Motor unit number estimation and neuromuscular fidelity in 3 stages of sarcopenia

Neuromuscular impairment at different stages of human sarcopenia

BACKGROUND

Degeneration of the motoneuron and neuromuscular junction (NMJ) and loss of motor units (MUs) contribute to age-related muscle wasting and weakness associated with sarcopenia. However, these features have not been comprehensively investigated in humans. This study aimed to compare neuromuscular system integrity and function at different stages of sarcopenia, with a particular focus on NMJ stability and MU properties.

METHODS

We recruited 42 young individuals (Y) (aged 25.98 ± 4.6 years; 57% females) and 88 older individuals (aged 75.9 ± 4.7 years; 55% females). The older group underwent a sarcopenia screening according to the revised guidelines of the European Working Group on Sarcopenia in Older People 2. In all groups, knee extensor muscle force was evaluated by isometric dynamometry, muscle morphology by ultrasound and MU potential properties by intramuscular electromyography (iEMG). MU number estimate (iMUNE) and blood samples were obtained. Muscle biopsies were collected in a subgroup of 16 Y and 52 older participants.

RESULTS

Thirty-nine older individuals were non-sarcopenic (NS), 31 pre-sarcopenic (PS) and 18 sarcopenic (S). A gradual decrease in quadriceps force, cross-sectional area and appendicular lean mass was observed across the different stages of sarcopenia (for all P < 0.0001). Handgrip force and the Short Physical Performance Battery score also showed a diminishing trend. iEMG analyses revealed elevated near fibre segment jitter in NS, PS and S compared with Y (Y vs. NS and S: P < 0.0001; Y vs. PS: P = 0.0169), suggestive of age-related impaired NMJ transmission. Increased C-terminal agrin fragment (P < 0.0001) and altered caveolin 3 protein expression were consistent with age-related NMJ instability in all the older groups. The iMUNE was lower in all older groups (P < 0.0001), confirming age-related loss of MUs. An age-related increase in MU potential complexity was also observed. These observations were accompanied by increased muscle denervation and axonal damage, evinced by the increase in neural cell adhesion molecule-positive fibres (Y vs. NS: P < 0.0001; Y vs. S: P = 0.02) and the increase in serum concentration of neurofilament light chain (P < 0.0001), respectively. Notably, most of these MU and NMJ parameters did not differ when comparing older individuals with or without sarcopenia.

CONCLUSIONS

Alterations in MU properties, axonal damage, an altered innervation profile and NMJ instability are prominent features of the ageing of the neuromuscular system. These neuromuscular alterations are accompanied by muscle wasting and weakness; however, they appear to precede clinically diagnosed sarcopenia, as they are already detectable in older NS individuals.

Neuromuscular consequences of spinal cord injury: New mechanistic insights and clinical considerations

The spinal cord facilitates communication between the brain and the body, containing intrinsic systems that work with lower motor neurons (LMNs) to manage movement. Spinal cord injuries (SCIs) can lead to partial paralysis and dysfunctions in muscles below the injury. While traditionally this paralysis has been attributed to disruptions in the corticospinal tract, a growing body of work demonstrates LMN damage is a factor. Motor units, comprising the LMN and the muscle fibers with which they connect, are essential for voluntary movement. Our understanding of their changes post-SCI is still emerging, but the health of motor units is vital, especially when considering innovative SCI treatments like nerve transfer surgery. This review seeks to collate current literature on how SCI impact motor units and explore neuromuscular clinical implications and treatment avenues. SCI reduced motor unit number estimates, and surviving motor units had impaired signal transmission at the neuromuscular junction, force-generating capacity, and excitability, which have the potential to recover chronically, yet the underlaying mechanisms are unclear. Furthermore, electrodiagnostic evaluations can aid in assessing the health lower and upper motor neurons, identify suitable targets for nerve transfer surgeries, and detect patients with time sensitive injuries. Lastly, many electrodiagnostic abnormalities occur in both chronic and acute SCI, yet factors contributing to these abnormalities are unknown. Future studies are required to determine how motor units adapt following SCI and the clinical implications of these adaptations.

Effects of a ketogenic diet on motor function and motor unit number estimation in aged C57BL/6 mice

OBJECTIVE

Pathological, age-related loss of muscle function, commonly referred to as sarcopenia, contributes to loss of mobility, impaired independence, as well as increased risk of adverse health events. Sarcopenia has been attributed to changes in both neural and muscular integrity during aging. Current treatment options are primarily limited to exercise and dietary protein fortification, but the therapeutic impact of these approaches are often inadequate. Prior work has suggested that a ketogenic diet (KD) might improve healthspan and lifespan in aging mice. Thus, we sought to investigate the effects of a KD on neuromuscular indices of sarcopenia in aged C57BL/6 mice.

DESIGN

A randomized, controlled pre-clinical experiment consisting of longitudinal assessments performed starting at 22-months of age (baseline) as well as 2, 6 and 10 weeks after the start of a KD vs. regular chow intervention.

SETTING

Preclinical laboratory study.

SAMPLE SIZE

Thirty-six 22-month-old mice were randomized into 2 dietary groups: KD [n = 22 (13 female and 9 male)], and regular chow [n = 15 (7 female and 8 male)].

MEASUREMENTS

Measures included body mass, hindlimb and all limb grip strength, rotarod for motor performance, plantarflexion muscle contractility, motor unit number estimations (MUNE), and repetitive nerve stimulation (RNS) as an index of neuromuscular junction transmission efficacy recorded from the gastrocnemius muscle. At end point, muscle wet weight and blood samples were collected to assess blood beta-hydroxybutyrate levels.

STATISTICAL ANALYSIS

Primary analyses were two-way mixed effects ANOVA (diet and time × diet) to determine the effect of a KD on indices of motor function (grip, rotarod) and indices of motor unit (MUNE) and muscle (contractility) function.

RESULTS

Beta-hydroxybutyrate (BHB) was significantly higher at 10 weeks in mice on a KD vs control group (0.83 ± 0.44 mmol/l versus 0.42 ± 0.21 mmol/l, η2 = 0.265, unpaired t-test, p = 0.0060). Mice on the KD intervention demonstrated significantly increased hindlimb grip strength (diet, p = 0.0001; time × diet, p = 0.0030), all limb grip strength (diet, p = 0.0005; time × diet, p = 0.0523), and rotarod latency to fall (diet, p = 0.0126; time × diet, p = 0.0021). Mice treated with the KD intervention also demonstrated increased MUNE (diet, p = 0.0465; time × diet, p = 0.0064), but no difference in muscle contractility (diet, p = 0.5248; time × diet, p = 0.5836) or RNS (diet, p = 0.3562; time × diet, p = 0.9871).

CONCLUSION

KD intervention improved neuromuscular and motor function in aged mice. This pre-clinical work suggests that further research is needed to assess the efficacy and physiological effects of a KD on indices of sarcopenia.

Exploration of a machine learning approach for diagnosing sarcopenia among Chinese community-dwelling older adults using sEMG-based data

BACKGROUND

In the practical application of sarcopenia screening, there is a need for faster, time-saving, and community-friendly detection methods. The primary purpose of this study was to perform sarcopenia screening in community-dwelling older adults and investigate whether surface electromyogram (sEMG) from hand grip could potentially be used to detect sarcopenia using machine learning (ML) methods with reasonable features extracted from sEMG signals. The secondary aim was to provide the interpretability of the obtained ML models using a novel feature importance estimation method.

METHODS

A total of 158 community-dwelling older residents (≥ 60 years old) were recruited. After screening through the diagnostic criteria of the Asian Working Group for Sarcopenia in 2019 (AWGS 2019) and data quality check, participants were assigned to the healthy group (n = 45) and the sarcopenic group (n = 48). sEMG signals from six forearm muscles were recorded during the hand grip task at 20% maximal voluntary contraction (MVC) and 50% MVC. After filtering recorded signals, nine representative features were extracted, including six time-domain features plus three time-frequency domain features. Then, a voting classifier ensembled by a support vector machine (SVM), a random forest (RF), and a gradient boosting machine (GBM) was implemented to classify healthy versus sarcopenic participants. Finally, the SHapley Additive exPlanations (SHAP) method was utilized to investigate feature importance during classification.

RESULTS

Seven out of the nine features exhibited statistically significant differences between healthy and sarcopenic participants in both 20% and 50% MVC tests. Using these features, the voting classifier achieved 80% sensitivity and 73% accuracy through a five-fold cross-validation. Such performance was better than each of the SVM, RF, and GBM models alone. Lastly, SHAP results revealed that the wavelength (WL) and the kurtosis of continuous wavelet transform coefficients (CWT_kurtosis) had the highest feature impact scores.

CONCLUSION

This study proposed a method for community-based sarcopenia screening using sEMG signals of forearm muscles. Using a voting classifier with nine representative features, the accuracy exceeds 70% and the sensitivity exceeds 75%, indicating moderate classification performance. Interpretable results obtained from the SHAP model suggest that motor unit (MU) activation mode may be a key factor affecting sarcopenia.

Postural balance disorders in sarcopenia based on surface electromyography

Background

Sarcopenia is an intrinsic factor that leads to balance disorders and falls in older adults. However, the characterization of sarcopenia-related postural balance deficits remains unclear.

Aims

This study aimed to explore the balance performance and postural control strategy in older adults with sarcopenia during static stance tasks using force platforms and surface electromyography.

Methods

Older adults with right-sided dominance were recruited, including 27 adults with sarcopenia and 27 healthy counterparts. Postural sway was measured with eyes open/closed on rigid/compliant surfaces. The time- and frequency-domain indexes of bilateral lower extremity muscle activity were simultaneously recorded.

Results

The postural sway and activity of multiple lower extremity muscles in the sarcopenia group were increased (P < 0.05). The amplitude contribution ratio of the right tibialis anterior muscle (larger in sarcopenia), co-contraction ratio of right ankle dorsiflexion (smaller in sarcopenia), and mean power frequency and median frequency of the left gluteus maximus muscle (smaller in sarcopenia) had main effects of grouping (P < 0.001, η2p = 0.06-0.10). All of them had discrimination for sarcopenia (area under the curve = 0.639-0.657, P < 0.001) and were correlated with balance function measurement in sarcopenia (|rs| = 0.22-0.44, P < 0.05).

Conclusion

The results of this study suggest that older adults with sarcopenia have decreased balance function and increased cost of electrophysiology. They were found to prefer the postural strategy of dominant ankle dorsiflexion and demonstrated overactivity of the dominant tibialis anterior muscles and fatigue vulnerability of the nondominant gluteus maximus. Improvements in these postural features may have balance benefits.

The MuSK-BMP pathway regulates synaptic Nav1.4 localization and muscle excitability

The neuromuscular junction (NMJ) is the linchpin of nerve-evoked muscle contraction. Broadly considered, the function of the NMJ is to transduce a nerve action potential into a muscle fiber action potential (MFAP). Efficient information transfer requires both cholinergic signaling, responsible for the generation of endplate potentials (EPPs), and excitation, the activation of postsynaptic voltage-gated sodium channels (Nav1.4) to trigger MFAPs. In contrast to the cholinergic apparatus, the signaling pathways that organize Nav1.4 and muscle fiber excitability are poorly characterized. Muscle-specific kinase (MuSK), in addition to its Ig1 domain-dependent role as an agrin-LRP4 receptor, is also a BMP co-receptor that binds BMPs via its Ig3 domain and shapes BMP-induced signaling and transcriptional output. Here we probed the function of the MuSK-BMP pathway at the NMJ using mice lacking the MuSK Ig3 domain ('ΔIg3-MuSK'). Synapses formed normally in ΔIg3-MuSK animals, but the postsynaptic apparatus was fragmented from the first weeks of life. Anatomical denervation was not observed at any age examined. Moreover, spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents were comparable to WT. However, trains of nerve-evoked MFAPs in ΔIg3-MuSK muscle were abnormal as revealed by increased jitter and blocking in single fiber electromyography. Further, nerve-evoked compound muscle action potentials (CMAPs), as well as twitch and tetanic muscle torque force production, were also diminished. Finally, Nav1.4 levels were reduced at ΔIg3-MuSK synapses but not at the extrajunctional sarcolemma, indicating that the observed excitability defects are the result of impaired localization of this voltage-gated ion channel at the NMJ. We propose that MuSK plays two distinct roles at the NMJ: as an agrin-LRP4 receptor necessary for establishing and maintaining cholinergic signaling, and as a BMP co-receptor required for maintaining proper Nav1.4 density, nerve-evoked muscle excitability and force production. The MuSK-BMP pathway thus emerges as a target for modulating excitability and functional innervation, which are defective in conditions such as congenital myasthenic syndromes and aging.

Ketogenic Diet Improves Motor Function and Motor Unit Connectivity in Aged C57BL/6 Mice

Objective

Pathological, age-related loss of muscle function, commonly referred to as sarcopenia, contributes to loss of mobility, impaired independence, as well as increased risk of adverse health events. Sarcopenia has been attributed to changes in both neural and muscular integrity during aging. Current treatment options are primarily limited to exercise and dietary protein fortification, but the therapeutic impact of these approaches are often inadequate. Prior work has suggested that a ketogenic diet (KD) might improve healthspan and lifespan in aging mice. Thus, we sought to investigate the effects of a KD on neuromuscular indices of sarcopenia in aged C57BL/6 mice.

Design

A randomized, controlled pre-clinical experiment consisting of longitudinal assessments performed starting at 22-months of age (baseline) as well as 2, 6 and 10 weeks after the start of a KD vs. regular chow intervention.

Setting

Preclinical laboratory study.

Sample size

Thirty-six 22-month-old mice were randomized into 2 dietary groups: KD [n = 22 (13 female and 9 male)], and regular chow [n = 15 (7 female and 8 male)].

Measurements

Measures included body mass, hindlimb and all limb grip strength, rotarod for motor performance, plantarflexion muscle contractility, motor unit number estimations (MUNE), and repetitive nerve stimulation (RNS) as an index of neuromuscular junction transmission efficacy recorded from the gastrocnemius muscle. At end point, blood samples were collected to assess blood beta-hydroxybutyrate levels.

Statistical Analysis

Two-way ANOVA mixed-effects analysis (time x diet) were performed to analyze grip, rotarod, MUNE, and muscle contractility data.

Results

Beta-hydroxybutyrate (BHB) was significantly higher at 10 weeks in mice on a KD vs control group (0.83 ± 0.44 mmol/l versus 0.42 ± 0.21 mmol/l, η2 = 0.265, unpaired t-test, p = 0.0060). Mice on the KD intervention demonstrated significantly increased hindlimb grip strength (time x diet, p = 0.0030), all limb grip strength (time x diet, p = 0.0523), and rotarod latency to fall (time x diet, p = 0.0021). Mice treated with the KD intervention also demonstrated significantly greater MUNE (time x diet, p = 0.0064), but no difference in muscle contractility (time x diet, p = 0.5836) or RNS (time x diet, p = 0.9871).

Conclusion

KD intervention improved neuromuscular and motor function in aged mice. This pre-clinical work suggests that further research is needed to assess the efficacy and physiological effects of a KD on indices of sarcopenia.

UBC-Nepal Expedition: Motor Unit Characteristics in Lowlanders Acclimatized to High Altitude and Sherpa

INTRODUCTION

With acclimatization to high altitude (HA), adaptations occur throughout the nervous system and at the level of the muscle, which may affect motor unit (MU) characteristics. However, despite the importance of MUs as the final common pathway for the control of voluntary movement, little is known about their adaptations with acclimatization.

METHODS

Ten lowlanders and Sherpa participated in this study 7 to 14 d after arrival at HA (5050 m), with seven lowlanders repeating the experiment at sea level (SL), 6 months after the expedition. The maximal compound muscle action potential (M max ) was recorded from relaxed biceps brachii. During isometric elbow flexions at 10% of maximal torque, a needle electrode recorded the MU discharge rate (MUDR) and MU potential (MUP) characteristics of single biceps brachii MUs.

RESULTS

Compared with SL, acclimatized lowlanders had ~10% greater MUDR, ~11% longer MUP duration, as well as ~18% lower amplitude and ~6% greater duration of the first phase of the M max (all P < 0.05). No differences were noted between SL and HA for variables related to MUP shape (e.g., jitter, jiggle; P > 0.08). Apart from lower near-fiber MUP area for Sherpa than acclimatized lowlanders ( P < 0.05), no M max or MU data were different between groups ( P > 0.10).

CONCLUSIONS

Like other components of the body, MUs in lowlanders adapt with acclimatization to HA. The absence of differences between acclimatized lowlanders and Sherpa suggests that evolutionary adaptations to HA are smaller for MUs than components of the cardiovascular or respiratory systems.

Quantitative motor unit potential analysis and nerve conduction studies for detection of subclinical peripheral nerve dysfunction in patients with compensated liver cirrhosis

Background

Peripheral neuropathy is an underestimated problem of compensated liver cirrhosis despite its negative effect on quality of life. The aim was to assess the role of meticulous electrophysiological screening (nerve conduction responses and quantitative motor unit potential analysis) of subclinical peripheral nerve dysfunction in patients with compensated cirrhosis and also to explore its relations with different characteristics of liver disease. Severity of cirrhosis was assessed by Child–Pugh and albumin-bilirubin (ALBI) scores.

Results

Prevalence of hepatic neuropathy (HN) was 82%. It involved sensory and motor fibers (66%), and its pathophysiology was axonal (53.7%) or mixed axonal and demyelinating (46.3). The most sensitive discriminating tests were ulnar sensory conduction velocity (area under curve (AUC) = 0.915) and peak latency (AUC = 0.887), peroneal motor conduction velocity (AUC = 0.885), ulnar distal motor latency (AUC = 0.842), and first dorsal interosseous number of phases (AUC = 0.736). HN showed significant correlation with the severity of liver disease assessed by both child (P = 0.029) and ALBI (P = 0.016) scores and also correlated with the low serum albumin level (P = 0.001).

Conclusions

Subclinical mild axonal polyneuropathy is very common in post-hepatitis C compensated cirrhosis picked up by meticulous electrophysiological testing, and it is related to severity of liver cirrhosis and low serum albumin level.

Postoperative management and rehabilitation after the supercharged end-to-side anterior interosseous nerve to ulnar motor nerve transfer: A report of 3 cases

INTRODUCTION

Compressive ulnar neuropathy at the elbow is the second most common compressive neuropathy. Nerve transfers are used for severe ulnar neuropathies as a means of facilitating recovery. Hand therapy and rehabilitation after nerve transfers have not been extensively explored.

PURPOSE OF THE STUDY

The aim of this repeated case study was to describe the responses, functional outcome, and neuromuscular health of three participants after the supercharged end-to-side (SETS) anterior interosseous nerve (AIN) to ulnar motor nerve transfer do describe the hand therapy and recovery of 3 cases reflecting different recovery potential mediators, trajectories, and outcomes.

STUDY DESIGN

Repeated case study.

METHODS

Three participants of similar age (76-80 years) that had severe ulnar neuropathy who underwent surgical treatment including a SETS AIN to ulnar motor nerve surgery were purposively selected from an ongoing clinical trial, based on their response to the surgical and the rehabilitation intervention (large, moderate, and small improvements). Clinical evaluations included measuring range of motion, strength testing, and clinical tests (ie, Egawa's sign) and, subjective assessment of rehabilitation adherence., Quick Disability of Arm, Shoulder and Hand and decomposition-based quantitative electromyography were performed at >23 months to evaluate patients.

RESULTS

All the three participants completed the surgical and hand therapy interventions, demonstrating a variable course of recovery and functional outcomes. The Quick Disability of Arm, Shoulder and Hand scores (>23 months) for participants A, B, and C were 68, 30, and 18, respectively. The person with the least improvement had idiopathic Parkinson's disease, dyslipidemia, history of depression, and gout. Comparison across cases suggested that the comorbidities, longer time from neuropathy to the surgical intervention, and psychosocial barriers to exercise and rehabilitation adherence influenced the recovery process. The participants with the best outcomes demonstrated improvements in his lower motor neurons or motor unit counts (109 and 18 motor units in the abductor digiti minimi (ADM) and first dorsal interosseous, respectively) and motor unit stability (39.5% and 37.6% near-fiber jiggle in the ADM and first dorsal interosseous, respectively). The participant with moderate response to the interventions had a motor unit count of 93 for the ADM muscle. We were unable to determine motor unit counts and measurements from the participant with the poorest outcomes due to his physical limitations.

CONCLUSIONS

SETS AIN to ulnar motor nerve followed by multimodal hand therapy provides measurable improvements in neurophysiology and function, although engagement in hand therapy and outcomes appear to be mediated by comorbid physical and psychosocial health.

Anconeus motor unit firing rates during isometric and muscle-shortening contractions comparing young and very old adults

With effects of aging, voluntary neural drive to the muscle, measured as motor unit (MU) firing rate, is lower in older adults during sustained isometric contractions compared with young adults, but differences remain unknown during limb movements. Therefore, our purpose was to compare MU firing rates during both isometric and shortening contractions between two adult age groups. We analyzed intramuscular electromyography of single-MU recordings in the anconeus muscle of young (n = 8, 19-33 yr) and very old (n = 13, 78-93 yr) male adults during maximal voluntary contractions (MVCs). In sustained isometric and muscle-shortening contractions during limb movement, MU trains were linked with elbow joint kinematic parameters throughout the contraction time course. The older group was 33% weaker and 10% slower during movements than the young group (P < 0.01). In isometric contractions, median firing rates were 42% lower (P < 0.01) in the older group (18 Hz) compared with the young group (31 Hz), but during shortening contractions firing rates were higher for both age groups and not statistically different between groups. As a function of contraction time, firing rates at MU recruitment threshold were 39% lower in the older group, but the firing rate decrease was attenuated threefold throughout shortening contraction compared with the young group. At the single-MU level, age-related differences during isometric contractions (i.e., pre-movement initiation) do not remain constant throughout movement that comprises greater effects of muscle shortening. Results indicate that neural drive is task dependent and during movement in older adults it is decreased minimally.NEW & NOTEWORTHY Changes of neural drive to the muscle with adult aging, measured as motor unit firing rates during limb movements, are unknown. Throughout maximal voluntary efforts we found that, in comparison with young adults, firing rates were lower during isometric contraction in older adults but not different during elbow extension movements. Despite the older group being ∼33% weaker across contractions, their muscles can receive neural drive during movements that are similar to that of younger adults.

Influence of IGF-I serum concentration on muscular regeneration capacity in patients with sarcopenia

Background

Previous research has described a neuroprotective effect of IGF-I, supporting neuronal survival, axon growth and proliferation of muscle cells. Therefore, the association between IGF-I concentration, muscle histology and electrophysiological markers in a cohort of patients with sarcopenia dares investigation.

Methods

Measurement of serum concentrations of IGF-I and binding partners, electromyographic measurements with the MUNIX (Motor Unit Number Index) method and muscle biopsies were performed in 31 patients with acute hip fracture older age 60 years. Molecular markers for denervation (neural cell adhesion molecule NCAM) and proliferation markers (Ki67) were assessed by immunofluorescence staining of muscle biopsy tissue. Skeletal muscle mass by bioelectrical impedance analysis and hand-grip strength were measured to assess sarcopenia status according to EWGSOP2 criteria.

Results

Thirty-one patients (20 women) with a mean age of 80.6 ± 7.4 years were included. Concentrations of IGF-I and its binding partners were significantly associated with sarcopenia (ß = − 0.360; p = 0.047) and MUNIX (ß = 0.512; p = 0.005). Further, expression of NCAM (ß = 0.380; p = 0.039) and Ki67 (ß = 0.424; p = 0.022) showed significant associations to IGF-I concentrations.

Conclusions

The findings suggest a pathogenetic role of IGF-I in sarcopenia based on muscle denervation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04699-3.

Neuromuscular Junction Aging: A Role for Biomarkers and Exercise

Age-related skeletal muscle degradation known as "sarcopenia" exerts considerable strain on public health systems globally. While the pathogenesis of such atrophy is undoubtedly multifactorial, disruption at the neuromuscular junction (NMJ) has recently gained traction as a key explanatory factor. The NMJ, an essential communicatory link between nerve and muscle, undergoes profound changes with advancing age. Ascertaining whether such changes potentiate the onset of sarcopenia would be paramount in facilitating a timely implementation of targeted therapeutic strategies. Hence, there is a growing level of importance to further substantiate the effects of age on NMJs, in parallel with developing measures to attenuate such changes. As such, this review aimed to establish the current standpoint on age-related NMJ deterioration and consequences for skeletal muscle, while illuminating a role for biomarkers and exercise in ameliorating these alterations. Recent insights into the importance of key biomarkers for NMJ stability are provided, while the stimulative benefits of exercise in preserving NMJ function are demonstrated. Further elucidation of the diagnostic and prognostic relevance of biomarkers, coupled with the therapeutic benefits of regular exercise may be crucial in combating age-related NMJ and skeletal muscle degradation.

Active older dancers have lower C-terminal Agrin fragment concentration, better balance and gait performance than sedentary peers

Motor neuron degeneration, denervation, neuromuscular junction (NMJ) fragmentation and loss of motor units (MUs), play a key-role in the development of sarcopenia. The aim of the present study was to investigate the beneficial effects of regular practice of dancing in physically active elders on concentration of C-terminal Agrin fragment (CAF), a marker of NMJ instability, muscle mass, strength, and physical performance in a group of 16 recreationally active older dancers (AOD; 70.1 ± 3.4 yr) compared to 15 age-matched sedentary peers (OS; 70.9 ± 6.2 yr). Circulating concentration of CAF was measured in serum, while morphology of the vastus lateralis and multifidus muscles was assessed by ultrasound imaging. In addition, the participants underwent two functional performance tests, the Timed Up and Go (TUG) and the 10-meter walk test (10-MWT), a lower and upper limb isometric strength test, a static and a dynamic balance test. Although no statistically significant differences were detected for both muscle morphology and isometric strength, higher CAF concentration (20%, p < 0.01) was found in OS. AOD showed a better performance in TUG (22%, p < 0.001), 10-MWT (17%, p < 0.001) and dynamic balance (25%, p < 0.01) than OS. Notably, CAF concentration correlated with dynamic balance performance (r = 0.3711, p < 0.05). Our results provide evidence that the regular practice of dancing in older age, together with non-structured light aerobic physical activities, is associated to lower CAF concentration and improved walking and balance performance. Our findings also suggest that NMJ instability, as indicated by elevated CAF serum concentration, seems to precede the loss of muscle size and alterations in muscle architecture normally associated with sarcopenia.

Neuroprotective effects of exercise on the aging human neuromuscular system

Human biological aging from maturity to senescence is associated with a gradual loss of muscle mass and neuromuscular function. It is not until very old age (>80 years) however, that these changes often manifest into functional impairments. A driving factor underlying the age-related loss of muscle mass and function is the reduction in the number and quality of motor units (MUs). A MU consists of a single motoneuron, located either in the spinal cord or the brain stem, and all of the muscle fibres it innervates via its peripheral axon. Throughout the adult lifespan, MUs are slowly, but progressively lost. The compensatory process of collateral reinnervation attempts to recapture orphaned muscle fibres following the death of a motoneuron. Whereas this process helps mitigate loss of muscle mass during the latter decades of adult aging, the neuromuscular system has fewer and larger MUs, which have lower quality connections between the axon terminal and innervated muscle fibres. Whether this process of MU death and degradation can be attenuated with habitual physical activity has been a challenging question of great interest. This review focuses on age-related alterations of the human neuromuscular system, with an emphasis on the MU, and presents findings on the potential protective effects of lifelong physical activity. Although there is some discrepancy across studies of masters athletes, if one considers all experimental limitations as well as the available literature in animals, there is compelling evidence of a protective effect of chronic physical training on human MUs. Our tenet is that high-levels of physical activity can mitigate the natural trajectory of loss of quantity and quality of MUs in old age.

Profiling age-related muscle weakness and wasting: neuromuscular junction transmission as a driver of age-related physical decline

Pathological age-related loss of skeletal muscle strength and mass contribute to impaired physical function in older adults. Factors that promote the development of these conditions remain incompletely understood, impeding development of effective and specific diagnostic and therapeutic approaches. Inconclusive evidence across species suggests disruption of action potential signal transmission at the neuromuscular junction (NMJ), the crucial connection between the nervous and muscular systems, as a possible contributor to age-related muscle dysfunction. Here we investigated age-related loss of NMJ function using clinically relevant, electrophysiological measures (single-fiber electromyography (SFEMG) and repetitive nerve stimulation (RNS)) in aged (26 months) versus young (6 months) F344 rats. Measures of muscle function (e.g., grip strength, peak plantarflexion contractility torque) and mass were assessed for correlations with physiological measures (e.g., indices of NMJ transmission). Other outcomes also included plantarflexion muscle contractility tetanic torque fade during 1-s trains of stimulation as well as gastrocnemius motor unit size and number. Profiling NMJ function in aged rats identified significant declines in NMJ transmission stability and reliability. Further, NMJ deficits were tightly correlated with hindlimb grip strength, gastrocnemius muscle weight, loss of peak contractility torque, degree of tetanic fade, and motor unit loss. Thus, these findings provide direct evidence for NMJ dysfunction as a potential mechanism of age-related muscle dysfunction pathogenesis and severity. These findings also suggest that NMJ transmission modulation may serve as a target for therapeutic development for age-related loss of physical function.

Screening for Sarcopenia (Physical Frailty) in the COVID-19 Era

Although the numbers of aged populations have risen considerably in the last few decades, the current coronavirus disease 2019 (COVID-19) has revealed an extensive vulnerability among these populations. Sarcopenia is an age-related disorder that increases hospitalization, dependencies, and mortality in older adults. It starts to develop in midlife or even earlier as a result of unbalanced diet/poor nutrition and low levels of physical activity, in addition to chronic disorders such as obesity and diabetes mellitus. Given that social isolation is adopted as the most protective measure against COVID-19, the level of physical activity and the intake of adequate diet have considerably declined, especially among older adults-denoting an increased possibility for developing sarcopenia. Research also shows a higher vulnerability of sarcopenic people to COVID-19 as well as the development of wasting disorders such as sarcopenia and cachexia in a considerable proportion of symptomatic and recovering COVID-19 patients. Muscular wasting in COVID-19 is associated with poor prognosis. Accordingly, early detection and proper management of sarcopenia and wasting conditions in older adults and COVID-19 patients may minimize morbidity and mortality during the current COVID-19 crisis. This review explored different aspects of screening for sarcopenia, stressing their relevance to the detection of altered muscular structure and performance in patients with COVID-19. Current guidelines recommend prior evaluation of muscle strength by simple measures such as grip strength to identify individuals with proven weakness who then would be screened for muscle mass loss. The latter is best measured by MRI and CT. However, due to the high cost and radiation risk entailed by these techniques, other simpler and cheaper techniques such as DXA and ultrasound are given preference. Muscle loss in COVID-19 patients was measured during the acute phase by CT scanning of the pectoralis muscle simultaneously during a routine check for lung fibrosis, which seems to be an efficient evaluation of sarcopenia among those patients with no additional cost. In recovering patients, muscle strength and physical performance have been evaluated by electromyography and traditional tests such as the six-minute walk test. Effective preventive and therapeutic interventions are necessary in order to prevent muscle loss and associated physical decline in COVID-19 patients.

Motor unit firing rates during constant isometric contraction: establishing and comparing an age-related pattern among muscles

Motor unit (MU) firing rates (FRs) are lower in aged adults, compared with young, at relative voluntary contraction intensities. However, from a variety of independent studies of disparate muscles, the age-related degree of difference in FR among muscles is unclear. Using a standardized statistical approach with data derived from primary studies, we quantified differences in FRs across several muscles between younger and older adults. The data set included 12 different muscles in young (18-35 yr) and older adults (62-93 yr) from 18 published and one unpublished study. Experiments recorded single MU activity from intramuscular electromyography during constant isometric contraction at different (step-like) voluntary intensities. For each muscle, FR ranges and FR variance explained by voluntary contraction intensity were determined using bootstrapping. Dissimilarity of FR variance among muscles was calculated by Euclidean distances. There were threefold differences in the absolute frequency of FR ranges across muscles in the young (soleus 8-16 and superior trapezius 20-49 Hz), but in the old, FR ranges were more similar and lower for nine out of 12 muscles. In contrast, the explained FR variance from voluntary contraction intensity in the older group had 1.6-fold greater dissimilarity among muscles than the young (P < 0.001), with FR variance differences being muscle dependent. Therefore, differences between muscle FR ranges were not explained by how FRs scale to changes in voluntary contraction intensity within each muscle. Instead, FRs were muscle dependent but were more dissimilar among muscles in the older group in their responsiveness to voluntary contraction intensity.NEW & NOTEWORTHY The mean frequency of motor unit firing rates were compared systematically among several muscles and between young and older adults from new and published data sets. Firing rates among muscles were lower and more similar during voluntary isometric contraction in older than younger adults. Firing rate responses from voluntary contraction intensity were muscle dependent and more dissimilar among muscles in the older than young adults.

Near-fiber electromyography

OBJECTIVE

Describe and evaluate the concepts of near fiber electromyography (NFEMG), the features used, including near fiber motor unit potential (NFMUP) duration and dispersion, which relate to motor unit distal axonal branch and muscle fiber conduction time dispersion, and NFMUP segment jitter, a new measure of the temporal variability of neuromuscular junction transmission (NMJ), and axonal branch and muscle fibre conduction for the near fibres (i.e. NF jitter), and the methods for obtaining their values.

METHODS

Trains of high-pass filtered motor unit potentials (MUPs) (i.e. NFMUP trains) were extracted from needle-detected EMG signals to assess changes in motor unit (MU) morphology and electrophysiology caused by neuromuscular disorders or ageing. Evaluations using simulated needle-detected EMG data were completed and example human data are presented.

RESULTS

NFEMG feature values can be used to detect axonal sprouting, conduction slowing and NMJ transmission delay as well as changes in MU fiber diameter variability, and NF jitter. These changes can be detected prior to alterations of MU size or numbers.

CONCLUSIONS

The evaluations clearly demonstrate and the example data support that NFMUP duration and dispersion reflect MU distal axonal branching, conduction slowing and NMJ transmission delay and/or MU fiber diameter variability and that NFMUP jiggle and segment jitter reflect NF jitter.

SIGNIFICANCE

NFEMG can detect early changes in MU morphology and/or electrophysiology and has the potential to augment clinical diagnosis and tracking of neuromuscular disorders.

Thirty years of translational research in Mobility Medicine: Collection of abstracts of the 2020 Padua Muscle Days

More than half a century of skeletal muscle research is continuing at Padua University (Italy) under the auspices of the Interdepartmental Research Centre of Myology (CIR-Myo), the European Journal of Translational Myology (EJTM) and recently also with the support of the A&CM-C Foundation for Translational Myology, Padova, Italy. The Volume 30(1), 2020 of the EJTM opens with the collection of abstracts for the conference "2020 Padua Muscle Days: Mobility Medicine 30 years of Translational Research". This is an international conference that will be held between March 18-21, 2020 in Euganei Hills and Padova in Italy. The abstracts are excellent examples of translational research and of the multidimensional approaches that are needed to classify and manage (in both the acute and chronic phases) diseases of Mobility that span from neurologic, metabolic and traumatic syndromes to the biological process of aging. One of the typical aim of Physical Medicine and Rehabilitation is indeed to reduce pain and increase mobility enough to enable impaired persons to walk freely, garden, and drive again. The excellent contents of this Collection of Abstracts reflect the high scientific caliber of researchers and clinicians who are eager to present their results at the PaduaMuscleDays. A series of EJTM Communications will also add to this preliminary evidence.

The motor unit and quantitative electromyography

Electromyography (EMG) assesses the anatomic motor unit (A-MU), but knowledge of its anatomy, physiology, and changes with pathology is limited. The electrophysiological motor unit (E-MU) and its motor unit potential (E-MUP) represents a fraction of the A-MU. Routine EMG assesses a limited number of E-MUP waveform characteristics (metrics) and their magnitudes qualitatively scaled in a nonlinear manner. Another approach is quantitative EMG (QEMG), whereby 20⁺ E-MUPs are extracted and both basic and derived metrics obtained and values expressed quantitatively. In diseased muscle, many E-MUP metrics may be normal, which complicates diagnostic interpretation. In QEMG, E-MUP metrics can be clustered and statistical analyses performed to assign probabilities that E-MUPs (and the muscle) are normal, neuropathic, or myopathic. In this article we review what is known about the A-MU, the restricted E-MU, E-MUP metrics, and what QEMG offers currently and in the future.

Test-retest reliability of near-fibre jiggle in the ulnar intrinsic hand muscles

OBJECTIVE

Near-fibre (NF) jiggle is one method of measuring the shape variability of motor unit potentials (MUPs) from successive firings during voluntary contractions. MUP shape variability has been associated with neuromuscular stability and health. The purpose of this study was to analyze the test-retest reliability of NF jiggle in the ulnar nerve innervated intrinsic hand muscles of healthy subjects.

METHODS

Twenty healthy adult were tested (Mean age = 23.2 ± 1.9; 8 females). Measurements of NF jiggle were assessed with a standard concentric needle during mild-moderate contractions from the first dorsal interosseous (FDI), the abductor digiti minimi (ADM), and the forth dorsal interosseous (4DI) muscles. Test-retest reliability were evaluated using intraclass-correlation coefficient (ICC).

RESULTS

NF jiggle showed good test-retest reliability in the FDI, ADM and 4DI muscles with ICC values of 0.86, 0.85, and 0.87, respectively. The SEM for the FDI, ADM, and 4DI were 1.9%, 2.1%, and 2.5%. Finally, the MDC of the FDI, ADM and 4DI were 4.4%, 5.0%, and 7.1%.

CONCLUSION

To date, this is the first investigation to explore NF jiggle in the intrinsic hand muscles. NF Jiggle demonstrates good test-retest reliability coefficients and with low measurement error.

Human motor unit characteristics of the superior trapezius muscle with age-related comparisons

Current understanding of human motor unit (MU) control and aging is mostly derived from hand and limb muscles that have spinal motor neuron innervations. The aim here was to characterize and test whether a muscle with a shared innervation supply from brainstem and spinal MU populations would demonstrate similar age-related adaptations as those reported for other muscles. In humans, the superior trapezius (ST) muscle acts to elevate and stabilize the scapula and has primary efferent supply from the spinal accessory nerve (cranial nerve XI) located in the brainstem. We compared electrophysiological properties obtained from intramuscular and surface recordings between 10 young (22-33 yr) and 10 old (77-88 yr) men at a range of voluntary isometric contraction intensities (from 15 to 100% of maximal efforts). The old group was 41% weaker with 43% lower MU discharge frequencies compared with the young (47.2 ± 9.6 Hz young and 26.7 ± 5.8 Hz old, P < 0.05) during maximal efforts. There was no difference in MU number estimation between age groups (228 ± 105 young and 209 ± 89 old, P = 0.33). Furthermore, there were no differences in needle detected near fiber (NF) stability parameters of jitter or jiggle. The old group had lower amplitude and smaller area of the stimulated compound muscle action potential and smaller NF MU potential area with higher NF counts. Thus, despite age-related ST weakness and lower MU discharge rates, there was minimal evidence of MU loss or compensatory reinnervation.NEW & NOTEWORTHY The human superior trapezius (ST) has shared spinal and brainstem motor neuron innervation providing a unique model to explore the impact of aging on motor unit (MU) properties. Although the ST showed higher MU discharge rates compared with most spinally innervated muscles, voluntary strength and mean MU rates were lower in old compared with young at all contraction intensities. There was no age-related difference in MU number estimates with minimal electrophysiological evidence of collateral reinnervation.

Reduced corticospinal responses in older compared with younger adults during submaximal isometric, shortening, and lengthening contractions

The aim of this study was to assess differences in motor performance, as well as corticospinal and spinal responses to transcranial magnetic and percutaneous nerve stimulation, respectively, during submaximal isometric, shortening, and lengthening contractions between younger and older adults. Fifteen younger [26 yr (SD 4); 7 women, 8 men] and 14 older [64 yr (SD 3); 5 women, 9 men] adults performed isometric and shortening and lengthening dorsiflexion on an isokinetic dynamometer (5°/s) at 25% and 50% of contraction type-specific maximums. Motor evoked potentials (MEPs) and H reflexes were recorded at anatomical zero. Maximal dorsiflexor torque was greater during lengthening compared with shortening and isometric contractions ( P < 0.001) but was not age dependent ( P = 0.158). However, torque variability was greater in older compared with young adults ( P < 0.001). Background electromyographic (EMG) activity was greater in older compared with younger adults ( P < 0.005) and was contraction type dependent ( P < 0.001). As evoked responses are influenced by both the maximal level of excitation and background EMG activity, the responses were additionally normalized {[MEP/maximum M wave (M_max)]/root-mean-square EMG activity (RMS) and [H reflex (H)/M_max]/RMS}. (MEP/M_max)/RMS and (H/M_max)/RMS were similar across contraction types but were greater in young compared with older adults ( P < 0.001). Peripheral motor conduction times were prolonged in older adults ( P = 0.003), whereas peripheral sensory conduction times and central motor conduction times were not age dependent ( P ≥ 0.356). These data suggest that age-related changes throughout the central nervous system serve to accommodate contraction type-specific motor control. Moreover, a reduction in corticospinal responses and increased torque variability seem to occur without a significant reduction in maximal torque-producing capacity during older age. NEW & NOTEWORTHY This is the first study to have explored corticospinal and spinal responses with aging during submaximal contractions of different types (isometric, shortening, and lengthening) in lower limb musculature. It is demonstrated that despite preserved maximal torque production capacity corticospinal responses are reduced in older compared with younger adults across contraction types along with increased torque variability during dynamic contractions. This suggests that the age-related corticospinal changes serve to accommodate contraction type-specific motor control.

Failure to expand the motor unit size to compensate for declining motor unit numbers distinguishes sarcopenic from non-sarcopenic older men

Key points

The age‐related loss of muscle mass is related to the loss of innervating motor neurons and denervation of muscle fibres.

Not all denervated muscle fibres are degraded; some may be reinnervated by an adjacent surviving neuron, which expands the innervating motor unit proportional to the numbers of fibres rescued.

Enlarged motor units have larger motor unit potentials when measured using electrophysiological techniques.

We recorded much larger motor unit potentials in relatively healthy older men compared to young men, but the older men with the smallest muscles (sarcopenia) had smaller motor unit potentials than healthy older men.

These findings suggest that healthy older men reinnervate large numbers of muscle fibres to compensate for declining motor neuron numbers, but a failure to do so contributes to muscle loss in sarcopenic men.

Abstract

Sarcopenia results from the progressive loss of skeletal muscle mass and reduced function in older age. It is likely to be associated with the well‐documented reduction of motor unit numbers innervating limb muscles and the increase in size of surviving motor units via reinnervation of denervated fibres. However, no evidence exists to confirm the extent of motor unit remodelling in sarcopenic individuals. The aim of the present study was to compare motor unit size and number between young (n = 48), non‐sarcopenic old (n = 13), pre‐sarcopenic (n = 53) and sarcopenic (n = 29) men. Motor unit potentials (MUPs) were isolated from intramuscular and surface EMG recordings. The motor unit numbers were reduced in all groups of old compared with young men (all P < 0.001). MUPs were higher in non‐sarcopenic and pre‐sarcopenic men compared with young men (P = 0.039 and 0.001 respectively), but not in the vastus lateralis of sarcopenic old (P = 0.485). The results suggest that extensive motor unit remodelling occurs relatively early during ageing, exceeds the loss of muscle mass and precedes sarcopenia. Reinnervation of denervated muscle fibres probably expands the motor unit size in the non‐sarcopenic and pre‐sarcopenic old, but not in the sarcopenic old. These findings suggest that a failure to expand the motor unit size distinguishes sarcopenic from pre‐sarcopenic muscles.

The age‐related loss of muscle mass is related to the loss of innervating motor neurons and denervation of muscle fibres.

Not all denervated muscle fibres are degraded; some may be reinnervated by an adjacent surviving neuron, which expands the innervating motor unit proportional to the numbers of fibres rescued.

Enlarged motor units have larger motor unit potentials when measured using electrophysiological techniques.

We recorded much larger motor unit potentials in relatively healthy older men compared to young men, but the older men with the smallest muscles (sarcopenia) had smaller motor unit potentials than healthy older men.

These findings suggest that healthy older men reinnervate large numbers of muscle fibres to compensate for declining motor neuron numbers, but a failure to do so contributes to muscle loss in sarcopenic men.

Skeletal muscle performance and ageing

The world population is ageing rapidly. As society ages, the incidence of physical limitations is dramatically increasing, which reduces the quality of life and increases healthcare expenditures. In western society, ~30% of the population over 55 years is confronted with moderate or severe physical limitations. These physical limitations increase the risk of falls, institutionalization, co-morbidity, and premature death. An important cause of physical limitations is the age-related loss of skeletal muscle mass, also referred to as sarcopenia. Emerging evidence, however, clearly shows that the decline in skeletal muscle mass is not the sole contributor to the decline in physical performance. For instance, the loss of muscle strength is also a strong contributor to reduced physical performance in the elderly. In addition, there is ample data to suggest that motor coordination, excitation-contraction coupling, skeletal integrity, and other factors related to the nervous, muscular, and skeletal systems are critically important for physical performance in the elderly. To better understand the loss of skeletal muscle performance with ageing, we aim to provide a broad overview on the underlying mechanisms associated with elderly skeletal muscle performance. We start with a system level discussion and continue with a discussion on the influence of lifestyle, biological, and psychosocial factors on elderly skeletal muscle performance. Developing a broad understanding of the many factors affecting elderly skeletal muscle performance has major implications for scientists, clinicians, and health professionals who are developing therapeutic interventions aiming to enhance muscle function and/or prevent mobility and physical limitations and, as such, support healthy ageing.

Aging Induces Changes in the Somatic Nerve and Postsynaptic Component without Any Alterations in Skeletal Muscles Morphology and Capacity to Carry Load of Wistar Rats

The present study aimed to analyze the morphology of the peripheral nerve, postsynaptic compartment, skeletal muscles and weight-bearing capacity of Wistar rats at specific ages. Twenty rats were divided into groups: 10 months-old (ADULT) and 24 months-old (OLD). After euthanasia, we prepared and analyzed the tibial nerve using transmission electron microscopy and the soleus and plantaris muscles for cytofluorescence and histochemistry. For the comparison of the results between groups we used dependent and independent Student's t-test with level of significance set at p ≤ 0.05. For the tibial nerve, the OLD group presented the following alterations compared to the ADULT group: larger area and diameter of both myelinated fibers and axons, smaller area occupied by myelinated and unmyelinated axons, lower numerical density of myelinated fibers, and fewer myelinated fibers with normal morphology. Both aged soleus and plantaris end-plate showed greater total perimeter, stained perimeter, total area and stained area compared to ADULT group (p < 0.05). Yet, aged soleus end-plate presented greater dispersion than ADULT samples (p < 0.05). For the morphology of soleus and plantaris muscles, density of the interstitial volume was greater in the OLD group (p < 0.05). No statistical difference was found between groups in the weight-bearing tests. The results of the present study demonstrated that the aging process induces changes in the peripheral nerve and postsynaptic compartment without any change in skeletal muscles and ability to carry load in Wistar rats.

Electrical impedance myography detects age-related muscle change in mice

Loss of muscle mass and strength represents one of the most significant contributors to impaired function in older adults. Convenient and non-invasive biomarkers are needed that can readily identify and track age-related muscle change. Previous data has suggested electrical impedance myography (EIM) has the potential to serve in this capacity. In this study we investigated how changes in EIM compared with other standard measures of muscle structure and function in aged compared with young mice. A total of 19 male mice aged approximately 25 months and 19 male mice aged 3 months underwent surface multifrequency EIM of the right gastrocnemius muscle using standard methods. Fore and hind limb grip strength, sciatic compound muscle action potential amplitude, and in-situ force of the gastrocnemius were also measured; after sacrifice, gastrocnemius myofiber size was assessed using standard histology. Spearman correlation coefficients were calculated to investigate the association between EIM and muscle characteristics. EIM in aged animals demonstrated significantly lower 50 kHz impedance phase (p<0.001) and reactance (p<0.01) values as well as reduced multifrequency parameters. In contrast, absolute gastrocnemius muscle mass was no different between young and aged mice (p = 0.58) but was reduced in aged mice after normalization to body mass (p<0.001). Median myofiber size in the aged mice was not different from that of young mice (p = 0.72). Aged mice showed reduced muscle function on the basis of normalized fore limb (p<0.001) and normalized hind limb (p<0.001) grip strength, as well as normalized gastrocnemius twitch (p<0.001) and normalized maximal isometric force (p<0.001). Sciatic compound muscle action potential amplitude was reduced in aged mice (p<0.05). EIM parameters showed good correlation with reduced standard physiological and electrophysiological measures of muscle health. Our study suggests that EIM is sensitive to aged-related muscle change and may represent a convenient and valuable method of quantifying loss of muscle health.

Mitochondria Initiate and Regulate Sarcopenia

We present the hypothesis that an accumulation of dysfunctional mitochondria initiates a signaling cascade leading to motor neuron and muscle fiber death and culminating in sarcopenia. Interactions between neural and muscle cells that contain dysfunctional mitochondria exacerbate sarcopenia. Preventing sarcopenia will require identifying mitochondrial sources of dysfunction that are reversible.