Muscle disuse alters skeletal muscle contractile function at the molecular and cellular levels in older adult humans in a sex-specific manner

Bioinformatics Analysis Identifies Key Genes in the Effect of Resistance Training on Female Skeletal Muscle Aging

Resistance training is used to combat skeletal muscle function decline in older adults. Few studies have been designed specific for females, resulting in very limited treatment options for skeletal muscle atrophy in aging women. Here, we analyzed the gene expression profiles of skeletal muscle samples from sedentary young women, sedentary older women, and resistance-trained older women, using microarray data from public database. A total of 45 genes that were differentially expressed during female muscle aging and reversed by resistance training were identified. Functional and pathway enrichment analysis, protein-protein interaction network analysis, and receiver operating characteristic analysis were performed to reveal the key genes and pathways involved in the effects of resistance training on female muscle aging. The collagen genes COL1A1, COL3A1, and COL4A1 were identified important regulators of female muscle aging and resistance training, by modulating multiple signaling pathways, such as PI3 kinase-Akt signaling, focal adhesions, extracellular matrix-receptor interactions, and relaxin signaling. Interestingly, the expression of CDKN1A and TP63 were increased during aging, and further upregulated by resistance training in older women, suggesting they may negatively affect resistance training outcomes. Our findings provide novel insights into the molecular mechanisms of resistance training on female muscle aging and identify potential biomarkers and targets for clinical intervention.

Age-related changes in human skeletal muscle transcriptome and proteome are more affected by chronic inflammation and physical inactivity than primary aging

Evaluation of the influence of primary and secondary aging on the manifestation of molecular and cellular hallmarks of aging is a challenging and currently unresolved issue. Our study represents the first demonstration of the distinct role of primary aging and chronic inflammation/physical inactivity - the most important drivers of secondary aging, in the regulation of transcriptomic and proteomic profiles in human skeletal muscle. To achieve this purpose, young healthy people (n = 15), young (n = 8) and older (n = 37) patients with knee/hip osteoarthritis, a model to study the effect of long-term inactivity and chronic inflammation on the vastus lateralis muscle, were included in the study. It was revealed that widespread and substantial age-related changes in gene expression in older patients relative to young healthy people (~4000 genes regulating mitochondrial function, proteostasis, cell membrane, secretory and immune response) were related to the long-term physical inactivity and chronic inflammation rather than primary aging. Primary aging contributed mainly to the regulation of genes (~200) encoding nuclear proteins (regulators of DNA repair, RNA processing, and transcription), mitochondrial proteins (genes encoding respiratory enzymes, mitochondrial complex assembly factors, regulators of cristae formation and mitochondrial reactive oxygen species production), as well as regulators of proteostasis. It was found that proteins associated with aging were regulated mainly at the post-transcriptional level. The set of putative primary aging genes and their potential transcriptional regulators can be used as a resource for further targeted studies investigating the role of individual genes and related transcription factors in the emergence of a senescent cell phenotype.

The association between obesity, knee pain, and gait during stair descent in older adults with knee osteoarthritis

BACKGROUND

Obesity and knee osteoarthritis adversely affect activities of daily living in older adults. Together, the complexities of their interaction on mobility, including stair negotiation, are unresolved. The purpose of this study was to determine the relationship between obesity, pain, and stair negotiation in older adults with knee osteoarthritis.

METHODS

Older adults with symptomatic knee osteoarthritis and overweight or obesity participated in the study (n = 28; age range = 57.0-78.0 yrs.; body mass index range = 26.6-42.8 kg•m-2). The Western Ontario and McMaster Universities Osteoarthritis Index pain subscale was used to measure knee pain. Measurements included a three-dimensional biomechanical analysis during descent on a set of force plate-instrumented stairs and a timed stair descent test. Pearson's r was used to determine associations between body mass index and pain, stair descent weight-acceptance phase vertical ground reaction force (vGRF) variables and lower extremity joint kinematics and kinetics, and timed stair descent performance.

FINDINGS

Significant correlations existed between body mass index and pain (r = 0.41; p = 0.03), peak vGRF (r = 0.39; p = 0.04), vertical impulse (r = 0.49; p = 0.008), and peak ankle plantar flexor moments (r = 0.50; p = 0.007) in older adults with knee osteoarthritis.

INTERPRETATION

Greater obesity in older adults with knee osteoarthritis was associated with greater knee pain and higher ankle joint loads during stair descent. These results support the recommendations of osteoarthritis treatment guidelines for weight-loss as a first-line of treatment for older adults with obesity and knee osteoarthritis.

Age-Dependent Changes in the Production of Mitochondrial Reactive Oxygen Species in Human Skeletal Muscle

A decrease in muscle mass and its functionality (strength, endurance, and insulin sensitivity) is one of the integral signs of aging. One of the triggers of aging is an increase in the production of mitochondrial reactive oxygen species. Our study was the first to examine age-dependent changes in the production of mitochondrial reactive oxygen species related to a decrease in the proportion of mitochondria-associated hexokinase-2 in human skeletal muscle. For this purpose, a biopsy was taken from m. vastus lateralis in 10 young healthy volunteers and 70 patients (26-85 years old) with long-term primary arthrosis of the knee/hip joint. It turned out that aging (comparing different groups of patients), in contrast to inactivity/chronic inflammation (comparing young healthy people and young patients), causes a pronounced increase in peroxide production by isolated mitochondria. This correlated with the age-dependent distribution of hexokinase-2 between mitochondrial and cytosolic fractions, a decrease in the rate of coupled respiration of isolated mitochondria and respiration when stimulated with glucose (a hexokinase substrate). It is discussed that these changes may be caused by an age-dependent decrease in the content of cardiolipin, a potential regulator of the mitochondrial microcompartment containing hexokinase. The results obtained contribute to a deeper understanding of age-related pathogenetic processes in skeletal muscles and open prospects for the search for pharmacological/physiological approaches to the correction of these pathologies.

Skeletal muscle adaptations in patients with lung cancer: Longitudinal observations from the whole body to cellular level

BACKGROUND

Cancer and its treatment can adversely affect skeletal muscle, impacting physical function, treatment response and survival. No studies, however, have comprehensively characterized these muscle adaptations longitudinally in human patients at the cellular level.

METHODS

We examined skeletal muscle size and function from the whole body to the sub-cellular level in 11 patients with non-small cell lung cancer (NSCLC; 6 male/5 female, mean age 58 ± 3 years) studied over a 2-month observation period starting during their first cycle of standard of care cancer treatment and in 11 age- and sex-matched healthy controls (HC) without a current or past history of cancer. Biopsies of the vastus lateralis were performed to assess muscle fibre size, contractility and mitochondrial content, along with assessments of physical function, whole muscle size and function, and circulating cytokines.

RESULTS

Body weight, composition and thigh muscle area and density were unaltered over time in patients with NSCLC, while muscle density was lower in patients with NSCLC versus HC (P = 0.03). Skeletal muscle fibre size decreased by 18% over time in patients (all P = 0.02) and was lower than HC (P = 0.02). Mitochondrial fractional area and density did not change over time in patients, but fractional area was lower in patients with NSCLC compared with HC (subsarcolemmal, P = 0.04; intermyofibrillar, P = 0.03). Patients with NSCLC had higher plasma concentrations of IL-6 (HC 1.40 ± 0.50; NSCLC 4.71 ± 4.22; P < 0.01), GDF-15 (HC 569 ± 166; NSCLC 2071 ± 1168; P < 0.01) and IL-8/CXCL8 (HC 4.9 ± 1.8; NSCLC 10.1 ± 6.0; P = 0.02) compared with HC, but there were no changes in inflammatory markers in patients with NSCLC over time. No changes were observed in markers of satellite cell activation or DNA damage in patients and no group differences were noted with HC. Whole-muscle strength was preserved over time in patients with NSCLC coincident with improved single fibre contractility.

CONCLUSIONS

This study is the first to comprehensively examine longitudinal alterations in skeletal muscle fibre size and function in patients with NSCLC and suggests that muscle fibre atrophy occurs during cancer treatment despite weight stability and no changes in conventional clinical measurements of whole body or thigh muscle size over this period.

Reambulation following hindlimb unloading attenuates disuse-induced changes in murine fracture healing

Patients with bone and muscle loss from prolonged disuse have higher risk of falls and subsequent fragility fractures. In addition, fracture patients with continued disuse and/or delayed physical rehabilitation have worse clinical outcomes compared to individuals with immediate weight-bearing activity following diaphyseal fracture. However, the effects of prior disuse followed by physical reambulation on fracture healing cellular processes and adjacent bone and skeletal muscle recovery post-injury remains poorly defined. To bridge this knowledge gap and inform future treatment and rehabilitation strategies for fractures, a preclinical model of fracture healing with a history of prior unloading with and without reambulation was employed. First, skeletally mature male and female C57BL/6J mice (18 weeks) underwent hindlimb unloading by tail suspension (HLU) for 3 weeks to induce significant bone and muscle loss modeling enhanced bone fragility. Next, mice had their right femur fractured by open surgical dissection (stabilized with 24-gauge pin). The, mice were randomly assigned to continued HLU or allowed normal weight-bearing reambulation (HLU + R). Mice given normal cage activity throughout the experiment served as healthy age-matched controls. All mice were sacrificed 4-days (DPF4) or 14-days (DPF14) following fracture to assess healing and uninjured hindlimb musculoskeletal properties (6-10 mice per treatment/biological sex). We found that continued disuse following fracture lead to severely diminished uninjured hindlimb skeletal muscle mass (gastrocnemius and soleus) and femoral bone volume adjacent to the fracture site compared to healthy age-matched controls across mouse sexes. Furthermore, HLU led to significantly decreased periosteal expansion (DPF4) and osteochondral tissue formation by DPF14, and trends in increased osteoclastogenesis (DPF14) and decreased woven bone vascular area (DPF14). In contrast, immediate reambulation for 2 weeks after fracture, even following a period of prolonged disuse, was able to increase hindlimb skeletal tissue mass and increase osteochondral tissue formation, albeit not to healthy control levels, in both mouse sexes. Furthermore, reambulation attenuated osteoclast formation seen in woven bone tissue undergoing disuse. Our results suggest that weight-bearing skeletal loading in both sexes immediately following fracture may improve callus healing and prevent further fall risk by stimulating skeletal muscle anabolism and decreasing callus resorption compared to minimal or delayed rehabilitation regimens.

Sex Differences in Inflammation and Muscle Wasting in Aging and Disease

Only in recent years, thanks to a precision medicine-based approach, have treatments tailored to the sex of each patient emerged in clinical trials. In this regard, both striated muscle tissues present significant differences between the two sexes, which may have important consequences for diagnosis and therapy in aging and chronic illness. In fact, preservation of muscle mass in disease conditions correlates with survival; however, sex should be considered when protocols for the maintenance of muscle mass are designed. One obvious difference is that men have more muscle than women. Moreover, the two sexes differ in inflammation parameters, particularly in response to infection and disease. Therefore, unsurprisingly, men and women respond differently to therapies. In this review, we present an up-to-date overview on what is known about sex differences in skeletal muscle physiology and disfunction, such as disuse atrophy, age-related sarcopenia, and cachexia. In addition, we summarize sex differences in inflammation which may underly the aforementioned conditions because pro-inflammatory cytokines deeply affect muscle homeostasis. The comparison of these three conditions and their sex-related bases is interesting because different forms of muscle atrophy share common mechanisms; for instance, those responsible for protein dismantling are similar although differing in terms of kinetics, severity, and regulatory mechanisms. In pre-clinical research, exploring sexual dimorphism in disease conditions could highlight new efficacious treatments or recommend implementation of an existing one. Any protective factors discovered in one sex could be exploited to achieve lower morbidity, reduce the severity of the disease, or avoid mortality in the opposite sex. Thus, the understanding of sex-dependent responses to different forms of muscle atrophy and inflammation is of pivotal importance to design innovative, tailored, and efficient interventions.

Effects of total knee arthroplasty on skeletal muscle structure and function at the cellular, organellar, and molecular levels

Total knee arthroplasty (TKA) is an important treatment option for knee osteoarthritis (OA) that improves self-reported pain and physical function, but objectively measured physical function typically remains reduced for years after surgery due, in part, to precipitous reductions in lower extremity neuromuscular function early after surgery. The present study examined intrinsic skeletal muscle adaptations during the first 5 weeks post-TKA to identify skeletal muscle attributes that may contribute to functional disability. Patients with advanced stage knee OA were evaluated prior to TKA and 5 weeks after surgery. Biopsies of the vastus lateralis were performed to assess muscle fiber size, contractility, and mitochondrial content, along with assessments of whole muscle size and function. TKA was accompanied by marked reductions in whole muscle size and strength. At the fiber (i.e., cellular) level, TKA caused profound muscle atrophy that was approximately twofold higher than that observed at the whole muscle level. TKA markedly reduced muscle fiber force production, contractile velocity, and power production, with force deficits persisting in myosin heavy chain (MHC) II fibers after expression relative to fiber size. Molecular level assessments suggest reduced strongly bound myosin-actin cross bridges and myofilament lattice stiffness as a mechanism underlying reduced force per unit fiber size. Finally, marked reductions in mitochondrial content were apparent and more prominent in the subsarcolemmal compartment. Our study represents the most comprehensive evaluation of skeletal muscle cellular adaptations to TKA and uncovers novel effects of TKA on muscle fiber size and intrinsic contractility early after surgery that may contribute to functional disability.NEW & NOTEWORTHY We report the first evaluation of the effects of total knee arthroplasty (TKA) on skeletal muscle at the cellular and subcellular levels. We found marked effects of TKA to cause skeletal muscle fiber atrophy and contractile dysfunction in older adults, as well as molecular mechanisms underlying impaired contractility. Our results reveal profound effects of TKA on muscle fiber size and intrinsic contractility early after surgery that may contribute to functional disability.

The Duration of Thigh Tourniquet Use Associated With Anterior Cruciate Ligament Reconstruction Does Not Produce Cellular-Level Contractile Dysfunction of the Quadriceps Muscle at 3 Weeks After Surgery

BACKGROUND

Anterior cruciate ligament (ACL) trauma and ACL reconstruction (ACLR) are associated with the loss of strength and function of the muscles that span the knee joint. The underlying mechanism associated with this is not completely understood.

PURPOSE

To determine whether the duration of tourniquet use during ACLR has an effect on knee extensor muscle contractile function and size at the cellular (ie, fiber) level 3 weeks after surgery and at the whole-muscle level at 6 months after surgery.

STUDY DESIGN

Descriptive laboratory study and case series; Level of evidence, 4.

METHODS

Study participants sustained an acute, first-time ACL injury. All participants underwent ACLR with the use of a tourniquet placed in a standardized location on the thigh; the tourniquet was inflated (pressure range, 250-275 mm Hg), and the time of tourniquet use during surgery was documented. Participants were evaluated 1 week before surgery (to measure patient function, strength, and subjective outcome with the Knee injury and Osteoarthritis Outcome Score [KOOS] and International Knee Documentation Committee [IKDC] score), at 3 weeks after ACLR surgery (to obtain muscle biopsy specimens of the vastus lateralis and assess muscle fiber cross-sectional area, contractile function, and mitochondrial content and morphometry), and at 6 months after ACLR (to evaluate patient function, strength, and subjective outcomes via KOOS and IKDC scores). Data were acquired on both the injured/surgical limb and the contralateral, normal side to facilitate the use of a within-subjects study design. Results are based on additional analysis of data acquired from previous research that had common entry criteria, treatments, and follow-up protocols.

RESULTS

At 3 weeks after ACLR, the duration of tourniquet use at the time of surgery did not explain the variation in single-muscle fiber contractile function or cross-sectional area (myosin heavy chain [MHC] I and II fibers) or subsarcolemmal and intermyofibrillar mitochondrial content or morphometry. At 6 months after ACLR, the duration of tourniquet use was not associated with the peak isometric and isokinetic torque measurements, patient function, or patient-reported outcomes.

CONCLUSION

The duration of tourniquet use at the time of ACLR surgery did not explain variation in muscle fiber size, contractile function, or mitochondrial content at 3 weeks after surgery or strength of the quadriceps musculature or patient-reported function or quality of life at 6-month follow-up.

Walking Speed Assessed by 4-Meter Walk Test in the Community-Dwelling Oldest Old Population in Vietnam

This study aims to provide data on usual walking speed in individuals aged 80 years or older and determine the association between walking speed and related factors in community-dwelling older adults. A cross-sectional study design was conducted to measure walking speed on community-dwelling elders aged 80 years or older in Soc Son district, Vietnam. Walking speed was assessed by a 4-Meter Walk Test with a usual-pace walking mode. Health-related characteristics of participants including risk of falls (The Timed Up and Go test, activities of daily living (ADL), instrumental activities of daily living (IADLs), cognitive impairment (Mini-Cog test) and frailty syndrome (The Reported Edmonton Frail Scale (REFS)). Multiple logistic regression was used to analyze the association between a slow walking speed and selected factors. A total of 364 older people were recruited, and the majority were female (65.4%). The overall average walking speed was 0.83 ± 0.27 m/s. The proportion of participants with a slow walking speed (<0.8 m/s) was 40.4%. Multiple logistic regression analyses showed that age, female, high fall risk (assessed by TUG test), ADL/IADL dependence and frailty syndrome had a negative effect on slow walking speed in this population. The results could provide useful reference data for further investigations and measures in clinical practice.

Diagnostic accuracy of calf circumference for decreased muscle mass in older adults with sarcopenia

Background: Increasing number of the older adults population results in increasing sarcopenia, a geriatric problem that may lead to poor quality of life, susceptibility to disease, malnutrition, and even death. Muscle mass is an important sarcopenia parameter that can be measured by Bioelectrical Impedance Analysis (BIA). Detection of decreased muscle mass can be done by measuring calf circumference, it is expected to provide an early diagnose of sarcopenia so that early intervention can be given and improve the quality of life of the older adults.Objective: To analyze the diagnostic accuracy of calf circumference for decreased muscle mass in older adults to provide simple way in diagnosing sarcopenia.Methods: This study involved 126 older adults, consisted of 57 men and 69 women aged 60-80 years in the community who met the inclusion criteria. Criteria of sarcopenia were defined based on the Asian Working Group for Sarcopenia (AWGS) Consensus, consisted of three components; muscle mass, handgrip strength, and walking speed. This study analyze the diagnostic accuracy of calf circumference for decreased muscle mass measured by single- frequency BIA and calf circumference was measured using a measuring tape. The analysis was carried out according to the receiver operating characteristic (ROC) curve to determine the cut-off point along with the sensitivity (Se) and specificity (Sp) values, positive and negative predictive values (PPV and NPV) of calf circumference as an indicator for low muscle mass.Results: Optimal cut-off point of calf circumference to indicate low muscle mass is 32.9 cm in women (Se 80.8%, Sp 79.1%, PPV 75.9%, NPV 87.5%) and 33.5 cm in men (Se 78.6%, Sp 74.4%, PPV 50%, NPV 91.4%). PPV in men is lower than women. This is due to a lower prevalence of decreased muscle mass in men than women. There were 49 participants with the calf circumference below cut-off point and 40 (31.7%) of the 126 participants had sarcopenia.Conclusion: Calf circumference has a diagnostic accuracy to find decreased muscle mass in  sarcopenia.

Skeletal muscle cellular contractile dysfunction after anterior cruciate ligament reconstruction contributes to quadriceps weakness at 6-month follow-up

Muscle dysfunction following anterior cruciate ligament reconstruction (ACLR) may evolve from alterations in muscle contractility at the myofilament protein level. Using a prospective, within-subject case-control design, we evaluated cellular-level contractility, cross-sectional area (CSA), and myosin heavy chain (MHC) isoform expression on single muscle fibers 3 weeks post ACLR, and evaluated their relationship to whole muscle strength and patient-oriented outcomes 6 months post operation. Biopsies of the vastus lateralis were performed 3 weeks post ACLR in 11 subjects (5 females, mean age ± SD = 24.7 ± 6.5 years, height = 172.7 ± 8.2 cm, mass = 75.7 ± 12.5 kg) following first-time ACL rupture and whole muscle strength and self-reported pain, function, and quality of life assessed 6 months post ACLR. At 3 weeks post ACLR, force production was reduced (p < 0.01) in MHC I (-36%) and IIA (-48%) fibers compared with the non-injured leg. When force production was expressed relative to CSA to account for fiber atrophy, reductions remained in MHC IIA fibers (-40%; p < 0.001), but MHC I fibers showed only a trend toward being lower (-13%; p = 0.09). Finally, skeletal muscle fiber functional deficits at 3 weeks post ACLR were associated with whole muscle weakness and less favorable patient-reported outcomes at 6-month follow-up. Thus, ACLR promotes early cellular contractile dysfunction that may contribute to decreased whole muscle strength and patient function, and increased patient-reported symptoms, at 6-month follow-up.

Skeletal muscle wasting: the estrogen side of sexual dimorphism

The importance of defining sex differences across various biological and physiological mechanisms is more pervasive now than it has been over the past 15-20 years. As the muscle biology field pushes to identify small molecules and interventions to prevent, attenuate, or even reverse muscle wasting, we must consider the effect of sex as a biological variable. It should not be assumed that a therapeutic will affect males and females with equal efficacy or equivalent target affinities under conditions where muscle wasting is observed. With that said, it is not surprising to find that we have an unclear or even a poor understanding of the effects of sex or sex hormones on muscle wasting conditions. Although recent investigations are beginning to establish experimental approaches that will allow investigators to assess the impact of sex-specific hormones on muscle wasting, the field still needs rigorous scientific tools that will allow the community to address critical hypotheses centered around sex hormones. The focus of this review is on female sex hormones, specifically estrogens, and the roles that these hormones and their receptors play in skeletal muscle wasting conditions. With the overall review goal of assembling the current knowledge in the area of sexual dimorphism driven by estrogens with an effort to provide insights to interested physiologists on necessary considerations when trying to assess models for potential sex differences in cellular and molecular mechanisms of muscle wasting.

Muscle damaging eccentric exercise attenuates disuse-induced declines in daily myofibrillar protein synthesis and transiently prevents muscle atrophy in healthy men

Short-term disuse leads to muscle loss driven by lowered daily myofibrillar protein synthesis (MyoPS). However, disuse commonly results from muscle damage, and its influence on muscle deconditioning during disuse is unknown. Twenty-one males [20 ± 1 yr, BMI = 24 ± 1 kg·m^-2 (± SE)] underwent 7 days of unilateral leg immobilization immediately preceded by 300 bilateral, maximal, muscle-damaging eccentric quadriceps contractions (DAM; subjects n = 10) or no exercise (CON; subjects n = 11). Participants ingested deuterated water and underwent temporal bilateral thigh MRI scans and vastus lateralis muscle biopsies of immobilized (IMM) and nonimmobilized (N-IMM) legs. N-IMM quadriceps muscle volume remained unchanged throughout both groups. IMM quadriceps muscle volume declined after 2 days by 1.7 ± 0.5% in CON (P = 0.031; and by 1.3 ± 0.6% when corrected to N-IMM; P = 0.06) but did not change in DAM, and declined equivalently in CON [by 6.4 ± 1.1% (5.0 ± 1.6% when corrected to N-IMM)] and DAM [by 2.6 ± 1.8% (4.0 ± 1.9% when corrected to N-IMM)] after 7 days. Immobilization began to decrease MyoPS compared with N-IMM in both groups after 2 days (P = 0.109), albeit with higher MyoPS rates in DAM compared with CON (P = 0.035). Frank suppression of MyoPS was observed between days 2 and 7 in CON (IMM = 1.04 ± 0.12, N-IMM = 1.86 ± 0.10%·day^-1; P = 0.002) but not DAM (IMM = 1.49 ± 0.29, N-IMM = 1.90 ± 0.30%·day^-1; P > 0.05). Declines in MyoPS and quadriceps volume after 7 days correlated positively in CON (r² = 0.403; P = 0.035) but negatively in DAM (r² = 0.483; P = 0.037). Quadriceps strength declined following immobilization in both groups, but to a greater extent in DAM. Prior muscle-damaging eccentric exercise increases MyoPS and prevents loss of quadriceps muscle volume after 2 (but not 7) days of disuse.NEW & NOTEWORTHY We investigated the impact of prior muscle-damaging eccentric exercise on disuse-induced muscle deconditioning. Two and 7 days of muscle disuse per se lowered quadriceps muscle volume in association with lowered daily myofibrillar protein synthesis (MyoPS). Prior eccentric exercise prevented the decline in muscle volume after 2 days and attenuated the decline in MyoPS after 2 and 7 days. These data indicate eccentric exercise increases MyoPS and transiently prevents quadriceps muscle atrophy during muscle disuse.

Reduced knee extensor torque production at low to moderate velocities in postmenopausal women with knee osteoarthritis

This study aimed to determine deficits in knee extensor muscle function through the torque-time and torque-velocity relationships and whether these deficits are associated with reduced functional performance in postmenopausal women with knee osteoarthritis (KOA). A clinical sample of postmenopausal women with established KOA (n = 18, ≥55 years) was compared to an age-matched healthy control sample (CON) (n = 26). The deficits in different parameters of the knee extensor torque-time (maximal isometric torque and rate of torque development) and torque-velocity relationship (maximum muscle power, maximal velocity and torque at 0-500°·s^-1 ) were assessed through a protocol consisting of isometric, isotonic and isokinetic tests. Functional performance was evaluated with sit-to-stand and stair-climbing tasks using a sensor-based technology (ie, time- and power-based outcomes). Postmenopausal women with KOA showed reduced maximal isometric torque (Hedge's g effect size (g) = 1.05, p = 0.001) and rate of torque development (g = 0.77-1.17, all p ≤ 0.02), combined with impaired torque production at slow to moderate velocities (g = 0.92-1.70, p ≤ 0.004), but not at high or maximal velocities (g = 0.16, p > 0.05). KOA were slower (g = 0.81-0.92, p ≤ 0.011) and less powerful (g = 1.11-1.29, p ≤ 0.001) during functional tasks. Additionally, knee extensor deficits were moderately associated with power deficits in stair climbing (r = 0.492-0.659). To conclude, knee extensor muscle weakness was presented in postmenopausal women with KOA, not only as limited maximal and rapid torque development during isometric contractions, but also dynamically at low to moderate velocities. These deficits were related to impaired functional performance. The assessment of knee extensor muscle weakness through the torque-time and torque-velocity relationships might enable individual targets for tailored exercise interventions in KOA.

Current perspectives on obesity and skeletal muscle contractile function in older adults

Obesity has become one of the most pressing public health issues of the 21st century and currently affects a substantial proportion of the older adult population. Although the cardiometabolic complications are well documented, research from the past 20 years has drawn attention to the detrimental effects of obesity on physical performance in older adults. Obesity-related declines in physical performance are due, in part, to compromised muscle strength and power. Recent evidence suggests there are a number of mechanisms potentially underlying reduced whole muscle function, including alterations in myofilament protein function and cellular contractile properties, and these may be related to morphological adaptations, such as shifts in fiber type composition and increased intramyocellular lipid content within skeletal muscle. To date, even less research has focused on how exercise and weight loss interventions for obese older adults affect these mechanisms. In light of this work, we provide an update on the current knowledge related to obesity and skeletal muscle contractile function and highlight a number of questions to address potential etiologic mechanisms as well as intervention strategies, which may help advance our understanding of how physical performance can be improved among obese older adults.

Influenza Infection has Fiber Type-Specific Effects on Cellular and Molecular Skeletal Muscle Function in Aged Mice

Skeletal muscle myopathies represent a common non-pulmonary manifestation of influenza infection, leading to reduced physical function and hospitalization in older adults. However, underlying mechanisms remain poorly understood. Our study examined the effects of influenza virus A pulmonary infection on contractile function at the cellular (single fiber) and molecular (myosin-actin interactions and myofilament properties) levels in soleus and extensor digitorum longus muscles of aged (20 months) C57BL/6 male mice that were healthy or flu-infected for 7 (7-days post-infection; 7-DPI) or 12 days (12-DPI). Cross-sectional area (CSA) of myosin heavy chain (MHC) IIA and IIB fibers was reduced at 12-DPI relative to 7-DPI and healthy. Maximal isometric force in MHC IIA fibers was also reduced at 12-DPI relative to 7-DPI and healthy, resulting in no change in specific force (maximal isometric force divided by CSA). In contrast, MHC IIB fibers produced greater isometric force and specific force at 7-DPI compared to 12-DPI or healthy. The increased specific force in MHC IIB fibers was likely due to greater myofilament lattice stiffness and/or an increased number or stiffness of strongly bound myosin-actin cross-bridges. At the molecular level, cross-bridge kinetics were slower in MHC IIA fibers with infection, while changes in MHC IIB fibers were largely absent. In both fiber types, greater myofilament lattice stiffness was positively related to specific force. This study provides novel evidence that cellular and molecular contractile function is impacted by influenza infection in a fiber type-specific manner, suggesting potential molecular mechanisms to help explain the impact of flu-induced myopathies.

The Association of an Increasing Waist Circumference and Risk of Incident Low Physical Function in Adults with Knee Osteoarthritis

OBJECTIVE

To investigate an 8-year change in waist circumference (WC) with the risk of incident low physical function over 1 year in adults with, or at risk of, knee osteoarthritis (OA).

METHODS

Data from the Osteoarthritis Initiative were used. Change in WC was measured from study enrollment (0 month) to the 96-month visit and classified as Increase (≥ 5cm gain) or Maintain (< 5cm gain). We identified World Health Organization (WHO) risk category based on WC at study enrollment as Large WC (males ≥ 102 cm, females ≥ 88 cm) or Small WC (males < 102 cm, females < 88 cm). The outcome was incident low physical function (≥ 28 Western Ontario and McMaster Universities Osteoarthritis Index physical function subscale) at the 108-month visit. To investigate the association of the 8-year change in WC with the risk of low physical function, we calculated risk ratios (95% CI) and adjusted for potential confounders. We repeated the analyses stratified by the WHO disease risk category.

RESULTS

The Increase WC group had 1.43 (95% CI 1.04-1.96) times the risk of incident low physical function compared to adults in the Maintain WC group. Adults with a Large WC at baseline who increased WC had 1.55 (95% CI 1.00-2.37) times the risk of incident low physical function compared to those who maintained WC. Adults with a Small WC at baseline who increased WC had 1.97 (95% CI 0.84-4.63) times the risk compared to those who maintained WC.

CONCLUSION

Increasing WC increases the risk of incident low physical function in the following year. Maintaining WC may mitigate developing low physical function.

Utility of Neuromuscular Electrical Stimulation to Preserve Quadriceps Muscle Fiber Size and Contractility After Anterior Cruciate Ligament Injuries and Reconstruction: A Randomized, Sham-Controlled, Blinded Trial

BACKGROUND

Anterior cruciate ligament (ACL) injuries and reconstruction (ACLR) promote quadriceps muscle atrophy and weakness that can persist for years, suggesting the need for more effective rehabilitation programs. Whether neuromuscular electrical stimulation (NMES) can be used to prevent maladaptations in skeletal muscle size and function is unclear.

PURPOSE

To examine whether early NMES use, started soon after an injury and maintained through 3 weeks after surgery, can preserve quadriceps muscle size and contractile function at the cellular (ie, fiber) level in the injured versus noninjured leg of patients undergoing ACLR.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 1.

METHODS

Patients (n = 25; 12 men/13 women) with an acute, first-time ACL rupture were randomized to NMES (5 d/wk) or sham (simulated microcurrent electrical nerve stimulation; 5 d/wk) treatment to the quadriceps muscles of their injured leg. Bilateral biopsies of the vastus lateralis were performed 3 weeks after surgery to measure skeletal muscle fiber size and contractility. Quadriceps muscle size and strength were assessed 6 months after surgery.

RESULTS

A total of 21 patients (9 men/12 women) completed the trial. ACLR reduced single muscle fiber size and contractility across all fiber types (P < .01 to P < .001) in the injured compared with noninjured leg 3 weeks after surgery. NMES reduced muscle fiber atrophy (P < .01) through effects on fast-twitch myosin heavy chain (MHC) II fibers (P < .01 to P < .001). NMES preserved contractility in slow-twitch MHC I fibers (P < .01 to P < .001), increasing maximal contractile velocity (P < .01) and preserving power output (P < .01), but not in MHC II fibers. Differences in whole muscle strength between groups were not discerned 6 months after surgery.

CONCLUSION

Early NMES use reduced skeletal muscle fiber atrophy in MHC II fibers and preserved contractility in MHC I fibers. These results provide seminal, cellular-level data demonstrating the utility of the early use of NMES to beneficially modify skeletal muscle maladaptations to ACLR.

CLINICAL RELEVANCE

Our results provide the first comprehensive, cellular-level evidence to show that the early use of NMES mitigates early skeletal muscle maladaptations to ACLR.

REGISTRATION

NCT02945553 (ClinicalTrials.gov identifier).

Quadriceps Lipid Content Has Sex-Specific Associations With Whole-Muscle, Cellular, and Molecular Contractile Function in Older Adults

Increased adiposity is associated with reduced skeletal muscle function in older adults, but the mechanisms underlying this relationship remain unclear. To explore whether skeletal muscle properties track with adiposity, whole-muscle, cellular, and molecular function were examined in relation to adiposity measured at various anatomical levels in healthy older (60-80 years) men and women. Although women had greater absolute and relative body and thigh fat than men, quadriceps muscle attenuation, an index of intramuscular lipid content, was similar between sexes. At the whole-muscle level, greater quadriceps attenuation was associated with reduced knee extensor function in women, but not men. In women, decreased myosin heavy chain I and IIA fiber-specific force was associated with higher intramuscular lipid content, which may be explained, in part, by the reduced myofilament lattice stiffness found in myosin heavy chain IIA fibers. Longer myosin attachment times in myosin heavy chain I fibers from men and women were associated with greater amounts of adipose tissue, suggesting that fat deposits lead to slower myosin-actin cross-bridge kinetics. Our results indicate greater quantities of adipose tissue alter myofilament properties and cross-bridge kinetics, which may partially explain the adiposity-induced decrements in single-fiber and whole-muscle function of older adults, especially women.

Effect of neuromuscular electrical stimulation on skeletal muscle size and function in patients with breast cancer receiving chemotherapy

Exercise has numerous benefits for patients with cancer, but implementation is challenging because of practical and logistical hurdles. This study examined whether neuromuscular electrical stimulation (NMES) can serve as a surrogate for classic exercise by eliciting an exercise training response in skeletal muscle of women diagnosed with breast cancer undergoing chemotherapy. Patients (n = 22) with histologically confirmed stage I, II, or III breast cancer scheduled to receive neoadjuvant or adjuvant chemotherapy were randomized to 8 wk of bilateral neuromuscular electrical stimulation (NMES; 5 days/wk) to their quadriceps muscles or control. Biopsy of the vastus lateralis was performed at baseline and after 8 wk of intervention to assess muscle fiber size, contractility, and mitochondrial content. Seventeen patients (8 control/9 NMES) completed the trial and were included in analyses. NMES promoted muscle fiber hypertrophy (P < 0.001), particularly in fast-twitch, myosin heavy chain (MHC) IIA fibers (P < 0.05) and tended to induce fiber type shifts in MHC II fibers. The effects of NMES on single-muscle fiber contractility were modest, and it was unable to prevent declines in the function in MHC IIA fibers. NMES did not alter intermyofibrillar mitochondrial content/structure but was associated with reductions in subsarcolemmal mitochondria. Our results demonstrate that NMES induces muscle fiber hypertrophy and fiber type shifts in MHC II fibers but had minimal effects on fiber contractility and promoted reductions in subsarcolemmal mitochondria. Further studies are warranted to evaluate the utility of NMES as an exercise surrogate in cancer patients and other conditions.NEW & NOTEWORTHY This is the first study to evaluate whether neuromuscular electrical stimulation (NMES) can be used as an exercise surrogate to improve skeletal muscle fiber size or function in cancer patients receiving treatment. We show that NMES promoted muscle fiber hypertrophy and fiber type shifts but had minimal effects on single-fiber contractility and reduced subsarcolemmal mitochondria.

Jumping Joints: The Complex Relationship Between Osteoarthritis and Jumping Mechanography

We investigated the relationship between lower limb osteoarthritis (OA) and muscle strength and power (assessed by jumping mechanography) in UK community-dwelling older adults. We recruited 249 older adults (144 males, 105 females). OA was assessed clinically at the knee according to ACR criteria and radiographically, at the knee and hip, using Kellgren and Lawrence grading. Two-footed jumping tests were performed using a Leonardo Mechanography Ground Reaction Force Platform to assess maximum muscle force, power and Esslinger Fitness Index. Linear regression was used to assess the relationship between OA and jumping outcomes. Results are presented as β (95% confidence interval). The mean age of participants was 75.2 years (SD 2.6). Males had a significantly higher maximum relative power during lift off (mean 25.7 W/kg vs. 19.9 W/kg) and maximum total force during lift off (mean 21.0 N/kg vs. 19.1 N/kg) than females. In adjusted models, we found significant associations in males between clinical knee OA and maximum relative power [- 6.00 (CI - 9.10, - 2.94)] and Esslinger Fitness Index [- 19.3 (- 29.0, - 9.7)]. In females, radiographic knee OA was associated with total maximum power [- 2.0 (- 3.9, - 0.1)] and Esslinger Fitness Index [- 8.2 (- 15.9, - 0.4)]. No significant associations were observed for maximum total force. We observed significant negative associations between maximum relative power and Esslinger Fitness Index and clinical knee OA in males and radiographic knee OA in females. We have used novel methodology to demonstrate relationships between muscle function and OA in older adults.

Short-term muscle disuse induces a rapid and sustained decline in daily myofibrillar protein synthesis rates

Short-term muscle disuse has been reported to lower both postabsorptive and postprandial myofibrillar protein synthesis rates. This study assessed the impact of disuse on daily myofibrillar protein synthesis rates following short-term (2 and 7 days) muscle disuse under free living conditions. Thirteen healthy young men (age: 20 ± 1 yr; BMI: 23 ± 1 kg/m^-2) underwent 7 days of unilateral leg immobilization via a knee brace, with the nonimmobilized leg acting as a control. Four days before immobilization participants ingested 400 mL of 70% deuterated water, with 50-mL doses consumed daily thereafter. Upper leg bilateral MRI scans and muscle biopsies were collected before and after 2 and 7 days of immobilization to determine quadriceps volume and daily myofibrillar protein synthesis rates. Immobilization reduced quadriceps volume in the immobilized leg by 1.7 ± 0.3 and 6.7 ± 0.6% after 2 and 7 days, respectively, with no changes in the control leg. Over the 1-wk immobilization period, myofibrillar protein synthesis rates were 36 ± 4% lower in the immobilized (0.81 ± 0.04%/day) compared with the control (1.26 ± 0.04%/day) leg (P < 0.001). Myofibrillar protein synthesis rates in the control leg did not change over time (P = 0.775), but in the immobilized leg they were numerically lower during the 0- to 2-day period (16 ± 6%, 1.11 ± 0.09%/day, P = 0.153) and were significantly lower during the 2- to 7-day period (44 ± 5%, 0.70 ± 0.06%/day, P < 0.001) when compared with the control leg. We conclude that 1 wk of muscle disuse induces a rapid and sustained decline in daily myofibrillar protein synthesis rates in healthy young men.

Fiber typing human skeletal muscle with fluorescent immunohistochemistry

Skeletal muscle myosin heavy chain (MyHC) fiber type composition is a critical determinant of overall muscle function and health. Various approaches interrogate fiber type at the single cell, but the two most commonly utilized are single-muscle fiber sodium dodecyl sulfate-polyacrylamide gel electrophoresis (smfSDS-PAGE) and fluorescent immunohistochemistry (IHC). Although smfSDS-PAGE is generally considered the "gold standard," IHC is more commonly used because of its time-effectiveness and relative ease. Unfortunately, there is lingering inconsistency on how best to accurately and quickly determine fiber type via IHC and an overall misunderstanding regarding pure fiber type proportions, specifically the abundance of fibers exclusively expressing highly glycolytic MyHC IIX in humans. We therefore 1) present information and data showing the low abundance of pure MyHC IIX muscle fibers in healthy human skeletal muscle and 2) leverage this information to provide straightforward protocols that are informed by human biology and employ inexpensive, easily attainable antibodies for the accurate determination of fiber type.

Resistance training-induced gains in knee extensor strength are related to increased neural cell adhesion molecule expression in older adults with knee osteoarthritis

Objective

Resistance training (RT) can improve whole muscle strength without increasing muscle fiber size or contractility. Neural adaptations, which lead to greater neural activation of muscle, may mediate some of these improvements, particularly in older adults, where motor neuron denervation is common. The purpose of this study was to explore the relationship of neural adaptations, as reflected by neural cell adhesion molecule (NCAM) expression, to improvements in (1) whole muscle strength and (2) muscle fiber size following RT in older adults with knee osteoarthritis. We performed whole muscle strength measurements and immunohistochemical analysis of fiber size, type, and NCAM expression before and after a 14-week RT program.

Results

RT increased whole-muscle strength as measured by 1-repetition maximum (1-RM) leg press (P = 0.01), leg extension (P = 0.03), and knee extensor peak torque (P = 0.050), but did not alter NCAM expression. Greater NCAM expression in myosin heavy chain (MHC) II fibers was associated with greater whole muscle strength gains (knee extensor peak torque r = 0.93; P < 0.01) and greater MHC II fiber size (r = 0.79; P < 0.01). Our results suggest that training-induced NCAM expression, and neural adaptations more generally, may be important for RT-induced morphological and functional improvements in older adults.

Trial registration NCT01190046

Muscle metabolism and atrophy: let's talk about sex

Skeletal muscle health is a strong predictor of overall health and longevity. Pathologies affecting skeletal muscle such as cancer cachexia, intensive care unit treatment, muscular dystrophies, and others are associated with decreased quality of life and increased mortality. Recent research has begun to determine that these muscular pathologies appear to present and develop differently between males and females. However, to our knowledge, there has yet to be a comprehensive review on musculoskeletal differences between males and females and how these differences may contribute to sex differences in muscle pathologies. Herein, we present a review of the current literature on muscle phenotype and physiology between males and females and how these differences may contribute to differential responses to atrophic stimuli. In general, females appear to be more susceptible to disuse induced muscle wasting, yet protected from inflammation induced (such as cancer cachexia) muscle wasting compared to males. These differences may be due in part to differences in muscle protein turnover, satellite cell content and proliferation, hormonal interactions, and mitochondrial differences between males and females. However, more works specifically examining muscle pathologies in females are necessary to more fully understand the inherent sex-based differences in muscle pathologies between the sexes and how they may correspond to different clinical treatments.

The Roles of Sex and Physical Activity in Gait and Knee Extensor Function With Age

Older females experience higher rates of disability than males, potentially due to sex-specific differences in gait and muscle function. The authors evaluated the effects of age and physical activity (PA) on gait mechanics and knee extensor muscle function in males and females. Three groups of 20 individuals (each 10 females) participated: young (21-35 y) and highly and less active older (55-70 y) adults. Knee extensor strength and joint mechanics during preferred speed gait were collected before and after 30 minutes of walking. Age by sex and PA by sex interactions indicated older and less active older females had lower concentric knee extensor muscle power and larger hip extension moments than males. After 30 minutes of walking, older less active adults had larger decreases in knee extensor power than their highly active older counterparts, and older adults of both sexes had decreases in ankle dorsiflexion moments while young adults did not. These results suggest that older, particularly less active, adults are susceptible to knee extensor muscle fatigue from moderate activity. For older adults, high levels of PA may be necessary to preserve gait mechanics in response to a bout of exercise. This new information may be important for targeting interventions in at-risk older adults.

Maintenance of skeletal muscle function following reduced daily physical activity in healthy older adults: a pilot trial

Older adults can experience periods of inactivity related to disease or illness, which can hasten the development of physical disability, in part, through reductions in skeletal muscle strength and power. To date no study has characterized adaptations in skeletal muscle physical function in response to reduced daily physical activity. Participants (15 men, aged 69 ± 2 years; 15 women, aged 68 ± 4 years) restricted their daily steps (<750 steps/day) while being energy restricted (-500 kcal/day) for 2 weeks before returning to normal activity levels during recovery (RC; 1 week). Before and after each phase, measures of knee extensor isometric maximum voluntary contraction (MVC), time-to-peak torque, and physical function were performed and muscle biopsies were taken from a subset of participants. Following the energy restriction and step-reduction phase (ER+SR), MVC was reduced by 9.1 and 6.1 Nm in men and women, respectively (p = 0.02), which returned to baseline after RC in men, but not women (p = 0.046). Maximum isometric tension in MHC IIA fibres (p < 0.01) and maximum power production in MHC I and IIA (p = 0.05) were increased by 14%, 25%, and 10%, respectively, following ER+SR. Reductions in muscle strength could not be explained by changes in single muscle fibre function in a subsample (n = 9 men) of volunteers. These data highlight the resilience of physical function in healthy older men in the face of an acute period of ER+SR and demonstrate sex-based differences in the ability to recover muscle strength upon resumption of physical activity.

Alterations in quadriceps muscle cellular and molecular properties in adults with moderate knee osteoarthritis

OBJECTIVE

Quadriceps muscle weakness is common in knee osteoarthritis (OA). While pain, disuse, and atrophy are commonly cited causes for muscle weakness in OA, emerging evidence suggests changes in muscle quality also occur. Alterations in muscle quality are not well understood, but likely include both cellular and morphologic adaptions. The purpose of this study was to conduct the first cellular-level analysis of the vastus lateralis in adults with moderate knee OA.

METHODS

Vastus lateralis biopsies were obtained from 24 subjects with moderate knee OA and 15 healthy controls. Quadriceps strength, muscle fiber cross sectional area (CSA), fiber type distribution, extracellular matrix (ECM) content, satellite cell abundance, and profibrotic gene expression were assessed.

RESULTS

Relative to controls, quadriceps strength was significantly lower in OA subjects (OA 62.23, 50.67-73.8 Nm vs 91.46, 75.91-107.0 Nm, P = 0.003) despite no difference in fiber CSA. OA subjects had significantly fewer Type I fibers (OA 41.51, 35.56-47.47% vs 53.07, 44.86-61.29%, P = 0.022) and more hybrid IIa/x fibers (OA 24.61, 20.61-28.61% vs 16.4, 11.60-21.20%, P = 0.009). Significantly greater ECM content, lower satellite cell density, and higher profibrotic gene expression was observed with OA, and muscle collagen content was inversely correlated to strength and satellite cell (SC) density.

CONCLUSION

Lower quadriceps function with moderate OA may not result from fiber size impairments, but is associated with ECM expansion. Impaired satellite cell density, high profibrotic gene expression, and a slow-to-fast fiber type transition may contribute to reduced muscle quality in OA. These findings can help guide therapeutic interventions to enhance muscle function with OA.

Age and sex related differences in shoulder abduction fatigue

BACKGROUND

Injury prevalence data commonly indicate trends of higher rates of work-related musculoskeletal disorders in older workers over their younger counterparts, and for females more than males. The purpose of this study was to investigate age and sex-related differences in manifestations of shoulder muscle fatigue in a cohort of young and older working age males and females, in a single experiment design allowing for direct comparison of the fatigue effects between the target groups.

METHODS

We report upper trapezius muscle fibre Conduction Velocity (CV) as an indicative measure of muscle fatigability, and isometric endurance time, at three levels of shoulder abduction lifting force set relative to participants' maximal strength.

RESULTS

Upper trapezius conduction velocity was significantly different between the young and old groups (p = 0.002) as well as between males and females (p = 0.016). Shoulder abduction endurance time was affected by age (P = 0.024) but not sex (p = 0.170).

CONCLUSIONS

The study identified age-related improvement in muscle fatigue resistance and increased resistance for females over males, contrary to injury prevalence trends. The muscle fatigue effects are most likely explained by muscle fibre type composition. Experimental fatigue treatments of the upper trapezius were tested at exposures relative to the participants' strength. Absolute strength is higher when young and is generally higher for males. The findings of this study point towards age and sex-related differences in strength rather than in muscle fatigue resistance as a primary cause for the differences in the injury trends.

Baicalin Attenuates Joint Pain and Muscle Dysfunction by Inhibiting Muscular Oxidative Stress in an Experimental Osteoarthritis Rat Model

Osteoarthritis (OA) is the most common degenerative joint disease, and causes major pain and disability in adults. It has been reported that muscle weakness and inflammation contribute to osteoarthritis development and progression. Oxidative stress plays important roles in muscle dysfunction and inflammation in osteomyelitis. Baicalin, the major active constituent of the isolated root of Scutellarialateriflora Georgi, has been shown to have anti-oxidative and anti-inflammatory effects. In this study, we evaluated the potential effects of baicalin on osteoarthritis. We established experimental osteoarthritis rat model, applied baicalin to the rats, and then explored the potential protective effect of baicalin on osteoarthritis severity, muscle dysfunction, and oxidative stress. Baicalin alleviated severity of OA in rats. Baicalin application attenuated muscle dysfunction in OA rats by increasing citrate synthase activity, myosin heavy chain IIa expression, and decreasing interleukin 6 production. Baicalin decreased muscular reactive oxygen species generation in OA rats. Baicalin inhibited nuclear factor erythroid-derived 2-like 2 expression in OA rats. Baicalin attenuated osteoarthritis in rat by inhibiting oxidative stress.

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

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

Age-related reduction in single muscle fiber calcium sensitivity is associated with decreased muscle power in men and women

Age-related declines in human skeletal muscle performance may be caused, in part, by decreased responsivity of muscle fibers to calcium (Ca²⁺). This study examined the contractile properties of single vastus lateralis muscle fibers with various myosin heavy chain (MHC) isoforms (I, I/IIA, IIA and IIAX) across a range of Ca²⁺ concentrations in 11 young (24.1±1.1years) and 10 older (68.8±0.8years) men and women. The normalized pCa-force curve shifted rightward with age, leading to decreased activation threshold (pCa₁₀) and/or Ca²⁺ sensitivity (pCa₅₀) for all MHC isoforms examined. In older adults, the slope of the pCa-force curve was unchanged in MHC I-containing fibers (I, I/IIA), but was steeper in MHC II-containing fibers (IIA, IIAX), indicating greater cooperativity compared to young adults. At sub-maximal [Ca²⁺], specific force was reduced in MHC I-containing fibers, but was minimally decreased in MHC IIA fibers as older adults produced greater specific forces at high [Ca²⁺] in these fibers. Lessor pCa₅₀ in MHC I fibers independently predicted reduced isokinetic knee extensor power across a range of contractile velocities, suggesting that the Ca²⁺ response of slow-twitch fibers contributes to whole muscle dysfunction. Our findings show that aging attenuates Ca²⁺ responsiveness across fiber types and that these cellular alterations may lead to age-related reductions in whole muscle power output.

Novel Sarcopenia-related Alterations in Sarcomeric Protein Post-translational Modifications (PTMs) in Skeletal Muscles Identified by Top-down Proteomics

Sarcopenia, the age-related loss of skeletal muscle mass and strength, is a significant cause of morbidity in the elderly and is a major burden on health care systems. Unfortunately, the underlying molecular mechanisms in sarcopenia remain poorly understood. Herein, we utilized top-down proteomics to elucidate sarcopenia-related changes in the fast- and slow-twitch skeletal muscles of aging rats with a focus on the sarcomeric proteome, which includes both myofilament and Z-disc proteins-the proteins that constitute the contractile apparatuses. Top-down quantitative proteomics identified significant changes in the post-translational modifications (PTMs) of critical myofilament proteins in the fast-twitch skeletal muscles of aging rats, in accordance with the vulnerability of fast-twitch muscles to sarcopenia. Surprisingly, age-related alterations in the phosphorylation of Cypher isoforms, proteins that localize to the Z-discs in striated muscles, were also noted in the fast-twitch skeletal muscle of aging rats. This represents the first report of changes in the phosphorylation of Z-disc proteins in skeletal muscle during aging. In addition, increased glutathionylation of slow skeletal troponin I, a novel modification that may help protect against oxidative damage, was observed in slow-twitch skeletal muscles. Furthermore, we have identified and characterized novel muscle type-specific proteoforms of myofilament proteins and Z-disc proteins, including a novel isoform of the Z-disc protein Enigma. The finding that the phosphorylation of Z-disc proteins is altered in response to aging in the fast-twitch skeletal muscles of aging rats opens new avenues for the investigation of the role of Z-discs in age-related muscle dysfunction.

The Relevance of Sex Differences in Performance Fatigability

Performance fatigability differs between men and women for a range of fatiguing tasks. Women are usually less fatigable than men, and this is most widely described for isometric fatiguing contractions and some dynamic tasks. The sex difference in fatigability is specific to the task demands so that one mechanism is not universal, including any sex differences in skeletal muscle physiology, muscle perfusion, and voluntary activation. However, there are substantial knowledge gaps about the task dependency of the sex differences in fatigability, the involved mechanisms, and the relevance to clinical populations and with advanced age. The knowledge gaps are in part due to the significant deficits in the number of women included in performance fatigability studies despite a gradual increase in the inclusion of women for the last 20 yr. Therefore, this review 1) provides a rationale for the limited knowledge about sex differences in performance fatigability, 2) summarizes the current knowledge on sex differences in fatigability and the potential mechanisms across a range of tasks, 3) highlights emerging areas of opportunity in clinical populations, and 4) suggests strategies to close the knowledge gap and understanding the relevance of sex differences in performance fatigability. The limited understanding about sex differences in fatigability in healthy and clinical populations presents as a field ripe with opportunity for high-impact studies. Such studies will inform on the limitations of men and women during athletic endeavors, ergonomic tasks, and daily activities. Because fatigability is required for effective neuromuscular adaptation, sex differences in fatigability studies will also inform on optimal strategies for training and rehabilitation in both men and women.

Moderate-intensity resistance exercise alters skeletal muscle molecular and cellular structure and function in inactive older adults with knee osteoarthritis

High-intensity resistance exercise (REX) training increases physical capacity, in part, by improving muscle cell size and function. Moderate-intensity REX, which is more feasible for many older adults with disease and/or disability, also increases physical function, but the mechanisms underlying such improvements are not understood. Therefore, we measured skeletal muscle structure and function from the molecular to the tissue level in response to 14 wk of moderate-intensity REX in physically inactive older adults with knee osteoarthritis (n = 17; 70 ± 1 yr). Although REX training increased quadriceps muscle cross-sectional area (CSA), average single-fiber CSA was unchanged because of reciprocal changes in myosin heavy chain (MHC) I and IIA fibers. Intermyofibrillar mitochondrial content increased with training because of increases in mitochondrial size in men, but not women, with no changes in subsarcolemmal mitochondria in either sex. REX increased whole muscle contractile performance similarly in men and women. In contrast, adaptations in single-muscle fiber force production per CSA (i.e., tension) and contractile velocity varied between men and women in a fiber type-dependent manner, with adaptations being explained at the molecular level by differential changes in myosin-actin cross-bridge kinetics and mechanics and single-fiber MHC protein expression. Our results are notable compared with studies of high-intensity REX because they show that the effects of moderate-intensity REX in older adults on muscle fiber size/structure and myofilament function are absent or modest. Moreover, our data highlight unique sex-specific adaptations due to differential cellular and subcellular structural and functional changes.NEW & NOTEWORTHY Moderate-intensity resistance training causes sex-specific adaptations in skeletal muscle structure and function at the cellular and molecular levels in inactive older adult men and women with knee osteoarthritis. However, these responses were minimal compared with high-intensity resistance training. Thus adjuncts to moderate-intensity training need to be developed to correct underlying cellular and molecular structural and functional deficits that are at the root of impaired physical function in this mobility-limited population.

DNA methylation assessment from human slow- and fast-twitch skeletal muscle fibers

A new application of the reduced representation bisulfite sequencing method was developed using low-DNA input to investigate the epigenetic profile of human slow- and fast-twitch skeletal muscle fibers. Successful library construction was completed with as little as 15 ng of DNA, and high-quality sequencing data were obtained with 32 ng of DNA. Analysis identified 143,160 differentially methylated CpG sites across 14,046 genes. In both fiber types, selected genes predominantly expressed in slow or fast fibers were hypomethylated, which was supported by the RNA-sequencing analysis. These are the first fiber type-specific methylation data from human skeletal muscle and provide a unique platform for future research.NEW & NOTEWORTHY This study validates a low-DNA input reduced representation bisulfite sequencing method for human muscle biopsy samples to investigate the methylation patterns at a fiber type-specific level. These are the first fiber type-specific methylation data reported from human skeletal muscle and thus provide initial insight into basal state differences in myosin heavy chain I and IIa muscle fibers among young, healthy men.

Human skeletal muscle fibre contractile properties and proteomic profile: adaptations to 3 weeks of unilateral lower limb suspension and active recovery

KEY POINTS

It is generally assumed that muscle fibres go through atrophy following disuse with a loss of specific force and an increase in unloaded shortening velocity. However, the underlying mechanisms remain to be clarified. Most studies have focused on events taking place during the development of disuse, whereas the subsequent recovery phase, which is equally important, has received little attention. Our findings support the hypotheses that the specific force of muscle fibres decreased following unilateral lower limb suspension (ULLS) and returned to normal after 3 weeks of active recovery as a result of a loss and recovery of myosin and actin content. Furthermore, muscle fibres went through extensive qualitative changes in muscle protein pattern following ULLS, and these were reversed by active recovery. Resistance training was very effective in restoring both muscle mass and qualitative muscle changes, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle.

ABSTRACT

Following disuse, muscle fibre function goes through adaptations such as a loss of specific force (PO /CSA) and an increase in unloaded shortening velocity, which could be a result of both quantitative changes (i.e. atrophy) and qualitative changes in protein pattern. The underlying mechanisms remain to be clarified. In addition, little is known about the recovery of muscle mass and strength following disuse. In the present study, we report an extensive dataset describing, in detail,the functional and protein content adaptations of skeletal muscle in response to both disuse and re-training. Eight young healthy subjects were subjected to 3 weeks of unilateral lower limb suspension (ULLS), a widely used human model of disuse skeletal muscle atrophy. Needle biopsies samples were taken from the vastus lateralis muscle Pre-ULLS, Post-ULLS and after 3 weeks of recovery during which heavy resistance training was performed. After disuse, cross-sectional area (CSA), PO /CSA and myosin concentration (MC) decreased in both type 1 and 2A skinned muscle fibres. After recovery, CSA and MC returned to levels comparable to those observed before disuse, whereas Po/CSA and unloaded shortening velocity reached a higher level. Myosin heavy chain isoform composition of muscle samples did not differ among the experimental groups. To study the mechanisms underlying such adaptations, a two-dimensional proteomic analysis was performed. ULLS induced a reduction of myofibrillar, metabolic (glycolytic and oxidative) and anti-oxidant defence system protein content. Resistance training was very effective in counteracting ULLS-induced alterations, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle.

Effect of 23-day muscle disuse on sarcoplasmic reticulum Ca2+ properties and contractility in human type I and type II skeletal muscle fibers

Inactivity negatively impacts on skeletal muscle function mainly through muscle atrophy. However, recent evidence suggests that the quality of individual muscle fibers is also altered. This study examined the effects of 23 days of unilateral lower limb suspension (ULLS) on specific force and sarcoplasmic reticulum (SR) Ca2+ content in individual skinned muscle fibers. Muscle biopsies of the vastus lateralis were taken from six young healthy adults prior to and following ULLS. After disuse, the endogenous SR Ca2+ content was ∼8% lower in type I fibers and maximal SR Ca2+ capacity was lower in both type I and type II fibers (−11 and −5%, respectively). The specific force, measured in single skinned fibers from three subjects, decreased significantly after ULLS in type II fibers (−23%) but not in type I fibers (−9%). Western blot analyses showed no significant change in the amounts of myosin heavy chain (MHC) I and MHC IIa following the disuse, whereas the amounts of sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA1) and calsequestrin increased by ∼120 and ∼20%, respectively, and the amount of troponin I decreased by ∼21%. These findings suggest that the decline in force and power occurring with muscle disuse is likely to be exacerbated in part by reductions in maximum specific force in type II fibers, and in the amount of releasable SR Ca2+ in both fiber types, the latter not being attributable to a reduced calsequestrin level. Furthermore, the ∼3-wk disuse in human elicits change in SR properties, in particular a more than twofold upregulation in SERCA1 density, before any fiber-type shift.

Men and Women Demonstrate Differences in Early Functional Recovery After Total Knee Arthroplasty

OBJECTIVE

To investigate whether sex affects the trajectory of functional recovery after total knee arthroplasty (TKA).

DESIGN

Retrospective analysis from a historical database containing data from 3 prospective clinical trials and a pilot study.

SETTING

Clinical laboratory setting.

PARTICIPANTS

Recruitment across studies was restricted to patients who underwent an elective unilateral TKA for the treatment of osteoarthritis and were between 50 and 85 years of age (N=301).

INTERVENTIONS

Across all 4 studies, patients received a TKA and physical therapy intervention. Measures of physical function and strength were assessed before TKA and 1, 3, and 6 months after TKA.

MAIN OUTCOME MEASURES

Using a repeated-measures maximum likelihood model, statistical inference was made to estimate the changes in outcomes from before surgery to 1, 3, and 6 months after TKA that were stratified by sex. Muscle strength was assessed during maximal isometric quadriceps and hamstrings contractions. Muscle activation was assessed in the quadriceps muscle. Physical function outcomes included timed Up and Go (TUG) test, stair climbing test, and 6-minute walk test (6MWT).

RESULTS

Women demonstrated less decline in quadriceps strength than did men at 1, 3, and 6 months after TKA (P<.04), whereas women demonstrated less decline in hamstrings strength 1 month after TKA (P<.0001). Women demonstrated a greater decline than did men on the TUG test (P=.001), stair climbing test (P=.004), and 6MWT (P=.001) 1 month after TKA. Sex differences in physical function did not persist at 3 and 6 months after TKA.

CONCLUSIONS

Sex affected early recovery of muscle and physical function in the first month after TKA. Women demonstrated better preservation of quadriceps strength but a greater decline on measures of physical function than did men.

Enteral sesame oil therapeutically relieves disease severity in rat experimental osteoarthritis

Background

Osteoarthritis (OA) is the most common cause of joint pain, affecting approximately 15% of the population. Recent studies indicate that quadriceps muscle weakness is directly involved in the pathogenesis of OA-associated joint pain. Oxidative stress plays an important role in skeletal muscle dysfunction. Sesame oil is a natural product with excellent antioxidative property. However, whether sesame oil can decrease OA-induced joint pain has never been investigated.

Objective

The aim of the present study was to examine the effect of sesame oil on OA-induced joint pain in rats.

Design

OA-associated joint pain in rats was induced by medial meniscal transection in rats. Sesame oil (0, 1, 2, or 4 ml/kg/day, orally) was given to rats 7 days after OA induction, while the parameters were determined 7 days after sesame oil administration.

Results

Daily sesame oil treatment for 7 days significantly decreased OA-associated joint pain. Sesame oil decreased muscular interleukin-6 and increased citrate synthase activity and myosin heavy chain IIa mRNA expression. Furthermore, sesame oil decreased muscular lipid peroxidation, nuclear Nrf2 protein expression, and reactive oxygen species generations as well as increased glutathione production and glutathione peroxidase activity in OA rats.

Conclusions

Sesame oil may relieve OA-associated joint pain by inhibiting quadriceps muscular oxidative stress, at least partially, in rats.

Skeletal muscle fiber size and fiber type distribution in human cancer: Effects of weight loss and relationship to physical function

BACKGROUND & AIMS

Cancer patients frequently experience weight loss, with negative consequences for functionality and prognosis. The extent to which muscle atrophy contributes to weight loss, however, is not clear, as few studies have directly measured muscle fiber morphology in cancer patients.

METHODS

Whole body and regional tissue composition were measured, along with the cross-sectional area (CSA) and fiber type of mechanically-isolated, single muscle fibers, in 19 cancer patients (8 with a history of weight loss, 11 weight-stable) and 15 non-diseased controls.

RESULTS

Whole body fat mass was reduced in cancer patients with a history of weight loss, but no differences in whole body or leg fat-free mass were apparent. In contrast, reductions (∼20%) in single muscle fiber CSA were found in both slow-twitch, myosin heavy chain (MHC) I and fast-twitch, MHC IIA fibers in both weight-stable patients and those with a history of weight loss. Fiber type distribution showed a shift towards a fast-twitch phenotype compared to controls, which may preserve muscle function in cancer patients despite atrophy, as positive relationships were found between the fractions of hybrid MHC IIAX and I/IIA fibers and 6-min walk performance.

CONCLUSIONS

Our results suggest that, although not apparent from whole body or regional measurements, cancer is associated with reduced skeletal muscle fiber size independent of weight loss history and a shift towards fast-twitch fibers, phenotypes that resemble adaptations to muscle disuse.

Skeletal muscle ultrastructure and function in statin-tolerant individuals

INTRODUCTION

Statins have well-known benefits on cardiovascular mortality, though up to 15% of patients experience side effects. With guidelines from the American Heart Association, American College of Cardiology, and American Diabetes Association expected to double the number of statin users, the overall incidence of myalgia and myopathy will increase.

METHODS

We evaluated skeletal muscle structure and contractile function at the molecular, cellular, and whole tissue levels in 12 statin tolerant and 12 control subjects.

RESULTS

Myosin isoform expression, fiber type distributions, single fiber maximal Ca(2+) -activated tension, and whole muscle contractile force were similar between groups. No differences were observed in myosin-actin cross-bridge kinetics in myosin heavy chain I or IIA fibers.

CONCLUSIONS

We found no evidence for statin-induced changes in muscle morphology at the molecular, cellular, or whole tissue levels. Collectively, our data show that chronic statin therapy in healthy asymptomatic individuals does not promote deleterious myofilament structural or functional adaptations.

Reduced rate of knee extensor torque development in older adults with knee osteoarthritis is associated with intrinsic muscle contractile deficits

We examined the effect of knee osteoarthritis on the rate of torque development (RTD) of the knee extensors in older adults with advanced-stage knee osteoarthritis (OA; n=15) and recreationally-active controls (n=15) of similar age, sex and health status, as well as the relationship between RTD and the size and contractility of single muscle fibers. OA participants had lower RTD when expressed in absolute terms (Nm/ms). There were sex differences in peak RTD (P<0.05), with greater RTD in men, but no group by sex interaction effects for any variables. The lower RTD in OA versus controls was not explained by variation between groups in the fiber type admixture of the muscle, and was mitigated when RTD was normalized to peak torque (PT). In knee OA volunteers, we found strong correlations between the RTD expressed relative to PT and the velocity of contraction of single myosin heavy chain (MHC) I and IIA/X muscle fibers (r=0.652 and 0.862; both P<0.05) and power output of MHC I fibers (r=0.642; P<0.05). In controls, RTD relative to PT was related to fiber cross-sectional area of MHC IIA/X fibers (r=0.707; P<0.05), but not measures of single fiber contractile performance. To our knowledge, these results represent the first demonstration that variation in whole muscle contractile kinetics in patients with advanced-stage knee osteoarthritis and healthy older adults is related, in part, to the size and function of single muscle fibers.

Effects of age and hindlimb immobilization and remobilization on fast troponin T precursor mRNA alternative splicing in rat gastrocnemius muscle

Fast skeletal muscle troponin T (TNNT3) is an important component of the skeletal muscle contractile machinery. The precursor mRNA (pre-mRNA) encoding TNNT3 is alternatively spliced, and changes in the pattern of TNNT3 splice form expression are associated with alterations in thin-filament calcium sensitivity and force production during muscle contraction and thereby regulate muscle function. Interestingly, during aging, the muscle force/cross-sectional area is reduced, suggesting that loss of mass does not completely account for the impaired muscle function that develops during the aging process. Therefore, in this study, we tested the hypothesis that age and changes in muscle loading are associated with alterations in Tnnt3 alternative splicing in the rat gastrocnemius muscle. We found that the relative abundance of several Tnnt3 splice forms varied significantly with age among 2-, 9-, and 18-month-old rats and that the pattern correlated with changes in body mass rather than muscle mass. Hindlimb immobilization for 7 days resulted in dramatic alterations in splice form relative abundance such that the pattern was similar to that observed in lighter animals. Remobilization for 7 days restored the splicing pattern toward that observed in the nonimmobilized limb, even though muscle mass had not yet begun to recover. In conclusion, the results suggest that Tnnt3 pre-mRNA alternative splicing is modulated rapidly (i.e., within days) in response to changes in the load placed on the muscle. Moreover, the results show that restoration of Tnnt3 alternative splicing to control patterns is initiated prior to an increase in muscle mass.

Oxidative stress participates in quadriceps muscle dysfunction during the initiation of osteoarthritis in rats

Osteoarthritis is the most common form of arthritis, affecting approximately 15% of the population. Quadriceps muscle weakness is one of the risk factors of osteoarthritis development. Oxidative stress has been reported to play an important role in the pathogenesis of various muscle dysfunction; however, whether it is involved in osteoarthritis-associated quadriceps muscle weakness has never been investigated. The aim of the present study is to examine the involvement of oxidative stress in quadriceps muscle dysfunction in the initiation of osteoarthritis in rats. Rat osteoarthritis was initiated by conducting meniscectomy (MNX). Quadriceps muscle dysfunction was evaluated by assessing muscular interleukin-6, citrate synthase activity, and myosin heavy chain IIa mRNA expression levels. Muscular oxidative stress was assessed by determining lipid peroxidation, Nrf2 expression, reactive oxygen species, and circulating antioxidants. Increased muscular interleukin-6 production as well as decreased citrate synthase activity and myosin heavy chain IIa mRNA expression were observed at 7 and 14 days after MNX. Biomarkers of oxidative stress were significantly increased after MNX. Muscular free radical counts were increased while glutathione and glutathione peroxidase expression were decreased in MNX-treated rats. We conclude that oxidative stress may be involved in the pathogenesis of muscle dysfunction in MNX-induced osteoarthritis.

In vivo and in vitro evidence that intrinsic upper- and lower-limb skeletal muscle function is unaffected by ageing and disuse in oldest-old humans

AIM

To parse out the impact of advanced ageing and disuse on skeletal muscle function, we utilized both in vivo and in vitro techniques to comprehensively assess upper- and lower-limb muscle contractile properties in 8 young (YG; 25 ± 6 years) and 8 oldest-old mobile (OM; 87 ± 5 years) and 8 immobile (OI; 88 ± 4 years) women.

METHODS

In vivo, maximal voluntary contraction (MVC), electrically evoked resting twitch force (RT), and physiological cross-sectional area (PCSA) of the quadriceps and elbow flexors were assessed. Muscle biopsies of the vastus lateralis and biceps brachii facilitated the in vitro assessment of single fibre-specific tension (Po).

RESULTS

In vivo, compared to the young, both the OM and OI exhibited a more pronounced loss of MVC in the lower limb [OM (-60%) and OI (-75%)] than the upper limb (OM = -51%; OI = -47%). Taking into account the reduction in muscle PCSA (OM = -10%; OI = -18%), only evident in the lower limb, by calculating voluntary muscle-specific force, the lower limb of the OI (-40%) was more compromised than the OM (-13%). However, in vivo, RT in both upper and lower limbs (approx. 9.8 N m cm(-2) ) and Po (approx. 123 mN mm(-2) ), assessed in vitro, implies preserved intrinsic contractile function in all muscles of the oldest-old and were well correlated (r = 0.81).

CONCLUSION

These findings suggest that in the oldest-old, neither advanced ageing nor disuse, per se, impacts intrinsic skeletal muscle function, as assessed in vitro. However, in vivo, muscle function is attenuated by age and exacerbated by disuse, implicating factors other than skeletal muscle, such as neuromuscular control, in this diminution of function.