Muscle fiber number in the biceps brachii muscle of young and old men

Association of Vitamin D Deficiency With Distal Biceps Injury: A Retrospective Analysis of 336,320 Patients

BACKGROUND

This study explores the association between vitamin D deficiency and distal biceps tendon injuries, illustrating that, although vitamin D deficiency is associated with prolonged hospital stays and various musculoskeletal problems, its connection to distal biceps tendon injuries is unknown.

HYPOTHESIS

Vitamin D deficiency is associated with an elevated risk of distal biceps injury but not with increased rates of subsequent surgery or revision surgery.

STUDY DESIGN

Case-control study.

LEVEL OF EVIDENCE

Level 3.

METHODS

A 1:1 matched retrospective comparative study of 336,320 vitamin-D-deficient patients was performed using PearlDiver data (between January 1, 2011 and October 31, 2018). Cohorts, with a mean age of 55.7 ± 13.2 years, underwent multivariate logistic regression to calculate distal biceps tendon injury and surgical repair incidence according to age and sex, while controlling for demographics and comorbidities.

RESULTS

The 1-year incidence of distal biceps tendinopathy in vitamin-D-deficient patients was 118 per 100,000 person-years (95% CI) compared with 44.3 per 100,000 person-years in matched controls. Male patients with vitamin D deficiency were at a greater risk for distal biceps tendinopathy after 1 and 2 years (adjusted odds ratio [aOR] = 2.81, 2.08-3.83; aOR = 2.80, 2.21-3.56). Female patients were also at a greater risk after both years (aOR = 1.69, 1.27-2.27; aOR = 1.57, 1.26-1.96). Vitamin D deficiency was not associated with an elevated risk of surgical repair or revision surgery.

CONCLUSION

In a nationwide cohort, a diagnosis of vitamin D deficiency elevated the risk of distal biceps tendinopathy but did not raise the rate of surgical repair or revision. As a result, prevention strategies in the form of vitamin supplementation should be increased for athletes.Clinical Relevance: These findings emphasize the clinical relevance of monitoring vitamin D levels in patients at risk for musculoskeletal injuries, and providing adequate care to those involved in high-demand physical activities.Strength of Recommendation: B.

Liquid Metal Actuators: A Comparative Analysis of Surface Tension Controlled Actuation

Liquid metals, with their unique combination of electrical and mechanical properties, offer great opportunities for actuation based on surface tension modulation. Thanks to the scaling laws of surface tension, which can be electrochemically controlled at low voltages, liquid metal actuators stand out from other soft actuators for their remarkable characteristics such as high contractile strain rates and higher work densities at smaller length scales. This review summarizes the principles of liquid metal actuators and discusses their performance as well as theoretical pathways toward higher performances. The objective is to provide a comparative analysis of the ongoing development of liquid metal actuators. The design principles of the liquid metal actuators are analyzed, including low-level elemental principles (kinematics and electrochemistry), mid-level structural principles (reversibility, integrity, and scalability), and high-level functionalities. A wide range of practical use cases of liquid metal actuators from robotic locomotion and object manipulation to logic and computation is reviewed. From an energy perspective, strategies are compared for coupling the liquid metal actuators with an energy source toward fully untethered robots. The review concludes by offering a roadmap of future research directions of liquid metal actuators.

The mechanoreceptor pezo-1 is required for normal crawling locomotion in the nematode C. elegans

The discovery in 2010 of the PIEZO family of mechanoreceptors revolutionized our understanding of the role of proprioceptive feedback in mammalian physiology. Much remains to be elucidated. This study looks at the role this receptor plays in normal locomotion. Like humans, the nematode C. elegans expresses PIEZO-type channels (encoded by the pezo-1 gene) throughout its somatic musculature. Here we use the unbiased automated behavioral software Tierpsy to characterize the effects that mutations removing PEZO-1 from body wall musculature have on C. elegans crawling. We find that loss of PEZO-1 results in disrupted locomotion and posture, consistent with phenotypes associated with loss of PIEZO2 in human musculature. C. elegans is thus an amenable system to study the role of mechanoreception on muscle physiology and function.

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.

Physiological-induced conductive response evaluation in specific muscle compartments under hybrid of electrical muscle stimulation and voluntary resistance training by electrical impedance tomography

Objective: The physiological-induced conductive response has been visualised for evaluation in specific muscle compartments under hybrid (hybridEMS) of electrical muscle stimulation (EMS) and voluntary resistance training (VRT) by electrical impedance tomography (EIT). Methods: In the experiments, tendency of conductivity distribution images σ over time was clearly detected for three specific muscle compartments, which are called AM 1 compartment composed of biceps brachii muscle, AM 2 compartment composed of triceps brachii muscle, and AM 3 compartment composed of brachialis muscle, under three training modalities. Results: From the experimental results, the tendency of physiological-induced conductive response are increased in all three training modalities with increasing training time. Correspondingly, the spatial-mean conductivity <σ>AM1,AM2,AM3 increased with the conductance value G and extracellular water ratio β of right arm by bio-impedance analysis (BIA) method. In addition, hybridEMS has the greatest effect on physiological-induced conductive response in AM 1, AM 2, and AM 3. Under hybridEMS, the spatial-mean conductivity increased from <σ pre > AM1 = 0.154 to <σ 23mins > AM1 = 0.810 in AM 1 muscle compartment (n = 8, p < 0.001); <σ pre > AM2 = 0.040 to <σ 23mins > AM2 = 0.254 in AM 2 muscle compartment (n = 8, p < 0.05); <σ pre > AM3 = 0.078 to <σ 23mins > AM3 = 0.497 in AM 3 muscle compartment (n = 8, p < 0.05). Conclusion: The paired-samples t-test results of <σ>AM1,AM2,AM3 under all three training modalities suggest hybridEMS has the most efficient elicitation on physiological induced conductive response compared to VRT and EMS. The effect of EMS on deep muscle compartment (AM 3) is slower compared to VRT and hybridEMS, with a significant difference after 15 min of training.

Human skeletal muscle-specific atrophy with aging: a comprehensive review

Age-related skeletal muscle atrophy appears to be a muscle group-specific process, yet only a few specific muscles have been investigated and our understanding in this area is limited. This review provides a comprehensive summary of the available information on age-related skeletal muscle atrophy in a muscle-specific manner, nearly half of which comes from the quadriceps. Decline in muscle-specific size over ∼50 yr of aging was determined from 47 cross-sectional studies of 982 young (∼25 yr) and 1,003 old (∼75 yr) individuals and nine muscle groups: elbow extensors (-20%, -0.39%/yr), elbow flexors (-19%, -0.38%/yr), paraspinals (-24%, -0.47%/yr), psoas (-29%, -0.58%/yr), hip adductors (-13%, -0.27%/yr), hamstrings (-19%, -0.39%/yr), quadriceps (-27%, -0.53%/yr), dorsiflexors (-9%, -0.19%/yr), and triceps surae (-14%, -0.28%/yr). Muscle-specific atrophy rate was also determined for each of the subcomponent muscles in the hamstrings, quadriceps, and triceps surae. Of all the muscles included in this review, there was more than a fivefold difference between the least (-6%, -0.13%/yr, soleus) to the most (-33%, -0.66%/yr, rectus femoris) atrophying muscles. Muscle activity level, muscle fiber type, sex, and timeline of the aging process all appeared to have some influence on muscle-specific atrophy. Given the large range of muscle-specific atrophy and the large number of muscles that have not been investigated, more muscle-specific information could expand our understanding of functional deficits that develop with aging and help guide muscle-specific interventions to improve the quality of life of aging women and men.

Robust morris screening method (RMSM) for complex physiological models

BACKGROUND AND OBJECTIVES

Morris screening sensitivity analysis (MSM) comes forth as the method needing the minimum number of model simulations to qualify the impact of input parameter variations on outputs of complex, nonlinear and overparametrized models. However, the reliability of MSM indices (mean and standard deviation) and the reproducibility of their results are rarely explored despite the input parameter tuning/identification needs. In fact, these models, such those used in medical applications as digital twins, often lie in this category and need efficient and robust tools to assess both sensitivity and reliability of the outputs to numerous input model parameters.

METHODS

In this study, a new Robust Morris Screening Method (RMSM) is proposed and based on new indices: the absolute median (χ^*) and the median absolute deviation (ρ). The proposed RMSM approach is evaluated on a complex multi-scales neuromuscular electrophysiological model simulating HD-sEMG (high density surface electromyography) signals at the skin surface. The reliability and stability of new RMSM indicators are evaluated at different trajectories within the parameter space and compared to classical MSM results. For this purpose, We propose a new methodology for parameter screening based on the ratio ρ/χ^* as a graphic indicator of (non)linearity and (non)monotonicity of parameter effects.

RESULTS

Firstly, the results demonstrated that the computed elementary effects (EE) of inputs are not normally distributed using MSM indices contrary to the proposed RMSM indices. Secondly, the ranking stability of RMSM indices was earlier obtained from 20 trajectories (T=20), while MSM ranking remained unstable until T = 100. Thirdly, The screening separation between influential and negligible input model parameters was more distinct and interpretable with RMSM than MSM.

CONCLUSION

The proposed RMSM approach ensures a fast, reliable and stable ranking of parameters for complex and overparametrized models compared to classical MSM. this allows a more precise exploration of the model parameter influence space for future application in parameter tuning and identification.

Does the early phase of aging affect the morphology of biceps brachii and torque and total work of elbow flexors in healthy volunteers?

Upper and lower limbs can be affected by several diseases and changes related to current life habits, such as the sedentarism, technological advances, and even eating habits. This cross-sectional study investigated morphological adaptations of the biceps brachii muscle and the performance of the elbow flexors in healthy individuals in the early phase of aging. Thirty-two volunteers were separated according to age range (3rd, 4th, and 5th decades of life) and sex. Smaller diameters and subtypes of fibers were evaluated using muscle biopsies, and peak torque and total work were assessed using an isokinetic dynamometer. The variables were compared considering sex and decade, using mixed-effects linear models. The smaller diameter of all fiber types did not differ significantly between age groups for either sex. The proportion of oxidative fibers was reduced in male participants in the 4th (-20%) and 5th (-6%) decades of life compared to the 3rd decade, and there was an increase in the number of oxidative fibers in women from the 4th (+14%) to the 5th decade of life. There were no significant changes in the peak torque and total work between the analyzed age groups. The early phase of aging starts with alterations in the proportion of fibers, with a decrease in oxidative fibers in men and an increase in oxidative fibers in women. Smaller diameter, torque, and total work did not change over these decades of life.

Adding heat stress to repeated-sprint training in hypoxia does not enhance performance improvements in canoe/kayak athletes

PURPOSE

The present study investigated the effects of adding heat stress to repeated-sprint training in hypoxia on performance and physiological adaptations in well-trained athletes.

METHODS

Sixteen canoe/kayak sprinters conducted 2 weeks of repeated-sprint training consisting of three sets of 5 × 10 s sprints with 20 s active recovery periods under conditions of either normobaric hypoxia (RSH, FiO₂: 14.5%, ambient temperature: 18 ℃, n = 8) or combined heat and normobaric hypoxia (RSHH, FiO₂: 14.5%, ambient temperature: 38 ℃, n = 8). Before and after training, the 10 × 10 s repeated-sprint ability (RSA) test and 500 m time trial were performed on a canoe/kayak ergometer.

RESULTS

Peak and average power outputs during the RSA test were significantly improved after training in both RSH (peak power: + 21.5 ± 4.6%, P < 0.001; average power: + 12.5 ± 1.9%, P < 0.001) and RSHH groups (peak power: + 18.8 ± 6.6%, P = 0.005; average power: + 10.9 ± 6.8%, P = 0.030). Indirect variables of skeletal muscle oxygen extraction (deoxygenated hemoglobin) and blood perfusion (total hemoglobin) during the RSA test were significantly increased after training in the RSH group (P = 0.041 and P = 0.034, respectively) but not in the RSHH group. In addition, finish time during the 500 m time trial was significantly shortened after the training only in the RSH group (RSH: - 3.9 ± 0.8%, P = 0.005; RSHH: - 3.1 ± 1.4%, P = 0.078).

CONCLUSION

Adding heat stress to RSH does not enhance performance improvement and may partially mask muscle tissue adaptation.

Associations of swallowing-related muscle quantity and quality with sarcopenic parameters

PURPOSE

To examine the associations between swallowing-related muscle characteristics and sarcopenic parameters.

METHODS

We included 147 community-dwelling older adults (age: 71.6 ± 4.7 years, body mass index: 23.0 ± 2.7 kg/m² (mean ± standard deviation), men: 50; women: 97) and categorized them into robust (n = 125), low-function (n = 17), and sarcopenia (n = 5) groups based on the diagnostic criteria of the Asia Working Group for Sarcopenia 2019. We evaluated the geniohyoid muscle (GHM) and tongue characteristics (muscle quantity and quality). The cross-sectional area (CSA) indicated the muscle quantity, and echo intensity (EI) values indicated the muscle quality. A multiple regression analysis was performed to clarify the relationship of swallowing-related muscle characteristics and strength with sarcopenic parameters.

RESULTS

The grip strength (CSA of GHM: β = 1.64, p = 0.03) and skeletal muscle mass index (CSA of tongue: β = 74.81, p = 0.003, EI of tongue: β = 1.92, p = 0.009) were better indicators of swallowing-related muscle characteristics.

CONCLUSION

These findings may facilitate the early detection of aging-related deterioration in swallowing-related musculature through the diagnostic process of sarcopenia and increase our understanding of muscle physiology.

Differential effects of vitamin D on upper and lower body fat-free mass: potential mechanisms

Vitamin D insufficiency is a global health concern and low vitamin D status is regularly associated with reduced muscle mass and sarcopenia in observational research. Recent research using Mendelian randomization (MR) has highlighted the potentially causal positive effect of serum vitamin D (25(OH)D) on total, trunk and upper body appendicular fat-free mass (FFM). However, no such effect was found in lower body FFM, a result that mirrors the outcomes of some vitamin D intervention studies. Here we review the current literature on vitamin D, muscle mass and strength and discuss some potential mechanisms for the differing effects of vitamin D on upper and lower body FFM. In particular, differences in distribution of the vitamin D receptor as well as androgen receptors, in the upper and lower body musculature, will be discussed.

Changes in vastus lateralis fibre cross-sectional area, pennation angle and fascicle length do not predict changes in muscle cross-sectional area

NEW FINDINGS

What is the central question of this study? Do changes in myofibre cross-sectional area, pennation angle and fascicle length predict vastus lateralis whole-muscle cross-sectional area changes following resistance training? What is the main finding and its importance? Changes in vastus lateralis mean myofibre cross-sectional area, fascicle length and pennation angle following a period of resistance training did not collectively predict changes in whole-muscle cross-sectional area. Despite the limited sample size in this study, these data reiterate that it remains difficult to generalize the morphological adaptations that predominantly drive tissue-level vastus lateralis muscle hypertrophy.

ABSTRACT

Myofibre hypertrophy during resistance training (RT) poorly associates with tissue-level surrogates of hypertrophy. However, it is underappreciated that, in pennate muscle, changes in myofibre cross-sectional area (fCSA), fascicle length (Lf ) and pennation angle (PA) likely coordinate changes in whole-muscle cross-sectional area (mCSA). Therefore, we determined if changes in fCSA, PA and Lf predicted vastus lateralis (VL) mCSA changes following RT. Thirteen untrained college-aged males (23 ± 4 years old, 25.4 ± 5.2 kg/m2 ) completed 7 weeks of full-body RT (twice weekly). Right leg VL ultrasound images and biopsies were obtained prior to (PRE) and 72 h following (POST) the last training bout. Regression was used to assess if training-induced changes in mean fCSA, PA and Lf predicted VL mCSA changes. Correlations were also performed between PRE-to-POST changes in obtained variables. Mean fCSA (+18%), PA (+8%) and mCSA (+22%) increased following RT (P < 0.05), but not Lf (0.1%, P = 0.772). Changes in fCSA, Lf and PA did not collectively predict changes in mCSA (R2 = 0.282, adjusted R2 = 0.013, F3,8  = 1.050, P = 0.422). Moderate negative correlations existed for percentage changes in PA and Lf (r = -0.548, P = 0.052) and changes in fCSA and Lf (r = -0.649, P = 0.022), and all other associations were weak (|r| < 0.500). Although increases in mean fCSA, PA and VL mCSA were observed, inter-individual responses for each variable and limitations for each technique make it difficult to generalize the morphological adaptations that predominantly drive tissue-level VL muscle hypertrophy. However, the small subject pool is a significant limitation, and more research in this area is needed.

Higher extracellular water/total body water ratio is associated with lower tongue and grip strength: a cross-sectional study

PURPOSE

This study aimed to investigate the relationship between tongue factors (tongue strength and tongue volume), systemic factors (grip strength and walking speed), and the extracellular water/total body water ratio.

METHODS

This cross-sectional study included community-dwelling adults. Body water composition and skeletal muscle mass index were measured using bioelectrical impedance analysis. Moreover, tongue grip strength, tongue volume, occlusal support condition (Eichner classification), and walking speed were measured. Multiple linear regression analysis was performed to investigate the relationship between the tongue and systemic factors as dependent variables and body water composition.

RESULTS

We included 171 community-dwelling adults (62 men and 109 women) aged 70.0 years (interquartile range: 8). Tongue strength (r=-0.22, p=0.004) and grip strength (r=-0.39, p<0.001) were correlated with the extracellular water/total body water ratio. In multiple linear regression analysis, tongue strength was associated with the extracellular water/total body water ratio (β=-0.20, p=0.034), and grip strength was associated with the extracellular water/total body water ratio (β=-0.12, p=0.047), sex, body mass index, skeletal muscle mass index, and occlusal support condition.

CONCLUSIONS

A higher extracellular water/total body water ratio indicates lower tongue and grip strength. Dentists and dental hygienists should be aware of systemic factors present in patients with lower tongue and grip strength. These findings may lead to further medical investigations and diagnosis of other systemic diseases.

Physical and electrophysiological motor unit characteristics are revealed with simultaneous high-density electromyography and ultrafast ultrasound imaging

Electromyography and ultrasonography provide complementary information about electrophysiological and physical (i.e. anatomical and mechanical) muscle properties. In this study, we propose a method to assess the electrical and physical properties of single motor units (MUs) by combining High-Density surface Electromyography (HDsEMG) and ultrafast ultrasonography (US). Individual MU firings extracted from HDsEMG were used to identify the corresponding region of muscle tissue displacement in US videos. The time evolution of the tissue velocity in the identified region was regarded as the MU tissue displacement velocity. The method was tested in simulated conditions and applied to experimental signals to study the local association between the amplitude distribution of single MU action potentials and the identified displacement area. We were able to identify the location of simulated MUs in the muscle cross-section within a 2 mm error and to reconstruct the simulated MU displacement velocity (cc > 0.85). Multiple regression analysis of 180 experimental MUs detected during isometric contractions of the biceps brachii revealed a significant association between the identified location of MU displacement areas and the centroid of the EMG amplitude distribution. The proposed approach has the potential to enable non-invasive assessment of the electrical, anatomical, and mechanical properties of single MUs in voluntary contractions.

The Tabula Sapiens: A multiple-organ, single-cell transcriptomic atlas of humans

Molecular characterization of cell types using single-cell transcriptome sequencing is revolutionizing cell biology and enabling new insights into the physiology of human organs. We created a human reference atlas comprising nearly 500,000 cells from 24 different tissues and organs, many from the same donor. This atlas enabled molecular characterization of more than 400 cell types, their distribution across tissues, and tissue-specific variation in gene expression. Using multiple tissues from a single donor enabled identification of the clonal distribution of T cells between tissues, identification of the tissue-specific mutation rate in B cells, and analysis of the cell cycle state and proliferative potential of shared cell types across tissues. Cell type-specific RNA splicing was discovered and analyzed across tissues within an individual.

Scapular Dynamic Muscular Stiffness Assessed through Myotonometry: A Narrative Review

Several tools have been used to assess muscular stiffness. Myotonometry stands out as an accessible, handheld, and easy to use tool. The purpose of this review was to summarize the psychometric properties and methodological considerations of myotonometry and its applicability in assessing scapular muscles. Myotonometry seems to be a reliable method to assess several muscles stiffness, as trapezius. This method has been demonstrated fair to moderate correlation with passive stiffness measured by shear wave elastography for several muscles, as well as with level of muscle contraction, pinch and muscle strength, Action Research Arm Test score and muscle or subcutaneous thickness. Myotonometry can detect scapular muscles stiffness differences between pre- and post-intervention in painful conditions and, sometimes, between symptomatic and asymptomatic subjects.

Exercise-mediated reinnervation of skeletal muscle in elderly people: An update

Sarcopenia is defined by the loss of muscle mass and function. In aging sarcopenia is due to mild chronic inflammation but also to fiber-intrinsic defects, such as mitochondrial dysfunction. Age-related sarcopenia is associated with physical disability and lowered quality of life. In addition to skeletal muscle, the nervous tissue is also affected in elderly people. With aging, type 2 fast fibers preferentially undergo denervation and are reinnervated by slow-twitch motor neurons. They spread forming new neuro-muscular junctions with the denervated fibers: the result is an increased proportion of slow fibers that group together since they are associated in the same motor unit. Grouping and fiber type shifting are indeed major histological features of aging skeletal muscle. Exercise has been proposed as an intervention for age-related sarcopenia due to its numerous beneficial effects on muscle mechanical and biochemical features. In 2013, a precursor study in humans was published in the European Journal of Translation Myology (formerly known as Basic and Applied Myology), highlighting the occurrence of reinnervation in the musculature of aged, exercise-trained individuals as compared to the matching control. This paper, entitled «Reinnervation of Vastus lateralis is increased significantly in seniors (70-years old) with a lifelong history of high-level exercise», is now being reprinted for the second issue of the «Ejtm Seminal Paper Series». In this short review we discuss those results in the light of the most recent advances confirming the occurrence of exercise-mediated reinnervation, ultimately preserving muscle structure and function in elderly people who exercise.

Genetically Determined Serum 25-Hydroxyvitamin D Is Associated with Total, Trunk, and Arm Fat-Free Mass: A Mendelian Randomization Study

PURPOSE

Low serum vitamin D status has been associated with reduced muscle mass in observational studies although the relationship is controversial and a causal association cannot be determined from such observations. Two-sample Mendelian randomization (MR) was applied to assess the association between serum vitamin D (25(OH)D) and total, trunk, arm and leg fat-free mass (FFM).

METHODS

MR was implemented using summary-level data from the largest genome-wide association studies (GWAS) on vitamin D (n=73,699) and total, trunk, arm and leg FFM. Inverse variance weighted method (IVW) was used to estimate the causal estimates. Weighted median (WM)-based method, and MR-Egger, leave-one-out were applied as sensitivity analysis.

RESULTS

Genetically higher serum 25(OH)D levels had a positive effect on total (IVW = Beta: 0.042, p = 0.038), trunk (IVW = Beta: 0.045, p = 0.023) and arm (right arm IVW = Beta: 0.044, p = 0.002; left arm IVW = Beta: 0.05, p = 0.005) FFM. However, the association with leg FFM was not significant (right leg IVW = Beta: 0.03, p = 0.238; left leg IVW = Beta: 0.039, p = 0.100). The likelihood of heterogeneity and pleiotropy was determined to be low (statistically non-significant), and the observed associations were not driven by single SNPs. Furthermore, MR pleiotropy residual sum and outlier test did not highlight any outliers.

CONCLUSIONS

Our results illustrate the potentially causal, positive effect of serum 25(OH)D concentration on total, trunk and upper body appendicular fat-free mass.

Effects of Muscle Architecture on Eccentric Exercise Induced Muscle Damage Responses

There is a need to investigate the role of muscle architecture on muscle damage responses induced by exercise. The aim of this study was to determine the effect of muscle architecture and muscle length on eccentric exercise-induced muscle damage responses. Eccentric exercise-induced muscle damage was performed randomly to the elbow flexor (EF), knee extensor (KE), and knee flexor (KF) muscle groups with two week intervals in 12 sedentary male subjects. Before and after each eccentric exercise (immediately after, on the 1st, 2nd, 3rd, and 7th days) range of motion, delayed onset muscle soreness, creatine kinase activity, myoglobin concentration and isometric peak torque in short and long muscle positions were evaluated. Furthermore, muscle volume and pennation angle of each muscle group was evaluated before initiating the eccentric exercise protocol. Pennation angle and muscle volume was significantly higher and the workload per unit muscle volume was significantly lower in the KE muscles compared with the KF and EF muscles (p < 0.01). EF muscles showed significantly higher pain levels at post-exercise days 1 and 3 compared with the KE (p < 0.01-0.001) and KF (p < 0.01) muscles. The deficits in range of motion were higher in the EF muscles compared to the KE and KF muscles immediately after (day 0, p < 0.01), day 1 (p < 0.05-0.01), and day 3 (p < 0.05) evaluations. The EF muscles represented significantly greater increases in CK and Mb levels at day 1, 3, and 7 than the KE muscles (p < 0.05-0.01). The CK and Mb levels were also significantly higher in the KF muscles compared with the KE muscles (p < 0.05, p < 0.01 respectively). The KF and EF muscles represented higher isometric peak torque deficits in all the post-exercise evaluations at muscle short position (p < 0.05-0.001) compared with the KE muscle after eccentric exercise. Isometric peak torque deficits in muscle lengthened position was significantly higher in EF in all the post-exercise evaluations compared with the KE muscle (p < 0.05-0.01). According to the results of this study it can be concluded that muscle structural differences may be one of the responsible factors for the different muscle damage responses following eccentric exercise in various muscle groups.

Age-related structural changes show that loss of fibers is not a significant contributor to muscle atrophy in old mice

Age-related loss of skeletal muscle mass is widely considered a consequence of both fiber atrophy and fiber death. Evidence for fiber death derives largely from an age-related reduction in fiber numbers in muscle cross-sections, however it is unclear how age-related alterations in muscle morphology affect accuracy of such counts. To explore this we performed an examination of muscle and tendon length, muscle mass and girth, and pennation angle, in addition to histological section fiber counts of parallel-fibered (sternomastoid), fusiform (biceps brachii), and pennate (tibialis anterior, extensor digitorum longus, soleus) muscles from 31 mice aged 6-32 months. Age-related decline in mass and girth occurred in soleus (p = 0.026; p = 0.040), tibialis anterior (p = 0.004; p = 0.039), and extensor digitorum longus (p = 0.040; p = 0.022) muscles, for which location of maximal girth also changed. Tendon length and pennation angle remained consistent across the lifespan in all except soleus which showed elongation of both proximal and distal tendons coupled with alterations in pennation angle. Age-related decreases in fiber number were observed in transversely sectioned soleus and extensor digitorum longus muscles however when age-related changes in morphology were accounted for via oblique sectioning the age-related decrease in fiber number was eliminated. Findings show loss of fibers is not a significant contributor to age-related muscle wasting in mice, and that age-related changes in connective tissue selectively impact muscle structure. Fiber shortening is a likely contributor to loss of mass and change in function in muscles of old mice.

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.

The Skeletal Muscle Emerges as a New Disease Target in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder that leads to progressive degeneration of motor neurons (MNs) and severe muscle atrophy without effective treatment. Most research on ALS has been focused on the study of MNs and supporting cells of the central nervous system. Strikingly, the recent observations of pathological changes in muscle occurring before disease onset and independent from MN degeneration have bolstered the interest for the study of muscle tissue as a potential target for delivery of therapies for ALS. Skeletal muscle has just been described as a tissue with an important secretory function that is toxic to MNs in the context of ALS. Moreover, a fine-tuning balance between biosynthetic and atrophic pathways is necessary to induce myogenesis for muscle tissue repair. Compromising this response due to primary metabolic abnormalities in the muscle could trigger defective muscle regeneration and neuromuscular junction restoration, with deleterious consequences for MNs and thereby hastening the development of ALS. However, it remains puzzling how backward signaling from the muscle could impinge on MN death. This review provides a comprehensive analysis on the current state-of-the-art of the role of the skeletal muscle in ALS, highlighting its contribution to the neurodegeneration in ALS through backward-signaling processes as a newly uncovered mechanism for a peripheral etiopathogenesis of the disease.

Maximal Fat Oxidation: Comparison between Treadmill, Elliptical and Rowing Exercises

Fat oxidation during exercise is associated with cardio-metabolic benefits, but the extent of which whole-body exercise modality elicits the greatest fat oxidation remains unclear. We investigated the effects of treadmill, elliptical and rowing exercise on fat oxidation in healthy individuals. Nine healthy males participated in three, peak oxygen consumption tests, on a treadmill, elliptical and rowing ergometer. Indirect calorimetry was used to assess maximal oxygen consumption (V̇O_2peak), maximal fat oxidation (MFO) rates, and the exercise intensity MFO occurred (Fat_max). Mixed venous blood was collected to assess lactate and blood gases concentrations. While V̇O_2peak was similar between exercise modalities, MFO rates were higher on the treadmill (mean ± SD; 0.61 ± 0.06 g·min^-1) compared to both the elliptical (0.41 ± 0.08 g·min^-1, p = 0.022) and the rower (0.40 ± 0.08 g·min^-1, p = 0.017). Fat_max values were also significantly higher on the treadmill (56.0 ± 6.2 %V̇O_2peak) compared to both the elliptical (36.8 ± 5.4 %V̇O_2peak, p = 0.049) and rower (31.6 ± 5.0 %V̇O_2peak, p = 0.021). Post-exercise blood lactate concentrations were also significantly lower following treadmill exercise (p = 0.021). Exercising on a treadmill maximizes fat oxidation to a greater extent than elliptical and rowing exercises, and remains an important exercise modality to improve fat oxidation, and consequently, cardio-metabolic health.

Impairment Mechanisms and Intervention Approaches for Aged Human Neuromuscular Junctions

The neuromuscular junction (NMJ) is a chemical synapse formed between a presynaptic motor neuron and a postsynaptic muscle cell. NMJs in most vertebrate species share many essential features; however, some differences distinguish human NMJs from others. This review will describe the pre- and postsynaptic structures of human NMJs and compare them to NMJs of laboratory animals. We will focus on age-dependent declines in function and changes in the structure of human NMJs. Furthermore, we will describe insights into the aging process revealed from mouse models of accelerated aging. In addition, we will compare aging phenotypes to other human pathologies that cause impairments of pre- and postsynaptic structures at NMJs. Finally, we will discuss potential intervention approaches for attenuating age-related NMJ dysfunction and sarcopenia in humans.

Muscle Oxygenation During Repeated Double-Poling Sprint Exercise in Normobaric Hypoxia and Normoxia

We compared upper limb muscle oxygenation responses during repeated double-poling sprint exercise in normobaric hypoxia and normoxia. Eight male kayakers completed a repeated double-poling sprint exercise (3 × 3 × 20-s maximal sprints, 40-s passive recovery, 5-min rest) in either hypoxia (HYP, FiO₂ = 14.5%) or normoxia (NOR, FiO₂ = 20.9%). Power output, muscle oxygenation of triceps brachii muscle (using near infrared spectroscopy), arterial oxygen saturation, and cardiorespiratory variables were monitored. Mean power output tended to be lower (-5.2%; P = 0.06) in HYP compared with NOR, while arterial oxygen saturation (82.9 ± 0.9% vs. 90.5 ± 0.8%) and systemic oxygen uptake (1936 ± 140 vs. 2408 ± 83 mL⋅min^-1) values were lower (P < 0.05). Exercise-induced increases in deoxygenated hemoglobin (241.7 ± 46.9% vs. 175.8 ± 27.2%) and total hemoglobin (138.0 ± 18.1% vs. 112.1 ± 6.7%) were greater in HYP in reference to NOR (P < 0.05). Despite moderate hypoxia exacerbating exercise-induced elevation in blood perfusion of active upper limb musculature, power output during repeated double-poling exercise only tended to be lower.

The age-related loss of skeletal muscle mass and function: Measurement and physiology of muscle fibre atrophy and muscle fibre loss in humans

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Loss of muscle mass with age is due to atrophy and loss of individual muscle fibres.

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Anabolic resistance is fundamental in age-related fibre atrophy.

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Fibre loss is associated with denervation and remodelling of motor units.

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The plasticity of both factors should be considered in future research.

Age-related loss of skeletal muscle mass and function, sarcopenia, is associated with physical frailty and increased risk of morbidity (chronic diseases), in addition to all-cause mortality. The loss of muscle mass occurs incipiently from middle-age (∼1%/year), and in severe instances can lead to a loss of ∼50% by the 8–9th decade of life. This review will focus on muscle deterioration with ageing and highlight the two underpinning mechanisms regulating declines in muscle mass and function: muscle fibre atrophy and muscle fibre loss (hypoplasia) – and their measurement. The mechanisms of muscle fibre atrophy in humans relate to imbalances in muscle protein synthesis (MPS) and breakdown (MPB); however, since there is limited evidence for basal alterations in muscle protein turnover, it would appear that “anabolic resistance” to fundamental environmental cues regulating diurnal muscle homeostasis (namely physical activity and nutrition), underlie age-related catabolic perturbations in muscle proteostasis. While the ‘upstream’ drivers of the desensitization of aged muscle to anabolic stimuli are poorly defined, they most likely relate to impaired efficiency of the conversion of nutritional/exercise stimuli into signalling impacting mRNA translation and proteolysis. Additionally, loss of muscle fibres has been shown in cadaveric studies using anatomical fibre counts, and from iEMG studies demonstrating motor unit loss, albeit with few molecular investigations of this in humans. We suggest that defining countermeasures against sarcopenia requires improved understandings of the co-ordinated regulation of muscle fibre atrophy and fibre loss, which are likely to be inextricably linked.

Skeletal Muscle Function in the Oldest-Old: The Role of Intrinsic and Extrinsic Factors

Although skeletal muscle function is diminished with advanced age, single muscle fiber function seems to be preserved. Therefore, this review examines the hypothesis that the skeletal muscle fiber, per se, is not the predominant factor responsible for the reduction in force-generating capacity in the oldest-old, but, rather, is attributable to a combination of factors external to the muscle fibers.

Speedup computation of HD-sEMG signals using a motor unit-specific electrical source model

Nowadays, bio-reliable modeling of muscle contraction is becoming more accurate and complex. This increasing complexity induces a significant increase in computation time which prevents the possibility of using this model in certain applications and studies. Accordingly, the aim of this work is to significantly reduce the computation time of high-density surface electromyogram (HD-sEMG) generation. This will be done through a new model of motor unit (MU)-specific electrical source based on the fibers composing the MU. In order to assess the efficiency of this approach, we computed the normalized root mean square error (NRMSE) between several simulations on single generated MU action potential (MUAP) using the usual fiber electrical sources and the MU-specific electrical source. This NRMSE was computed for five different simulation sets wherein hundreds of MUAPs are generated and summed into HD-sEMG signals. The obtained results display less than 2% error on the generated signals compared to the same signals generated with fiber electrical sources. Moreover, the computation time of the HD-sEMG signal generation model is reduced to about 90% compared to the fiber electrical source model. Using this model with MU electrical sources, we can simulate HD-sEMG signals of a physiological muscle (hundreds of MU) in less than an hour on a classical workstation. Graphical Abstract Overview of the simulation of HD-sEMG signals using the fiber scale and the MU scale. Upscaling the electrical source to the MU scale reduces the computation time by 90% inducing only small deviation of the same simulated HD-sEMG signals.

Unwavering Pathobiology of Volumetric Muscle Loss Injury

Volumetric muscle loss (VML) resulting from extremity trauma presents chronic and persistent functional deficits which ultimately manifest disability. Acellular biological scaffolds, or decellularized extracellular matrices (ECMs), embody an ideal treatment platform due to their current clinical use for soft tissue repair, off-the-shelf availability, and zero autogenous donor tissue burden. ECMs have been reported to promote functional skeletal muscle tissue remodeling in small and large animal models of VML injury, and this conclusion was reached in a recent clinical trial that enrolled 13 patients. However, numerous other pre-clinical reports have not observed ECM-mediated skeletal muscle regeneration. The current study was designed to reconcile these discrepancies. The capacity of ECMs to orchestrate functional muscle tissue remodeling was interrogated in a porcine VML injury model using unbiased assessments of muscle tissue regeneration and functional recovery. Here, we show that VML injury incites an overwhelming inflammatory and fibrotic response that leads to expansive fibrous tissue deposition and chronic functional deficits, which ECM repair does not augment.

Investigation of the HD-sEMG probability density function shapes with varying muscle force using data fusion and shape descriptors

This work presents an evaluation of the High Density surface Electromyogram (HD-sEMG) Probability Density Function (PDF) shape variation according to contraction level. On that account, using PDF shape descriptors: High Order Statistics (HOS) and Shape Distances (SD), we try to address the absence of a consensus for the sEMG non-Gaussianity evolution with force variation. This is motivated by the fact that PDF shape information are relevant in physiological assessment of the muscle architecture and function, such as contraction level classification, in complement to classical amplitude parameters. Accordingly, both experimental and simulation studies are presented in this work. For data fusion, the watershed image processing technique was used. This technique allowed us to find the dominant PDF shape variation profiles from the 64 signals. The experimental protocol consisted of three isometric isotonic contractions of 30, 50 and 70% of the Maximum Voluntary Contraction (MVC). This protocol was performed by six subjects and recorded using an 8 × 8 HD-sEMG grid. For the simulation study, the muscle modeling was done using a fast computing cylindrical HD-sEMG generation model. This model was personalized by morphological parameters obtained by sonography. Moreover, a set of the model parameter configurations were compared as a focused sensitivity analysis of the PDF shape variation. Further, monopolar, bipolar and Laplacian electrode configurations were investigated in both experimental and simulation studies. Results indicated that sEMG PDF shape variations according to force increase are mainly dependent on the Motor Unit (MU) spatial recruitment strategy, the MU type distribution within the muscle, and the used electrode arrangement. Consequently, these statistics can give us an insight into non measurable parameters and specifications of the studied muscle primarily the MU type distribution.

Analysis of the sEMG/force relationship using HD-sEMG technique and data fusion: A simulation study

The relationship between the surface Electromyogram (sEMG) signal and the force of an individual muscle is still ambiguous due to the complexity of experimental evaluation. However, understanding this relationship should be useful for the assessment of neuromuscular system in healthy and pathological contexts. In this study, we present a global investigation of the factors governing the shape of this relationship. Accordingly, we conducted a focused sensitivity analysis of the sEMG/force relationship form with respect to neural, functional and physiological parameters variation. For this purpose, we used a fast generation cylindrical model for the simulation of an 8×8 High Density-sEMG (HD-sEMG) grid and a twitch based force model for the muscle force generation. The HD-sEMG signals as well as the corresponding force signals were simulated in isometric non-fatiguing conditions and were based on the Biceps Brachii (BB) muscle properties. A total of 10 isometric constant contractions of 5s were simulated for each configuration of parameters. The Root Mean Squared (RMS) value was computed in order to quantify the sEMG amplitude. Then, an image segmentation method was used for data fusion of the 8×8 RMS maps. In addition, a comparative study between recent modeling propositions and the model proposed in this study is presented. The evaluation was made by computing the Normalized Root Mean Squared Error (NRMSE) of their fitting to the simulated relationship functions. Our results indicated that the relationship between the RMS (mV) and muscle force (N) can be modeled using a 3rd degree polynomial equation. Moreover, it appears that the obtained coefficients are patient-specific and dependent on physiological, anatomical and neural parameters.

Age-Related Changes in Morphology and Function of Scapular Muscles in Asymptomatic People

BACKGROUND

Aging is a contributing factor to rotator cuff tears. Dysfunction of the scapular muscles can be a potential cause of rotator cuff tears. Although aging can contribute to dysfunction of the scapular muscles and subsequently result in rotator cuff tears, the effects of aging on scapular muscles are unclear.

OBJECTIVE

To determine changes in sizes and electromyographic activities of the scapular muscles with age in asymptomatic subjects.

DESIGN

Cross-sectional study.

PATIENTS

Fifty-four asymptomatic subjects without shoulder pain and dysfunction (18 male and 36 female) were recruited for this study. Subjects consisted of 19 people aged 20-39 years (young adult group, 27.2 ± 5.2 years), 17 people aged 40-59 years (middle age group, 49.9 ± 6.6 years), and 18 people aged 60-79 years (elderly group, 68.0 ± 7.1 years).

METHODS

Using ultrasonography, we measured the muscle thickness of the trapezius (upper and lower portions), serratus anterior, and middle deltoid muscles during rest with the subject in a sitting position. Activities of the same muscles were measured with surface electromyography during shoulder elevation in the scapular plane up to 120°. The electromyographic data were analyzed at ranges 0°-30°, 30°-60°, and 60°-90°. Analysis of variance and Bonferroni multiple comparisons were used for statistical analysis.

RESULTS

Muscle thickness of the lower trapezius in the elderly group was decreased significantly compared with that in the young adult group (95% confidence interval -0.40, 2.24; P < .001) and middle age group (95% confidence interval -0.34, 1.89; P = .012). There were no significant differences in muscle activity and activity ratio between groups.

CONCLUSION

In scapular muscles, muscle thickness of the lower trapezius significantly decreased with aging, although there were no decreases in muscle thicknesses of the other muscles between different age groups. For the functional change, aging may not affect the scapular muscle activities and balance of the activities between the deltoid and scapular muscles.

LEVEL OF EVIDENCE

IV.

Latent mitochondrial DNA deletion mutations drive muscle fiber loss at old age

With age, somatically derived mitochondrial DNA (mtDNA) deletion mutations arise in many tissues and species. In skeletal muscle, deletion mutations clonally accumulate along the length of individual fibers. At high intrafiber abundances, these mutations disrupt individual cell respiration and are linked to the activation of apoptosis, intrafiber atrophy, breakage, and necrosis, contributing to fiber loss. This sequence of molecular and cellular events suggests a putative mechanism for the permanent loss of muscle fibers with age. To test whether mtDNA deletion mutation accumulation is a significant contributor to the fiber loss observed in aging muscle, we pharmacologically induced deletion mutation accumulation. We observed a 1200% increase in mtDNA deletion mutation-containing electron transport chain-deficient muscle fibers, an 18% decrease in muscle fiber number and 22% worsening of muscle mass loss. These data affirm the hypothesized role for mtDNA deletion mutation in the etiology of muscle fiber loss at old age.

Chemical delivery array with millisecond neurotransmitter release

Technologies that restore or augment dysfunctional neural signaling represent a promising route to deeper understanding and new therapies for neurological disorders. Because of the chemical specificity and subsecond signaling of the nervous system, these technologies should be able to release specific neurotransmitters at specific locations with millisecond resolution. We have previously demonstrated an organic electronic lateral electrophoresis technology capable of precise delivery of charged compounds, such as neurotransmitters. However, this technology, the organic electronic ion pump, has been limited to a single delivery point, or several simultaneously addressed outlets, with switch-on speeds of seconds. We report on a vertical neurotransmitter delivery device, configured as an array with individually controlled delivery points and a temporal resolution of 50 ms. This is achieved by supplementing lateral electrophoresis with a control electrode and an ion diode at each delivery point to allow addressing and limit leakage. By delivering local pulses of neurotransmitters with spatiotemporal dynamics approaching synaptic function, the high-speed delivery array promises unprecedented access to neural signaling and a path toward biochemically regulated neural prostheses.

Mitochondrial quality control in insulin resistance and diabetes

Diabetes is increasingly prevalent and a primary contributor to the major causes of disability and death. Despite the central role of mitochondria in metabolism, the relationship between mitochondrial quality and insulin action remains unclear. An increasing number of genetically-engineered and aging rodent models are shedding additional light on the mitochondrion's role in regulating glucose metabolism and insulin sensitivity by modulating mitochondrial morphology, function and quality control pathways. Clarification of the role of mitochondria in regulating key cellular processes including metabolic flux, autophagy, and apoptosis will drive the development of novel therapeutic strategies for maintaining mitochondrial quality and improving human health.

Early reduction in toe flexor strength is associated with physical activity in elderly men

[Purpose] To compare the toe flexor, hand grip and knee extensor strengths of young and elderly men, and to examine the association between toe flexor strength and physical activity or inactivity levels. [Subjects and Methods] Young (n=155, 18–23 years) and elderly (n=60, 65–88 years) men participated in this study. Toe flexor, hand grip, and knee extensor strength were measured. Physical activity (time spent standing/walking per day) and inactivity (time spent sitting per day) were assessed using a self-administered questionnaire. [Results] Toe flexor, hand grip, and knee extensor strength of the elderly men were significantly lower than those of the young men. Standing/walking and sitting times of the elderly men were lower than those of the young men. Toe flexor strength correlated with hand grip and knee extensor strength in both groups. In elderly men, toe flexor strength correlated with standing/walking time. In comparison to the young men’s mean values, toe flexor strength was significantly lower than knee extensor and hand grip strength in the elderly group. [Conclusion] The results suggest that age-related reduction in toe flexor strength is greater than those of hand grip and knee extensor strengths. An early loss of toe flexor strength is likely associated with reduced physical activity in elderly men.

Effect of age on human-computer-interface control via neck electromyography

The purpose of this study was to determine the effect of age on visuomotor tracking using submental and anterior neck surface electromyography (sEMG) to assess feasibility of computer control via neck musculature, which allows people with little remaining motor function to interact with computers. Thirty-two healthy adults participated: sixteen younger adults aged 18 - 29 years and sixteen older adults aged 69 - 85 years. Participants modulated sEMG to achieve targets presented at different amplitudes using real-time visual feedback. Root-mean-squared (RMS) error was used to quantify tracking performance. RMS error was increased for older adults relative to younger adults. Older adults demonstrated more RMS error than younger adults as a function of increasing target amplitude. The differential effects of age found on static tracking performance in anterior neck musculature suggest more difficult translation of human-computer-interfaces controlled using anterior neck musculature for static tasks to older populations.

Age-Associated Methylation Suppresses SPRY1, Leading to a Failure of Re-quiescence and Loss of the Reserve Stem Cell Pool in Elderly Muscle

The molecular mechanisms by which aging affects stem cell number and function are poorly understood. Murine data have implicated cellular senescence in the loss of muscle stem cells with aging. Here, using human cells and by carrying out experiments within a strictly pre-senescent division count, we demonstrate an impaired capacity for stem cell self-renewal in elderly muscle. We link aging to an increased methylation of the SPRY1 gene, a known regulator of muscle stem cell quiescence. Replenishment of the reserve cell pool was modulated experimentally by demethylation or siRNA knockdown of SPRY1. We propose that suppression of SPRY1 by age-associated methylation in humans inhibits the replenishment of the muscle stem cell pool, contributing to a decreased regenerative response in old age. We further show that aging does not affect muscle stem cell senescence in humans.

Repeated double-poling sprint training in hypoxia by competitive cross-country skiers

PURPOSE

Repeated-sprint training in hypoxia (RSH) was recently shown to improve repeated-sprint ability (RSA) in cycling. This phenomenon is likely to reflect fiber type-dependent, compensatory vasodilation, and therefore, our hypothesis was that RSH is even more beneficial for activities involving upper body muscles, such as double poling during cross-country skiing.

METHODS

In a double-blinded fashion, 17 competitive cross-country skiers performed six sessions of repeated sprints (each consisting of four sets of five 10-s sprints, with 20-s intervals of recovery) either in normoxia (RSN, 300 m; FiO2, 20.9%; n = 8) or normobaric hypoxia (RSH, 3000 m; FiO2, 13.8 %; n = 9). Before (pre) and after (post) training, performance was evaluated with an RSA test (10-s all-out sprints-20-s recovery, until peak power output declined by 30%) and a simulated team sprint (team sprint, 3 × 3-min all-out with 3-min rest) on a double-poling ergometer. Triceps brachii oxygenation was measured by near-infrared spectroscopy.

RESULTS

From pretraining to posttraining, peak power output in the RSA was increased (P < 0.01) to the same extent (29% ± 13% vs 26% ± 18%, nonsignificant) in RSH and in RSN whereas the number of sprints performed was enhanced in RSH (10.9 ± 5.2 vs 17.1 ± 6.8, P < 0.01) but not in RSN (11.6 ± 5.3 vs 11.7 ± 4.3, nonsignificant). In addition, the amplitude in total hemoglobin variations during sprints throughout RSA rose more in RSH (P < 0.01). Similarly, the average power output during all team sprints improved by 11% ± 9% in RSH and 15% ± 7% in RSN.

CONCLUSIONS

Our findings reveal greater improvement in the performance of repeated double-poling sprints, together with larger variations in the perfusion of upper body muscles in RSH compared with those in RSN.

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

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

Sex differences with aging in the fatigability of dynamic contractions

This study determined the sex difference with aging in fatigability of the elbow flexor muscles during a dynamic fatiguing task, and explored the associated mechanisms. We compared fatigability of the elbow flexor muscles in 18 young (20.2 ± 1 years: 9 men) and 36 old adults (73.5 ± 1 years: 16 men) during and in recovery from repeated dynamic contractions (~60°/s) with a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque until failure. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (an increase in the superimposed twitch, SIT) and the peak rate of muscle relaxation. Time to failure was briefer for the men than the women (6.1 ± 2.1 vs. 9.7 ± 5.5 min, respectively; P=0.02) with no difference between young and old adults (7.2 ± 2.9 vs. 8.4 ± 5.2 min, respectively, P=0.45) and no interaction (P>0.05). The relative decline in peak relaxation rate with fatigability was similar for young and old adults (P=0.11), but greater for men than women (P=0.046). Supraspinal fatigue increased for all groups and was associated with the time to failure (P<0.05). Regression analysis however, indicated that the time to failure was best predicted by the peak relaxation rate (baseline values and slowing with fatigability) (r(2)=0.55). Rate-limiting contractile mechanisms (e.g. excitation-contraction coupling) were responsible for the increased fatigability of the elbow flexors of men compared with women for a dynamic fatiguing task of slow angular velocity, and this sex difference was maintained with aging. The age difference in fatigability for the dynamic task was diminished for both sexes relative to what is typically observed with isometric fatiguing contractions.

Aging related changes in determinants of muscle force generating capacity: a comparison of muscle aging in men and male rodents

Human aging is associated with a progressive decline in skeletal muscle mass and force generating capacity, however the exact mechanisms underlying these changes are not fully understood. Rodents models have often been used to enhance our understanding of mechanisms of age-related changes in human skeletal muscle. However, to what extent age-related alterations in determinants of muscle force generating capacity observed in rodents resemble those in humans has not been considered thoroughly. This review compares the effect of aging on muscle force generating determinants (muscle mass, fiber size, fiber number, fiber type distribution and muscle specific tension), in men and male rodents at similar relative age. It appears that muscle aging in male F344*BN rat resembles that in men most; 32-35-month-old rats exhibit similar signs of muscle weakness to those of 70-80-yr-old men, and the decline in 36-38-month-old rats is similar to that in men aged over 80 yrs. For male C57BL/6 mice, age-related decline in muscle force generating capacity seems to occur only at higher relative age than in men. We conclude that the effects on determinants of muscle force differ between species as well as within species, but qualitatively show the same pattern as that observed in men.

The influences of obesity and age on functional performance during intermittent upper extremity tasks

In this study, the main and interactive effects of obesity and age on functional performance were assessed during intermittent exertions involving the upper extremity. The prevalence of obesity has doubled over the past 30 years and this increase is associated with higher health care costs, rates of workplace injury, and lost workdays. Obesity and aging can modify job demands and affect worker capacity in terms of muscular and psychomotor function. However, there is a lack of empirical studies quantifying the work-relevant (or ergonomic) impacts related to task demands, capacities, and their potential imbalance. Eight obese and eight non-obese participants from each of two age groups (18-25 and 50-65 years) completed three endurance tasks involving fixed levels of task demands: hand grip, shoulder flexion, and a simulated assembly task using the upper extremity. Measures of functional performance including endurance, discomfort, motor control, and task performance were recorded for each of the task conditions. Endurance times were ∼60% longer for the non-obese group, and older participants had longer endurance times; however there was no evidence of interactive effects of obesity and age. Obesity also impaired functional performance, as indicated by higher rates of strength loss, increases in discomfort, and declines in task performance. These observed impairments may reflect underlying physiological differences among individuals who are obese, but that are independent of age. Obesity-related impairments may have implications for the design of work duration and demand level to prevent fatigue development for workers who are obese.

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

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