Increased neuromuscular transmission instability and motor unit remodelling with diabetic neuropathy as assessed using novel near fibre motor unit potential parameters

Can non-invasive motor unit analysis reveal distinct neural strategies of force production in young with uncomplicated type 1 diabetes?

PURPOSE

to investigate the early consequences of type 1 diabetes (T1D) on the neural strategies of muscle force production.

METHODS

motor unit (MU) activity was recorded from the vastus lateralis muscle with High-Density surface Electromyography during isometric knee extension at 20 and 40% of maximum voluntary contraction (MVC) in 8 T1D (4 males, 4 females, 30.5 ± 3.6 years) and 8 matched control (4 males, 4 females, 27.3 ± 5.9 years) participants. Muscle biopsies were also collected from vastus lateralis for fiber type analysis, including myosin heavy chain (MyHC) isoform content via protein and mRNA expression.

RESULTS

MVC was comparable between groups as well as MU conduction velocity, action potentials' amplitude and proportions of MyHC protein isoforms. Nonetheless, MU discharge rate, relative derecruitment thresholds and mRNA expression of MyHC isoform I were lower in T1D.

CONCLUSIONS

young people with uncomplicated T1D present a different neural control of muscle force production. Furthermore, differences are detectable non-invasively in absence of any functional manifestation (i.e., force production and fiber type distribution). These novel findings suggest that T1D has early consequences on the neuromuscular system and highlights the necessity of a better characterization of neural control in this population.

Neuromuscular junction transmission failure in aging and sarcopenia: The nexus of the neurological and muscular systems

Sarcopenia, or age-related decline in muscle form and function, exerts high personal, societal, and economic burdens when untreated. Integrity and function of the neuromuscular junction (NMJ), as the nexus between the nervous and muscular systems, is critical for input and dependable neural control of muscle force generation. As such, the NMJ has long been a site of keen interest in the context of skeletal muscle function deficits during aging and in the context of sarcopenia. Historically, changes of NMJ morphology during aging have been investigated extensively but primarily in aged rodent models. Aged rodents have consistently shown features of NMJ endplate fragmentation and denervation. Yet, the presence of NMJ changes in older humans remains controversial, and conflicting findings have been reported. This review article describes the physiological processes involved in NMJ transmission, discusses the evidence that supports NMJ transmission failure as a possible contributor to sarcopenia, and speculates on the potential of targeting these defects for therapeutic development. The technical approaches that are available for assessment of NMJ transmission, whether each approach has been applied in the context of aging and sarcopenia, and the associated findings are summarized. Like morphological studies, age-related NMJ transmission deficits have primarily been studied in rodents. In preclinical studies, isolated synaptic electrophysiology recordings of endplate currents or potentials have been mostly used, and paradoxically, have shown enhancement, rather than failure, with aging. Yet, in vivo assessment of single muscle fiber action potential generation using single fiber electromyography and nerve-stimulated muscle force measurements show evidence of NMJ failure in aged mice and rats. Together these findings suggest that endplate response enhancement may be a compensatory response to post-synaptic mechanisms of NMJ transmission failure in aged rodents. Possible, but underexplored, mechanisms of this failure are discussed including the simplification of post-synaptic folding and altered voltage-gated sodium channel clustering or function. In humans, there is limited clinical data that has selectively investigated single synaptic function in the context of aging. If sarcopenic older adults turn out to exhibit notable impairments in NMJ transmission (this has yet to be examined but based on available evidence appears to be plausible) then these NMJ transmission defects present a well-defined biological mechanism and offer a well-defined pathway for clinical implementation. Investigation of small molecules that are currently available clinically or being testing clinically in other disorders may provide a rapid route for development of interventions for older adults impacted by sarcopenia.

Acute adaptation of central and peripheral motor unit features to exercise-induced fatigue differs with concentric and eccentric loading

NEW FINDINGS

What is the central question of this study? Conflicting evidence exists on motor unit (MU) firing rate in response to exercise-induced fatigue, possibly due to the contraction modality used: Do MU properties adapt similarly following concentric and eccentric loading? What is the main finding and its importance? MU firing rate increased following eccentric loading only despite a decline in absolute force. Force steadiness deteriorated following both loading methods. Central and peripheral MU features are altered in a contraction type-dependant manner, which is an important consideration for training interventions.

ABSTRACT

Force output of muscle is partly mediated by the adjustment of motor unit (MU) firing rate (FR). Disparities in MU features in response to fatigue may be influenced by contraction type, as concentric (CON) and eccentric (ECC) contractions demand variable amounts of neural input, which alters the response to fatigue. This study aimed to determine the effects of fatigue following CON and ECC loading on MU features of the vastus lateralis (VL). High-density surface (HD-sEMG) and intramuscular (iEMG) electromyography were used to record MU potentials (MUPs) from bilateral VLs of 12 young volunteers (six females) during sustained isometric contractions at 25% and 40% of the maximum voluntary contraction (MVC), before and after completing CON and ECC weighted stepping exercise. Multi-level mixed effects linear regression models were performed with significance assumed as P < 0.05. MVC decreased in both CON and ECC legs post-exercise (P < 0.0001), as did force steadiness at both 25% and 40% MVC (P < 0.004). MU FR increased in ECC at both contraction levels (P < 0.001) but did not change in CON. FR variability increased in both legs at 25% and 40% MVC following fatigue (P < 0.01). From iEMG measures at 25% MVC, MUP shape did not change (P > 0.1) but neuromuscular junction transmission instability increased in both legs (P < 0.04), and markers of fibre membrane excitability increased following CON only (P = 0.018). These data demonstrate that central and peripheral MU features are altered following exercise-induced fatigue and differ according to exercise modality. This is important when considering interventional strategies targeting MU function.

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.

Differences in motor unit behavior during isometric contractions in patients with diabetic peripheral neuropathy at various disease severities

The aim of this study was to determine whether HD-sEMG is sensitive to detecting changes in motor unit behavior amongst healthy adults and type 2 diabetes mellitus (T2DM) patients presenting diabetic peripheral neuropathy (DPN) at different levels. Healthy control subjects (CON, n = 8) and T2DM patients presenting no DPN symptoms (ABS, n = 8), moderate DPN (MOD, n = 18), and severe DPN (SEV, n = 12) performed isometric ankle dorsiflexion at 30 % maximum voluntary contraction while high-density surface EMG (HD-sEMG) was recorded from the tibialis anterior muscle. HD-sEMG signals were decomposed, providing estimates of discharge rate, motor unit conduction velocity (MUCV), and motor unit territory area (MUTA). As a result, the ABS group presented reduced MUCV compared to CON. The groups with diabetes presented significantly larger MUTA compared to the CON group (p < 0.01), and the SEV group presented a significantly lower discharge rate compared to CON and ABS (p < 0.01). In addition, the SEV group presented significantly higher CoV_force compared to CON and MOD. These results support the use of HD-SEMG as a method to detect peripheral and central changes related to DPN.

Motor unit dysregulation following 15 days of unilateral lower limb immobilisation

Disuse atrophy, caused by situations of unloading such as limb immobilisation, causes a rapid yet diverging reduction in skeletal muscle function when compared to muscle mass. While mechanistic insight into the loss of mass is well studied, deterioration of muscle function with a focus towards the neural input to muscle remains underexplored. This study aimed to determine the role of motor unit adaptation in disuse-induced neuromuscular deficits. Ten young, healthy male volunteers underwent 15 days of unilateral lower limb immobilisation with intramuscular electromyography (iEMG) bilaterally recorded from the vastus lateralis (VL) during knee extensor contractions normalised to maximal voluntary contraction (MVC), pre and post disuse. Muscle cross-sectional area was determined by ultrasound. Individual MUs were sampled and analysed for changes in motor unit (MU) discharge and MU potential (MUP) characteristics. VL CSA was reduced by approximately 15% which was exceeded by a two-fold decrease of 31% in muscle strength in the immobilised limb, with no change in either parameter in the non-immobilised limb. Parameters of MUP size were reduced by 11% to 24% with immobilisation, while neuromuscular junction (NMJ) transmission instability remained unchanged, and MU firing rate decreased by 8% to 11% at several contraction levels. All adaptations were observed in the immobilised limb only. These findings highlight impaired neural input following immobilisation reflected by suppressed MU firing rate which may underpin the disproportionate reductions of strength relative to muscle size. KEY POINTS: Muscle mass and function decline rapidly in situations of disuse such as bed rest and limb immobilisation. The reduction in muscle function commonly exceeds that of muscle mass, which may be associated with the dysregulation of neural input to muscle. We have used intramuscular electromyography to sample individual motor unit and near fibre potentials from the vastus lateralis following 15 days of unilateral limb immobilisation. Following disuse, the disproportionate loss of muscle strength when compared to size coincided with suppressed motor unit firing rate. These motor unit adaptations were observed at multiple contraction levels and in the immobilised limb only. Our findings demonstrate neural dysregulation as a key component of functional loss following muscle disuse in humans.

Effects of short-term unloading and active recovery on human motor unit properties, neuromuscular junction transmission and transcriptomic profile

Electrophysiological alterations of the neuromuscular junction (NMJ) and motor unit potential (MUP) with unloading are poorly studied. We aimed to investigate these aspects and the underlying molecular mechanisms with short-term unloading and active recovery (AR). Eleven healthy males underwent a 10-day unilateral lower limb suspension (ULLS) period, followed by 21-day AR based on resistance exercise. Quadriceps femoris (QF) cross-sectional area (CSA) and isometric maximum voluntary contraction (MVC) were evaluated. Intramuscular electromyographic recordings were obtained during 10% and 25% MVC isometric contractions from the vastus lateralis (VL). Biomarkers of NMJ molecular instability (serum c-terminal agrin fragment, CAF), axonal damage (neurofilament light chain) and denervation status were assessed from blood samples and VL biopsies. NMJ and ion channel transcriptomic profiles were investigated by RNA-sequencing. QF CSA and MVC decreased with ULLS. Increased CAF and altered NMJ transcriptome with unloading suggested the emergence of NMJ molecular instability, which was not associated with impaired NMJ transmission stability. Instead, increased MUP complexity and decreased motor unit firing rates were found after ULLS. Downregulation of ion channel gene expression was found together with increased neurofilament light chain concentration and partial denervation. The AR period restored most of these neuromuscular alterations. In conclusion, the human NMJ is destabilized at the molecular level but shows functional resilience to a 10-day unloading period at least at relatively low contraction intensities. However, MUP properties are altered by ULLS, possibly due to alterations in ion channel dynamics and initial axonal damage and denervation. These changes are fully reversed by 21 days of AR. KEY POINTS: We used integrative electrophysiological and molecular approaches to comprehensively investigate changes in neuromuscular integrity and function after a 10-day unilateral lower limb suspension (ULLS), followed by 21 days of active recovery in young healthy men, with a particular focus on neuromuscular junction (NMJ) and motor unit potential (MUP) properties alterations. After 10-day ULLS, we found significant NMJ molecular alterations in the absence of NMJ transmission stability impairment. These findings suggest that the human NMJ is functionally resilient against insults and stresses induced by short-term disuse at least at relatively low contraction intensities, at which low-threshold, slow-type motor units are recruited. Intramuscular electromyography analysis revealed that unloading caused increased MUP complexity and decreased motor unit firing rates, and these alterations could be related to the observed changes in skeletal muscle ion channel pool and initial and partial signs of fibre denervation and axonal damage. The active recovery period restored these neuromuscular changes.

Neuromuscular recruitment strategies of the vastus lateralis according to sex

AIM

Despite males typically exhibiting greater muscle strength and fatigability than females, it remains unclear if there are sex-based differences in neuromuscular recruitment strategies e.g. recruitment and modulation of motor unit firing rate (MU FR) at normalized forces and during progressive increases in force.

METHODS

The study includes 29 healthy male and 31 healthy female participants (18-35 years). Intramuscular electromyography (iEMG) was used to record individual motor unit potentials (MUPs) and near-fibre MUPs from the vastus lateralis (VL) during 10% and 25% maximum isometric voluntary contractions (MVC), and spike-triggered averaging was used to obtain motor unit number estimates (MUNE) of the VL.

RESULTS

Males exhibited greater muscle strength (P < .001) and size (P < .001) than females, with no difference in force steadiness at 10% or 25% MVC. Females had 8.4% and 6.5% higher FR at 10% and 25% MVC, respectively (both P < .03), while the MUP area was 33% smaller in females at 10% MVC (P < .02) and 26% smaller at 25% MVC (P = .062). However, both sexes showed similar increases in MU size and FR when moving from low- to mid-level contractions. There were no sex differences in any near-fibre MUP parameters or in MUNE.

CONCLUSION

In the vastus lateralis, females produce muscle force via different neuromuscular recruitment strategies to males which is characterized by smaller MUs discharging at higher rates. However, similar strategies are employed to increase force production from low- to mid-level contractions. These findings of similar proportional increases between sexes support the use of mixed sex cohorts in studies of this nature.

Effect of nitrate supplementation on skeletal muscle motor unit activity during isometric blood flow restriction exercise

Background

Nitrate (NO₃⁻) supplementation has been reported to lower motor unit (MU) firing rate (MUFR) during dynamic resistance exercise; however, its impact on MU activity during isometric and ischemic exercise is unknown.

Purpose

To assess the effect of NO₃⁻ supplementation on knee extensor MU activities during brief isometric contractions and a 3 min sustained contraction with blood flow restriction (BFR).

Methods

Sixteen healthy active young adults (six females) completed two trials in a randomized, double-blind, crossover design. Trials were preceded by 5 days of either NO₃⁻ (NIT) or placebo (PLA) supplementation. Intramuscular electromyography was used to determine the M. vastus lateralis MU potential (MUP) size, MUFR and near fibre (NF) jiggle (a measure of neuromuscular stability) during brief (20 s) isometric contractions at 25% maximal strength and throughout a 3 min sustained BFR isometric contraction.

Results

Plasma nitrite (NO₂⁻) concentration was elevated after NIT compared to PLA (475 ± 93 vs. 198 ± 46 nmol L⁻¹, p < 0.001). While changes in MUP area, NF jiggle and MUFR were similar between NIT and PLA trials (all p > 0.05), MUP duration was shorter with NIT compared to PLA during brief isometric contractions and the sustained ischemic contraction (p < 0.01). In addition, mean MUP duration, MUP area and NF jiggle increased, and MUFR decreased over the 3 min sustained BFR isometric contraction for both conditions (all p < 0.05).

Conclusions

These findings provide insight into the effect of NO₃⁻ supplementation on MUP properties and reveal faster MUP duration after short-term NO₃⁻ supplementation which may have positive implications for skeletal muscle contractile performance.

Myelination, axonal loss and Schwann cell characteristics in axonal polyneuropathy compared to controls

Introduction

Polyneuropathy is a debilitating condition characterized by distal sensory and motor deficits. Schwann cell dysfunction and axonal loss are integral factors in pathophysiology and disease progression of polyneuropathy.

Aims

The aim of this study was the assessment of Schwann cell characteristics, nerve fibers and myelination parameters in polyneuropathy patients compared to controls.

Methods

Nerve tissue was obtained from polyneuropathy patients (n = 10) undergoing diagnostic sural nerve biopsies. Biopsies of healthy peripheral nerves (n = 5) were harvested during elective sural nerve grafting for chronic peripheral nerve lesions. Exclusion criteria for the healthy control group were recent neurological trauma, diabetes, neurological and cardiovascular disease, as well as active malignancies and cytotoxic medication within the last 12 months. The over-all architecture of nerve sections and myelination parameters were histomorphometrically analyzed. Immunofluorescent imaging was used to evaluate Schwann cell phenotypes, senescence markers and myelination parameters.

Results

Histomorphometric analysis of nerve biopsies showed significant axonal loss in polyneuropathy patients compared to controls, which was in accordance with the neuropathological findings. Immunofluorescent staining of Schwann cells and myelin basic protein indicated a significant impairment of myelination and lower Schwann cell counts compared to controls. Phenotypic alterations and increased numbers of non-myelinating p75-positive Schwann cells were found in polyneuropathy patients.

Discussion

This study provided quantitative data of axonal loss, reduced myelination and Schwann cell dysfunction of polyneuropathy patients compared to neurologically healthy controls. Phenotypic alterations of Schwann cells were similar to those seen after peripheral nerve injury, highlighting the clinical relevance of Schwann cell dysfunction.

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.

Feasibility of a home-based foot-ankle exercise programme for musculoskeletal dysfunctions in people with diabetes: randomised controlled FOotCAre (FOCA) Trial II

This study sought to assess the feasibility of design, adherence, satisfaction, safety and changes in outcomes followed by a home-based foot–ankle exercise guided by a booklet in individuals with diabetic peripheral neuropathy (DPN). 20 participants were allocated usual care [control group (CG)] or usual care plus home-based foot–ankle exercises [intervention group (IG)] for 8 weeks. For feasibility, we assessed contact, preliminary screening and recruitment rates, adherence, and using a 5-point Likert scale to satisfaction and safety of the booklet. In the IG, we assessed preliminary changes in DPN symptoms, DPN severity (classified by a fuzzy model) and foot–ankle range of motion between baseline and Week 8. In the first 20 weeks, 1310 individuals were screened for eligibility by phone contact. Contact rate was 89% (contacted participants/20w), preliminary screening success 28% (participants underwent screening/20w), and recruitment rate 1.0 participants/week (eligible participants/20w). The recruitment rate was less than the ideal rate of 5 participants/week. The adherence to the exercises programme was 77%, and the dropout was 11% and 9% for the IG and CG, respectively. In the IG, participants’ median level of satisfaction was 4 (IQR: 4–5) and perceived safety was 3 (IQR: 3–5). IG significantly decreased the DPN severity (p = 0.020), increased hallux relative to forefoot (first metatarsal) range of motion (ROM) (p < 0.001) and decreased maximum forefoot relative to hindfoot (midfoot motion) dorsiflexion during gait (p = 0.029). The home-based programme was feasible, satisfactory, safe and showed preliminary positive changes in DPN severity and foot motion during gait.

Trial Registration ClinicalTrials.gov, NCT04008745. Registered 02/07/2019. https://clinicaltrials.gov/ct2/show/NCT04008745.

Greater fatigability and motor unit discharge variability in human type 2 diabetes

This study determined the discharge characteristics of motor units from two lower limb muscles before and after fatiguing exercise in people with type 2 diabetes (T2D) with no symptoms of polyneuropathy and activity‐matched controls. Seventeen people with T2D (65.0 ± 5.6 years; 8 women) and 17 controls (63.6 ± 4.5 years; 8 women) performed: (a) intermittent, isometric contractions at 50% maximal voluntary isometric contraction (MVIC) sustained to failure with the ankle dorsiflexors, and (b) a dynamic fatiguing task (30% MVIC load) for 6 min with the knee extensors. Before and after the fatiguing tasks, motor unit characteristics (including coefficient of variation (CV) of interspike intervals (ISI)) were quantified from high‐density electromyography and muscle contractile properties were assessed via electrical stimulation. Fatigability was ~50% greater for people with T2D than controls for the dorsiflexors (time‐to‐failure: 7.3 ± 4.1 vs. 14.3 ± 9.1 min, p = .010) and knee extensors (power reduction: 56.7 ± 11.9 vs. 31.5 ± 25.5%, p < .001). The CV of ISI was greater for the T2D than control group for the tibialis anterior (23.1 ± 11.0 vs. 21.3 ± 10.7%, p < .001) and vastus lateralis (27.8 ± 20.2 vs. 24.5 ± 16.1%, p = .011), but these differences did not change after the fatiguing exercises. People with T2D had greater reductions in the electrically evoked twitch amplitude of the dorsiflexors (8.5 ± 5.1 vs. 4.0 ± 3.4%·min^‐1, p = .013) and knee extensors (49.1 ± 10.0 vs. 31.8 ± 15.9%, p = .004) than controls. Although motor unit activity was more variable in people with T2D than controls, the greater fatigability of the T2D group for lower limb muscles was due to mechanisms involving disruption of contractile function of the exercising muscles rather than motor unit behavior.

Sensory-Motor Mechanisms Increasing Falls Risk in Diabetic Peripheral Neuropathy

Diabetic peripheral neuropathy (DPN) is associated with peripheral sensory and motor nerve damage that affects up to half of diabetes patients and is an independent risk factor for falls. Clinical implications of DPN-related falls include injury, psychological distress and physical activity curtailment. This review describes how the sensory and motor deficits associated with DPN underpin biomechanical alterations to the pattern of walking (gait), which contribute to balance impairments underpinning falls. Changes to gait with diabetes occur even before the onset of measurable DPN, but changes become much more marked with DPN. Gait impairments with diabetes and DPN include alterations to walking speed, step length, step width and joint ranges of motion. These alterations also impact the rotational forces around joints known as joint moments, which are reduced as part of a natural strategy to lower the muscular demands of gait to compensate for lower strength capacities due to diabetes and DPN. Muscle weakness and atrophy are most striking in patients with DPN, but also present in non-neuropathic diabetes patients, affecting not only distal muscles of the foot and ankle, but also proximal thigh muscles. Insensate feet with DPN cause a delayed neuromuscular response immediately following foot–ground contact during gait and this is a major factor contributing to increased falls risk. Pronounced balance impairments measured in the gait laboratory are only seen in DPN patients and not non-neuropathic diabetes patients. Self-perception of unsteadiness matches gait laboratory measures and can distinguish between patients with and without DPN. Diabetic foot ulcers and their associated risk factors including insensate feet with DPN and offloading devices further increase falls risk. Falls prevention strategies based on sensory and motor mechanisms should target those most at risk of falls with DPN, with further research needed to optimise interventions.

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.

Influence of sex on the age-related adaptations of neuromuscular function and motor unit properties in elite masters athletes

KEY POINTS

Masters athletes maintain high levels of activity into older age and allow an examination of the effects of aging dissociated from the effects of increased sedentary behaviour. Evidence suggests masters athletes are more successful at motor unit remodelling, the reinnervation of denervated fibres acting to preserve muscle fibre number, but little data are available in females. Here we used intramuscular electromyography to demonstrate that motor units sampled from the tibialis anterior show indications of remodelling from middle into older age and which does not differ between males and females. The age-related trajectory of motor unit discharge characteristic differs according to sex, with female athletes progressing to a slower firing pattern that was not observed in males. Our findings indicate motor unit remodelling from middle to older age occurs to a similar extent in male and female athletes, with discharge rates progressively slowing in females only.

ABSTRACT

Motor unit (MU) remodelling acts to minimise loss of muscle fibres following denervation in older age, which may be more successful in masters athletes. Evidence suggests performance and neuromuscular function decline with age in this population, although the majority of studies have focused on males, with little available data on female athletes. Functional assessments of strength, balance and motor control were performed in 30 masters athletes (16 male) aged 44-83 years. Intramuscular needle electrodes were used to sample individual motor unit potentials (MUPs) and near-fibre MUPs in the tibialis anterior (TA) during isometric contractions at 25% maximum voluntary contraction, and used to determine discharge characteristics (firing rate, variability) and biomarkers of peripheral MU remodelling (MUP size, complexity, stability). Multilevel mixed-effects linear regression models examined effects of age and sex. All aspects of neuromuscular function deteriorated with age (P < 0.05) with no age × sex interactions, although males were stronger (P < 0.001). Indicators of MU remodelling also progressively increased with age to a similar extent in both sexes (P < 0.05), whilst MU firing rate progressively decreased with age in females (p = 0.029), with a non-significant increase in males (p = 0.092). Masters athletes exhibit age-related declines in neuromuscular function that are largely equal across males and females. Notably, they also display features of MU remodelling with advancing age, probably acting to reduce muscle fibre loss. The age trajectory of MU firing rate assessed at a single contraction level differed between sexes, which may reflect a greater tendency for females to develop a slower muscle phenotype.

The impact of sensory and/or sensorimotor neuropathy on lower limb muscle endurance, explosive and maximal muscle strength in patients with type 2 diabetes mellitus

AIMS

The purpose of this study was to investigate the impact of diabetic neuropathy (dNP) on lower limb endurance, explosive and maximal muscle strength in patients with Type 2 Diabetes Mellitus (T2DM).

METHODS

Fifty-four participants, aged between 55 and 85, were enrolled in this observational comparative study. The patients with T2DM had an average HbA1c of 7.4% (±1.03) and diabetes duration of 13 years. Participants were classified by means of electroneuromyography as T2DM without dNP (dNP-; n = 8), T2DM with sensory dNP (dNPs; n = 13), T2DM with sensorimotor dNP (dNPsm; n = 14), and healthy controls without neuropathy (C; n = 19). Maximal muscle strength and muscle endurance of the dominant knee and ankle were measured by dynamometry, while explosive muscle strength was evaluated by mechanography.

RESULTS

Muscle endurance "total work" in knee extension and ankle plantar flexion was higher in the healthy controls compared to dNP-, dNPs and dNPsm, in knee flexion compared to dNPs and dNPsm, and in ankle dorsiflexion compared to dNPsm only (p<0.05). Furthermore, relative explosive muscle strength "total power/body weight" and relative maximal muscle strength "peak torque/lean body mass of the dominant leg" considering knee flexion, ankle plantar flexion and dorsiflexion, were higher in healthy controls compared to the dNPsm group, and for maximal muscle strength ankle dorsiflexion even between dNP- and dNPsm (p < 0.05).

CONCLUSIONS

Muscle endurance is impaired in patients with T2DM, independent of the presence of dNP. Explosive and maximal muscle strength are more likely affected by the presence and severity of dNP.

Effect of an educational booklet for prevention and treatment of foot musculoskeletal dysfunctions in people with diabetic neuropathy: the FOotCAre (FOCA) trial II, a study protocol of a randomized controlled trial

Background

This study is a part of a series of two clinical trials. We consider diabetic polyneuropathy (DPN), a common chronic and progressive complication of diabetes mellitus that has several impacts on individuals’ foot health and quality of life. Based on the current trends of self-monitoring and self-care, providing a tool with foot-related exercises and educational care may help patients to avoid or reduce the musculoskeletal complications resulting from DPN, improving autonomous performance in daily living tasks. The aim of this trial is to evaluate the effects of an educational booklet for foot care and foot muscle strengthening on DPN symptoms and severity, clinical outcomes, and gait biomechanics in patients with DPN.

Methods/design

The FOotCAre (FOCA) trial II study has been designed as a single-blind, two-parallel-arm randomized controlled trial. It will include 48 patients with DPN who will be randomly allocated to a control (recommended foot care by international consensus with no foot exercises) group or an intervention (foot-related exercises using an educational booklet three times/week at home for 8 weeks) group. Participants from both groups will be assessed at baseline, after 8 weeks, and at 16 weeks for follow-up. The primary outcomes are the DPN symptoms and severity, and the secondary outcomes are foot–ankle kinematics, gait kinetics, plantar pressure distribution during gait, tactile and vibratory sensitivities, foot strength, functional balance, and foot health and functionality.

Discussion

The booklet is a management tool that allows users to be autonomous in their treatment by choosing how and where to perform the exercises. This allows the patients to perform the exercises regularly as a continuous habit for foot care and health, which is an important element in the management of the diabetic foot. As the booklet focuses on specific foot–ankle exercises, we expect that it will improve the clinical aspects of DPN and produce beneficial biomechanical changes during gait, becoming a powerful self-management tool that can be easily implemented to improve the performance of daily living tasks.

Trial registration

ClinicalTrials.gov, NCT04008745. Registered on 2 July 2019.

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.

The reliability of methods to estimate the number and size of human motor units and their use with large limb muscles

Purpose

Current methods for estimating muscle motor unit (MU) number provide values which are remarkably similar for muscles of widely differing size, probably because surface electrodes sample from similar and relatively small volumes in each muscle. We have evaluated an alternative means of estimating MU number that takes into account differences in muscle size.

Methods

Intramuscular motor unit potentials (MUPs) were recorded and muscle cross-sectional area (CSA) was measured using MRI to provide a motor unit number estimate (iMUNE). This was compared to the traditional MUNE method, using compound muscle action potentials (CMAP) and surface motor unit potentials (sMUPs) recorded using surface electrodes. Data were collected from proximal and distal regions of the vastus lateralis (VL) in young and old men while test–retest reliability was evaluated with VL, tibialis anterior and biceps brachii.

Results

MUPs, sMUPs and CMAPs were highly reliable (r = 0.84–0.91). The traditional MUNE, based on surface recordings, did not differ between proximal and distal sites of the VL despite the proximal CSA being twice the distal CSA. iMUNE, however, gave values that differed between young and old and were proportional to the muscle size.

Conclusion

When evaluating the contribution that MU loss makes to muscle atrophy, such as in disease or ageing, it is important to have a method such as iMUNE, which takes into account any differences in total muscle size.

Role of Oxidative Stress as Key Regulator of Muscle Wasting during Cachexia

Skeletal muscle atrophy is a pathological condition mainly characterized by a loss of muscular mass and the contractile capacity of the skeletal muscle as a consequence of muscular weakness and decreased force generation. Cachexia is defined as a pathological condition secondary to illness characterized by the progressive loss of muscle mass with or without loss of fat mass and with concomitant diminution of muscle strength. The molecular mechanisms involved in cachexia include oxidative stress, protein synthesis/degradation imbalance, autophagy deregulation, increased myonuclear apoptosis, and mitochondrial dysfunction. Oxidative stress is one of the most common mechanisms of cachexia caused by different factors. It results in increased ROS levels, increased oxidation-dependent protein modification, and decreased antioxidant system functions. In this review, we will describe the importance of oxidative stress in skeletal muscles, its sources, and how it can regulate protein synthesis/degradation imbalance, autophagy deregulation, increased myonuclear apoptosis, and mitochondrial dysfunction involved in cachexia.

The Association between Glucose Control and Functional Indices in Older People with Diabetes

Diabetes may be viewed as a disease of accelerated aging as it is a risk factor for physical disability and impairment in simple and complex activities, and is a higher risk for falls and fractures. Data from the last several years suggests that this increased risk is due not only to recognized diabetes complications but also to an accelerated decline in physical capacity due to lower muscle quality and a more rapid decline in muscle mass and lower extremity strength over time. Aim. To find the association between glucose control and functional indices. Methods. A cross-sectional study conducted at the Center for Successful Aging with Diabetes at the Sheba Medical Center. Individuals with a diagnosis of type 2 diabetes over the age of 60 were included. Functional status was assessed using tools that assess aerobic, strength, and balance capacities. Medical assessment was conducted through interview, physical examination, and collection of information from medical records. The association between functional indices and A1C was assessed using linear regression. Results. 153 consecutive individuals were evaluated. There was an inverse association between A1C and score achieved on the 6-minute walk; with increasing meters walked on the 6-minute walk test, there was a reduction in A1C (p = 0.003). There was also an inverse association with the 10-meter walk (p = 0.007), BERG balance test (p = 0.0006), and timed up and go (p = 0.01). Conclusion. In this cohort of older people with diabetes, there was an association between A1C and measures of functional indices. Future studies of glucose-lowering medication should include physical and functional assessment.

Declining Skeletal Muscle Function in Diabetic Peripheral Neuropathy

PURPOSE

The present review highlights current concepts regarding the effects of diabetic peripheral neuropathy (DPN) in skeletal muscle. It discusses the lack of effective pharmacologic treatments and the role of physical exercise intervention in limb protection and symptom reversal. It also highlights the importance of magnetic resonance imaging (MRI) techniques in providing a mechanistic understanding of the disease and helping develop targeted treatments.

METHODS

This review provides a comprehensive reporting on the effects of DPN in the skeletal muscle of patients with diabetes. It also provides an update on the most recent trials of exercise intervention targeting DPN pathology. Lastly, we report on emerging MRI techniques that have shown promise in providing a mechanistic understanding of DPN and can help improve the design and implementation of clinical trials in the future.

FINDINGS

Impairments in lower limb muscles reduce functional capacity and contribute to altered gait, increased fall risk, and impaired balance in patients with DPN. This finding is an important concern for patients with DPN because their falls are likely to be injurious and lead to bone fractures, poorly healing wounds, and chronic infections that may require amputation. Preliminary studies have shown that moderate-intensity exercise programs are well tolerated by patients with DPN. They can improve their cardiorespiratory function and partially reverse some of the symptoms of DPN. MRI has the potential to bring new mechanistic insights into the effects of DPN as well as to objectively measure small changes in DPN pathology as a result of intervention.

IMPLICATIONS

Noninvasive exercise intervention is particularly valuable in DPN because of its safety, low cost, and potential to augment pharmacologic interventions. As we gain a better mechanistic understanding of the disease, more targeted and effective interventions can be designed.

Neuromuscular contributions to the age-related reduction in muscle power: Mechanisms and potential role of high velocity power training

Although much of the literature on neuromuscular changes with aging has focused on loss of muscle mass and isometric strength, deficits in muscle power are more pronounced with aging and may be a more sensitive measure of neuromuscular degeneration. This review aims to identify the adaptations to the neuromuscular system with aging, with specific emphasis on changes that result in decreased muscle power. We discuss how these changes in neuromuscular performance can affect mobility, and ultimately contribute to an increased risk for falls in older adults. Finally, we evaluate the literature regarding high-velocity muscle power training (PT), and its potential advantages over conventional strength training for improving functional performance and mitigating fall risk in older adults.

Reduced complexity of force and muscle activity during low level isometric contractions of the ankle in diabetic individuals

BACKGROUND

This study evaluated the structure and amount of variability of surface electromyography (sEMG) patterns and ankle force data during low-level isometric contractions in diabetic subjects with different degrees of neuropathy.

METHODS

We assessed 10 control subjects and 38 diabetic patients, classified as absent, mild, moderate, or severe neuropathy, by a fuzzy system based on clinical variables. Multichannel sEMG (64-electrode matrix) of tibialis anterior and gastrocnemius medialis muscles were acquired during isometric contractions at 10%, 20%, and 30% of the maximum voluntary contraction, and force levels during dorsi- and plantarflexion were recorded. Standard deviation and sample entropy of force signals were calculated and root mean square and sample entropy were calculated from sEMG signals. Differences among groups of force and sEMG variables were verified using a multivariate analysis of variance.

FINDINGS

Overall, during dorsiflexion contractions, moderate and severe subjects had higher force standard deviation and moderate subjects had lower force sample entropy. During plantarflexion, moderate subjects had higher force standard deviation and all diabetic subjects had lower entropy. Tibialis anterior presented higher root mean square in absent group and lower entropy in mild subjects. For gastrocnemius medialis, entropy was higher in severe and lower in moderate subjects.

INTERPRETATION

Diabetic neuropathy affects the complexity of the neuromuscular system during low-level isometric contractions, reducing the system's capacity to adapt to challenging mechanical demands. The observed patterns of neuromuscular complexity were not associated with disease severity, with the majority of alterations recorded in moderate subject.

Muscle fiber conduction velocity in different gait phases of early and late-stage diabetic neuropathy

We investigated the muscle fiber conduction velocity (MFCV) during gait phases of the lower limb muscles in individuals with various degrees of diabetic peripheral neuropathy (DPN). Forty-five patients were classified into severity degrees of DPN by a fuzzy model. The stages were absent (n=11), mild (n=14), moderate (n=11) and severe (n=9), with 10 matched healthy controls. While walking, all subjects had their sEMG (4 linear electrode arrays) recorded for tibialis anterior (TA), gastrocnemius medialis (GM), vastus lateralis (VL) and biceps femoris (BF). MFCV was calculated using a maximum likelihood algorithm with 30ms standard deviation Gaussian windows. In general, individuals in the earlier stages of DPN showed lower MFCV of TA, GM and BF, whilst individuals with severe DPN presented higher MFCV of the same muscles. We observed that mild patients already showed lower MFCV of TA at early stance and swing, and lower MFCV of BF at swing. All diabetic groups showed a markedly reduction in MFCV of VL, irrespective of DPN. Severe patients presented higher MFCV mainly in distal muscles, TA at early and swing phases and GM at propulsion and midstance. The absent group already showed MFCV of VL and GM reductions at the propulsion phase and of VL at early stance. Although MFCV changes were not as progressive as the DPN was, we clearly distinguished diabetic patients from controls, and severe patients from all others.

Insulin signaling controls neurotransmission via the 4eBP-dependent modification of the exocytotic machinery

Altered insulin signaling has been linked to widespread nervous system dysfunction including cognitive dysfunction, neuropathy and susceptibility to neurodegenerative disease. However, knowledge of the cellular mechanisms underlying the effects of insulin on neuronal function is incomplete. Here, we show that cell autonomous insulin signaling within the Drosophila CM9 motor neuron regulates the release of neurotransmitter via alteration of the synaptic vesicle fusion machinery. This effect of insulin utilizes the FOXO-dependent regulation of the thor gene, which encodes the Drosophila homologue of the eif-4e binding protein (4eBP). A critical target of this regulatory mechanism is Complexin, a synaptic protein known to regulate synaptic vesicle exocytosis. We find that the amounts of Complexin protein observed at the synapse is regulated by insulin and genetic manipulations of Complexin levels support the model that increased synaptic Complexin reduces neurotransmission in response to insulin signaling.

Electrodiagnostic Evaluation of Individuals Implanted With Extracellular Matrix for the Treatment of Volumetric Muscle Injury: Case Series

BACKGROUND

Electrodiagnosis can reveal the nerve and muscle changes following surgical placement of an extracellular matrix (ECM) bioscaffold for treatment of volumetric muscle loss (VML).

OBJECTIVE

The purpose of this study was to characterize nerve conduction study (NCS) and electromyography (EMG) changes following ECM bioscaffold placement in individuals with VML. The ability of presurgical NCS and EMG to be used as a tool to help identify candidates who are likely to display improvements postsurgically also was explored.

DESIGN

A longitudinal case series design was used.

METHODS

The study was conducted at the McGowan Institute for Regenerative Medicine at the University of Pittsburgh. Eight individuals with a history of chronic VML participated. The intervention was surgical placement of an ECM bioscaffold at the site of VML. The strength of the affected region was measured using a handheld dynamometer, and electrophysiologic evaluation was conducted on the affected limb with standard method of NCS and EMG. All measurements were obtained the day before surgery and repeated 6 months after surgery.

RESULTS

Seven of the 8 participants had a preoperative electrodiagnosis of incomplete mononeuropathy within the site of VML. After ECM treatment, 5 of the 8 participants showed improvements in NCS amplitude or needle EMG parameters. The presence of electrical activity within the scaffold remodeling site was concomitant with clinical improvement in muscle strength.

LIMITATIONS

This study had a small sample size, and participants served as their own controls. The electromyographers and physical therapists performing the evaluation were not blinded.

CONCLUSIONS

Electrodiagnostic data provide objective evidence of physiological improvements in muscle function following ECM placement at sites of VML. Future studies are warranted to further investigate the potential of needle EMG as a predictor of successful outcomes following ECM treatment for VML.

Motor unit number and transmission stability in octogenarian world class athletes: Can age-related deficits be outrun?

Our group has shown a greater number of functioning motor units (MU) in a cohort of highly active older (∼65 yr) masters runners relative to age-matched controls. Because of the precipitous loss in the number of functioning MUs in the eighth and ninth decades of life it is unknown whether older world class octogenarian masters athletes (MA) would also have greater numbers of functioning MUs compared with age-matched controls. We measured MU numbers and neuromuscular transmission stability in the tibialis anterior of world champion MAs (∼80 yr) and compared the values with healthy age-matched controls (∼80 yr). Decomposition-enhanced spike-triggered averaging was used to collect surface and intramuscular electromyography signals during dorsiflexion at ∼25% of maximum voluntary isometric contraction. Near fiber (NF) MU potential analysis was used to assess neuromuscular transmission stability. For the MAs compared with age-matched controls, the amount of excitable muscle mass (compound muscle action potential) was 14% greater (P < 0.05), there was a trend (P = 0.07) toward a 27% smaller surface-detected MU potential representative of less collateral reinnervation, and 28% more functioning MUs (P < 0.05). Additionally, the MAs had greater MU neuromuscular stability than the controls, as indicated by lower NF jitter and jiggle values (P < 0.05). These results demonstrate that high-performing octogenarians better maintain neuromuscular stability of the MU and mitigate the loss of MUs associated with aging well into the later decades of life during which time the loss of muscle mass and strength becomes functionally relevant. Future studies may identify the concomitant roles genetics and exercise play in neuroprotection.

Physiology in Medicine: neuromuscular consequences of diabetic neuropathy

Diabetic polyneuropathy (DPN) refers to peripheral nerve dysfunction as a complication of diabetes mellitus. This condition is relatively common and is likely a result of vascular and/or metabolic disturbances related to diabetes. In the early or less severe stages of DPN it typically results in sensory impairments but can eventually lead to major dysfunction of the neuromuscular system. Some of these impairments may include muscle atrophy and weakness, slowing of muscle contraction, and loss of power and endurance. Combined with sensory deficits these changes in the motor system can contribute to decreased functional capacity, impaired mobility, altered gait, and increased fall risk. There is no pharmacological disease-modifying therapy available for DPN and the mainstay of treatment is linked to treating the diabetes itself and revolves around strict glycemic control. Exercise therapy (including aerobic, strength, or balance training-based exercise) appears to be a promising preventative and treatment strategy for patients with DPN and those at risk. The goal of this Physiology in Medicine article is to highlight important and overlooked dysfunction of the neuromuscular system as a result of DPN with an emphasis on the physiologic basis for that dysfunction. Additionally, we sought to provide information that clinicians can use when following patients with diabetes or DPN including support for the inclusion of exercise-based therapy as an effective, accessible, and inexpensive form of treatment.

Innervation and neuromuscular control in ageing skeletal muscle

Changes in the neuromuscular system affecting the ageing motor unit manifest structurally as a reduction in motor unit number secondary to motor neuron loss; fibre type grouping due to repeating cycles of denervation-reinnervation; and instability of the neuromuscular junction that may be due to either or both of a gradual perturbation in postsynaptic signalling mechanisms necessary for maintenance of the endplate acetylcholine receptor clusters or a sudden process involving motor neuron death or traumatic injury to the muscle fibre. Functionally, these changes manifest as a reduction in strength and coordination that precedes a loss in muscle mass and contributes to impairments in fatigue. Regular muscle activation in postural muscles or through habitual physical activity can attenuate some of these structural and functional changes up to a point along the ageing continuum. On the other hand, regular muscle activation in advanced age (>75 years) loses its efficacy, and at least in rodents may exacerbate age-related motor neuron death. Transgenic mouse studies aimed at identifying potential mechanisms of motor unit disruptions in ageing muscle are not conclusive due to many different mechanisms converging on similar motor unit alterations, many of which phenocopy ageing muscle. Longitudinal studies of ageing models and humans will help clarify the cause and effect relationships and thus, identify relevant therapeutic targets to better preserve muscle function across the lifespan.

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.

Diabetes and Frailty: Two Converging Conditions?

Diabetes mellitus is a disabling, chronic cardiovascular and medical disease with a tremendous health, social and economic burden in our ageing communities. It has a prevalence of 10% to 30% in people older than 65 years of age, and more than half of all subjects with diabetes in the United States are older than 60 years of age. The main impact of diabetes in older adults stems from its effect on function, both physical and cognitive, that finally impairs their quality of life, although the impact on survival is modest. Frailty has emerged during the past 2 decades as the most powerful predictor of disability and other adverse outcomes, including mortality, disability and institutionalization in older adults. In this article we explore the relationship between diabetes and frailty, and we recognize that they are intimately related chronic medical conditions that result in huge societal and personal health burdens.

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.

Decreased muscle endurance associated with diabetic neuropathy may be attributed partially to neuromuscular transmission failure

Diabetic polyneuropathy (DPN) can cause muscle atrophy, weakness, contractile slowing, and neuromuscular transmission instability. Our objective was to assess the response of the impaired neuromuscular system of DPN in humans when stressed with a sustained maximal voluntary contraction (MVC). Baseline MVC and evoked dorsiflexor contractile properties were assessed in DPN patients (n = 10) and controls (n = 10). Surface electromyography was used to record tibialis anterior evoked maximal compound muscle action potentials (CMAPs) and neuromuscular activity during MVCs. Participants performed a sustained isometric dorsiflexion MVC for which task termination was determined by the inability to sustain ≥60% MVC torque. The fatigue protocol was immediately followed by a maximal twitch, with additional maximal twitches and MVCs assessed at 30 s and 2 min postfatigue. DPN patients fatigued ∼21% more quickly than controls (P < 0.05) and featured less relative electromyographic activity during the first one-third of the fatigue protocol compared with controls (P < 0.05). Immediately following fatigue, maximal twitch torque was reduced similarly (∼20%) in both groups, and concurrently CMAPs were reduced (∼12%) in DPN patients, whereas they were unaffected in controls (P > 0.05). Twitch torque and CMAP amplitude recovered to baseline 30 s postfatigue. Additionally, at 30 s postfatigue, both groups had similar (∼10%) reductions in MVC torque relative to baseline, and MVC strength recovered by 2 min postfatigue. We conclude DPN patients possess less endurance than controls, and neuromuscular transmission failure may contribute to this greater fatigability.