The numbers of limb motor neurons in the human lumbosacral cord throughout life

Incidence and mortality of ALS: a 42-year population-based nationwide study

BACKGROUND AND AIM

Recent studies have suggested that the incidence rate (IR) and the rate of death (MR) of amyotrophic lateral sclerosis (ALS) are increasing. Still, it remains unclear whether this is due to improved case ascertainment or represents a true increase. We examined the development in the incidence and mortality of ALS in Denmark for 42 years.

METHODS

We retrieved individual-level data of all patients aged above 18 years with first-time ALS diagnosed at any Danish department of neurology. The IR and MR were calculated based on data from 1980 to 2021, stratified by gender and age.

RESULTS

We identified 5,943 patients with ALS and identified a total of 5,069 deaths in the nationwide population. Overall, the IR was 3.4 per 100,000 persons per year (95% CI 3.4-3.5). ALS incidence rose gradually during the study period, and the IR was 2.8 times higher (95% CI 2.4-3.2) when comparing the latest period (2018-2021) with the first (1980-1983). Parallel to the IR, the MR increased over time and was associated with male gender and rose with age at diagnosis, peaking in the 70-79-year age group.

CONCLUSION

In Denmark, the IR and MR of ALS increased threefold from 1980 to 2021, with steadily increasing risk related to male gender and in particular to higher age. Considering our aging societies, the number of elderly patients with ALS can be expected to increase considerably.

Muscle fibroblasts and stem cells stimulate motor neurons in an age and exercise-dependent manner

Exercise preserves neuromuscular function in aging through unknown mechanisms. Skeletal muscle fibroblasts (FIB) and stem cells (MuSC) are abundant in skeletal muscle and reside close to neuromuscular junctions, but their relative roles in motor neuron maintenance remain undescribed. Using direct cocultures of embryonic rat motor neurons with either human MuSC or FIB, RNA sequencing revealed profound differential regulation of the motor neuron transcriptome, with FIB generally favoring neuron growth and cell migration and MuSC favoring production of ribosomes and translational machinery. Conditioned medium from FIB was superior to MuSC in preserving motor neurons and increasing their maturity. Lastly, we established the importance of donor age and exercise status and found an age-related distortion of motor neuron and muscle cell interaction that was fully mitigated by lifelong physical activity. In conclusion, we show that human muscle FIB and MuSC synergistically stimulate the growth and viability of motor neurons, which is further amplified by regular exercise.

Reactive oxygen species in age-related musculoskeletal decline: implications for nutritional intervention

Musculoskeletal disorders and age-related musculoskeletal decline are major contributors to the burden of ill health seen in older subjects. Despite this increased burden, these chronic disorders of old age receive a relatively small proportion of national research funds. Much has been learned about fundamental processes involved in ageing from basic science research and this is leading to identification of key pathways that mediate ageing which may help the search for interventions to reduce age-related musculoskeletal decline. This short review will focus on the role of reactive oxygen species in age-related skeletal muscle decline and on the implications of this work for potential nutritional interventions in sarcopenia. The key physiological role of reactive oxygen species is now known to be in mediating redox signalling in muscle and other tissues and ageing leads to disruption of such pathways. In muscle, this is reflected in an age-related attenuation of specific adaptations and responses to contractile activity that impacts the ability of skeletal muscle from ageing individuals to respond to exercise. These pathways provides potential targets for identification of logical interventions that may help maintain muscle mass and function during ageing.

Age-related increase in the excitability of mouse layer V pyramidal neurons in the primary motor cortex is accompanied by an increased persistent inward current

Sarcopenia, or pathological age-related loss of muscle strength and mass, contributes to physical function impairment in older adults. While current understanding of sarcopenia is centered mostly on neuromuscular mechanisms, mounting evidence supports that deficits at the level of the primary motor cortex (PMC) play a significant role. Despite the importance of the PMC to initiate movement, understanding of how age affects the excitability of layer V pyramidal neurons (LVPNs) of the PMC is limited. To address this, we used the whole-cell patch clamp technique to measure the excitability of LVPNs of the PMC in young, late adulthood, and old mice. Old LVPNs had increased firing frequency and membrane input resistance, but no differences in action potential kinetics versus young and late adulthood mice. Since changes in the persistent inward current (PIC) are known to contribute to changes in motor neuron excitability, we measured LVPN PICs as a putative contributor to LVPN excitability. The PIC amplitude was increased in old LVPN via increases in Na+ and Ca2+ PICs, in addition to being active across a wider voltage range. Given that LVPN function is integral to initiation of voluntary muscle contraction, altered LVPN excitability likely contributes to age-related impairment of physical function.

Ageing leads to selective type II myofibre deterioration and denervation independent of reinnervative capacity in human skeletal muscle

Age-related loss of muscle mass and function is underpinned by changes at the myocellular level. However, our understanding of the aged muscle phenotype might be confounded by factors secondary to ageing per se, such as inactivity and adiposity. Here, using healthy, lean, recreationally active, older men, we investigated the impact of ageing on myocellular properties in skeletal muscle. Muscle biopsies were obtained from young men (22 ± 3 years, n = 10) and older men (69 ± 3 years, n = 11) matched for health status, activity level and body mass index. Immunofluorescence was used to assess myofibre composition, morphology (size and shape), capillarization, the content of satellite cells and myonuclei, the spatial relationship between satellite cells and capillaries, denervation and myofibre grouping. Compared with young muscle, aged muscle contained 53% more type I myofibres, in addition to smaller (-32%) and misshapen (3%) type II myofibres (P < 0.05). Aged muscle manifested fewer capillaries (-29%) and satellite cells (-38%) surrounding type II myofibres (P < 0.05); however, the spatial relationship between these two remained intact. The proportion of denervated myofibres was ∼2.6-fold higher in old than young muscle (P < 0.05). Aged muscle had more grouped type I myofibres (∼18-fold), primarily driven by increased size of existing groups rather than increased group frequency (P < 0.05). Aged muscle displayed selective deterioration of type II myofibres alongside increased denervation and myofibre grouping. These data are key to understanding the cellular basis of age-related muscle decline and reveal a pressing need to fine-tune strategies to preserve type II myofibres and innervation status in ageing populations.

Delayed neuromuscular fatigue recovery unveils reduced fatigue tolerance in elderly following maximal intermittent exercise

The aim of the study was to assess the impact of aging on neuromuscular fatigue and recovery. Ten young (23.08 ± 1.43 years) and older (61.19 ± 1.80 years) males performed an intermittent maximal isometric exercise with the knee extensors followed by 27 min of recovery. Maximal voluntary contraction (MVC), total work (W'), voluntary activation (VA), potentiated resting twitch (Ptw), and electromyography (EMG) were recorded and then analyzed. Peripheral and central fatigue following exercise were lower in old compared to young (- 29.99% vs.  - 42.68% and  - 14.55 vs.  - 20.02%; P < 0.05, respectively). Despite old performing 50% less work, RMS/Mmax reduction was similar between old and young (- 26.46% vs.  - 29.93%; P > 0.05, respectively). During the recovery period, our results showed that recovery of the MVC was impaired for old (14.93% for old vs. 30.66% for young) and still incomplete until 27 min.VA increased significantly compared to post exercise after 1 min only for young (P = 0.001), potentially affecting the recovery pattern of MVC during the early phase due to their significant correlation (r2 = 0.58, P = 0.01). Peripheral fatigue recovery was also lower for old (11.18% vs. 18.72%; P < 0.001), and both groups failed to recover their baseline value (both P < 0.005). The lower peripheral and central fatigue observed in elderly following exercise appears for the first instance as a fatigue resistance. However, the delayed neuromuscular recovery reveals instead a reduced fatigue tolerance reflecting age-related alteration within contractile properties and/or within central nervous system.

Morphometric analysis of spinal motor neuron degeneration in sporadic amyotrophic lateral sclerosis

OBJECTIVES

This study aimed to clarify the relationship between 43-kDa TAR DNA-binding protein (TDP-43) pathology and spinal cord anterior horn motor neuron (AHMN) atrophy in sporadic amyotrophic lateral sclerosis (SALS).

METHODS

Eight patients with SALS and 12 controls were included in this study. Formalin-fixed specimens of lumbar spinal cord samples were paraffin-embedded and sectioned at the level of the fourth lumbar spinal cord with a 4 μm thickness. Using a microscope, the long diameters of the neurons with nucleoli were measured in spinal AHMNs stained with an anti-SMI-32 antibody. AHMNs were divided into medial and lateral nuclei for statistical analysis. We also used previously reported data to measure the long diameter of AHMNs with initial TDP-43 pathology, in which TDP-43 was present both in the nucleus and cytoplasm.

RESULTS

The long diameter of the lumbar spinal AHMNs in patients with SALS was smaller in the medial nucleus (42.54 ± 9.33 μm, n = 24) and the lateral nucleus (49.41 ± 13.86 μm, n = 129) than in controls (medial nucleus: 55.84 ± 13.49 μm, n = 85, p < 0.001; lateral nucleus: 62.39 ± 13.29 μm, n = 756, p < 0.001, Mann-Whitney U test). All 21 motor neurons with initial TDP-43 pathology were in the lateral nucleus, and their long diameter (67.60 ± 18.3 μm, p = 0.352) was not significantly different from that of controls.

CONCLUSION

Motor neuron atrophy in SALS does not occur during the initial stages of TDP-43 pathology, and TDP-43 pathology is already advanced in the atrophied motor neurons.

Autophagy markers LC3 and p62 in aging lumbar motor neurons

Autophagy is a ubiquitous process through which damaged cytoplasmic structures are recycled and degraded within cells. Aging can affect autophagy regulation in different steps leading to the accumulation of damaged organelles and proteins, which can contribute to cell dysfunction and death. Motor neuron (MN) loss and sarcopenia are prominent features of neuromuscular aging. Previous studies on phrenic MNs showed increased levels of the autophagy proteins LC3 and p62 in 24 month compared to 6 month old mice, consistent with the onset of diaphragm muscle sarcopenia. In the present study, we hypothesized that aging leads to increased expression of the autophagy markers LC3 and p62 in single lumbar MNs. Expression of LC3 and p62 in lumbar MNs (spinal levels L1-L6) was assessed using immunofluorescence and confocal imaging of male and female mice at 6, 18 and 24 months of age, reflecting 100 %, 90 % and 75 % survival, respectively. A mixed linear model with animal as a random effect was used to compare relative LC3 and p62 expression in choline acetyl transferase-positive MNs across age groups. Expression of LC3 and p62 decreased in the white matter of the lumbar spinal cord with aging, with ~29 % decrease in LC3 and ~ 7 % decrease in p62 expression at 24 months of age compared to 6 months of age. There was no change in LC3 or p62 expression in the gray matter with age. LC3 expression in MNs relative to white matter increased significantly with age, with 150 % increase at 24 months of age compared to 6 months of age. Similarly, p62 expression in MNs relative to white matter increased significantly with age, with ~14 % increase at 24 months of age compared to 6 months of age. No effect of sex or MN pool was observed in LC3 and p62 expression in MNs. Overall, these data suggest autophagy impairment during elongation (increased LC3) and degradation (increased p62) phases with aging in lumbar MNs.

Differences in muscle activity of extrinsic and intrinsic foot muscles in toe grip and push-down movements of the great toe

Toe flexor strength is generated primarily by the flexor hallucis longus (FHL) of the extrinsic foot muscles (EFMs) and the plantar intrinsic foot muscles (PIFMs) of the great toe. Toe flexion methods can be broadly classified into toe grip (TG) and toe push-down (TP). Additionally, TP's interphalangeal joint (IPJ) position may influence the FHL and PIFMs activity ratios. This study aimed to elucidate the differences in the muscle activity and muscle activity ratios of the FHL and AbdH during TG, TP with IPJ flexion (TPIF), and TP with IPJ extension (TPIE). Surface electromyography and a custom-made instrument were used to measure the FHL and AbdH muscle activity during TG, TPIF, and TPIE of the great toe in 28 healthy men. The muscle activity and AbdH/FHL muscle activity ratio in the three conditions were statistically compared. The FHL activity was significantly higher during TG and TPIF than during TPIE. The AbdH muscle activity was significantly higher during TPIF and TPIE than that during TG. The AbdH/FHL muscle activity ratio was significantly higher for TPIE, TPIF, and TG in that order. This study showed that the FHL and AbdH muscle activity differed depending on the TG and TP of the great toe, and that the AbdH/FHL muscle activity ratio was different in the IPJ position. These results suggest that selecting a toe flexion method according to the target muscle when measuring and training the great toe flexor strength is important.

Cortical and spinal responses to short-term strength training and detraining in young and older adults in rectus femoris muscle

INTRODUCTION

Strength training mitigates the age-related decline in strength and muscle activation but limited evidence exists on specific motor pathway adaptations.

METHODS

Eleven young (22-34 years) and ten older (66-80 years) adults underwent five testing sessions where lumbar-evoked potentials (LEPs) and motor-evoked potentials (MEPs) were measured during 20 and 60% of maximum voluntary contraction (MVC). Ten stimulations, randomly delivered, targeted 25% of maximum compound action potential for LEPs and 120, 140, and 160% of active motor threshold (aMT) for MEPs. The 7-week whole-body resistance training intervention included five exercises, e.g., knee extension (5 sets) and leg press (3 sets), performed twice weekly and was followed by 4 weeks of detraining.

RESULTS

Young had higher MVC (~ 63 N·m, p = 0.006), 1-RM (~ 50 kg, p = 0.002), and lower aMT (~ 9%, p = 0.030) than older adults at baseline. Young increased 1-RM (+ 18 kg, p < 0.001), skeletal muscle mass (SMM) (+ 0.9 kg, p = 0.009), and LEP amplitude (+ 0.174, p < 0.001) during 20% MVC. Older adults increased MVC (+ 13 N·m, p = 0.014), however, they experienced decreased LEP amplitude (- 0.241, p < 0.001) during 20% MVC and MEP amplitude reductions at 120% (- 0.157, p = 0.034), 140% (- 0.196, p = 0.026), and 160% (- 0.210, p = 0.006) aMT during 60% MVC trials. After detraining, young and older adults decreased 1-RM, while young adults decreased SMM.

CONCLUSION

Higher aMT and MEP amplitude in older adults were concomitant with lower baseline strength. Training increased strength in both groups, but divergent modifications in cortico-spinal activity occurred. Results suggest that the primary locus of adaptation occurs at the spinal level.

Common synaptic inputs and persistent inward currents of vastus lateralis motor units are reduced in older male adults

Although muscle atrophy may partially account for age-related strength decline, it is further influenced by alterations of neural input to muscle. Persistent inward currents (PIC) and the level of common synaptic inputs to motoneurons influence neuromuscular function. However, these have not yet been described in the aged human quadriceps. High-density surface electromyography (HDsEMG) signals were collected from the vastus lateralis of 15 young (mean ± SD, 23 ± 5 y) and 15 older (67 ± 9 y) men during submaximal sustained and 20-s ramped contractions. HDsEMG signals were decomposed to identify individual motor unit discharges, from which PIC amplitude and intramuscular coherence were estimated. Older participants produced significantly lower knee extensor torque (p < 0.001) and poorer force tracking ability (p < 0.001) than young. Older participants also had lower PIC amplitude (p = 0.001) and coherence estimates in the alpha frequency band (p < 0.001) during ramp contractions when compared to young. Persistent inward currents and common synaptic inputs are lower in the vastus lateralis of older males when compared to young. These data highlight altered neural input to the clinically and functionally important quadriceps, further underpinning age-related loss of function which may occur independently of the loss of muscle mass.

Molecular hallmarks of ageing in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal, severely debilitating and rapidly progressing disorder affecting motor neurons in the brain, brainstem, and spinal cord. Unfortunately, there are few effective treatments, thus there remains a critical need to find novel interventions that can mitigate against its effects. Whilst the aetiology of ALS remains unclear, ageing is the major risk factor. Ageing is a slowly progressive process marked by functional decline of an organism over its lifespan. However, it remains unclear how ageing promotes the risk of ALS. At the molecular and cellular level there are specific hallmarks characteristic of normal ageing. These hallmarks are highly inter-related and overlap significantly with each other. Moreover, whilst ageing is a normal process, there are striking similarities at the molecular level between these factors and neurodegeneration in ALS. Nine ageing hallmarks were originally proposed: genomic instability, loss of telomeres, senescence, epigenetic modifications, dysregulated nutrient sensing, loss of proteostasis, mitochondrial dysfunction, stem cell exhaustion, and altered inter-cellular communication. However, these were recently (2023) expanded to include dysregulation of autophagy, inflammation and dysbiosis. Hence, given the latest updates to these hallmarks, and their close association to disease processes in ALS, a new examination of their relationship to pathophysiology is warranted. In this review, we describe possible mechanisms by which normal ageing impacts on neurodegenerative mechanisms implicated in ALS, and new therapeutic interventions that may arise from this.

Sex- and age-specific normative values of lower extremity muscle power in Italian community-dwellers

BACKGROUND

Muscle power is associated with health-related parameters. Simple equations were validated to estimate lower extremity muscle power measures based on the time to complete the five-repetition sit-to-stand test. The present study was conducted to provide lower extremity muscle power estimates and produce centile values in a large and relatively unselected population across a wide age spectrum.

METHODS

Data were from the Longevity Check-up 7+ (Lookup 7+) project, an ongoing initiative conducted in unconventional settings (e.g., exhibitions, shopping centres and health promotion campaigns) across Italy to foster adoption of healthy lifestyles. Absolute, relative, allometric and specific muscle power measures of the lower extremities were estimated using validated formulas. Cross-sectional centile and normative values for muscle power measures from 18 to 81+ years were produced for the two sexes. Smoothed normative curves for men and women were constructed using the lambda-mu-sigma method.

RESULTS

From 1 June 2015 to 31 October 2021, 13 515 participants were enrolled of whom 12 864 were eligible for the present study. Mean age was 55.9 years (standard deviation: 14.8 years; range: 18-98 years), and 7217 (56.%) were women. Absolute, relative, allometric and specific muscle power declined significantly with age. Specific patterns of decline were observed according to sex and muscle power parameter. Absolute muscle power peaked at 41-50 and 31-40 years in men and women, respectively. Afterwards, a decline rate of approximately 12% per decade was observed, regardless of sex. Relative muscle power showed the largest reduction with age, such that it was 40.6% and 46.4% smaller in men and women older than 80, respectively, compared with those aged 18-30 years. Age-related changes in allometric and specific muscle power measures were similar between men and women.

CONCLUSIONS

Data from the Lookup 7+ project indicate that lower extremity muscle power estimated using simple equations is significantly associated with age. Sex-specific patterns of decline in absolute and relative muscle power were observed with age. Allometric and specific muscle power declined at a similar rate in men and women.

Sarcopenia of the longitudinal tongue muscles in rats

The tongue is a muscular hydrostat, with lingual movements occurring during breathing, chewing, swallowing, vocalization, vomiting, coughing and grooming/sexual activities. In the elderly, reduced lingual dysfunction and weakness contribute to increased risks of obstructive sleep apnea and aspiration pneumonia. In Fischer 344 (F344) rats, a validated model of aging, hypoglossal motor neuron death is apparent, although there is no information regarding tongue strength. The intrinsic tongue muscles, the superior and inferior longitudinal, transversalis and verticalis exist in an interdigitated state. Recently, we established a method to measure the specific force of individual intrinsic tongue muscle, accounting for the tissue bulk that is not in the direction of uniaxial force. In the longitudinal muscles of 6- (n = 10), 18- (n = 9) and 24-month-old (n = 12) female and male F344 rats, we assessed specific force, fatigability, fiber type dependent cross-sectional area (CSA) and overall CSA. Muscle force and fatigue was assessed ex vivo using platinum plate simulation electrodes. Tongue muscles were frozen in melting isopentane, and transverse sections cut at 10 µm. Muscle fiber type was classified based on immunoreactivity to myosin heavy chain (MyHC) isoform antibodies. In H&E stained muscle, CSA and uniaxial muscle contributions to total tongue bulk was assessed. We observed a robust ∼30% loss of longitudinal specific force, with reductions in overall longitudinal muscle fiber CSA and specific atrophy of type IIx/IIb fibers. It will be important to investigate the mechanistic underpinnings of hypoglossal motor neuron death and tongue muscle weakness to eventually provide therapies for age-associated lingual dysfunctions.

Neuromuscular Dysfunction Precedes Cognitive Impairment in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) develops along a continuum that spans years prior to diagnosis. Decreased muscle function and mitochondrial respiration occur years earlier in those that develop AD; however, it is unknown what causes these peripheral phenotypes in a disease of the brain. Exercise promotes muscle, mitochondria, and cognitive health and is proposed to be a potential therapeutic for AD, but no study has investigated how skeletal muscle adapts to exercise training in an AD-like context. Utilizing 5xFAD mice, an AD model that develops ad-like pathology and cognitive impairments around 6 mo of age, we examined in vivo neuromuscular function and exercise adapations (mitochondrial respiration and RNA sequencing) before the manifestation of overt cognitive impairment. We found 5xFAD mice develop neuromuscular dysfunction beginning as early as 4 mo of age, characterized by impaired nerve-stimulated muscle torque production and compound nerve action potential of the sciatic nerve. Furthermore, skeletal muscle in 5xFAD mice had altered, sex-dependent, adaptive responses (mitochondrial respiration and gene expression) to exercise training in the absence of overt cognitive impairment. Changes in peripheral systems, specifically neural communication to skeletal muscle, may be harbingers for AD and have implications for lifestyle interventions, like exercise, in AD.

Hallmarks of ageing in human skeletal muscle and implications for understanding the pathophysiology of sarcopenia in women and men

Ageing is a complex biological process associated with increased morbidity and mortality. Nine classic, interdependent hallmarks of ageing have been proposed involving genetic and biochemical pathways that collectively influence ageing trajectories and susceptibility to pathology in humans. Ageing skeletal muscle undergoes profound morphological and physiological changes associated with loss of strength, mass, and function, a condition known as sarcopenia. The aetiology of sarcopenia is complex and whilst research in this area is growing rapidly, there is a relative paucity of human studies, particularly in older women. Here, we evaluate how the nine classic hallmarks of ageing: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication contribute to skeletal muscle ageing and the pathophysiology of sarcopenia. We also highlight five novel hallmarks of particular significance to skeletal muscle ageing: inflammation, neural dysfunction, extracellular matrix dysfunction, reduced vascular perfusion, and ionic dyshomeostasis, and discuss how the classic and novel hallmarks are interconnected. Their clinical relevance and translational potential are also considered.

Regional recruitment and differential behavior of motor units during postural control in older adults

Aging is associated with neuromuscular system changes that may have implications for the recruitment and firing behaviors of motor units (MUs). In previous studies, we observed that young adults recruit subpopulations of triceps surae MUs during tasks that involved leaning in five directions: common units that were active during different leaning directions and unique units that were active in only one leaning direction. Furthermore, the MU subpopulation firing behaviors [average firing rate (AFR), coefficient of variation (CoVISI), and intermittent firing] modulated with leaning direction. The purpose of this study was to examine whether older adults exhibited this regional recruitment of MUs and firing behaviors. Seventeen older adults (aged 74.8 ± 5.3 yr) stood on a force platform and maintained their center of pressure leaning in five directions. High-density surface electromyography recordings from the triceps surae were decomposed into single MU action potentials. A MU tracking analysis identified groups of MUs as being common or unique across the leaning directions. Although leaning in different directions did not affect the AFR and CoVISI of common units (P > 0.05), the unique units responded to the leaning directions by increasing AFR and CoVISI, albeit modestly (F = 18.51, P < 0.001). The unique units increased their intermittency with forward leaning (F = 9.22, P = 0.003). The mediolateral barycenter positions of MU activity in both subpopulations were found in similar locations for all leaning directions (P > 0.05). These neuromuscular changes may contribute to the reduced balance performance seen in older adults.NEW & NOTEWORTHY In this study, we observed differences in motor unit recruitment and firing behaviors of distinct subpopulations of motor units in the older adult triceps surae muscle from those observed in the young adult. Our results suggest that the older adult central nervous system may partially lose the ability to regionally recruit and differentially control motor units. This finding may be an underlying cause of balance difficulties in older adults during directionally challenging leaning tasks.

A Computer Interface Controlled by Upper Limb Muscles: Effects of a Two Weeks Training on Younger and Older Adults

As the population worldwide ages, there is a growing need for assistive technology and effective human-machine interfaces to address the wider range of motor disabilities that older adults may experience. Motor disabilities can make it difficult for individuals to perform basic daily tasks, such as getting dressed, preparing meals, or using a computer. The goal of this study was to investigate the effect of two weeks of training with a myoelectric computer interface (MCI) on motor functions in younger and older adults. Twenty people were recruited in the study: thirteen younger (range: 22-35 years old) and seven older (range: 61-78 years old) adults. Participants completed six training sessions of about 2 hours each, during which the activity of right and left biceps and trapezius were mapped into a control signal for the cursor of a computer. Results highlighted significant improvements in cursor control, and therefore in muscle coordination, in both groups. All participants with training became faster and more accurate, although people in different age range learned with a different dynamic. Results of the questionnaire on system usability and quality highlighted a general consensus about easiness of use and intuitiveness. These findings suggest that the proposed MCI training can be a powerful tool in the framework of assistive technologies for both younger and older adults. Further research is needed to determine the optimal duration and intensity of MCI training for different age groups and to investigate long-term effects of training on physical and cognitive function.

Age-related differences in the neuromuscular performance of fatigue-provoking exercise under severe whole-body hyperthermia conditions

PURPOSE

The purpose of this study was to determine if aging would lead to greater decline in neuromuscular function during a fatiguing task under severe whole-body hyperthermia conditions.

METHODS

Twelve young (aged 19-21 years) and 11 older (aged 65-80 years) males were enrolled in the study, which comprised a randomized control trial under a thermoneutral condition at an ambient temperature of 23°C (CON) and an experimental trial with passive lower body heating in 43°C water (HWI-43°C). Changes in neuromuscular function and fatigability, and physical performance-influencing factors such as psychological, thermoregulatory, neuroendocrine, and immune responses to whole-body hyperthermia were measured.

RESULTS

A slower increase in rectal temperature, and a lower heart rate, thermal sensation, and sweating rate were observed in older males than young males in response to HWI-43°C trial (p < 0.05). Nevertheless, prolactin increased more in response to hyperthermia in young males, while interleukin-6 and cortisol levels increased more in older males (p < 0.05). Peripheral dopamine levels decreased in older males and increased in young males in response to hyperthermia (p < 0.05). Surprisingly, older males demonstrated greater neuromuscular fatigability resistance and faster maximal voluntary contraction (MVC) torque recovery after a 2-min sustained isometric MVC task under thermoneutral and severe hyperthermic conditions (p < 0.05).

CONCLUSION

Neuromuscular performance during fatigue-provoking sustained isometric exercise under severe whole-body hyperthermia conditions appears to decline in both age groups, but a lower relative decline in torque production for older males may relate to lower psychological and thermophysiological strain along with a diminished dopamine response and prolactin release.

Muscle power-related parameters in middle-aged and older Brazilian women: a cross-sectional study

The present study was conducted to provide normative values for lower-limb muscle power estimated through equations based on the 5 times sit-to-stand (5STS) test in Brazilian older women. In addition, we investigated the association between muscle power parameters and age. The study followed a cross-sectional design. Participants were community-dwelling women. Candidates were considered eligible if they were 18 years or older, lived independently, and possessed sufficient physical and cognitive abilities to perform all measurements required by the protocol. The 5STS test was performed as fast as possible using a standard protocol. Absolute, relative, and allometric muscle power measures were estimated using 5STS-based equations. Two thousand four-hundred seventy-one women participated in the present study. Results indicated that muscle power-related parameters decreased linearly with age. Women 60-69 years showed a marginal reduction in absolute (- 5.2%), relative (- 7.9%), and allometric (- 4.0%) muscle power. A larger reduction was observed in those 70-79 years and reached ¼ of loss in participants ≥ 80, in comparison to middle-aged participants. Pearson's correlation and linear regression analyses indicated that power-related parameters were negatively associated with age. In conclusion, data of the present study provide normative values for lower-limb muscle power parameters according to 5STS-based equations. We observed that muscle power-related parameters declined with age, such that participants 60-69, 70-79, and ≥ 80 years displayed lower absolute and relative muscle power compared middle-aged women. A later decline was observed in allometric muscle power. Relative muscle power declined to a greater extent than other parameters, suggesting a possible window of opportunity for interventions.

The Brain and Spinal Microvasculature in Normal Aging

Changes in the brain and spinal cord microvasculature during normal aging contribute to the "sensitive" nature of aged central nervous system tissue to ischemic insults. In this review, we will examine alterations in the central nervous system microvasculature during normal aging, which we define as aging without a dominant pathology such as neurodegenerative processes, vascular injury or disease, or trauma. We will also discuss newer technologies to improve the study of central nervous system microvascular structure and function. Microvasculature within the brain and spinal cord will be discussed separately as anatomy and physiology differ between these compartments. Lastly, we will identify critical areas for future studies as well as key unanswered questions.

Loss of larger hypoglossal motor neurons in aged Fischer 344 rats

The intrinsic (longitudinal, transversalis and verticalis) and extrinsic (genioglossus, styloglossus, hyoglossus and geniohyoid) tongue muscles are innervated by hypoglossal motor neurons (MNs). Tongue muscle activations occur during many behaviors: maintaining upper airway patency, chewing, swallowing, vocalization, vomiting, coughing, sneezing and grooming/sexual activities. In the tongues of the elderly, reduced oral motor function and strength contribute to increased risk of obstructive sleep apnoea. Tongue muscle atrophy and weakness is also described in rats, yet hypoglossal MN numbers are unknown. In young (6-months, n=10) and old (24-months, n=8) female and male Fischer 344 (F344) rats, stereological assessment of hypoglossal MN numbers and surface areas were performed on 16µm Nissl-stained brainstem cryosections. We observed a robust loss of ~15% of hypoglossal MNs and a modest ~8% reduction in their surface areas with age. In the larger size tertile of hypoglossal MNs, age-associated loss of hypoglossal MNs approached ~30% These findings uncover a potential neurogenic locus of pathology for age-associated tongue dysfunctions.

Pathogenesis, Intervention, and Current Status of Drug Development for Sarcopenia: A Review

Sarcopenia refers to the loss of muscle strength and mass in older individuals and is a major determinant of fall risk and impaired ability to perform activities of daily living, often leading to disability, loss of independence, and death. Owing to its impact on morbidity, mortality, and healthcare expenditure, sarcopenia in the elderly has become a major focus of research and public policy debates worldwide. Despite its clinical importance, sarcopenia remains under-recognized and poorly managed in routine clinical practice, partly owing to the lack of available diagnostic testing and uniform diagnostic criteria. Since the World Health Organization and the United States assigned a disease code for sarcopenia in 2016, countries worldwide have assigned their own disease codes for sarcopenia. However, there are currently no approved pharmacological agents for the treatment of sarcopenia; therefore, interventions for sarcopenia primarily focus on physical therapy for muscle strengthening and gait training as well as adequate protein intake. In this review, we aimed to examine the latest information on the epidemiology, molecular mechanisms, interventions, and possible treatments with new drugs for sarcopenia.

Aging alters mechanisms underlying voluntary movements in spinal motor neurons of mice, primates, and humans

Spinal motor neurons have been implicated in the loss of motor function that occurs with advancing age. However, the cellular and molecular mechanisms that impair the function of these neurons during aging remain unknown. Here, we show that motor neurons do not die in old female and male mice, rhesus monkeys, and humans. Instead, these neurons selectively and progressively shed excitatory synaptic inputs throughout the soma and dendritic arbor during aging. Thus, aged motor neurons contain a motor circuitry with a reduced ratio of excitatory to inhibitory synapses that may be responsible for the diminished ability to activate motor neurons to commence movements. An examination of the motor neuron translatome (ribosomal transcripts) in male and female mice reveals genes and molecular pathways with roles in glia-mediated synaptic pruning, inflammation, axonal regeneration, and oxidative stress that are upregulated in aged motor neurons. Some of these genes and pathways are also found altered in motor neurons affected with amyotrophic lateral sclerosis (ALS) and responding to axotomy, demonstrating that aged motor neurons are under significant stress. Our findings show mechanisms altered in aged motor neurons that could serve as therapeutic targets to preserve motor function during aging.

Plantar intrinsic foot muscle activation during functional exercises compared to isolated foot exercises in younger adults

BACKGROUND

Training the plantar intrinsic foot muscles (PIFMs) has the potential to benefit patients with lower extremity musculoskeletal conditions as well as the aged population. Isolated foot exercises, often standard in clinical practice, are difficult to perform, whereas functional exercises are much easier to accomplish. However, it is unclear whether functional exercises are comparable to isolated foot exercises in activating the PIFMs.

OBJECTIVE

This study aims to compare the activation of PIFMs between functional exercises versus isolated foot exercises.

METHODS

Using surface electromyography (EMG), muscle activation of three PIFMs was measured in four functional exercises (i.e. normal/unstable toe stance, toe walking, and hopping) versus a muscle-specific isolated foot exercise in 29 younger adults, resulting in 12 comparisons.

RESULTS

Functional exercises showed larger mean EMG amplitudes than the isolated foot exercises in 25% of the 12 comparisons, while there was no difference in the remaining 75%.

CONCLUSION

Functional exercises provoked comparable or even more activation of the PIFMs than isolated foot exercises. Given that functional exercises are easier to perform, this finding indicates the need to further investigate the effectiveness of functional exercises in physical therapy to improve muscle function and functional task performance in populations that suffer from PIFM weakness or dysfunction.

Modification of the locomotor pattern when deviating from the characteristic heel-to-toe rolling pattern during walking

PURPOSE

Humans are amongst few animals that step first on the heel, and then roll on the ball of the foot and toes. While this heel-to-toe rolling pattern has been shown to render an energetic advantage during walking, the effect of different foot contact strategies, on the neuromuscular control of adult walking gaits has received less attention. We hypothesised that deviating from heel-to-toe rolling pattern affects the energy transduction and weight acceptance and re-propulsive phases in gait along with the modification of spinal motor activity.

METHODS

Ten subjects walked on a treadmill normally, then placed their feet flat on the ground at each step and finally walked on the balls of the feet.

RESULTS

Our results show that when participants deviate from heel-to-toe rolling pattern strategy, the mechanical work increases on average 85% higher (F = 15.5; p < 0.001), mainly linked to a lack of propulsion at late stance. This modification of the mechanical power is related to a differential involvement of lumbar and sacral segment activation. Particularly, the delay between the major bursts of activation is on average 65% smaller, as compared to normal walking (F = 43.2; p < 0.001).

CONCLUSION

Similar results are observable in walking plantigrade animals, but also at the onset of independent stepping in toddlers, where the heel-to-toe rolling pattern is not yet established. These indications seem to bring arguments to the fact that the rolling of the foot during human locomotion has evolved to optimise gait, following selective pressures from the evolution of bipedal posture.

Deletion of Sod1 in Motor Neurons Exacerbates Age-Related Changes in Axons and Neuromuscular Junctions in Mice

Whole-body knock-out of Cu,Zn superoxide dismutase (Sod1KO) results in accelerated, age-related loss of muscle mass and function associated with neuromuscular junction (NMJ) breakdown similar to sarcopenia. In order to determine whether altered redox in motor neurons underlies this phenotype, an inducible neuron-specific deletion of Sod1 (i-mnSod1KO) was compared with wild-type (WT) mice of different ages (adult, mid-age, and old) and whole-body Sod1KO mice. Nerve oxidative damage, motor neuron numbers and structural changes to neurons and NMJ were examined. Tamoxifen-induced deletion of neuronal Sod1 from two months of age. No specific effect of a lack of neuronal Sod1 was seen on markers of nerve oxidation (electron paramagnetic resonance of an in vivo spin probe, protein carbonyl, or protein 3-nitrotyrosine contents). i-mnSod1KO mice showed increased denervated NMJ, reduced numbers of large axons and increased number of small axons compared with old WT mice. A large proportion of the innervated NMJs in old i-mnSod1KO mice displayed a simpler structure than that seen in adult or old WT mice. Thus, previous work showed that neuronal deletion of Sod1 induced exaggerated loss of muscle in old mice, and we report that this deletion leads to a specific nerve phenotype including reduced axonal area, increased proportion of denervated NMJ, and reduced acetyl choline receptor complexity. Other changes in nerve and NMJ structure seen in the old i-mnSod1KO mice reflect aging of the mice.

Aging affects the number and morphological heterogeneity of rat phrenic motor neurons and phrenic motor axons

Diaphragm muscle (DIAm) motor units comprise a phrenic motor neuron (PhMN), the phrenic nerve and the muscle fibers innervated, with the size of PhMNs and axons characteristic of motor unit type. Smaller PhMNs and their axons comprise slow (type S) and fatigue-resistant (type FR) DIAm motor units, while larger PhMNs and their axons comprise more fatigable (type FF) motor units. With aging, we have shown a loss of larger PhMNs, consistent with selective atrophy of type IIx/IIb DIAm fibers and reduced maximum DIAm force. In the present study, we hypothesized that with aging there is a loss of larger myelinated phrenic α motor axons. Female and male young (6 months) and old (24 months) Fischer 344 rats were studied. PhMNs were retrogradely labeled by intrapleural injection of 488-conjugated CTB. The phrenic nerves were excised ~1 cm from the DIAm insertion and mounted in resin, and phrenic α motor axons were delineated based on size (i.e., >4 μm diameters). In older rats, the number of larger PhMNs and larger phrenic α motor axons were reduced. There were no differences in non-α axons. In addition, there was evidence of demyelination of larger phrenic α motor axons in older rats. Together, these findings are consistent with the selective age-related vulnerability of larger PhMNs and denervation of type FF motor units, which may underlie DIAm sarcopenia.

Acylated Ghrelin Receptor Agonist HM01 Decreases Lean Body and Muscle Mass, but Unacylated Ghrelin Protects against Redox-Dependent Sarcopenia

Sarcopenia, the progressive loss of muscle mass and dysfunction, universally affects the elderly and is closely associated with frailty and reduced quality of life. Despite the inevitable consequences of sarcopenia and its relevance to healthspan, no pharmacological therapies are currently available. Ghrelin is a gut-released hormone that increases appetite and body weight upon acylation, which activates its receptor GHSR1a. Recent studies have demonstrated that acyl and unacylated ghrelin are protective against acute pathological conditions of skeletal muscle. We hypothesized that both acyl ghrelin receptor agonist (HM01) and unacylated ghrelin ameliorate muscle atrophy and contractile dysfunction in oxidative stress-induced sarcopenia. HM01, unacylated ghrelin, or saline was delivered via osmotic pump. HM01 increased food consumption transiently, while the body weight remained elevated. It also decreased lean body mass and muscle mass of wildtype and Sod1KO. In contrast, unacylated ghrelin ameliorated loss of muscle mass by 15-30% in Sod1KO mice without changes in food consumption or body weights. Contractile force was decreased by ~30% in Sod1KO mice, but unacylated ghrelin prevented the force deficit by ~80%. We identified downregulation of transcription factor FoxO3a and its downstream E3 ligase MuRF1 by unacylated ghrelin. Our data show a direct role of unacylated ghrelin in redox-dependent sarcopenia independent of changes of food consumption or body weight.

Bioenergetic Evaluation of Muscle Fatigue in Murine Tongue

Muscle fatigue is the diminution of force required for a particular action over time. Fatigue may be particularly pronounced in aging muscles, including those used for swallowing actions. Because risk for swallowing impairment (dysphagia) increases with aging, the contribution of muscle fatigue to age-related dysphagia is an emerging area of interest. The use of animal models, such as mice and rats (murine models) allows experimental paradigms for studying the relationship between muscle fatigue and swallowing function with a high degree of biological precision that is not possible in human studies. The goal of this article is to review basic experimental approaches to the study of murine tongue muscle fatigue related to dysphagia. Traditionally, murine muscle fatigue has been studied in limb muscles through direct muscle stimulation and behavioral exercise paradigms. As such, physiological and bioenergetic markers of muscle fatigue that have been validated in limb muscles may be applicable in studies of cranial muscle fatigue with appropriate modifications to account for differences in muscle architecture, innervation ratio, and skeletal support. Murine exercise paradigms may be used to elicit acute fatigue in tongue muscles, thereby enabling study of putative muscular adaptations. Using these approaches, hypotheses can be developed and tested in mice and rats to allow for future focused studies in human subjects geared toward developing and optimizing treatments for age-related dysphagia.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Evaluation of Spatiotemporal Patterns of the Spinal Muscle Coordination Output during Walking in the Exoskeleton

Recent advances in the performance and evaluation of walking in exoskeletons use various assessments based on kinematic/kinetic measurements. While such variables provide general characteristics of gait performance, only limited conclusions can be made about the neural control strategies. Moreover, some kinematic or kinetic parameters are a consequence of the control implemented on the exoskeleton. Therefore, standard indicators based on kinematic variables have limitations and need to be complemented by performance measures of muscle coordination and control strategy. Knowledge about what happens at the spinal cord output level might also be critical for rehabilitation since an abnormal spatiotemporal integration of activity in specific spinal segments may result in a risk for abnormalities in gait recovery. Here we present the PEPATO software, which is a benchmarking solution to assess changes in the spinal locomotor output during walking in the exoskeleton with respect to reference data on normal walking. In particular, functional and structural changes at the spinal cord level can be mapped into muscle synergies and spinal maps of motoneuron activity. A user-friendly software interface guides the user through several data processing steps leading to a set of performance indicators as output. We present an example of the usage of this software for evaluating walking in an unloading exoskeleton that allows a person to step in simulated reduced (the Moon's) gravity. By analyzing the EMG activity from lower limb muscles, the algorithms detected several performance indicators demonstrating differential adaptation (shifts in the center of activity, prolonged activation) of specific muscle activation modules and spinal motor pools and increased coactivation of lumbar and sacral segments. The software is integrated at EUROBENCH facilities to benchmark the performance of walking in the exoskeleton from the point of view of changes in the spinal locomotor output.

Changes in neuromuscular function in elders: Novel techniques for assessment of motor unit loss and motor unit remodeling with aging

Functional muscle fiber denervation is a major contributor to the decline in physical function observed with aging and is now a recognized cause of sarcopenia, a muscle disorder characterized by progressive and generalized degenerative loss of skeletal muscle mass, quality, and strength. There is an interrelationship between muscle strength, motor unit (MU) number, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functional MUs. During normal aging, there is a time-related progression of MU loss, an adaptive sprouting followed by a maladaptive sprouting, and continuing recession of terminal Schwann cells leading to a reduced capacity for compensatory reinnervation in elders. In amyotrophic lateral sclerosis, increasing age at onset predicts worse survival ALS and it is possible that age-related depletion of the motor neuron pool may worsen motor neuron disease. MUNE methods are used to estimate the number of functional MU, data from MUNIX arguing for motor neuron loss with aging will be reviewed. Recently, a new MRI technique MU-MRI could be used to assess the MU recruitment or explore the activity of a single MU. This review presents published studies on the changes of neuromuscular function with aging, then focusing on these two novel techniques for assessment of MU loss and MU remodeling.

Human skeletal muscle acetylcholine receptor gene expression in elderly males performing heavy resistance exercise

Muscle fiber denervation is a major contributor to the decline in muscle mass and function during aging. Heavy resistance exercise is an effective tool for increasing muscle mass and strength, but whether it can rescue denervated muscle fibers remains unclear. Therefore, the purpose of this study was to investigate the potential of heavy resistance exercise to modify indices of denervation in healthy elderly individuals. 38 healthy elderly men (72±5 years) underwent 16 weeks of heavy resistance exercise while 20 healthy elderly men (72±6 years) served as non-exercising sedentary controls. Muscle biopsies were obtained pre and post training, and midway at eight weeks. Biopsies were analysed by immunofluorescence for the prevalence of myofibers expressing embryonic myosin (MyHCe), neonatal myosin (MyHCn), nestin, and neural cell adhesion molecule (NCAM), and by RT-qPCR for gene expression levels of acetylcholine receptor (AChR) subunits, MyHCn, MyHCe, p16 and Ki67. In addition to increases in strength and type II fiber hypertrophy, heavy resistance exercise training led to a decrease in AChR α1 and ε subunit mRNA (at eight weeks). Changes in gene expression levels of the α1 and ε AChR subunits with eight weeks of heavy resistance exercise supports the role of this type of exercise in targeting stability of the neuromuscular junction. The number of fibers positive for NCAM, nestin, and MyHCn was not affected, suggesting that a longer timeframe is needed for adaptations to manifest at the protein level.

Redox Control of Signalling Responses to Contractile Activity and Ageing in Skeletal Muscle

Research over almost 40 years has established that reactive oxygen species are generated at different sites in skeletal muscle and that the generation of these species is increased by various forms of exercise. Initially, this was thought to be potentially deleterious to skeletal muscle and other tissues, but more recent data have identified key roles of these species in muscle adaptations to exercise. The aim of this review is to summarise our current understanding of these redox signalling roles of reactive oxygen species in mediating responses of muscle to contractile activity, with a particular focus on the effects of ageing on these processes. In addition, we provide evidence that disruption of the redox status of muscle mitochondria resulting from age-associated denervation of muscle fibres may be an important factor leading to an attenuation of some muscle responses to contractile activity, and we speculate on potential mechanisms involved.

Enhancing Locomotor Learning With Transcutaneous Spinal Electrical Stimulation and Somatosensory Augmentation: A Pilot Randomized Controlled Trial in Older Adults

This study investigated locomotor learning of a complex terrain walking task in older adults, when combined with two adjuvant interventions: transcutaneous spinal direct current stimulation (tsDCS) to increase lumbar spinal cord excitability, and textured shoe insoles to increase somatosensory feedback to the spinal cord. The spinal cord has a crucial contribution to control of walking, and is a novel therapeutic target for rehabilitation of older adults. The complex terrain task involved walking a 10-meter course consisting of nine obstacles and three sections of compliant (soft) walking surface. Twenty-three participants were randomly assigned to one of the following groups: sham tsDCS and smooth insoles (sham/smooth; control group), sham tsDCS and textured insoles (sham/textured), active tsDCS and smooth insoles (active/smooth), and active tsDCS and textured insoles (active/textured). The first objective was to assess the feasibility, tolerability, and safety of the interventions. The second objective was to assess preliminary efficacy for increasing locomotor learning, as defined by retention of gains in walking speed between a baseline visit of task practice, and a subsequent follow-up visit. Variability of the center of mass while walking over the course was also evaluated. The change in executive control of walking (prefrontal cortical activity) between the baseline and follow-up visits was measured with functional near infrared spectroscopy. The study results demonstrated feasibility based on enrollment and retention of participants, tolerability based on self-report, and safety based on absence of adverse events. Preliminary efficacy was supported based on trends showing larger gains in walking speed and more pronounced reductions in mediolateral center of mass variability at the follow-up visit in the groups randomized to active tsDCS or textured insoles. These data justify future larger studies to further assess dosing and efficacy of these intervention approaches. In conclusion, rehabilitation interventions that target spinal control of walking present a potential opportunity for enhancing walking function in older adults.

Scavenging mitochondrial hydrogen peroxide by peroxiredoxin 3 overexpression attenuates contractile dysfunction and muscle atrophy in a murine model of accelerated sarcopenia

Age-related muscle atrophy and weakness, or sarcopenia, are significant contributors to compromised health and quality of life in the elderly. While the mechanisms driving this pathology are not fully defined, reactive oxygen species, neuromuscular junction (NMJ) disruption, and loss of innervation are important risk factors. The goal of this study is to determine the impact of mitochondrial hydrogen peroxide on neurogenic atrophy and contractile dysfunction. Mice with muscle-specific overexpression of the mitochondrial H₂ O₂  scavenger peroxiredoxin3 (mPRDX3) were crossed to Sod1KO mice, an established mouse model of sarcopenia, to determine whether reduced mitochondrial H₂ O₂ can prevent or delay the redox-dependent sarcopenia. Basal rates of H₂ O₂  generation were elevated in isolated muscle mitochondria from Sod1KO, but normalized by mPRDX3 overexpression. The mPRDX3 overexpression prevented the declines in maximum mitochondrial oxygen consumption rate and calcium retention capacity in Sod1KO. Muscle atrophy in Sod1KO was mitigated by ~20% by mPRDX3 overexpression, which was associated with an increase in myofiber cross-sectional area. With direct muscle stimulation, maximum isometric specific force was reduced by ~20% in Sod1KO mice, and mPRDX3 overexpression preserved specific force at wild-type levels. The force deficit with nerve stimulation was exacerbated in Sod1KO compared to direct muscle stimulation, suggesting NMJ disruption in Sod1KO. Notably, this defect was not resolved by overexpression of mPRDX3. Our findings demonstrate that muscle-specific PRDX3 overexpression reduces mitochondrial H₂ O₂  generation, improves mitochondrial function, and mitigates loss of muscle quantity and quality, despite persisting NMJ impairment in a murine model of redox-dependent sarcopenia.

Reinnervation of Vastus lateralis is increased significantly in seniors (70-years old) with a lifelong history of high-level exercise (2013, revisited here in 2022)

In 2013 we presented results showing that at the histological level lifelong increased physical activity promotes reinnervation of muscle fibers in aging muscles. Indeed, in muscle biopsies from 70-year old men with a lifelong history of high-level physical activity, we observed a considerable increase in fiber-type groupings (F-TG), almost exclusively of the slow type. Slow-type transformation by denervation-reinnervation in senior sportsmen seems to fluctuate from those with scarce fiber-type transformation and groupings to almost fully transformed muscle, going through a process in which isolated fibers co-expressing fast and slow Myosin Heavy Chains (MHCs) seems to fill the gaps. Taken together, our results suggest that, beyond the direct effects of aging on the muscle fibers, changes occurring in skeletal muscle tissue appear to be largely, although not solely, a result of sparse denervation-reinnervation. The lifelong exercise allows the body to adapt to the consequences of the age-related denervation and to preserve muscle structure and function by saving otherwise lost muscle fibers through recruitment to different, mainly slow, motor units. These beneficial effects of high-level life-long exercise on motoneurons, specifically on the slow type motoneurones that are those with higher daily activity, and on muscle fibers, serve to maintain size, structure and function of muscles, delaying the functional decline and loss of independence that are commonly seen in late aging. Several studies of independent reserchers with independent analyses confirmed and cited our 2013 results. Thus, the results we presented in our paper in 2013 seem to have held up rather well. Trial Registration: ClinicalTrials.gov: NCT01679977

Preserved stem cell content and innervation profile of elderly human skeletal muscle with lifelong recreational exercise

Abstract

Muscle fibre denervation and declining numbers of muscle stem (satellite) cells are defining characteristics of ageing skeletal muscle. The aim of this study was to investigate the potential for lifelong recreational exercise to offset muscle fibre denervation and compromised satellite cell content and function, both at rest and under challenged conditions. Sixteen elderly lifelong recreational exercisers (LLEX) were studied alongside groups of age‐matched sedentary (SED) and young subjects. Lean body mass and maximal voluntary contraction were assessed, and a strength training bout was performed. From muscle biopsies, tissue and primary myogenic cell cultures were analysed by immunofluorescence and RT‐qPCR to assess myofibre denervation and satellite cell quantity and function. LLEX demonstrated superior muscle function under challenged conditions. When compared with SED, the muscle of LLEX was found to contain a greater content of satellite cells associated with type II myofibres specifically, along with higher mRNA levels of the beta and gamma acetylcholine receptors (AChR). No difference was observed between LLEX and SED for the proportion of denervated fibres or satellite cell function, as assessed in vitro by myogenic cell differentiation and fusion index assays. When compared with inactive counterparts, the skeletal muscle of lifelong exercisers is characterised by greater fatigue resistance under challenged conditions in vivo, together with a more youthful tissue satellite cell and AChR profile. Our data suggest a little recreational level exercise goes a long way in protecting against the emergence of classic phenotypic traits associated with the aged muscle.

Key points

The detrimental effects of ageing can be partially offset by lifelong self‐organized recreational exercise, as evidence by preserved type II myofibre‐associated satellite cells, a beneficial muscle innervation status and greater fatigue resistance under challenged conditions.

Satellite cell function (in vitro), muscle fibre size and muscle fibre denervation determined by immunofluorescence were not affected by recreational exercise.

Individuals that are recreationally active are far more abundant than master athletes, which sharply increases the translational perspective of the present study. Future studies should further investigate recreational activity in relation to muscle health, while also including female participants.

DO MOTONEURON DISCHARGE RATES SLOW WITH AGING? A SYSTEMATIC REVIEW AND META-ANALYSIS

Nervous system maladaptation is linked to the loss of maximal strength and motor control with aging. Motor unit discharge rates are a critical determinant of force production; thus, lower discharge rates could be a mechanism underpinning maximal strength and motor control losses during aging. This meta-analysis summarized the findings of studies comparing motor unit discharge rates between young and older adults, and examined the effects of the selected muscle and contraction intensity on the magnitude of discharge rates difference between these two groups. Estimates from 29 studies, across a range of muscles and contraction intensities, were combined in a multilevel meta-analysis, to investigate whether discharge rates differed between young and older adults. Motor unit discharge rates were higher in younger than older adults, with a pooled standardized mean difference (SMD) of 0.66 (95%CI= 0.29-1.04). Contraction intensity had a significant effect on the pooled SMD, with a 1% increase in intensity associated with a 0.009 (95%CI= 0.003-0.015) change in the pooled SMD. These findings suggest that reductions in motor unit discharge rates, especially at higher contraction intensities, may be an important mechanism underpinning age-related losses in maximal force production.

Pharmacologic treatment with OKN-007 reduces alpha-motor neuron loss in spinal cord of aging mice

Aging is associated with molecular and functional declines in multiple physiologic systems. We have previously reported age-related changes in spinal cord that included a decline in α-motor neuron numbers, axonal loss, and demyelination associated with increased inflammation and blood-spinal cord barrier (BSCB) permeability. These changes may influence other pathologies associated with aging, in particular loss of muscle mass and function (sarcopenia), which we and others have shown is accompanied by neuromuscular junction disruption and loss of innervation. Interventions to protect and maintain motor neuron viability and function in aging are currently lacking and could have a significant impact on improving healthspan. Here we tested a promising compound, OKN-007, that has known antioxidant, anti-inflammatory and neuroprotective properties, as a potential intervention in age-related changes in the spinal cord. OKN-007 is a low molecular weight disulfonyl derivative of (N-tert Butyl-α-phenylnitrone) (PBN) that can easily cross the blood-brain barrier. We treated middle age (16 month) wild-type male mice with OKN-007 in drinking water at a dose of 150 mg/kg/day until 25 months of age. OKN-007 treatment exerted a number of beneficial effects in the aging spinal cord, including a 35% increase in the number of lumbar α-motor neurons in OKN-treated old mice compared to age-matched controls. Brain spinal cord barrier permeability, which is increased in aging spinal cord, was also blunted by OKN-007 treatment. Age-related changes in microglia proliferation and activation are blunted by OKN-007, while we found no effect on astrocyte proliferation. Transcriptome analysis identified expression changes in a number of genes that are involved in neuronal structure and function and revealed a subset of genes whose changes in response to aging are reversed by OKN-007 treatment. Overall, our findings suggest that OKN-007 exerts neuroprotective and anti-inflammatory effects on the aging spinal cord and support OKN-007 as a potential therapeutic to improve α-motor neuron health.

The Influence of Motor Unit Number and Muscle Activation on Early Phase Rate of Torque Development in Younger and Older Men

This study examined the influence of muscle activation and motor unit number estimation (MUNE) on early phase voluntary rate of torque development (RTD) in younger (YM) and older (OM) men. Thirty-two YM (n = 17; Age = 22 yrs) and OM (n = 15; Age = 74 yrs) volunteered to participate in this study. Early phase RTD (first 50 ms of a rapid isometric contraction; RTD₅₀) and normalized surface electromyography (first 50 ms of muscle excitation; nEMG₅₀) were recorded from the right quadricep muscle group. MUNE was examined from the right vastus lateralis. Multiple linear regression analyses revealed that nEMG₅₀ had a significant effect on RTD₅₀ independent of age group (p ≤ 0.001). nEMG₅₀ had a significant effect on RTD₅₀ in the OM group (p = 0.037). MUNE had no effect on RTD₅₀ independent of age. Older adults may depend more on muscle activation at contraction onset of early phase RTD compared to younger adults.

Neuromuscular Age-Related Adjustment of Gait When Moving Upwards and Downwards

Locomotor movements are accommodated to various surface conditions by means of specific locomotor adjustments. This study examined underlying age-related differences in neuromuscular control during level walking and on a positive or negative slope, and during stepping upstairs and downstairs. Ten elderly and eight young adults walked on a treadmill at two different speeds and at three different inclinations (0°, +6°, and −6°). They were also asked to ascend and descend stairs at self-selected speeds. Full body kinematics and surface electromyography of 12 lower-limb muscles were recorded. We compared the intersegmental coordination, muscle activity, and corresponding modifications of spinal motoneuronal output in young and older adults. Despite great similarity between the neuromuscular control of young and older adults, our findings highlight subtle age-related differences in all conditions, potentially reflecting systematic age-related adjustments of the neuromuscular control of locomotion across various support surfaces. The main distinctive feature of walking in older adults is a significantly wider and earlier activation of muscles innervated by the sacral segments. These changes in neuromuscular control are reflected in a reduction or lack of propulsion observed at the end of stance in older adults at different slopes, with the result of a delay in the timing of redirection of the centre-of-mass velocity and of an unanticipated step-to-step transition strategy.

Variation in muscle and neuromuscular junction morphology between atrophy-resistant and atrophy-prone muscles supports failed re-innervation in aging muscle atrophy

In advanced age, there is an accelerated decline in skeletal muscle mass that appears to be secondary to repeated cycles of denervation-reinnervation and eventually, failed reinnervation. However, whether variation in reinnervation capacity explains why some muscles are less vulnerable to age-related atrophy has not been addressed. In this study we examined changes in neuromuscular junction (NMJ) morphology, fiber cross-sectional area (CSA) and fiber type, accumulation of severely atrophied myofibers, and expression of a marker of denervation in four muscles that exhibit differences in the degree of age-related atrophy and which span the extremes of fiber type composition in 8 mo old (8 M) and 34 mo old (34 M) male Fischer 344 Brown Norway F1 hybrid rats. Aging muscle atrophy was most pronounced in the fast twitch gastrocnemius (Gas; 25%) and similar between extensor digitorum longus (EDL) and slow-twitch soleus (Sol) muscle (14-15%), whereas the slow-twitch adductor longus (AL) increased in mass by 21% between 8 M and 34 M (P < 0.05 for all). Only the Sol exhibited significant alterations in fiber type with aging, and there was a decrease in fiber CSA in the Gas, EDL, and Sol (P < 0.05) with aging that was not seen in the AL. Muscles that atrophied had an increased fraction of severely atrophic myofibers (P < 0.05), but this was not observed in the AL. The Gas and EDL both demonstrated a similar degree of age-related remodeling of pre- and post-synaptic NMJ components. On the other hand, pre- and post-synaptic morphology underwent greater changes with aging in the AL, and many of these same morphological variables were already greater in the Sol vs AL at 8 M, suggesting the Sol had already undergone substantial remodeling and may be nearing its adaptive limits. Consistent with this idea, analysis of NMJ morphology in Sol from 3 M rats exhibited similar values as 8 M AL, and the Sol demonstrated greater expression of the denervation marker neural cell adhesion molecule (NCAM) compared to the AL at 34 M. Collectively, our results are consistent with NMJ remodeling capacity being finite with aging and that maintained remodeling potential confers atrophy protection in aging skeletal muscle by reducing the degree of persistent denervation.

Shared and distinct mechanisms of skeletal muscle atrophy: A narrative review

Maintenance of skeletal muscle mass and function is an incredibly nuanced balance of anabolism and catabolism that can become distorted within different pathological conditions. In this paper we intend to discuss the distinct intracellular signaling events that regulate muscle protein atrophy for a given clinical occurrence. Aside from the common outcome of muscle deterioration, several conditions have at least one or more distinct mechanisms that creates unique intracellular environments that facilitate muscle loss. The subtle individuality to each of these given pathologies can provide both researchers and clinicians with specific targets of interest to further identify and increase the efficacy of medical treatments and interventions.

Deletion of Neuronal CuZnSOD Accelerates Age-Associated Muscle Mitochondria and Calcium Handling Dysfunction That Is Independent of Denervation and Precedes Sarcopenia

Skeletal muscle suffers atrophy and weakness with aging. Denervation, oxidative stress, and mitochondrial dysfunction are all proposed as contributors to age-associated muscle loss, but connections between these factors have not been established. We examined contractility, mitochondrial function, and intracellular calcium transients (ICTs) in muscles of mice throughout the life span to define their sequential relationships. We performed these same measures and analyzed neuromuscular junction (NMJ) morphology in mice with postnatal deletion of neuronal Sod1 (i-mn-Sod1^-/- mice), previously shown to display accelerated age-associated muscle loss and exacerbation of denervation in old age, to test relationships between neuronal redox homeostasis, NMJ degeneration and mitochondrial function. In control mice, the amount and rate of the decrease in mitochondrial NADH during contraction was greater in middle than young age although force was not reduced, suggesting decreased efficiency of NADH utilization prior to the onset of weakness. Declines in both the peak of the ICT and force were observed in old age. Muscles of i-mn-Sod1^-/- mice showed degeneration of mitochondrial and calcium handling functions in middle-age and a decline in force generation to a level not different from the old control mice, with maintenance of NMJ morphology. Together, the findings support the conclusion that muscle mitochondrial function decreases during aging and in response to altered neuronal redox status prior to NMJ deterioration or loss of mass and force suggesting mitochondrial defects contribute to sarcopenia independent of denervation.

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.

Effect of Caloric Restriction on Aging: Fixing the Problems of Nutrient Sensing in Postmitotic Cells?

The review discusses the role of metabolic disorders (in particular, insulin resistance) in the development of age-related diseases and normal aging with special emphasis on the changes in postmitotic cells of higher organisms. Caloric restriction helps to prevent such metabolic disorders, which could probably explain its ability to prolong the lifespan of laboratory animals. Maintaining metabolic homeostasis is especially important for the highly differentiated long-lived body cells, whose lifespan is comparable to the lifespan of the organism itself. Normal functioning of these cells can be ensured only upon correct functioning of the cytoplasm clean-up system and availability of all required nutrients and energy sources. One of the central problems in gerontology is the age-related disruption of glucose metabolism leading to obesity, diabetes, metabolic syndrome, and other related pathologies. Along with the adipose tissue, skeletal muscles are the main consumers of insulin; hence the physical activity of muscles, which supports their energy metabolism, delays the onset of insulin resistance. Insulin resistance disrupts the metabolism of cardiomyocytes, so that they fail to utilize the nutrients to perform their functions even being surrounded by a nutrient-rich environment, which contributes to the development of age-related cardiovascular diseases. Metabolic pathologies also alter the nutrient sensitivity of neurons, thus disrupting the action of insulin in the central nervous system. In addition, there is evidence that neurons can develop insulin resistance as well. It has been suggested that affecting nutritional sensors (e.g., AMPK) in postmitotic cells might improve the state of the entire multicellular organism, slow down its aging, and increase the lifespan.

Influence of muscle fatigue on contractile twitch characteristics in persons with parkinson's disease and older adults: A pilot study

Introduction

It is widely accepted that pathophysiological changes to the central nervous system of persons with Parkinson's disease (PD) result in negative effects on motor function. However, less information is known regarding the pathology of PD on skeletal muscle. The purpose of this study was to determine the effect of a fatiguing isometric knee extension protocol on muscle mechanics using evoked twitch contractions in persons with PD and in non-impaired older adults (OLD).

Methods

Evoked twitch contractions were examined during a fatiguing protocol in PD (66 ± 9 yr, n = 8) and OLD (65 ± 10 yr, n = 5). Participants performed 5-sec maximal isometric voluntary contractions of the quadriceps femoris with 5-sec rest for 3-min. Every 30-sec during rest intervals, a maximal transcutaneous electrical stimulus was administered to the quadriceps femoris to quantify evoked peak twitch torque (pTT), peak relaxation rate (pRR), and peak rate of torque development (pRTD).

Results

A large effect of voluntary fatigue (%decline) was observed (g = 1.58). There were no significant differences in pTT (p = 0.09; 95% CI:-3.6, 0.28) or pRR (p = 0.11; 95% CI:-31, 3.6). However, the slope decline of pRTD in OLD (-35.4 ± 24.7) was greater than PD (-11.5 ± 11.4; p = 0.03), indicating that skeletal muscle in persons with PD is less fatigable compared to non-impaired older adults.

Conclusion

The rate, not the maximum capacity, of torque generation of the muscle during a fatiguing knee extension protocol was affected by PD. Future studies are warranted to identify the mechanism(s) responsible for the observed differences in skeletal muscle contractile characteristics and potential myofiber distribution variation in PD.

Blood-spinal cord barrier leakage is independent of motor neuron pathology in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease involving progressive degeneration of upper and lower motor neurons. The pattern of lower motor neuron loss along the spinal cord follows the pattern of deposition of phosphorylated TDP-43 aggregates. The blood-spinal cord barrier (BSCB) restricts entry into the spinal cord parenchyma of blood components that can promote motor neuron degeneration, but in ALS there is evidence for barrier breakdown. Here we sought to quantify BSCB breakdown along the spinal cord axis, to determine whether BSCB breakdown displays the same patterning as motor neuron loss and TDP-43 proteinopathy. Cerebrospinal fluid hemoglobin was measured in living ALS patients (n = 87 control, n = 236 ALS) as a potential biomarker of BSCB and blood–brain barrier leakage. Cervical, thoracic, and lumbar post-mortem spinal cord tissue (n = 5 control, n = 13 ALS) were then immunolabelled and semi-automated imaging and analysis performed to quantify hemoglobin leakage, lower motor neuron loss, and phosphorylated TDP-43 inclusion load. Hemoglobin leakage was observed along the whole ALS spinal cord axis and was most severe in the dorsal gray and white matter in the thoracic spinal cord. In contrast, motor neuron loss and TDP-43 proteinopathy were seen at all three levels of the ALS spinal cord, with most abundant TDP-43 deposition in the anterior gray matter of the cervical and lumbar cord. Our data show that leakage of the BSCB occurs during life, but at end-stage disease the regions with most severe BSCB damage are not those where TDP-43 accumulation is most abundant. This suggests BSCB leakage and TDP-43 pathology are independent pathologies in ALS.