels (r(2)=.4017, p=.049). Thus, motor unit numbers in ulnar hand muscles may be decreased with CP. MUSIX values are associated with greater hand impairment. Therefore, peripheral motor unit loss as a component of the weakness found with CP deserves further evaluation.

Increased motor unit potential shape variability across consecutive motor unit discharges in the tibialis anterior and vastus medialis muscles of healthy older subjects

OBJECTIVE

To study the potential utility of using near fiber (NF) jiggle as an assessment of neuromuscular transmission stability in healthy older subjects using decomposition-based quantitative electromyography (DQEMG).

METHODS

The tibialis anterior (TA) and vastus medialis (VM) muscles were tested in 9 older men (77 ± 5 years) and 9 young male control subjects (23 ± 0.3 years). Simultaneous surface and needle-detected electromyographic (EMG) signals were collected during voluntary contractions, and then analyzed using DQEMG. Motor unit potential (MUP) and NF MUP parameters were analyzed.

RESULTS

NF jiggle was significantly increased for both the TA and VM in the old age group relative to the younger controls (P<0.05). NF jiggle was significantly higher in the TA compared to VM (P<0.05). For TA, NF jiggle was negatively correlated with MUNE, and positively correlated with S-MUP amplitude, NF count, MUP duration, MUP peak-to-peak voltage, and MUP area (P<0.05). For VM, NF jiggle was positively correlated with NF count and MUP area (P<0.05), and no significant correlations were found between NF jiggle and S-MUP amplitude, MUP duration, or MUP peak-to-peak voltage (MUNE was not calculated for VM, so no correlation could be made).

CONCLUSIONS

Healthy aging is associated with neuromuscular transmission instability (increased NF jiggle) and MU remodeling, which can be measured using DQEMG.

SIGNIFICANCE

NF jiggle derived from DQEMG can be a useful method of identifying neuromuscular dysfunction at various stages of MU remodeling and aging.

Motor Unit Number Estimation in Neuromuscular Disease

Motor unit number estimation (MUNE) is an electrophysiological method designed to quantify motor unit loss in target muscles of interest. Most of the techniques are noninvasive and are therefore well suited for longitudinal monitoring. In this brief review, we describe the more commonly used techniques and their applications in amyotrophic lateral sclerosis, poliomyelitis, spinal muscular atrophy and hereditary sensorimotor neuropathies. Findings in some of these studies offer important pathophysiological insights. Since conventional electrophysiologic methods are not sensible measures of motor neuronal loss, MUNE could play a potentially important role in the diagnosis, monitoring of disease progression and response to treatment in neuromuscular diseases in which motor unit loss is a major feature.

Influence of needle electrode depth on DE-STA motor unit number estimation

INTRODUCTION

To assess a potential source of technique-associated error, we evaluated the influence of needle electrode depth on decomposition-enhanced spike-triggered averaging (DE-STA) motor unit number estimation (MUNE) and quantitative motor unit analysis in the upper trapezius (UT).

METHODS

The DE-STA MUNE protocol was performed at superficial, intermediate, and deep needle electrode depths in 18 control subjects.

RESULTS

Mean surface-detected motor unit potential amplitudes were significantly smaller for intermediate versus superficial (P<0.05), deep versus superficial (P<0.001), and deep versus intermediate (P<0.05). MUNE was significantly larger for deep versus superficial (P<0.001), with statistical trends toward larger MUNE values at greater depths for the remaining comparisons. No significant differences were found among needle electrode depths for quantitative motor unit potential parameters.

CONCLUSIONS

These results demonstrate the important influence of needle electrode depth on DE-STA MUNE in the UT. Suggestions are made for improved standardization of the protocol.

The potential use of spectral electromyographic fatigue as a screening and outcome monitoring tool of sarcopenic back muscle alterations

Background

To examine whether or not median frequency surface electromyographic (MF-EMG) back muscle fatigue monitoring would be able to identify alterations in back muscle function in elderly muscles, if a protocol was used that allowed optimum standardization of the processes underlying electromyographic fatigue, and whether these tests were reliable from day to day.

Methods

A total of 42 older (21 females; 67 (±10.5) years old) and 44 younger persons (19 females; 33 (±10) years) performed maximum isometric back extensions which were followed by one 30 s lasting 80% submaximum extension. Participants were seated on a dynamometer with their trunks 30° anteflexed, and they repeated all tests after 1-2 days and 6 weeks. SEMG was recorded bilaterally from the L1 (iliocostalis lumborum), L2 (longissimus), and L5 (multifidus) recording sites. Outcome variables included maximum back extension torque, initial MF-EMG (IMF-EMG), MF-EMG slope declines, and individual MF-EMG muscular imbalance scores. Two-factorial ANOVAs served to examine the age and gender-specific effects, and models from Generalizability Theory (G-Theory) were used for assessing retest-reliability.

Results

Maximum back extension moment was non-significantly smaller in elders. IMF-EMG was overall higher in elders, with significant differences at the L5 recordings sites. In the elderly, MF-EMG fatigue declines were significantly smaller in L5, in the recording with the most negative slope, or if the slope of all electrodes was considered. Retest reliability was unanimous in young and older persons. ICC-type measurements from G-Theory of both the IMF and the fatigue slopes ranged from 0.7 to 0.85. Absolute SEM values were found clinically acceptable for the IMF-EMG, but relatively high for the fatigue slope declines.

Conclusions

The MF-EMG fatigue method is able to elucidate alterations of aging back muscles. This method, thus, might be suggested as a potential biomarker to objectively identify persons at risk for sarcopenia. Considering the clinical relevance of the IMF-EMG relative to the MF-EMG slope declines, spectral EMG may also be used as an outcome monitoring tool in elderly populations.

Respiratory chain deficiency in aged spinal motor neurons

Sarcopenia, muscle wasting, and strength decline with age, is an important cause of loss of mobility in the elderly individuals. The underlying mechanisms are uncertain but likely to involve defects of motor nerve, neuromuscular junction, and muscle. Loss of motor neurons with age and subsequent denervation of skeletal muscle has been recognized as one of the contributing factors. This study investigated aspects of mitochondrial biology in spinal motor neurons from elderly subjects. We found that protein components of complex I of mitochondrial respiratory chain were reduced or absent in a proportion of aged motor neurons–a phenomenon not observed in fetal tissue. Further investigation showed that complex I-deficient cells had reduced mitochondrial DNA content and smaller soma size. We propose that mitochondrial dysfunction in these motor neurons could lead to the cell loss and ultimately denervation of muscle fibers.

Human neuromuscular structure and function in old age: A brief review

Natural adult aging is associated with many functional impairments of the human neuromuscular system. One of the more observable alterations is the loss of contractile muscle mass, termed sarcopenia. The loss of muscle mass occurs primarily due to a progressive loss of viable motor units, and accompanying atrophy of remaining muscle fibers. Not only does the loss of muscle mass contribute to impaired function in old age, but alterations in fiber type and myosin heavy chain isoform expression also contribute to weaker, slower, and less powerful contracting muscles. This review will focus on motor unit loss associated with natural adult aging, age-related fatigability, and the age-related differences in strength across contractile muscle actions.

Age-related neuromuscular function and dynamic balance control during slow and fast balance perturbations

This study investigated age-related differences in dynamic balance control and its connection to reflexes and explosive isometric plantar flexor torque in 19 males (9 Young aged 20–33 yr, 10 Elderly aged 61–72 yr). Dynamic balance was measured during Slow (15 cm/s) and Fast (25 cm/s) anterior and posterior perturbations. H/M-ratio was measured at 20% of maximal M-wave (H/M20%) 10, 30, and 90 ms after perturbations. Stretch reflexes were measured from tibialis anterior and soleus during anterior and posterior perturbations, respectively. In Slow, Elderly exhibited larger peak center-of-pressure (COP) displacement (15%; P < 0.05) during anterior perturbations. In Fast, Young showed a trend for faster recovery (37%; P = 0.086) after anterior perturbations. M-wave latency was similar between groups (6.2 ± 0.7 vs. 6.9 ± 1.2 ms), whereas Elderly showed a longer H-reflex latency (33.7 ± 2.3 vs. 36.4 ± 1.7 ms; P < 0.01). H/M20% was higher in Young 30 ms after Fast anterior (50%; P < 0.05) and posterior (51%; P < 0.05) perturbations. Plantar flexor rapid torque was also higher in Young (26%; P < 0.05). After combining both groups' data, H/M20% correlated negatively with Slow peak COP displacement ( r = −0.510, P < 0.05) and positively with Fast recovery time ( r = 0.580, P < 0.05) for anterior perturbations. Age-related differences in balance control seem to be more evident in anterior than posterior perturbations, and rapid sensory feedback is generally important for balance perturbation recovery.

Reliability of a modified motor unit number index (MUNIX) technique

INTRODUCTION

The purpose of this study was to examine the relative and absolute between-day reliability of the motor unit number index (MUNIX).

METHODS

Young, healthy adults (n=19) attended two testing sessions separated by 4-weeks where their maximal pinch-grip strength, MUNIX, and motor unit size index (MUSIX) were assessed in the abductor pollicis brevis muscle. Reliability was assessed by intraclass correlation coefficients (ICC), coefficient of variation (CV) and limits of agreement (LOA).

RESULTS

No mean differences were observed for MUNIX or MUSIX. The CV for the MUNIX and MUSIX measures were between 13.5% and 17.5%. The ICC for both measures were moderate to moderately-high (0.73-0.76), The LOA for both indicated a homoscedastic relationship.

DISCUSSION

Our findings indicate moderate to moderately-high reliability for both MUNIX and MUSIX. Future work is needed to ensure both measures are reliable in other muscles and cohorts, and further investigations are required to examine the validity of MUNIX.

Reliability of spike triggered averaging of the surface electromyogram for motor unit action potential estimation

INTRODUCTION

The reliability of estimated motor unit parameters using spike triggered averaging (STA) of the surface electromyogram (sEMG) has not been tested thoroughly. We investigated factors that may induce amplitude bias in estimated motor unit action potentials (MUAPs) and shape variations.

METHODS

An sEMG record was simulated. MUAPs were then estimated from the STA of the simulated EMG.

RESULTS

Variations in MUAP duration led to under-estimation of real MUAP amplitude, while synchronized firing led to over-estimation of amplitude. Spurious firing resulted in over-estimation of the amplitude of small motor units but under-estimation of the amplitude of large ones. Variability in amplitude and high firing rates had minimal influence on amplitude estimation. High firing rates and variation in MUAP duration led to large variations in MUAP shape. Estimation errors also correlated with shape variations.

CONCLUSIONS

Recommendations to enhance the accuracy of the STA estimates have been proposed.

Interrelationship between muscle strength, motor units, and aging

The interrelationship between muscle strength, motor unit (MU) number, and age is poorly understood, and in this study we sought to determine whether age-related differences in muscle strength are moderated by estimates of functioning MU number and size. Eighteen older adults (OA; 67 ± 1.20 years) and 24 young adults (YA; 22 ± 0.74 years) participated in this study. Maximum voluntary pinch-grip strength of the nondominant hand was determined and estimates of MU number were obtained from the abductor pollicis brevis muscle using the noninvasive motor unit number index (MUNIX) technique. The MUNIX technique was also utilized to derive a motor unit size index (MUSIX). An analysis of covariance (Age Group × MUNIX or MUSIX) was used to test heterogeneity of regression slopes, with body mass and gender serving as covariates. We observed that the slope of pinch-grip strength on the estimated number of MUs between YA and OA differed, indicated by an Age Group × MUNIX interaction (p = 0.04). Specifically, after controlling for the effect of body mass and gender, the slope in OA was significantly positive (0.14 ± 0.06 N/MUs, p = 0.03), whereas no such relationship was found in YA (-0.08 ± 0.09 N/MUs, p = 0.35). A significant Age Group × MUSIX interaction was also observed for strength (p < 0.01). In contrast to MUNIX, the slope in younger adults was significantly positive (0.48 ± 0.11 N/μV, p < 0.01), whereas no such relationship was found in older adults (-0.30 ± 0.22 N/μV, p = 0.18). These findings indicate that there is an interrelationship between muscle strength, MU numbers, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functioning motor units.

Role of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) in denervation-induced atrophy in aged muscle: facts and hypotheses

Aging-related loss of muscle mass, a biological process named sarcopenia, contributes to mobility impairment, falls, and physical frailty, resulting in an impaired quality of life in older people. In view of the aging of our society, understanding the underlying mechanisms of sarcopenia is a major health-care imperative. Evidence obtained from human and rodent studies demonstrates that skeletal muscle denervation/reinnervation cycles occur with aging, and that progressive failure of myofiber reinnervation is a major cause of the accelerating phase of sarcopenia in advanced age. However, the mechanisms responsible for the loss of myofiber innervation with aging remain unknown. The two major strategies that counteract sarcopenia, that is, caloric restriction and endurance training, are well known to protect neuromuscular junction (NMJ) integrity, albeit through undefined mechanisms. Interestingly, both of these interventions better preserve PGC-1α expression with aging, a transcriptional coactivator which has recently been shown to regulate key proteins involved in maintaining NMJ integrity. We therefore propose that the aging-related decline in PGC-1α may be a central mechanism promoting instability of the NMJ and consequently, aging-related alterations of myofiber innervation in sarcopenia. Similarly, the promotion of PGC-1α expression by both caloric restriction and exercise training may be fundamental to their protective benefits for aging muscle by better preserving NMJ integrity.

Adaptive and maladaptive motor axonal sprouting in aging and motoneuron disease

Motor unit (MU) enlargement by sprouting is an important compensatory mechanism for loss of functional MUs during normal aging and neuromuscular disease. Perisynaptic Schwann cells at neuromuscular junctions extend processes that bridge between denervated and reinnervated endplates, and guide axonal sprouts to reinnervate the denervated endplates. In a rat model of partial denervation, high levels of daily neuromuscular activity have been shown to inhibit the outgrowth of sprouts by preventing Schwann cell bridging. In this review, we consider (1) the relative roles of increasing levels of oxidative stress and neuromuscular activity to the destabilization of neuromuscular junctions with age and disease, and (2) how a progressive increase in the neuromuscular activity of declining numbers of functional MUs contributes to the progressive failure of adaptive sprouting and, in turn, to the progressive muscle weakness in the motoneuron diseases of post-polio syndrome and amyotrophic lateral sclerosis. We conclude that there is a time-related progression of MU loss, adaptive sprouting followed by maladaptive sprouting, and continuing recession of terminals during normal aging. The progression is accelerated in motoneuron disease, progressing more rapidly in the post-polio syndrome after prolonged denervation and extremely rapidly in ALS.

Intra-rater reliability of motor unit number estimation and quantitative motor unit analysis in subjects with amyotrophic lateral sclerosis

OBJECTIVES

To assess the intra-rater reliability of decomposition-enhanced spike-triggered averaging (DE-STA) motor unit number estimation (MUNE) and quantitative motor unit potential analysis in the upper trapezius (UT) and biceps brachii (BB) of subjects with amyotrophic lateral sclerosis (ALS) and to compare the results from the UT to control data.

METHODS

Patients diagnosed with clinically probable or definite ALS completed the experimental protocol twice with the same evaluator for the UT (n=10) and BB (n=9).

RESULTS

Intra-rater reliability for the UT was good for the maximum compound muscle action potential (CMAP) (ICC=0.88), mean surface-detected motor unit potential (S-MUP) (ICC=0.87) and MUNE (ICC=0.88), and for the BB was moderate for maximum CMAP (ICC=0.61), and excellent for mean S-MUP (ICC=0.94) and MUNE (ICC=0.93). A significant difference between tests was found for UT MUNE. Comparing subjects with ALS to control subjects, UT maximum CMAP (p<0.01) and MUNE (p<0.001) values were significantly lower, and mean S-MUP values significantly greater (p<0.05) in subjects with ALS.

CONCLUSIONS

This study has demonstrated the ability of the DE-STA MUNE technique to collect highly reliable data from two separate muscle groups and to detect the underlying pathophysiology of the disease.

SIGNIFICANCE

This was the first study to examine the reliability of this technique in subjects with ALS, and demonstrates its potential for future use as an outcome measure in ALS clinical trials and studies of ALS disease severity and natural history.

Unique aspects of competitive weightlifting: performance, training and physiology

Weightlifting is a dynamic strength and power sport in which two, multijoint, whole-body lifts are performed in competition; the snatch and clean and jerk. During the performance of these lifts, weightlifters have achieved some of the highest absolute and relative peak power outputs reported in the literature. The training structure of competitive weightlifters is characterized by the frequent use of high-intensity resistance exercise movements. Varied coaching and training philosophies currently exist around the world and further research is required to substantiate the best type of training programme for male and female weightlifters of various age groups. As competitive weightlifting is contested over eight male and seven female body weight categories, the anthropometric characteristics of the athletes widely ranges. The body compositions of weightlifters are similar to that of athletes of comparable body mass in other strength and power sports. However, the shorter height and limb lengths of weightlifters provide mechanical advantages when lifting heavy loads by reducing the mechanical torque and the vertical distance that the barbell must be displaced. Furthermore, the shorter body dimensions coincide with a greater mean skeletal muscle cross-sectional area that is advantageous to weightlifting performance. Weightlifting training induces a high metabolic cost. Although dietary records demonstrate that weightlifters typically meet their required daily energy intake, weightlifters have been shown to over consume protein and fat at the expense of adequate carbohydrate. The resulting macronutrient imbalance may not yield optimal performance gains. Cross-sectional data suggest that weightlifting training induces type IIX to IIA fibre-type transformation. Furthermore, weightlifters exhibit hypertrophy of type II fibres that is advantageous to weightlifting performance and maximal force production. As such, the isometric peak force and contractile rate of force development of weightlifters is ~15-20% and ~13-16% greater, respectively, than in other strength and power athletes. In addition, weightlifting training has been shown to reduce the typical sex-related difference in the expression of neuromuscular strength and power. However, this apparent sex-related difference appears to be augmented with increasing adult age demonstrating that women undergo a greater age-related decline in muscle shortening velocity and peak power when compared with men. Weightlifting training and competition has been shown to induce significant structural and functional adaptations of the cardiovascular system. The collective evidence shows that these adaptations are physiological as opposed to pathological. Finally, the acute exercise-induced testosterone, cortisol and growth hormone responses of weightlifters have similarities to that of following conventional strength and hypertrophy protocols involving large muscle mass exercises. The routine assessment of the basal testosterone : cortisol ratio may be beneficial when attempting to quantify the adaptive responses to weightlifting training. As competitive weightlifting is becoming increasingly popular around the world, further research addressing the physiological responses and adaptations of female weightlifters and younger (i.e. ≤17 years of age) and older (i.e. ≥35 years of age) weightlifters of both sexes is required.

Changes of cortico-muscular coherence: an early marker of healthy aging?

Cortico-muscular coherence (CMC) at beta frequency (13-30 Hz) occurs particularly during weak to moderate isometric contraction. It is a well-established measure of communication between the primary motor cortex (M1) and corresponding muscles revealing information about the integrity of the pyramidal system. Although the slowing of brain and muscle dynamics during healthy aging has been evidenced, functional communication as determined by CMC has not been investigated so far. Since decline of motor functions at higher age is likely to be associated with CMC changes, the present study aims at shedding light on the functionality of the motor system from a functional interaction perspective. To this end, CMC was investigated in 27 healthy subjects aging between 22 and 77 years during isometric contraction of their right forearm. Neuromagnetic activity was measured using whole-head magnetoencephalography (MEG). Muscle activity was measured by means of surface electromyography (EMG) of the right extensor digitorum communis (EDC) muscle. Additionally, MEG-EMG phase lags were calculated in order to estimate conducting time. The analysis revealed CMC and M1 power amplitudes to be increased with age accompanied by slowing of M1, EMG, and CMC. Frequency changes were particularly found in subjects aged above 40 years suggesting that at this middle age, neurophysiological changes occur, possibly reflecting an early neurophysiological marker of seniority. Since MEG-EMG phase lags did not vary with age, changes cannot be explained by alterations of nerve conduction. We argue that the M1 power amplitude increase and the shift towards lower frequencies might represent a neurophysiological marker of healthy aging which is possibly compensated by increased CMC amplitude.

Motor unit survival in lifelong runners is muscle dependent

A contributing factor to the loss of muscle mass and strength with adult aging is the reduction in the number of functioning motor units (MUs). Recently, we reported that lifelong competitive runners (master runners = ~66 yr) had greater numbers of MUs in a leg muscle (tibialis anterior) than age-matched recreationally active controls. This suggested that long-term exposure to high levels of physical activity may limit the loss of MU numbers with adult aging. However, it is unknown if this finding is the result of long-term activation of the specifically exercised motoneuron pool (i.e., tibialis anterior) or an overall systemic neuroprotective effect of high levels of physical activity.

PURPOSE

The purpose was to estimate the number of functioning MUs (MUNEs) in the biceps brachii (an upper body muscle not directly loaded by running) of nine young (27 ± 5 yr) and nine old (70 ± 5 yr) men and nine lifelong competitive master runners (67 ± 4 yr).

METHODS

Decomposition-enhanced spike-triggered averaging was used to measure surface and intramuscular EMG signals during elbow flexion at 10% of maximum voluntary isometric contraction.

RESULTS

Derived MUNEs were lower in the biceps brachii of runners (185 ± 69 MUs) and old men (133 ± 69 MUs) than the young (354 ± 113 MUs), but the old and master runners were similar.

CONCLUSIONS

Although there were no significant differences in MUNE between both older groups in the biceps brachii muscle, with the number of subjects tested here, we cannot eliminate the possibility of some whole-body neuroprotective effect. However, when compared with the remote biceps muscle, a greater influence on age-related spinal motoneuron survival was found in a chronically activated MN pool specific to the exercised muscle.

Mechanisms of muscle fatigue: Central factors and task dependency

Muscle fatigue can be caused by a number of different mechanisms that result in an acute reduction in the ability to perform a motor task. These mechanisms include the physiological processes that range from the motivation associated with performing the task through to the force exerted by the contractile proteins once they are activated. Two issues are examined in this brief review. First, the role of mechanisms located in the central nervous system (central factors) in the fatigue experienced by human subjects. Second, the importance of task conditions (task dependency) on the fatigue mechanisms involved in a particular performance. The literature documents a prominent role for central factors in the development of muscle fatigue. This role is examined by considering subject motivation, the descending signals sent by suprasegmental centres (central command) and motor unit behaviour. The significance of these or other mechanisms, however, appears to depend on the details of the task. Variation in such requirements as contraction intensity or duration, speed of movement, or type of muscle contraction alters the role of the different mechanisms in limiting performance. Unfortunately, few studies have examined these effects systematically. The field of muscle fatigue would benefit substantially from studies that determined the boundary conditions for the different fatigue mechanisms.

Unique aspects of competitive weightlifting: performance, training and physiology

Weightlifting is a dynamic strength and power sport in which two, multijoint, whole-body lifts are performed in competition; the snatch and clean and jerk. During the performance of these lifts, weightlifters have achieved some of the highest absolute and relative peak power outputs reported in the literature. The training structure of competitive weightlifters is characterized by the frequent use of high-intensity resistance exercise movements. Varied coaching and training philosophies currently exist around the world and further research is required to substantiate the best type of training programme for male and female weightlifters of various age groups. As competitive weightlifting is contested over eight male and seven female body weight categories, the anthropometric characteristics of the athletes widely ranges. The body compositions of weightlifters are similar to that of athletes of comparable body mass in other strength and power sports. However, the shorter height and limb lengths of weightlifters provide mechanical advantages when lifting heavy loads by reducing the mechanical torque and the vertical distance that the barbell must be displaced. Furthermore, the shorter body dimensions coincide with a greater mean skeletal muscle cross-sectional area that is advantageous to weightlifting performance. Weightlifting training induces a high metabolic cost. Although dietary records demonstrate that weightlifters typically meet their required daily energy intake, weightlifters have been shown to over consume protein and fat at the expense of adequate carbohydrate. The resulting macronutrient imbalance may not yield optimal performance gains. Cross-sectional data suggest that weightlifting training induces type IIX to IIA fibre-type transformation. Furthermore, weightlifters exhibit hypertrophy of type II fibres that is advantageous to weightlifting performance and maximal force production. As such, the isometric peak force and contractile rate of force development of weightlifters is ~15-20% and ~13-16% greater, respectively, than in other strength and power athletes. In addition, weightlifting training has been shown to reduce the typical sex-related difference in the expression of neuromuscular strength and power. However, this apparent sex-related difference appears to be augmented with increasing adult age demonstrating that women undergo a greater age-related decline in muscle shortening velocity and peak power when compared with men. Weightlifting training and competition has been shown to induce significant structural and functional adaptations of the cardiovascular system. The collective evidence shows that these adaptations are physiological as opposed to pathological. Finally, the acute exercise-induced testosterone, cortisol and growth hormone responses of weightlifters have similarities to that of following conventional strength and hypertrophy protocols involving large muscle mass exercises. The routine assessment of the basal testosterone : cortisol ratio may be beneficial when attempting to quantify the adaptive responses to weightlifting training. As competitive weightlifting is becoming increasingly popular around the world, further research addressing the physiological responses and adaptations of female weightlifters and younger (i.e. ≤17 years of age) and older (i.e. ≥35 years of age) weightlifters of both sexes is required.

Surface EMG model of the bicep during aging: a preliminary study

Reduction in the median frequency and the amplitude of surface electromyogram (sEMG) has been observed among older subjects compared with the younger cohort. These changes in sEMG have been associated with a reduction in the number of muscle fibers and a drop in the ratio of type II muscle fibers. However, the details of this association are not known. This paper has experimentally determined the difference between the magnitude and spectrum of sEMG of the younger and older cohorts, and estimated the changes to the muscle by populating a lifelike model with the experimental data. Experiments were conducted on subjects belonging to younger (20-28 years) and older (61-69) age groups. From the simulated results, it is shown that experimental sEMG signals are matched by the model representing the older cohort with a substantially reduced number of motor units compared to the younger people. In the model, the best match with experimental results was observed when the ratio of the bicep motor units between the older and the younger subjects was 0.5. The results also indicate a substantial reduction in the ratio of fast fibers, from 0.45 in the younger cohort to 0.11 in the older cohort.

Loss of synaptic vesicles from neuromuscular junctions in aged MRF4-null mice

MRF4 belongs to the basic helix-loop-helix class of transcription factors and this and other members of its family profoundly influence skeletal muscle development. Less is known about the role of these factors in aging. As MRF4 is preferentially expressed in subsynaptic nuclei, we postulated it might play a role in maintenance of the neuromuscular junction. To test this hypothesis, we examined the junctional regions of 19-20-month-old mice and found decreased levels of SV2B, a marker of synaptic vesicles, in MRF4-null mice relative to controls. There was a corresponding decrease in grip strength in MRF4-null mice. Taken together, these data suggest that the intrinsic muscle factor, MRF4 plays an important role in maintenance of neuromuscular junctions.

Prevention of falling risk in elderly people: the relevance of muscular strength and symmetry of lower limbs in postural stability

Falls are one of the major health problems affecting the quality of life among older adults. The aging process is associated with decreasing muscle strength and an increasing risk of falling. The variables and techniques adopted to quantify muscular strength and postural stability were different in each protocol; a great number of reports analyzed the risk factors and predictors of falls, but the results appear still uncertain. To date, there is no clear, definitive statement or review that has examined the effect of the quadriceps strength on static balance performances in different sensory conditions. This contribution aims to provide an overview of experimental works to increase the comprehension and prevention of falls and fall-related injuries in the elderly. Based on a review of the literature, this work was designed to explore the relationship among risk of falls, postural stability, and muscular strength of lower limbs in older adults.

Pharyngeal pressure generation during tongue-hold swallows across age groups

PURPOSE

To compare the effects of the tongue-hold swallowing maneuver on pharyngeal pressure generation in healthy young and elderly research volunteers.

METHOD

Sixty-eight healthy research volunteers (young, n = 34, mean age = 26.8 years, SD = 5.5; elderly, n = 34, mean age = 72.6 years, SD = 4.8; sex equally represented) performed 5 noneffortful saliva swallows and 5 tongue-hold swallows each. Amplitude and duration of pharyngeal pressure were investigated during both swallowing conditions with solid-state pharyngeal manometry at the level of the oropharynx, hypopharynx, and upper esophageal sphincter (UES).

RESULTS

At both pharyngeal levels, tongue-hold swallows produced lower peak pressure compared with saliva swallows. During tongue-hold swallows, UES relaxation pressure was increased in the elders, whereas the younger group displayed a trend toward reduced relaxation pressure. Elderly individuals produced pressure longer during control swallows in the oropharynx and hypopharynx than young individuals.

CONCLUSIONS

The tongue-hold maneuver affects oropharyngeal and hypopharyngeal pressure in the young and elders in similar ways, whereas effects on UES peak relaxation pressure differ between age groups. Reduced pharyngeal peak pressure and increased UES relaxation pressure underscore the notion that tongue-hold swallows should not be performed when bolus is present. Long-term training effects remain to be investigated.

Age affects reciprocal cellular interactions in neuromuscular synapses following peripheral nerve injury

Studies of the influence of age on regeneration and reinnervation in the peripheral nervous system (PNS) and neuromuscular junction (NMJ) are reviewed, with a particular focus on aged and denervated skeletal muscles. The morphological and functional features of incomplete regeneration and reinnervation are compared between adult and aged animals. In addition, some possible mechanisms of the age-related defects will be discussed. Increased fragmentation or damage in individual components of the NMJ (terminal Schwann cells (TSCs), axon terminals and acetylcholine receptor sites occurs during muscle reinnervation following PNS injury in the aged animals. The capacity to produce ultraterminal sprouting or multiple innervation secondary to PNS injury is maintained, but not the capacity to eliminate such anomalous axonal profiles. The frequency and accuracy of reoccupation of the synaptic sites by TSCs and axon terminals are impaired. Thus, despite the capability of extending neural processes, the rate at which regenerating nerve fibers grow, mature and precisely appose the postsynaptic muscle fiber is impaired, resulting in the failure of re-establishment of the normal single motor innervation in the NMJ. A complex set of cellular interactions in the NMJ are known to participate in the neurotrophism and neurotrophism to support growth of the regenerating and sprouting axons and their pathfinding to direct the target muscle fiber. Besides the capability of α-motoneurons, signaling originating from the TSCs and muscle may be impaired during aging.

Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure

Aging is characterized by loss of spinal motor neurons (MNs) due to apoptosis, reduced insulin-like growth factor I signaling, elevated amounts of circulating cytokines, and increased cell oxidative stress. The age-related loss of spinal MNs is paralleled by a reduction in muscle fiber number and size (sarcopenia), resulting in impaired mechanical muscle performance that in turn leads to a reduced functional capacity during everyday tasks. Concurrently, maximum muscle strength, power, and rate of force development are decreased with aging, even in highly trained master athletes. The impairment in muscle mechanical function is accompanied and partly caused by an age-related loss in neuromuscular function that comprise changes in maximal MN firing frequency, agonist muscle activation, antagonist muscle coactivation, force steadiness, and spinal inhibitory circuitry. Strength training appears to elicit effective countermeasures in elderly individuals even at a very old age (>80 years) by evoking muscle hypertrophy along with substantial changes in neuromuscular function, respectively. Notably, the training-induced changes in muscle mass and nervous system function leads to an improved functional capacity during activities of daily living.

Assessing motor deficits in compressive neuropathy using quantitative electromyography

Background

Studying the changes that occur in motor unit potential trains (MUPTs) may provide insight into the extent of motor unit loss and neural re-organization resulting from nerve compression injury. The purpose of this study was to determine the feasibility of using decomposition-based quantitative electromyography (DQEMG) to study the pathophysiological changes associated with compression neuropathy.

Methods

The model used to examine compression neuropathy was carpal tunnel syndrome (CTS) due to its high prevalence and ease of diagnosis. Surface and concentric needle electromyography data were acquired simultaneously from the abductor pollicis brevis muscle in six individuals with severe CTS, eight individuals with mild CTS and nine healthy control subjects. DQEMG was used to detect intramuscular MUPTs during constant-intensity contractions and to estimate parameters associated with the surface- and needle-detected motor unit potentials (SMUPs and MUPs, respectively). MUP morphology and stability, SMUP morphology and motor unit number estimates (MUNEs) were compared among the groups using Kruskal-Wallis tests.

Results

The severe CTS group had larger amplitude and longer duration MUPs and smaller MUNEs than the mild CTS and control groups, suggesting that the individuals with severe CTS had motor unit loss with subsequent collateral reinnervation, and that DQEMG using a constant-intensity protocol was sensitive to these changes. SMUP morphology and MUP complexity and stability did not significantly differ among the groups.

Conclusions

These results provide evidence that MUP amplitude parameters and MUNEs obtained using DQEMG, may be a valuable tool to investigate pathophysiological changes in muscles affected by compressive motor neuropathy to augment information obtained from nerve conduction studies. Although there were trends in many of these measures, in this study, MUP complexity and stability and SMUP parameters were, of limited value.

Pilot study to test the safety of an exercise machine on healthy adult females

AIM

To assess the safety and effectiveness of a chair-type training machine developed for the elderly and to conduct a training program in healthy women using this machine.

METHODS

Twenty-four healthy women (mean age: 27.0 +/- 5.4 years; range: 21-38 years) were randomly assigned to the exercise program (intervention) group (n = 12) or the control group (n = 12).

INTERVENTION

Training program using the chair-type training machine for the elderly in 60-min sessions twice a week for 12 weeks.

MEASUREMENTS

Safety standards for machinery (ISO 12100) and physiological phenomena (blood pressure, heart rate, and subjective symptoms and objective symptoms) were assessed. The primary endpoints were isometric muscle strength (knee joint extension strength (KJES)), ankle dorsal flexion strength (ADFS), and ankle plantar flexion strength (APFS); these were assessed using standardized protocols at the beginning and the end of intervention.

RESULTS

There were two dropouts in the intervention group. The safety of the machine was judged as acceptable based on the standard. No training-related medical problems occurred. It was observed that 12 weeks after the end of the training program, the isometric muscle strengths of all the left muscles and the right APFS were reinforced in the intervention group (P < 0.05). The right KJES and right ADFS of the intervention group did not show significant change when compared to the control.

CONCLUSION

The chair-type training machine and the training program with this machine are safe and effective for increasing muscle strength.

Effects of ageing on motor unit activation patterns and reflex sensitivity in dynamic movements

Both contraction type and ageing may cause changes in H-reflex excitability. H reflex is partly affected by presynaptic inhibition that may also be an important factor in the control of MU activation. The purpose of the study was to examine age related changes in H-reflex excitability and motor unit activation patterns in dynamic and in isometric contractions. Ten younger (YOUNG) and 13 elderly (OLD) males performed isometric (ISO), concentric (CON) and eccentric (ECC) plantarflexions with submaximal activation levels (20% and 40% of maximal soleus surface EMG). Intramuscular EMG data was analyzed utilizing an intramuscular spike amplitude frequency histogram method. Average H/M ratio was always lowest in ECC (n.s.). Mean spike amplitude increased with activation level (P<.05), whereas no significant differences were found between contraction types. Both H-reflex excitability, which may be due to an increase in presynaptic inhibition, and mean spike frequency were higher in YOUNG compared to OLD. In OLD the mean spike frequency was significantly smaller in CON compared to ISO. Lack of difference in mean spike amplitude and frequency across contraction types in YOUNG would imply a similar activation strategy, whereas the lower frequency in dynamic contractions in OLD could be related to synergist muscle behavior.

Modeling age-related neuromuscular changes in humans

With aging, motoneurons and muscle tissue undergo significant changes, which influence function in terms of strength, mobility, and overall independence. Mathematical modeling provides a practical method of studying the relationships among recruitment, rate-coding, and force output in motor units, and may be used to predict functional neuromuscular changes related to aging. For this study, the Heckman-Binder model was used to examine changes in human quadriceps motor units. Relationships among current input, firing frequency, and force output were defined for both a younger and an older individual. Included in the model were age-related effects associated with reduced muscle contractile speed; reduced muscle-fibre number, size, and specific tension; reduced gain of the frequency-current relationship; decreased size of motoneurons; and altered motor unit remodeling. Adjustment of this model to reflect age-related changes resulted in a leftward shift of the force-frequency function, lower firing frequency for any given current injected into the motoneuron, and a reduction in maximal force output. The model suggests that older individuals are capable of reaching force levels up to approximately 50% of those attained by younger individuals, with relatively similar or even slightly lower levels of current input. This could mean that the sense of effort and the contribution of factors other than degree of effort from afferent inputs to the pool, including conscious supraspinal centres, might be different in the older adult.

Neurophysiological measures in amyotrophic lateral sclerosis: Markers of progression in clinical trials

In this review we evaluate clinical neurophysiological methods, originally described for use in diagnosis that can be applied to measurement of change during the progress of amyotrophic lateral sclerosis (ALS). Such measurements are potentially important in clinical trials, and also in clinical practice. We have assessed methods for lower and upper motor neuron function, including conventional EMG, nerve conduction and F-wave studies, the derived Neurophysiological Index, motor unit counting methods (MUNE), and transcranial magnetic motor cortex stimulation. We have also addressed the validity of measurements of electromechanical coupling. Methods for measuring muscle strength are beyond the scope of this review. We conclude that MUNE, M-wave amplitude and the Neurophysiological Index are sufficiently reliable, sensitive, and relevant to the clinical problem of ALS, to be used in clinical trials in the disease. Transcranial magnetic stimulation is of limited value, but a combination of the measurements made as part of this technique may also be useful. We conclude that clinical neurophysiological techniques should now be used in measuring change in clinical trials in ALS.