Exercise intolerance and rapid skeletal muscle energetic decline in human age-associated frailty

Reliability of In Vivo Creatine-Weighted Chemical Exchange Saturation Transfer (CrCEST) MRI in Calf Skeletal Muscle of Healthy Volunteers at 3 T

BACKGROUND

Skeletal muscle mitochondrial oxidative phosphorylation (mtOXPHOS) is important for ATP generation and its dysfunction leads to exercise intolerance. Phosphorus magnetic resonance spectroscopy (31P-MRS) is a useful, noninvasive technique for mtOXPHOS assessment but has limitations. Creatine-weighted chemical exchange saturation transfer (CrCEST) MRI is a potential alternative to assess muscle bioenergetics.

PURPOSE

To evaluate the interscan repeatability, intra- and interobserver reproducibility of CrCEST during mild plantar flexion exercise.

STUDY TYPE

Retrospective.

SUBJECTS

Twenty healthy volunteers (age 37.6 ± 12.4 years, 11 females).

FIELD STRENGTH/SEQUENCE

3 T/CEST imaging using gradient echo readout.

ASSESSMENT

τCrCEST (postexercise Cr recovery time) was assessed in two scans for each participant, following mild plantar flexion exercises targeting the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (Sol) muscles. Three observers measured τCrCEST for interobserver reproducibility. Three readings by one observer were used to measure intraobserver reproducibility. Two scans were used for within-participant interscan repeatability.

STATISTICAL TESTS

Paired t tests, intraclass correlation coefficient (ICC), and Pearson correlation were conducted. Bland-Altman plots were used to analyze the interobserver variability. A P-value of 0.05 was considered statistically significant.

RESULTS

There was excellent intra- (ICC∈ 0.94 - 0.98 $$ \in \left[0.94-0.98\right] $$ ) and interobserver (ICC∈ 0.9 - 0.98 $$ \in \left[0.9-0.98\right] $$ ) reproducibility, with moderate interscan repeatability for τCrCEST in LG and MG (ICC∈ 0.54 - 0.74 $$ \in \left[0.54-0.74\right] $$ ) and poor-to-moderate interscan repeatability in Sol (ICC∈ 0.24 - 0.53 $$ \in \left[0.24-0.53\right] $$ ). Excellent interobserver reproducibility was confirmed by Bland-Altman plots (fixed bias P-value∈ 0.08 - 0.87 $$ \in \left[0.08-0.87\right] $$ ).

DATA CONCLUSION

CrCEST MRI shows promise in assessing muscle bioenergetics by evaluating τCrCEST during mild plantar flexion exercise with reasonable reliability, particularly in LG and MG.

LEVEL OF EVIDENCE

4 TECHNICAL EFFICACY: Stage 1.

Decoding the decline: unveiling drivers of sarcopenia

There remains a critical need to define molecular pathways underlying sarcopenia to identify putative therapeutic targets. Research in the mechanisms of aging and sarcopenia relies heavily on preclinical rodent models. In this issue of the JCI, Kerr et al. implemented a clinically-relevant sarcopenia classification system of aged C57BL/6J mice, capturing sarcopenia prevalence across both sexes. The authors performed detailed physiological, molecular, and energetic analyses and demonstrated that mitochondrial biogenesis, oxidative capacity, and AMPK-autophagy signaling decreased as sarcopenia progressed in male mice. Sarcopenia was less prevalent in female mice with fewer alterations compared with the male-affected processes. The findings highlight factors beyond age as necessary for classifying the sarcopenic phenotype in rodent models, reveal sexual dimorphism across the trajectory of age-related declines in muscle mass and function in a commonly used rodent model, and provide insight into sex-dependent molecular alterations associated with sarcopenia progression.

Metabolic dysfunction and the development of physical frailty: an aging war of attrition

The World Health Organization recently declared 2021-2030 the decade of healthy aging. Such emphasis on healthy aging requires an understanding of the biologic challenges aging populations face. Physical frailty is a syndrome of vulnerability that puts a subset of older adults at high risk for adverse health outcomes including functional and cognitive decline, falls, hospitalization, and mortality. The physiology driving physical frailty is complex with age-related biological changes, dysregulated stress response systems, chronic inflammatory pathway activation, and altered energy metabolism all likely contributing. Indeed, a series of recent studies suggests circulating metabolomic distinctions can be made between frail and non-frail older adults. For example, marked restrictions on glycolytic and mitochondrial energy production have been independently observed in frail older adults and collectively appear to yield a reliance on the highly fatigable ATP-phosphocreatine (PCr) energy system. Further, there is evidence that age-associated impairments in the primary ATP generating systems (glycolysis, TCA cycle, electron transport) yield cumulative deficits and fail to adequately support the ATP-PCr system. This in turn may acutely contribute to several major components of the physical frailty phenotype including muscular fatigue, weakness, slow walking speed and, over time, result in low physical activity and accelerate reductions in lean body mass. This review describes specific age-associated metabolic declines and how they can collectively lead to metabolic inflexibility, ATP-PCr reliance, and the development of physical frailty. Further investigation remains necessary to understand the etiology of age-associated metabolic deficits and develop targeted preventive strategies that maintain robust metabolic health in older adults.

Association between dietary phosphate intake and skeletal muscle energetics in adults without cardiovascular disease

Highly bioavailable inorganic phosphate (Pi) is present in large quantities in the typical Western diet and represents a large fraction of total phosphate intake. Dietary Pi excess induces exercise intolerance and skeletal muscle mitochondrial dysfunction in normal mice. However, the relevance of this to humans remains unknown. The study was conducted on 13 individuals without a history of cardiopulmonary disease (46% female, 15% Black participants) enrolled in the pilot-phase of the Dallas Heart and Mind Study. Total dietary phosphate was estimated from 24-h dietary recall (ASA24). Muscle ATP synthesis was measured at rest, and phosphocreatinine (PCr) dynamics was measured during plantar flexion exercise using 7-T 31P magnetic resonance (MR) spectroscopy in the calf muscle. Correlation was assessed between dietary phosphate intake normalized to total caloric intake, resting ATP synthesis, and PCr depletion during exercise. Higher dietary phosphate intake was associated with lower resting ATP synthesis (r = -0.62, P = 0.03), and with higher levels of PCr depletion during plantar flexion exercise relative to the resting period (r = -0.72; P = 0.004). These associations remain significant after adjustment for age and estimated glomerular filtration rate (both P < 0.05). High dietary phosphate intake was also associated with lower serum Klotho levels, and Klotho levels are in turn associated with PCr depletion and higher ADP accumulation post exercise. Our study suggests that higher dietary phosphate is associated with reduced skeletal muscle mitochondrial function at rest and exercise in humans providing new insight into potential mechanisms linking the Western diet to impaired energy metabolism.NEW & NOTEWORTHY This is the first translational research study directly demonstrating the adverse effects of dietary phosphate on muscle energy metabolism in humans. Importantly, our data show that dietary phosphate is associated with impaired muscle ATP synthesis at rest and during exercise, independent of age and renal function. This is a new biologic paradigm with significant clinical dietary implications.

Energetics and clinical factors for the time required to walk 400 m: The Study of Muscle, Mobility and Aging (SOMMA)

BACKGROUND

Walking slows with aging often leading to mobility disability. Mitochondrial energetics has been found to be associated with gait speed over short distances. Additionally, walking is a complex activity but few clinical factors that may be associated with walk time have been studied.

METHODS

We examined 879 participants ≥70 years and measured the time to walk 400 m. We tested the hypothesis that decreased mitochondrial energetics by respirometry in muscle biopsies and magnetic resonance spectroscopy in the thigh and is associated with longer time to walk 400 m. We also used cardiopulmonary exercise testing to assess the energetic costs of walking: maximum oxygen consumption (VO2peak) and energy cost-capacity (the ratio of VO2, at a slow speed to VO2peak). In addition, we tested the hypothesis that selected clinical factors would also be associated with 400-m walk time.

RESULTS

Lower Max OXPHOS was associated with longer walk time, and the association was explained by the energetic costs of walking, leg power, and weight. Additionally, a multivariate model revealed that longer walk time was also significantly associated with lower VO2peak, greater cost-capacity ratio, weaker leg power, heavier weight, hip and knee stiffness, peripheral neuropathy, greater perceived exertion while walking slowly, greater physical fatigability, less moderate-to-vigorous exercise, less sedentary time, and anemia. Significant associations between age, sex, muscle mass, and peripheral artery disease with 400-m walk time were explained by other clinical and physiologic factors.

CONCLUSIONS

Lower mitochondrial energetics is associated with needing more time to walk 400 m. This supports the value of developing interventions to improve mitochondrial energetics. Additionally, doing more moderate-to-vigorous exercise, increasing leg power, reducing weight, treating hip and knee stiffness, and screening for and treating anemia may reduce the time required to walk 400 m and reduce the risk of mobility disability.

HDL-C and apolipoprotein A-I are independently associated with skeletal muscle mitochondrial function in healthy humans

Prior animal and cell studies have demonstrated a direct role of high-density lipoprotein (HDL) and apolipoprotein A-I (ApoA-I) in enhancing skeletal muscle mitochondrial function and exercise capacity. However, the relevance of these animal and cell investigations in humans remains unknown. Therefore, a cross-sectional study was conducted in 48 adults (67% female, 8% Black participants, age 39 ± 15.4 yr old) to characterize the associations between HDL measures, ApoA-I, and muscle mitochondrial function. Forearm muscle oxygen recovery time (tau) from postexercise recovery kinetics was used to assess skeletal muscle mitochondrial function. Lipoprotein measures were assessed by nuclear magnetic resonance. HDL efflux capacity was assessed using J774 macrophages, radiolabeled cholesterol, and apolipoprotein B-depleted plasma both with and without added cyclic adenosine monophosphate. In univariate analyses, faster skeletal muscle oxygen recovery time (lower tau) was significantly associated with higher levels of HDL cholesterol (HDL-C), ApoA-I, and larger mean HDL size, but not HDL cholesterol efflux capacity. Slower recovery time (higher tau) was positively associated with body mass index (BMI) and fasting plasma glucose (FPG). In multivariable linear regression analyses, higher levels of HDL-C and ApoA-I, as well as larger HDL size, were independently associated with faster skeletal muscle oxygen recovery times that persisted after adjusting for BMI and FPG (all P < 0.05). In conclusion, higher levels of HDL-C, ApoA-I, and larger mean HDL size were independently associated with enhanced skeletal muscle mitochondrial function in healthy humans.NEW & NOTEWORTHY Our study provides the first direct evidence supporting the beneficial role of HDL-C and ApoA-I on enhanced skeletal muscle mitochondrial function in healthy young to middle-aged humans without cardiometabolic disease.

Lower muscle mitochondrial energetics is associated with greater phenotypic frailty in older women and men: the Study of Muscle, Mobility and Aging

BACKGROUND

Phenotypic frailty syndrome identifies older adults at greater risk for adverse health outcomes. Despite the critical role of mitochondria in maintaining cellular function, including energy production, the associations between muscle mitochondrial energetics and frailty have not been widely explored in a large, well-phenotyped, older population.

METHODS

The Study of Muscle, Mobility and Aging (SOMMA) assessed muscle energetics in older adults (N = 879, mean age = 76.3 years, 59.2% women). 31Phosporous magnetic resonance spectroscopy measured maximal production of adenosine triphosphate (ATPmax) in vivo, while ex vivo high-resolution respirometry of permeabilized muscle fibers from the vastus lateralis measured maximal oxygen consumption supported by fatty acids and complex I- and II-linked carbohydrates (e.g., Max OXPHOSCI+CII). Five frailty criteria, shrinking, weakness, exhaustion, slowness, and low activity, were used to classify participants as robust (0, N = 397), intermediate (1-2, N = 410), or frail (≥ 3, N = 66). We estimated the proportional odds ratio (POR) for greater frailty, adjusted for multiple potential confounders.

RESULTS

One-SD decrements of most respirometry measures (e.g., Max OXPHOSCI+CII, adjusted POR = 1.5, 95%CI [1.2,1.8], p = 0.0001) were significantly associated with greater frailty classification. The associations of ATPmax with frailty were weaker than those between Max OXPHOSCI+CII and frailty. Muscle energetics was most strongly associated with slowness and low physical activity components.

CONCLUSIONS

Our data suggest that deficits in muscle mitochondrial energetics may be a biological driver of frailty in older adults. On the other hand, we did observe differential relationships between measures of muscle mitochondrial energetics and the individual components of frailty.

Energetics and Clinical Factors for the Time Required to Walk 400 Meters The Study of Muscle, Mobility and Aging (SOMMA)

Background

Walking slows with aging often leading to mobility disability. Mitochondrial energetics has been found to influence gait speed over short distances. Additionally, walking is a complex activity but few clinical factors that may influence walk time have been studied.

Methods

We examined 879 participants ≥70 years and measured the time to walk 400m. We tested the hypothesis that decreased mitochondrial energetics by respirometry in muscle biopsies and magnetic resonance spectroscopy in the thigh, is associated with longer time to walk 400m. We also used cardiopulmonary exercise testing to assess the energetic costs of walking: maximum oxygen consumption (VO 2 peak) and energy cost-capacity (the ratio of VO2, at a slow speed to VO 2 peak). In addition, we tested the hypothesis that selected clinical factors would also be associated with 400m walk time.

Results

Lower Max OXPHOS was associated with longer walk time and the association was explained by the energetics costs of walking, leg power and weight. Additionally, a multivariate model revealed that longer walk time was also significantly associated with lower VO 2 peak, greater cost-capacity ratio, weaker leg power, heavier weight, hip and knee stiffness, peripheral neuropathy, greater perceived exertion while walking slowly, greater physical fatigability, less moderate-to-vigorous exercise, less sedentary time and anemia. Significant associations between age, sex, muscle mass, and peripheral artery disease with 400m walk time were explained by other clinical and physiologic factors.

Conclusions

Lower mitochondrial energetics is associated with needing more time to walk 400m. This supports the value of developing interventions to improve mitochondrial energetics. Additionally, doing more moderate-to-vigorous exercise, increasing leg power, reducing weight, treating hip and knee stiffness, and screening for and treating anemia may reduce the time required to walk 400m and reduce the risk of mobility disability.

Native lamin A/C proteomes and novel partners from heart and skeletal muscle in a mouse chronic inflammation model of human frailty

Clinical frailty affects ∼10% of people over age 65 and is studied in a chronically inflamed (Interleukin-10 knockout; "IL10-KO") mouse model. Frailty phenotypes overlap the spectrum of diseases ("laminopathies") caused by mutations in LMNA. LMNA encodes nuclear intermediate filament proteins lamin A and lamin C ("lamin A/C"), important for tissue-specific signaling, metabolism and chromatin regulation. We hypothesized that wildtype lamin A/C associations with tissue-specific partners are perturbed by chronic inflammation, potentially contributing to dysfunction in frailty. To test this idea we immunoprecipitated native lamin A/C and associated proteins from skeletal muscle, hearts and brains of old (21-22 months) IL10-KO versus control C57Bl/6 female mice, and labeled with Tandem Mass Tags for identification and quantitation by mass spectrometry. We identified 502 candidate lamin-binding proteins from skeletal muscle, and 340 from heart, including 62 proteins identified in both tissues. Candidates included frailty phenotype-relevant proteins Perm1 and Fam210a, and nuclear membrane protein Tmem38a, required for muscle-specific genome organization. These and most other candidates were unaffected by IL10-KO, but still important as potential lamin A/C-binding proteins in native heart or muscle. A subset of candidates (21 in skeletal muscle, 30 in heart) showed significantly different lamin A/C-association in an IL10-KO tissue (p < 0.05), including AldoA and Gins3 affected in heart, and Lmcd1 and Fabp4 affected in skeletal muscle. To screen for binding, eleven candidates plus prelamin A and emerin controls were arrayed as synthetic 20-mer peptides (7-residue stagger) and incubated with recombinant purified lamin A "tail" residues 385-646 under relatively stringent conditions. We detected strong lamin A binding to peptides solvent exposed in Lmcd1, AldoA, Perm1, and Tmem38a, and plausible binding to Csrp3 (muscle LIM protein). These results validated both proteomes as sources for native lamin A/C-binding proteins in heart and muscle, identified four candidate genes for Emery-Dreifuss muscular dystrophy (CSRP3, LMCD1, ALDOA, and PERM1), support a lamin A-interactive molecular role for Tmem38A, and supported the hypothesis that lamin A/C interactions with at least two partners (AldoA in heart, transcription factor Lmcd1 in muscle) are altered in the IL10-KO model of frailty.

Interleukin-6 Drives Mitochondrial Dysregulation and Accelerates Physical Decline: Insights From an Inducible Humanized IL-6 Knock-In Mouse Model

Chronic activation of inflammatory pathways (CI) and mitochondrial dysfunction are independently linked to age-related functional decline and early mortality. Interleukin 6 (IL-6) is among the most consistently elevated chronic activation of inflammatory pathways markers, but whether IL-6 plays a causative role in this mitochondrial dysfunction and physical deterioration remains unclear. To characterize the role of IL-6 in age-related mitochondrial dysregulation and physical decline, we have developed an inducible human IL-6 (hIL-6) knock-in mouse (TetO-hIL-6mitoQC) that also contains a mitochondrial-quality control reporter. Six weeks of hIL-6 induction resulted in upregulation of proinflammatory markers, cell proliferation and metabolic pathways, and dysregulated energy utilization. Decreased grip strength, increased falls off the treadmill, and increased frailty index were also observed. Further characterization of skeletal muscles postinduction revealed an increase in mitophagy, downregulation of mitochondrial biogenesis genes, and an overall decrease in total mitochondrial numbers. This study highlights the contribution of IL-6 to mitochondrial dysregulation and supports a causal role of hIL-6 in physical decline and frailty.

The overall and domain-specific quality of life of Chinese community-dwelling older adults: the role of intrinsic capacity and disease burden

Objective

This study aimed to investigate the impact of the different domains of intrinsic capacity (IC) and chronic disease burden on health-related quality of life (HRQoL) and domain-specific HRQoL in Chinese community-dwelling older adults.

Design

A cross-sectional observational study of a community-based cohort.

Participants

We evaluated Chinese older adults (n = 429, mean age, 72.91 ± 7.014 years; female proportion, 57.30%).

Measurements

IC contains five domains, namely locomotion, vitality, cognition, psychological, and sensory capacity. Locomotion dysfunction was defined as grip and/or gait decline. Vitality decline was defined if two of the following three parameters were present: fatigue, physical inactivity, and weight loss or overweight. Cognition was classified into normal cognition, pre-mild cognitive impairment (pre-MCI), and MCI according to the normative z-scores of the neuropsychological test battery. Psychological dysfunction was diagnosed based on depressive symptoms. Sensory dysfunction was defined as hearing and/or vision impairment. HRQoL was assessed using the AQoL-8D scale, which comprised physical (including independent living, senses, and pain) and psychosocial (including mental health, happiness, self-worth, coping, and relationships) dimensions. Low HRQoL (HRQoL score or subscores in the highest quintile) was used as a dependent variable in logistic regression analyses adjusted for demographic, health-related, and psychological confounders.

Results

Sensory impairment was an independent determinant of senses, and locomotion impairment was significantly associated with overall HRQoL, independent living, and pain in the physical dimension of HRQoL. Cognition was an independent determinant of the senses. Vitality was independently associated with overall HRQoL, senses, and pain in the physical dimension and mental health and relationships in the psychological dimension of HRQoL. The psychological domain of IC was independently associated with overall and domain-specific HRQoL apart from senses after adjustment for all confounders. The number of multimorbidities mainly had a significant impact on independent living after adjustment for all confounders.

Conclusion

IC domains and chronic disease burden had heterogeneous influences on overall and domain-specific HRQoL. The impairment of sensory and locomotion domains had a synergistic impact on the overall and physical dimensions of HRQoL. The vitality and psychological domains of IC had more profound effects on HRQoL. Older people with high morbidity might have a higher risk of poor independent living.

WASF3 disrupts mitochondrial respiration and may mediate exercise intolerance in myalgic encephalomyelitis/chronic fatigue syndrome

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is characterized by various disabling symptoms including exercise intolerance and is diagnosed in the absence of a specific cause, making its clinical management challenging. A better understanding of the molecular mechanism underlying this apparent bioenergetic deficiency state may reveal insights for developing targeted treatment strategies. We report that overexpression of Wiskott-Aldrich Syndrome Protein Family Member 3 (WASF3), here identified in a 38-y-old woman suffering from long-standing fatigue and exercise intolerance, can disrupt mitochondrial respiratory supercomplex formation and is associated with endoplasmic reticulum (ER) stress. Increased expression of WASF3 in transgenic mice markedly decreased their treadmill running capacity with concomitantly impaired respiratory supercomplex assembly and reduced complex IV levels in skeletal muscle mitochondria. WASF3 induction by ER stress using endotoxin, well known to be associated with fatigue in humans, also decreased skeletal muscle complex IV levels in mice, while decreasing WASF3 levels by pharmacologic inhibition of ER stress improved mitochondrial function in the cells of the patient with chronic fatigue. Expanding on our findings, skeletal muscle biopsy samples obtained from a cohort of patients with ME/CFS showed increased WASF3 protein levels and aberrant ER stress activation. In addition to revealing a potential mechanism for the bioenergetic deficiency in ME/CFS, our study may also provide insights into other disorders associated with fatigue such as rheumatic diseases and long COVID.

Intermittent treatment with elamipretide preserves exercise tolerance in aged female mice

The pathology of aging impacts multiple organ systems, including the kidney and skeletal and cardiac muscles. Long-term treatment with the mitochondrial-targeted peptide elamipretide has previously been shown to improve in vivo mitochondrial function in aged mice, which is associated with increased fatigue resistance and treadmill performance, improved cardiovascular diastolic function, and glomerular architecture of the kidney. However, elamipretide is a short tetrameric peptide that is not orally bioavailable, limiting its routes of administration. This study tested whether twice weekly intermittent injections of elamipretide could recapitulate the same functional improvements as continuous long-term infusion. We found that intermittent treatment with elamipretide for 8 months preserved exercise tolerance and left ventricular mass in mice with modest protection of diastolic function and skeletal muscle force production but did not affect kidney function as previously reported using continuous treatment.

The influence of acute dietary nitrate supplementation on skeletal muscle fatigue and recovery in older women

Older individuals fatigue more rapidly during, and recover more slowly from, dynamic exercise. Women are particularly vulnerable to these deleterious effects of aging, which increases their risk of falling. We have shown that dietary nitrate (NO3 - ), a source of nitric oxide (NO) via the NO3 -  → nitrite (NO2 - ) → NO pathway, enhances muscle speed and power in older individuals in the non-fatigued state; however, it is unclear if it reduces fatigability and/or improves recoverability in this population. Using a double-blind, placebo-controlled, crossover design, we studied 18 older (age 70 ± 4 years) women who were administered an acute dose of beetroot juice (BRJ) containing either 15.6 ± 3.6 or <0.05 mmol of NO3 - . Blood samples were drawn throughout each ~3 h visit for plasma NO3 - and NO2 - analysis. Peak torque was measured during, and periodically for 10 min after, 50 maximal knee extensions performed at 3.14 rad/s on an isokinetic dynamometer. Ingestion of NO3 - -containing BRJ increased plasma NO3 - and NO2 - concentrations by 21 ± 8 and 4 ± 4 fold, respectively. However, there were no differences in muscle fatigue or recovery. Dietary NO3 - increases plasma NO3 - and NO2 - concentrations but does not reduce fatigability during or enhance recoverability after high intensity exercise in older women.

Effects of Low- and High-Frequency Cardiac Rehabilitation on Risk Factors, Physical Fitness and Quality of Life in Middle-Aged Women with Coronary Heart Disease

Cardiac rehabilitation (CR) is a system that comprehensively manages risk factors to reduce the recurrence rate after cardiovascular disease treatment. This study compared the effects of home-based low-frequency CR (1-2 times/week) and center-based high-frequency CR (3-5 times/week) for 12 weeks. This study was conducted as an observational case-control study. Ninety women, ages 45 to 60, who underwent coronary artery stenting were enrolled. Measurement variables were waist circumference, body mass index (BMI), blood pressure (BP), total cholesterol (TC), low-density lipoprotein cholesterol (LDLC), high-density lipoprotein cholesterol (HDLC), triglycerides (TG), glucose, VO₂ peak, body composition, and quality of life. Significant changes were observed in systolic BP, TC, LDLC, TG, VO₂ peak, exercise duration, and quality of life in both groups. However, BMI, waist circumference, body fat percentage, HDLC, and blood glucose only exhibited significant changes with HFT. The interaction effects according to time and group were as follows: systolic BP, waist circumference, body fat, BMI, HDLC, and glucose (p < 0.05). Therefore, in CR participants, HFT improved more than LFT on obesity factors, HDLC, and glucose change. As well as center-based HFT, home-based LFT also improved risk factors for cardiovascular disease, fitness, and quality of life. For female patients who have difficulty visiting the CR center frequently, home-based LFT may be a CR program that can be presented as an alternative.

Multisystem physiological perspective of human frailty and its modulation by physical activity

"Frailty" is a term used to refer to a state characterized by enhanced vulnerability to, and impaired recovery from, stressors compared with a nonfrail state, which is increasingly viewed as a loss of resilience. With increasing life expectancy and the associated rise in years spent with physical frailty, there is a need to understand the clinical and physiological features of frailty and the factors driving it. We describe the clinical definitions of age-related frailty and their limitations in allowing us to understand the pathogenesis of this prevalent condition. Given that age-related frailty manifests in the form of functional declines such as poor balance, falls, and immobility, as an alternative we view frailty from a physiological viewpoint and describe what is known of the organ-based components of frailty, including adiposity, the brain, and neuromuscular, skeletal muscle, immune, and cardiovascular systems, as individual systems and as components in multisystem dysregulation. By doing so we aim to highlight current understanding of the physiological phenotype of frailty and reveal key knowledge gaps and potential mechanistic drivers of the trajectory to frailty. We also review the studies in humans that have intervened with exercise to reduce frailty. We conclude that more longitudinal and interventional clinical studies are required in older adults. Such observational studies should interrogate the progression from a nonfrail to a frail state, assessing individual elements of frailty to produce a deep physiological phenotype of the syndrome. The findings will identify mechanistic drivers of frailty and allow targeted interventions to diminish frailty progression.

The influence of acute dietary nitrate supplementation on skeletal muscle fatigue and recovery in older women

Older individuals fatigue more rapidly during, and recover more slowly from, dynamic exercise. Women are particularly vulnerable to these deleterious effects of aging, which increases their risk of falling. We have shown that dietary nitrate (NO ₃ ^- ), a source of nitric oxide (NO) via the NO ₃ ^- → nitrite (NO ₂ ^- ) → NO pathway, enhances muscle speed and power in older individuals in the non-fatigued state; however, it is unclear if it reduces fatigability and/or improves recoverability in this population. Using a double-blind, placebo-controlled, crossover design, we studied 18 older (age 70 ± 4 y) women who were administered an acute dose of beetroot juice (BRJ) containing either 15.6±3.6 or <0.05 mmol of NO ₃ ^- . Blood samples were drawn throughout each ∼3 h visit for plasma NO ₃ ^- and NO ₂ ^- analysis. Peak torque was measured during, and periodically for 10 min after, 50 maximal knee extensions performed at 3.14 rad/s on an isokinetic dynamometer. Ingestion of NO ₃ ^- -containing BRJ increased plasma NO ₃ ^- and NO ₂ ^- concentrations by 21±8 and 4±4 fold, respectively. However, there were no differences in muscle fatigue or recovery. Dietary NO ₃ ^- increases plasma NO ₃ ^- and NO ₂ ^- concentrations but does not reduce fatigability during or enhance recoverability after high intensity exercise in older women.

Skeletal muscle mitochondrial inertia is associated with carnitine acetyltransferase activity and physical function in humans

BACKGROUNDAt the onset of exercise, the speed at which phosphocreatine (PCr) decreases toward a new steady state (PCr on-kinetics) reflects the readiness to activate mitochondrial ATP synthesis, which is secondary to Acetyl-CoA availability in skeletal muscle. We hypothesized that PCr on-kinetics are slower in metabolically compromised and older individuals and are associated with low carnitine acetyltransferase (CrAT) protein activity and compromised physical function.METHODSWe applied 31P-magnetic resonance spectroscopy (31P-MRS) to assess PCr on-kinetics in 2 cohorts of volunteers. Cohort 1 included patients who had type 2 diabetes, were obese, were lean trained (VO2max > 55 mL/kg/min), and were lean untrained (VO2max < 45 mL/kg/min). Cohort 2 included young (20-30 years) and older (65-80 years) individuals with normal physical activity and older, trained individuals. Previous results of CrAT protein activity and acetylcarnitine content in muscle tissue were used to explore the underlying mechanisms of PCr on-kinetics, along with various markers of physical function.RESULTSPCr on-kinetics were significantly slower in metabolically compromised and older individuals (indicating mitochondrial inertia) as compared with young and older trained volunteers, regardless of in vivo skeletal muscle oxidative capacity (P < 0.001). Mitochondrial inertia correlated with reduced CrAT protein activity, low acetylcarnitine content, and functional outcomes (P < 0.001).CONCLUSIONPCr on-kinetics are significantly slower in metabolically compromised and older individuals with normal physical activity compared with young and older trained individuals, regardless of in vivo skeletal muscle oxidative capacity, indicating greater mitochondrial inertia. Thus, PCr on-kinetics are a currently unexplored signature of skeletal muscle mitochondrial metabolism, tightly linked to functional outcomes. Skeletal muscle mitochondrial inertia might emerge as a target of intervention to improve physical function.TRIAL REGISTRATIONNCT01298375 and NCT03666013 (clinicaltrials.gov).FUNDINGRM and MH received an EFSD/Lilly grant from the European Foundation for the Study of Diabetes (EFSD). VS was supported by an ERC starting grant (grant 759161) "MRS in Diabetes."

Frailty in older people living with HIV: current status and clinical management

This paper will update care providers on the clinical and scientific aspects of frailty which affects an increasing proportion of older people living with HIV (PLWH). The successful use of combination antiretroviral therapy has improved long-term survival in PLWH. This has increased the proportion of PLWH older than 50 to more than 50% of the HIV population. Concurrently, there has been an increase in the premature development of age-related comorbidities as well as geriatric syndromes, especially frailty, which affects an important minority of older PLWH. As the number of frail older PLWH increases, this will have an important impact on their health care delivery. Frailty negatively affects a PLWH's clinical status, and increases their risk of adverse outcomes, impacting quality of life and health-span. The biologic constructs underlying the development of frailty integrate interrelated pathways which are affected by the process of aging and those factors which accelerate aging. The negative impact of sarcopenia in maintaining musculoskeletal integrity and thereby functional status may represent a bidirectional interaction with frailty in PLWH. Furthermore, there is a growing body of literature that frailty states may be transitional. The recognition and management of related risk factors will help to mitigate the development of frailty. The application of interdisciplinary geriatric management principles to the care of older PLWH allows reliable screening and care practices for frailty. Insight into frailty, increasingly recognized as an important marker of biologic age, will help to understand the diversity of clinical status occurring in PLWH, which therefore represents a fundamentally new and important aspect to be evaluated in their health care.

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.

Multisystem derangements in frailty and sarcopenia: a source for biomarker discovery

PURPOSE OF REVIEW

Multisystem derangements, encompassing metabolic, musculoskeletal and stress-response systems, occur during aging and are associated with the development of physical frailty and sarcopenia. These modular changes are relevant sources for the identification of biomarkers for the two conditions. Here, we provide an up-to-date overview on existing biomarkers of physical frailty and sarcopenia and discuss emerging approaches for biomarker discovery.

RECENT FINDINGS

Inflammatory, metabolic and hematologic markers are shared between physical frailty and sarcopenia. Gut microbial derivatives and damage-associated molecular patterns transferred via extracellular vesicles have been indicated as possible gut-muscle axis regulators and candidate markers of physical frailty and sarcopenia.

SUMMARY

Mediators of metabolic, musculoskeletal and stress-response system dysregulation are shared by physical frailty and sarcopenia and indicate the existence of common pathophysiological pathways. Multiplatform biomarker analyses have been proposed as an innovating approach for tracking the multifaceted and dynamic nature of physical frailty and sarcopenia. Upon validation, the identified biomarkers may support diagnostic makeup and tracking of the two conditions in both research and clinical settings.

Physiological Systems in Promoting Frailty

Frailty is a complex syndrome affecting a growing sector of the global population as medical developments have advanced human mortality rates across the world. Our current understanding of frailty is derived from studies conducted in the laboratory as well as the clinic, which have generated largely phenotypic information. Far fewer studies have uncovered biological underpinnings driving the onset and progression of frailty, but the stage is set to advance the field with preclinical and clinical assessment tools, multiomics approaches together with physiological and biochemical methodologies. In this article, we provide comprehensive coverage of topics regarding frailty assessment, preclinical models, interventions, and challenges as well as clinical frameworks and prevalence. We also identify central biological mechanisms that may be at play including mitochondrial dysfunction, epigenetic alterations, and oxidative stress that in turn, affect metabolism, stress responses, and endocrine and neuromuscular systems. We review the role of metabolic syndrome, insulin resistance and visceral obesity, focusing on glucose homeostasis, adenosine monophosphate-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), and nicotinamide adenine dinucleotide (NAD+ ) as critical players influencing the age-related loss of health. We further focus on how immunometabolic dysfunction associates with oxidative stress in promoting sarcopenia, a key contributor to slowness, weakness, and fatigue. We explore the biological mechanisms involved in stem cell exhaustion that affect regeneration and may contribute to the frailty-associated decline in resilience and adaptation to stress. Together, an overview of the interplay of aging biology with genetic, lifestyle, and environmental factors that contribute to frailty, as well as potential therapeutic targets to lower risk and slow the progression of ongoing disease is covered. © 2022 American Physiological Society. Compr Physiol 12:1-46, 2022.

Interleukin-6 potentiates endurance training adaptation and improves functional capacity in old mice

BACKGROUND

Interventions to preserve functional capacities at advanced age are becoming increasingly important. So far, exercise provides the only means to counteract age-related decrements in physical performance and muscle function. Unfortunately, the effectiveness of exercise interventions in elderly populations is hampered by reduced acceptance and compliance as well as disuse complications. We therefore studied whether application of interleukin-6 (IL-6), a pleiotropic myokine that is induced by skeletal muscle activity and exerts broad systemic effects in response to exercise, affects physical performance and muscle function alone or in combination with training in aged mice.

METHODS

Sedentary old male mice (Sed+Saline, n = 15) were compared with animals that received recombinant IL-6 (rIL-6) in an exercise-mimicking pulsatile manner (Sed+IL-6, n = 16), were trained with a moderate-intensity, low-volume endurance exercise regimen (Ex+Saline, n = 13), or were exposed to a combination of these two interventions (Ex+IL-6, n = 16) for 12 weeks. Before and at the end of the intervention, mice underwent a battery of tests to quantify endurance performance, muscle contractility in situ, motor coordination, and gait and metabolic parameters.

RESULTS

Mice exposed to enhanced levels of IL-6 during endurance exercise bouts showed superior improvements in endurance performance (33% more work and 12% greater peak power compared with baseline), fatigue resistance in situ (P = 0.0014 vs. Sed+Saline; P = 0.0199 vs. Sed+IL-6; and P = 0.0342 vs. Ex+Saline), motor coordination (rotarod performance, P = 0.0428), and gait (gait speed, P = 0.0053) following training. Pulsatile rIL-6 treatment in sedentary mice had only marginal effects on glucose tolerance and some gait parameters. No increase in adverse events or mortality related to rIL-6 treatment was observed.

CONCLUSIONS

Administration of rIL-6 paired with treadmill running bouts potentiates the adaptive response to a moderate-intensity low-volume endurance exercise regimen in old mice, while being safe and well tolerated.

Multicomponent Physical Exercise Training in Multimorbid and Palliative Oldest Adults

Exercise counteracts aging and pathology symptoms, but there is still scarce research on exercise programs for multimorbid and/or palliative old patients (MPO-Ps). In order to analyze whether the multicomponent physical-cognitive training is beneficial for this population, 17 MPO-Ps (81.59 ± 5.63 years) completed a >26 weeks home-based intervention (20-50 min/session, three sessions/week). Twenty-eight supervised and thirty-two autonomous sessions were gradually distributed along three phases: supervised training (ST), reduced supervision training (RST), and autonomous training (AT). Physical function (gait speed, hand grip and lower-limb strength, balance, and agility), mental status (MMSE), and autonomy in daily living (the Barthel Index) were assessed. Categorical analyses regarding the changes in the walking aids used in the test were added to improve the assessment of strength and agility along the intervention. Despite important study limitations, such as the small sample size and lack of a control group, and despite the MPO-Ps' very low baseline fitness and initial exercise intolerance, they benefited from the dual-tasking approach, especially in autonomy, lower-limb strength, and balance. Agility improvements were shown only by categorical analyses. As expected, most benefits increased the supervision (ST phase). Gait speed and cognitive status maintained despite the total autonomy in training in the last phase. Further research with larger samples should confirm if multicomponent physical-cognitive exercise, individualized and tailored on daily-basis, together with technical assistance and medical supervision, benefits this MPO-Ps population, and if it can be prescribed to them with security, in spite some of them already being palliative patients.

Skeletal muscle-specific Keap1 disruption modulates fatty acid utilization and enhances exercise capacity in female mice

Skeletal muscle health is important for the prevention of various age-related diseases. The loss of skeletal muscle mass, which is known as sarcopenia, underlies physical disability, poor quality of life and chronic diseases in elderly people. The transcription factor NRF2 plays important roles in the regulation of the cellular defense against oxidative stress, as well as the metabolism and mitochondrial activity. To determine the contribution of skeletal muscle NRF2 to exercise capacity, we conducted skeletal muscle-specific inhibition of KEAP1, which is a negative regulator of NRF2, and examined the cell-autonomous and non-cell-autonomous effects of NRF2 pathway activation in skeletal muscles. We found that NRF2 activation in skeletal muscles increased slow oxidative muscle fiber type and improved exercise endurance capacity in female mice. We also observed that female mice with NRF2 pathway activation in their skeletal muscles exhibited enhanced exercise-induced mobilization and β-oxidation of fatty acids. These results indicate that NRF2 activation in skeletal muscles promotes communication with adipose tissues via humoral and/or neuronal signaling and facilitates the utilization of fatty acids as an energy source, resulting in increased mitochondrial activity and efficient energy production during exercise, which leads to improved exercise endurance.

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Systemic Keap1 knockdown enhances exercise endurance capacity in mice.

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Keap1 deficiency in skeletal muscle activates NRF2 pathway.

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Keap1 deficiency in skeletal muscle enhances endurance capacity in female mice.

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Keap1 deficiency in skeletal muscle promotes exercise-induced fatty acid utilization.

Dynamic 31P-MRI and 31P-MRS of lower leg muscles in heart failure patients

Impaired oxidative metabolism is one of multi-variate factors leading to exercise intolerance in heart failure patients. The purpose of the study was to demonstrate the use of dynamic ³¹P magnetic resonance spectroscopy (MRS) and ³¹P magnetic resonance imaging (MRI) techniques to measure PCr resynthesis rate post-exercise as a biomarker for oxidative metabolism in skeletal muscle in HF patients and controls. In this prospective imaging study, we recruited six HF patients and five healthy controls. The imaging protocol included ³¹P-MRS, spectrally selective 3D turbo spin echo for ³¹P-MRI, and Dixon multi-echo GRE for fat–water imaging on a 3 T clinical MRI scanner. All the subjects were scanned pre-exercise, during plantar flexion exercise, and post-exercise recovery, with two rounds of exercise for ³¹P -MRS and ³¹P-MRI, respectively. Unpaired t-tests were used to compare ³¹P-MRS and ³¹P-MRI results between the HF and control cohorts. The results show that PCr resynthesis rate was significantly slower in the HF cohort compared to the controls using ³¹P-MRS (P = 0.0003) and ³¹P-MRI (P = 0.0014). ³¹P-MRI showed significant differences between the cohorts in muscle groups (soleus (P = 0.0018), gastrocnemius lateral (P = 0.0007) and gastrocnemius medial (P = 0.0054)). The results from this study suggest that ³¹P-MRS/³¹P-MRI may be used to quantify lower leg muscle oxidative metabolism in HF patients, with ³¹P-MRI giving an additional advantage of allowing further localization of oxidative metabolism deficits. Upon further validation, these techniques may serve as a potentially useful clinical imaging biomarker for staging and monitoring therapies in HF-patients.

The physical frailty syndrome as a transition from homeostatic symphony to cacophony

Frailty in aging marks a state of decreased reserves resulting in increased vulnerability to adverse outcomes when exposed to stressors. This Perspective synthesizes the evidence on the aging-related pathophysiology underpinning the clinical presentation of physical frailty as a phenotype of a clinical syndrome that is distinct from the cumulative-deficit-based frailty index. We focus on integrating the converging evidence on the conceptualization of physical frailty as a state, largely independent of chronic diseases, that emerges when the dysregulation of multiple interconnected physiological and biological systems crosses a threshold to critical dysfunction, severely compromising homeostasis. Our exegesis posits that the physiology underlying frailty is a critically dysregulated complex dynamical system. This conceptual framework implies that interventions such as physical activity that have multisystem effects are more promising to remedy frailty than interventions targeted at replenishing single systems. We then consider how this framework can drive future research to further understanding, prevention and treatment of frailty, which will likely preserve health and resilience in aging populations.