Skeletal muscle oxidative capacity in young and older women and men

Associations of accelerometry-measured and self-reported physical activity and sedentary behavior with skeletal muscle energetics: The Study of Muscle, Mobility and Aging (SOMMA)

BACKGROUND

Skeletal muscle energetics decline with age, and physical activity (PA) has been shown to offset these declines in older adults. Yet, many studies reporting these effects were based on self-reported PA or structured exercise interventions. Therefore, we examined the associations of accelerometry-measured and self-reported PA and sedentary behavior (SB) with skeletal muscle energetics and explored the extent to which PA and sedentary behavior would attenuate the associations of age with muscle energetics.

METHODS

As part of the Study of Muscle, Mobility and Aging, enrolled older adults (n = 879), 810 (age = 76.4 ± 5.0 years old, mean ± SD; 58% women) had maximal muscle oxidative capacity measured ex vivo via high-resolution respirometry of permeabilized myofibers (maximal oxidative phosphorylation (maxOXPHOS)) and in vivo by 31phosphorus magnetic resonance spectroscopy (maximal adenosine triphosphate (ATPmax)). Accelerometry-measured sedentary behavior, light activity, and moderate-to-vigorous PA (MVPA) were assessed using a wrist-worn ActiGraph GT9X over 7 days. Self-reported sedentary behavior, MVPA, and all PA were assessed with the Community Healthy Activities Model Program for Seniors (CHAMPS) questionnaire. Linear regression models with progressive covariate adjustments evaluated the associations of sedentary behavior and PA with muscle energetics, as well as the attenuation of the age/muscle energetics association by MVPA and sedentary behavior. As a sensitivity analysis, we also examined activPAL-measured daily step count and time spent in sedentary behavior and their associations with muscle energetics.

RESULTS

Every 30 min/day more of ActiGraph-measured MVPA was associated with 0.65 pmol/(s × mg) higher maxOXPHOS and 0.012 mM/s higher ATPmax after adjusting for age, site/technician, and sex (p < 0.05). Light activity was not associated with maxOXPHOS or ATPmax. Meanwhile, every 30 min/day spent in ActiGraph-measured sedentary behavior was associated with 0.39 pmol/s × mg lower maxOXPHOS and 0.006 mM/s lower ATPmax (p < 0.05). Only associations with ATPmax held after further adjusting for socioeconomic status, body mass index, lifestyle factors, and multimorbidity. CHAMPS MVPA and all PA yielded similar associations with maxOXPHOS and ATPmax (p < 0.05), but sedentary behavior did not. Higher activPAL step count was associated with higher maxOXHPOS and ATPmax (p < 0.05), but time spent in sedentary behavior was not. Additionally, age was significantly associated with muscle energetics for men only (p < 0.05); adjusting for time spent in ActiGraph-measured MVPA attenuated the age association with ATPmax by 58% in men.

CONCLUSION

More time spent in accelerometry-measured or self-reported daily PA, especially MVPA, was associated with higher skeletal muscle energetics. Interventions aimed specifically at increasing higher intensity activity might offer potential therapeutic interventions to slow age-related decline in muscle energetics. Our work also emphasizes the importance of taking PA into consideration when evaluating associations related to skeletal muscle energetics.

Nutritional approaches targeting mitochondria for the prevention of sarcopenia

A decline in function and loss of mass, a condition known as sarcopenia, is observed in the skeletal muscles with aging. Sarcopenia has a negative effect on the quality of life of elderly. Individuals with sarcopenia are at particular risk for adverse outcomes, such as reduced mobility, fall-related injuries, and type 2 diabetes mellitus. Although the pathogenesis of sarcopenia is multifaceted, mitochondrial dysfunction is regarded as a major contributor for muscle aging. Hence, the development of preventive and therapeutic strategies to improve mitochondrial function during aging is imperative for sarcopenia treatment. However, effective and specific drugs that can be used for the treatment are not yet approved. Instead studies on the relationship between food intake and muscle aging have suggested that nutritional intake or dietary control could be an alternative approach for the amelioration of muscle aging. This narrative review approaches various nutritional components and diets as a treatment for sarcopenia by modulating mitochondrial homeostasis and improving mitochondria. Age-related changes in mitochondrial function and the molecular mechanisms that help improve mitochondrial homeostasis are discussed, and the nutritional components and diet that modulate these molecular mechanisms are addressed.

Mitochondrial function in peripheral blood cells across the human lifespan

Mitochondrial dysfunction is considered a hallmark of aging. Up to now, a gradual decline of mitochondrial respiration with advancing age has mainly been demonstrated in human muscle tissue. A handful of studies have examined age-related mitochondrial dysfunction in human blood cells, and only with small sample sizes and mainly in platelets. In this study, we analyzed mitochondrial respiration in peripheral blood mononuclear cells (PBMCs) and platelets from 308 individuals across the human lifespan (0-86 years). In regression analyses, with adjustment for false discovery rate (FDR), we found age-related changes in respiratory measurements to be either small or absent. The main significant changes were an age-related relative decline in complex I-linked respiration and a corresponding rise of complex II-linked respiration in PBMCs. These results add to the understanding of mitochondrial dysfunction in aging and to its possible role in immune cell and platelet senescence.

Associations of Objective and Self-Reported Physical Activity and Sedentary Behavior with Skeletal Muscle Energetics: The Study of Muscle, Mobility and Aging (SOMMA)

Background

Skeletal muscle energetics decline with age, and physical activity (PA) has been shown to counteract these declines in older adults. Yet, many studies were based on self-reported PA or structured exercise interventions. We examined the associations of objective daily PA and sedentary behavior (SB) with skeletal muscle energetics and also compared with self-reported PA and SB. We also explored the extent to which PA would attenuate the associations of age with muscle energetics.

Methods

Among the Study of Muscle, Mobility and Aging (SOMMA) enrolled older adults, 810 (mean age=76±5, 58% women) had maximal muscle oxidative capacity measured ex vivo via high-resolution respirometry of permeabilized myofibers (maxOXPHOS) and in vivo by 31 Phosphorus magnetic resonance spectroscopy (ATP max ). Objective PA was measured using the wrist-worn ActiGraph GT9X over 7-days to capture sedentary behavior (SB), light, and moderate-to-vigorous PA (MVPA). Self-reported SB, MVPA, and all exercise-related PA were assessed with The Community Healthy Activities Model Program for Seniors questionnaire. Linear regression models with progressive covariate adjustments evaluated the associations between SB, PA and muscle energetics, and the attenuation of the age / muscle energetic association by PA.

Results

Every 30 minutes more objective MVPA was associated with 0.65 pmol/s*mg higher maxOXPHOS and 0.012 mM/sec higher ATP max , after adjustment for age, site/technician and sex. More time spent in objective light+MVPA was significantly associated with higher ATP max , but not maxOXPHOS. In contrast, every 30 minutes spent in objective SB was associated with 0.43 pmol/s*mg lower maxOXPHOS and 0.004 mM/sec lower ATP max . Only associations with ATP max held after further adjusting for socioeconomic status, body mass index, lifestyle factors and multimorbidities. Self-reported MVPA and all exercise-related activities, but not SB, yielded similar associations with maxOXPHOS and ATP max . Lastly, age was only significantly associated with muscle energetics in men. Adjusting for objective time spent in MVPA attenuated the age association with ATP max by nearly 60% in men.

Conclusion

More time spent in daily PA, especially MVPA, were associated with higher muscle energetics. Interventions that increase higher intensity activity might offer potential therapeutic interventions to slow the age-related decline in muscle energetics. Our work also emphasizes the importance of taking PA into consideration when evaluating associations related to skeletal muscle energetics.

Age-related changes of skeletal muscle metabolic response to contraction are also sex-dependent

Mitochondria adapt to increased energy demands during muscle contraction by acutely altering metabolite fluxes and substrate oxidation. With age, an impaired mitochondrial metabolic response may contribute to reduced exercise tolerance and decreased skeletal muscle mass, specific force, increased overall fatty depositions in the skeletal muscle, frailty and depressed energy maintenance. We hypothesized that elevated energy stress in mitochondria with age alters the capacity of mitochondria to utilize different substrates following muscle contraction. To test this hypothesis, we used in vivo electrical stimulation to simulate high-intensity intervals (HII) or low intensity steady-state (LISS) exercise in young (5-7 months) and aged (27-29 months) male and female mice to characterize effects of age and sex on mitochondrial substrate utilization in skeletal muscle following contraction. Mitochondrial respiration using glutamate decreased in aged males following HII and glutamate oxidation was inhibited following HII in both the contracted and non-stimulated muscle of aged female muscle. Analyses of the muscle metabolome of female mice indicated that changes in metabolic pathways induced by HII and LISS contractions in young muscle are absent in aged muscle. To test improved mitochondrial function on substrate utilization following HII, we treated aged females with elamipretide (ELAM), a mitochondrially-targeted peptide shown to improve mitochondrial bioenergetics and restore redox status in aged muscle. ELAM removed inhibition of glutamate oxidation and showed increased metabolic pathway changes following HII, suggesting rescuing redox status and improving bioenergetic function in mitochondria from aged muscle increases glutamate utilization and enhances the metabolic response to muscle contraction in aged muscle. KEY POINTS: Acute local contraction of gastrocnemius can systemically alter mitochondrial respiration in non-stimulated muscle. Age-related changes in mitochondrial respiration using glutamate or palmitoyl carnitine following contraction are sex-dependent. Respiration using glutamate after high-intensity contraction is inhibited in aged female muscle. Metabolite level and pathway changes following muscle contraction decrease with age in female mice. Treatment with the mitochondrially-targeted peptide elamipretide can partially rescue metabolite response to muscle contraction.

Phytochemical compound PB125 attenuates skeletal muscle mitochondrial dysfunction and impaired proteostasis in a model of musculoskeletal decline

Impaired mitochondrial function and disrupted proteostasis contribute to musculoskeletal dysfunction. However, few interventions simultaneously target these two drivers to prevent musculoskeletal decline. Nuclear factor erythroid 2-related factor 2 (Nrf2) activates a transcriptional programme promoting cytoprotection, metabolism, and proteostasis. We hypothesized daily treatment with a purported Nrf2 activator, PB125, in Hartley guinea pigs, a model of musculoskeletal decline, would attenuate the progression of skeletal muscle mitochondrial dysfunction and impaired proteostasis and preserve musculoskeletal function. We treated 2- and 5-month-old male and female Hartley guinea pigs for 3 and 10 months, respectively, with the phytochemical compound PB125. Longitudinal assessments of voluntary mobility were measured using Any-MazeTM open-field enclosure monitoring. Cumulative skeletal muscle protein synthesis rates were measured using deuterium oxide over the final 30 days of treatment. Mitochondrial oxygen consumption in soleus muscles was measured using high resolution respirometry. In both sexes, PB125 (1) increased electron transfer system capacity; (2) attenuated the disease/age-related decline in coupled and uncoupled mitochondrial respiration; and (3) attenuated declines in protein synthesis in the myofibrillar, mitochondrial and cytosolic subfractions of the soleus. These effects were not associated with statistically significant prolonged maintenance of voluntary mobility in guinea pigs. Collectively, treatment with PB125 contributed to maintenance of skeletal muscle mitochondrial respiration and proteostasis in a pre-clinical model of musculoskeletal decline. Further investigation is necessary to determine if these documented effects of PB125 are also accompanied by slowed progression of other aspects of musculoskeletal dysfunction. KEY POINTS: Aside from exercise, there are no effective interventions for musculoskeletal decline, which begins in the fifth decade of life and contributes to disability and cardiometabolic diseases. Targeting both mitochondrial dysfunction and impaired protein homeostasis (proteostasis), which contribute to the age and disease process, may mitigate the progressive decline in overall musculoskeletal function (e.g. gait, strength). A potential intervention to target disease drivers is to stimulate nuclear factor erythroid 2-related factor 2 (Nrf2) activation, which leads to the transcription of genes responsible for redox homeostasis, proteome maintenance and mitochondrial energetics. Here, we tested a purported phytochemical Nrf2 activator, PB125, to improve mitochondrial function and proteostasis in male and female Hartley guinea pigs, which are a model for musculoskeletal ageing. PB125 improved mitochondrial respiration and attenuated disease- and age-related declines in skeletal muscle protein synthesis, a component of proteostasis, in both male and female Hartley guinea pigs.

Muscle Delivery of Mitochondria-Targeted Drugs for the Treatment of Sarcopenia: Rationale and Perspectives

An impairment in mitochondrial homeostasis plays a crucial role in the process of aging and contributes to the incidence of age-related diseases, including sarcopenia, which is defined as an age-dependent loss of muscle mass and strength. Mitochondrial dysfunction exerts a negative impact on several cellular activities, including bioenergetics, metabolism, and apoptosis. In sarcopenia, mitochondria homeostasis is disrupted because of reduced oxidative phosphorylation and ATP generation, the enhanced production of reactive species, and impaired antioxidant defense. This review re-establishes the most recent evidence on mitochondrial defects that are thought to be relevant in the pathogenesis of sarcopenia and that may represent promising therapeutic targets for its prevention/treatment. Furthermore, we describe mechanisms of action and translational potential of promising mitochondria-targeted drug delivery systems, including molecules able to boost the metabolism and bioenergetics, counteract apoptosis, antioxidants to scavenge reactive species and decrease oxidative stress, and target mitophagy. Even though these mitochondria-delivered strategies demonstrate to be promising in preclinical models, their use needs to be promoted for clinical studies. Therefore, there is a compelling demand to further understand the mechanisms modulating mitochondrial homeostasis, to characterize powerful compounds that target muscle mitochondria to prevent sarcopenia in aged people.

Benefits of a plant-based diet and considerations for the athlete

Individuals may opt to follow a plant-based diet for a variety of reasons, such as religious practices, health benefits or concerns for animal or environmental welfare. Such diets offer a broad spectrum of health benefits including aiding in the prevention and management of chronic diseases. In addition to health benefits, a plant-based diet may provide performance-enhancing effects for various types of exercise due to high carbohydrate levels and the high concentration of antioxidants and phytochemicals found in a plant-based diet. However, some plant-based foods also contain anti-nutrional factors, such as phytate and tannins, which decrease the bioavailability of key nutrients, such as iron, zinc, and protein. Thus, plant-based diets must be carefully planned to ensure adequate intake and absorption of energy and all essential nutrients. The current narrative review summarizes the current state of the research concerning the implications of a plant-based diet for health and exercise performance. It also outlines strategies to enhance the bioavailability of nutrients, sources of hard-to-get nutrients, and sport supplements that could interest plant-based athletes.

Mitochondrial Impairment in Sarcopenia

Sarcopenia is defined by the age-related loss of skeletal muscle quality, which relies on mitochondrial homeostasis. During aging, several mitochondrial features such as bioenergetics, dynamics, biogenesis, and selective autophagy (mitophagy) are altered and impinge on protein homeostasis, resulting in loss of muscle mass and function. Thus, mitochondrial dysfunction contributes significantly to the complex pathogenesis of sarcopenia, and mitochondria are indicated as potential targets to prevent and treat this age-related condition. After a concise presentation of the age-related modifications in skeletal muscle quality and mitochondrial homeostasis, the present review summarizes the most relevant findings related to mitochondrial alterations in sarcopenia.

Mitochondria and the thermal limits of ectotherms

Temperature is a critical abiotic factor shaping the distribution and abundance of species, but the mechanisms that underpin organismal thermal limits remain poorly understood. One possible mechanism underlying these limits is the failure of mitochondrial processes, as mitochondria play a crucial role in animals as the primary site of ATP production. Conventional measures of mitochondrial performance suggest that these organelles can function at temperatures much higher than those that limit whole-organism function, suggesting that they are unlikely to set organismal thermal limits. However, this conclusion is challenged by recent data connecting sequence variation in mitochondrial genes to whole-organism thermal tolerance. Here, we review the current state of knowledge of mitochondrial responses to thermal extremes and ask whether they are consistent with a role for mitochondrial function in shaping whole-organism thermal limits. The available data are fragmentary, but it is possible to draw some conclusions. There is little evidence that failure of maximal mitochondrial oxidative capacity as assessed in vitro sets thermal limits, but there is some evidence to suggest that temperature effects on ATP synthetic capacity may be important. Several studies suggest that loss of mitochondrial coupling is associated with the thermal limits for organismal growth, although this needs to be rigorously tested. Most studies have utilized isolated mitochondrial preparations to assess the effects of temperature on these organelles, and there remain many untapped opportunities to address these questions using preparations that retain more of their biological context to better connect these subcellular processes with whole-organism thermal limits.

NIRS-derived skeletal muscle oxidative capacity is correlated with aerobic fitness and independent of sex

Near-infrared spectroscopy (NIRS) provides a simple and reliable measure of skeletal muscle oxidative capacity; however, its relationship to aerobic fitness and sex are unclear. We hypothesized that NIRS-derived oxidative capacity in the vastus lateralis (VL) and medial gastrocnemius (MG) would be correlated with indices of aerobic fitness and independent of sex. Twenty-six participants (13 males, 13 females) performed ramp- and step-incremental tests to volitional exhaustion on separate days to establish maximal oxygen uptake (V̇o2max), peak power output (PPO), lactate threshold (LT), gas exchange threshold (GET), respiratory compensation point (RCP), and maximal fat oxidation (MFO). Data were normalized to lean body mass to account for sex-based differences in body composition. Exercise tests were preceded by duplicate measurements of NIRS-derived oxidative capacity on the VL and MG muscles (i.e., repeated arterial occlusions following a brief set of muscle contractions). Skeletal muscle oxidative capacity for the VL (means ± SD: 21.9 ± 4.6 s) and MG (22.5 ± 6.1 s) were similar but unrelated (r2 = 0.03, P = 0.39). Skeletal muscle oxidative capacity for the VL, but not the MG (P > 0.05 for all variables), was significantly correlated with V̇o2max (r2 = 0.24; P = 0.01), PPO (r2 = 0.23; P = 0.01), LT (r2 = 0.23; P = 0.01), GET (r2 = 0.23; P = 0.01), and RCP (r2 = 0.27; P = 0.006). MFO was not correlated with VL or MG skeletal muscle oxidative capacity (P > 0.05). Females (54.9 ± 4.5 mL·kg LBM-1·min-1) and males (56.0 ± 6.2 mL·kg LBM-1·min-1), matched for V̇o2max (P = 0.62), had similar NIRS-derived oxidative capacities for VL (20.7 ± 4.4 vs. 23.2 ± 4.6 s; P = 0.18) and MG (24.4 ± 6.8 vs. 20.5 ± 4.8 s; P = 0.10). Overall, NIRS-derived skeletal muscle oxidative capacity in VL is indicative of aerobic fitness and independent of sex in humans.NEW & NOTEWORTHY Near-infrared spectroscopy (NIRS) can be used to measure skeletal muscle oxidative capacity. Here, we demonstrated that NIRS-derived skeletal muscle oxidative capacity of the vastus lateralis was independent of sex, reliable across and within days, and correlated with maximal and submaximal indices of aerobic fitness, including maximal oxygen uptake, lactate threshold, and respiratory compensation point. These findings highlight the utility of NIRS for investigating skeletal muscle oxidative capacity in females and males.

Nutritional Supplements to Support Resistance Exercise in Countering the Sarcopenia of Aging

Skeletal muscle plays an indispensable role in metabolic health and physical function. A decrease in muscle mass and function with advancing age exacerbates the likelihood of mobility impairments, disease development, and early mortality. Therefore, the development of non-pharmacological interventions to counteract sarcopenia warrant significant attention. Currently, resistance training provides the most effective, low cost means by which to prevent sarcopenia progression and improve multiple aspects of overall health. Importantly, the impact of resistance training on skeletal muscle mass may be augmented by specific dietary components (i.e., protein), feeding strategies (i.e., timing, per-meal doses of specific macronutrients) and nutritional supplements (e.g., creatine, vitamin-D, omega-3 polyunsaturated fatty acids etc.). The purpose of this review is to provide an up-to-date, evidence-based account of nutritional strategies to enhance resistance training-induced adaptations in an attempt to combat age-related muscle mass loss. In addition, we provide insight on how to incorporate the aforementioned nutritional strategies that may support the growth or maintenance of skeletal muscle and subsequently extend the healthspan of older individuals.

Older adults with sarcopenia have distinct skeletal muscle phosphodiester, phosphocreatine, and phospholipid profiles

The loss of skeletal muscle mass and function with age (sarcopenia) is a critical healthcare challenge for older adults. 31‐phosphorus magnetic resonance spectroscopy (³¹P‐MRS) is a powerful tool used to evaluate phosphorus metabolite levels in muscle. Here, we sought to determine which phosphorus metabolites were linked with reduced muscle mass and function in older adults. This investigation was conducted across two separate studies. Resting phosphorus metabolites in skeletal muscle were examined by ³¹P‐MRS. In the first study, fifty‐five older adults with obesity were enrolled and we found that resting phosphocreatine (PCr) was positively associated with muscle volume and knee extensor peak power, while a phosphodiester peak (PDE2) was negatively related to these variables. In the second study, we examined well‐phenotyped older adults that were classified as nonsarcopenic or sarcopenic based on sex‐specific criteria described by the European Working Group on Sarcopenia in Older People. PCr content was lower in muscle from older adults with sarcopenia compared to controls, while PDE2 was elevated. Percutaneous biopsy specimens of the vastus lateralis were obtained for metabolomic and lipidomic analyses. Lower PCr was related to higher muscle creatine. PDE2 was associated with glycerol‐phosphoethanolamine levels, a putative marker of phospholipid membrane damage. Lipidomic analyses revealed that the major phospholipids, (phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol) were elevated in sarcopenic muscle and were inversely related to muscle volume and peak power. These data suggest phosphorus metabolites and phospholipids are associated with the loss of skeletal muscle mass and function in older adults.

Age-related degeneration of the lumbar paravertebral muscles: Systematic review and three-level meta-regression

BACKGROUND

Morphological changes of the lumbar spine muscles are not well characterised with ageing. To further the understanding of age-related degeneration of the lumbar spine musculature, normative morphological changes that occur within the paravertebral muscles must first be established.

METHODS

A systematic review and meta-regressions were conducted adhering to PRISMA guidelines. Searches for published and unpublished data were completed in June 2019.

RESULTS

Searches returned 4781 articles. 34 articles were included in the quantitative analysis. Three-level meta-analyses showed age-related atrophy (r = -0.26; 95% CI: -0.33, -0.17) and fat infiltration (r = 0.39; 95% CI: 0.28, 0.50) in the lumbar paravertebral muscles. Degenerative changes were muscle-specific and men (r = -0.32; 95% CI: -0.61, 0.01) exhibited significantly greater muscle atrophy than women (r = -0.24; 95% CI: -0.47, 0.03). Imaging modality, specifically ultrasound, also influenced age-related muscle atrophy. Measurements taken across all lumbar levels revealed the greatest fat infiltration with ageing (r = 0.58, 95% CI: 0.35, 0.74). Moderators explained a large proportion of between-study variance in true effects for muscle atrophy (72.6%) and fat infiltration (79.8%) models.

CONCLUSIONS

Lumbar paravertebral muscles undergo age-related degeneration in healthy adults with muscle, lumbar level and sex-specific responses. Future studies should use high-resolution imaging modalities to quantify muscle atrophy and fat infiltration.

In vivo assessment of mitochondrial capacity using NIRS in locomotor muscles of young and elderly males with similar physical activity levels

Mitochondrial capacity is pivotal to skeletal muscle function and is suggested to decline with age. However, there is large heterogeneity in current data, possibly due to effect modifiers such as physical activity, sex and muscle group. Yet, few studies have compared multiple muscle groups in different age groups with comparable physical activity levels. Here, we newly used near-infrared spectroscopy (NIRS) to characterise mitochondrial capacity in three different locomotor muscles in young (19-25 year) and older (65-71 year), healthy males with similar physical activity levels. Mitochondrial capacity and reperfusion after arterial occlusion was measured in the vastus lateralis (VL), the gastrocnemius (GA) and the tibialis anterior (TA). Physical activity was verified using accelerometry and was not different between the age groups (404.3 ± 214.9 vs 494.9 ± 187.0 activity kcal per day, p = 0.16). Mitochondrial capacity was significantly lower in older males in the GA and VL, but not in the TA (p = 0.048, p = 0.036 and p = 0.64, respectively). Reperfusion rate was not significantly different for the GA (p = 0.55), but was significantly faster in the TA and VL in the young group compared to the older group (p = 0.0094 and p = 0.039, respectively). In conclusion, we identified distinct modes of mitochondrial ageing in different locomotor muscles in a young and older population with similar physical activity patterns. Furthermore, we show that NIRS is suitable for relatively easy application in ageing research and can reveal novel insights into mitochondrial functioning with age.

Muscular Atrophy and Sarcopenia in the Elderly: Is There a Role for Creatine Supplementation?

Sarcopenia is characterized by a loss of muscle mass, quality, and function, and negatively impacts health, functionality, and quality of life for numerous populations, particularly older adults. Creatine is an endogenously produced metabolite, which has the theoretical potential to counteract many of the morphological and metabolic parameters underpinning sarcopenia. This can occur through a range of direct and indirect mechanisms, including temporal and spatial functions that accelerate ATP regeneration during times of high energy demand, direct anabolic and anti-catabolic functions, and enhanced muscle regenerating capacity through positively impacting muscle stem cell availability. Studies conducted in older adults show little benefit of creatine supplementation alone on muscle function or mass. In contrast, creatine supplementation as an adjunct to exercise training seems to augment the muscle adaptive response to the training stimulus, potentially through increasing capacity for higher intensity exercise, and/or by enhancing post-exercise recovery and adaptation. As such, creatine may be an effective dietary strategy to combat age-related muscle atrophy and sarcopenia when used to complement the benefits of exercise training.

The clinical impact and biological mechanisms of skeletal muscle aging

Skeletal muscle is a highly plastic tissue that remarkably adapts to diverse stimuli including exercise, injury, disuse, and, as discussed here, aging. Humans achieve peak skeletal muscle mass and strength in mid-life and then experience a progressive decline of up to 50% by the ninth decade. The loss of muscle mass and function with aging is a phenomenon termed sarcopenia. It is evidenced by the loss and atrophy of muscle fibers and the concomitant accretion of fat and fibrous tissue. Sarcopenia has been recognized as a key driver of limitations in physical function and mobility, but is perhaps less appreciated for its role in age-related metabolic dysfunction and loss of organismal resilience. Similar to other tissues, muscle is prone to multiple forms of age-related molecular and cellular damage, including disrupted protein turnover, impaired regenerative capacity, cellular senescence, and mitochondrial dysfunction. The objective of this review is to highlight the clinical consequences of skeletal muscle aging, and provide insights into potential biological mechanisms. In light of population aging, strategies to improve muscle health in older adults promise to have a profound public health impact.

Skeletal Muscle Mitochondrial Function is Determined by Burn Severity, Sex, and Sepsis, and is Associated With Glucose Metabolism and Functional Capacity in Burned Children

BACKGROUND

Restoring normal mitochondrial function represents a new target for strategies aimed at mitigating the stress response to severe burn trauma and hastening recovery. Our objective was to investigate the determinants of skeletal muscle mitochondrial respiratory capacity and function and its association with glucose metabolism and functional capacity in burned children.

METHODS

Data from burned children enrolled in the placebo arm of an ongoing prospective clinical trial were analyzed. Mitochondrial respiratory capacity was determined in permeabilized myofibers by high-resolution respirometry on at least one occasion per participant. In subsets of patients, glucose kinetics and cardiorespiratory fitness (VO2peak) were also determined. Mixed multiple regression models were used to identify the determinants of mitochondrial respiratory function and to assess the relationship between mitochondrial respiration and both glucose control and functional capacity (VO2peak).

MAIN RESULTS

Increasing full-thickness burn size was associated with greater adjusted coupled (ATP-producing) respiration, adjusted for age, sex, sepsis, and time of testing (P < 0.01; n = 55, obs = 97). Girls had on average 23.3% lower coupled respiration (adjusted mean and 95% confidence of interval [CI], -7.1; -12.6 to -1.7 pmol/s/mg; P < 0.025) and 29.8% lower respiratory control than boys (adjusted mean and 95% CI, -0.66; -1.07 to -0.25; P < 0.01; n = 55, obs = 97). The presence of sepsis was associated with lower respiration coupled to ATP production by an average of 25.5% compared with nonsepsis (adjusted mean and 95% CI, -6.9; -13.0 to -0.7 pmol/s/mg; P < 0.05; n = 55, obs = 97), after adjustment for age, sex, full-thickness burn size, and time of testing. During a hyperinsulinemic euglycemic clamp, hepatic glucose release was associated with greater coupled respiration and respiratory control (P < 0.05; n = 42, obs = 73), independent of age, sepsis, full-thickness burn size, and time postinjury testing. Coupled respiration was positively associated with VO2peak after adjustment for age, full-thickness burn size, and time of exercise testing (P < 0.025; n = 18, obs = 25).

CONCLUSIONS

Burn severity, sex, and sepsis influence skeletal muscle mitochondrial function in burned children. Glucose control and functional capacity are associated with altered mitochondrial respiratory function in muscle of burn survivors, highlighting the relationship of altered muscle bioenergetics with the clinical sequelae accompanying severe burn trauma.

Age-Associated Impairments in Mitochondrial ADP Sensitivity Contribute to Redox Stress in Senescent Human Skeletal Muscle

It remains unknown if mitochondrial bioenergetics are altered with aging in humans. We established an in vitro method to simultaneously determine mitochondrial respiration and H₂O₂ emission in skeletal muscle tissue across a range of biologically relevant ADP concentrations. Using this approach, we provide evidence that, although the capacity for mitochondrial H₂O₂ emission is not increased with aging, mitochondrial ADP sensitivity is impaired. This resulted in an increase in mitochondrial H₂O₂ and the fraction of electron leak to H₂O₂, in the presence of virtually all ADP concentrations examined. Moreover, although prolonged resistance training in older individuals increased muscle mass, strength, and maximal mitochondrial respiration, exercise training did not alter H₂O₂ emission rates in the presence of ADP, the fraction of electron leak to H₂O₂, or the redox state of the muscle. These data establish that a reduction in mitochondrial ADP sensitivity increases mitochondrial H₂O₂ emission and contributes to age-associated redox stress.

Variables Influencing the Effectiveness of Creatine Supplementation as a Therapeutic Intervention for Sarcopenia

Sarcopenia is an age-related muscle condition characterized by a reduction in muscle quantity, force generating capacity and physical performance. Sarcopenia occurs in 8-13% of adults ≥ 60 years of age and can lead to disability, frailty, and various other diseases. Over the past few decades, several leading research groups have focused their efforts on developing strategies and recommendations for attenuating sarcopenia. One potential nutritional intervention for sarcopenia is creatine supplementation. However, research is inconsistent regarding the effectiveness of creatine on aging muscle. The purpose of this perspective paper is to: (1) propose possible reasons for the inconsistent responsiveness to creatine in aging adults, (2) discuss the potential mechanistic actions of creatine on muscle biology, (3) determine whether the timing of creatine supplementation influences aging muscle, (4) evaluate the evidence investigating the effects of creatine with other compounds (protein, conjugated linoleic acid) in aging adults, and (5) provide insight regarding the safety of creatine for aging adults.

Improving mitochondrial function with SS-31 reverses age-related redox stress and improves exercise tolerance in aged mice

Sarcopenia and exercise intolerance are major contributors to reduced quality of life in the elderly for which there are few effective treatments. We tested whether enhancing mitochondrial function and reducing mitochondrial oxidant production with SS-31 (elamipretide) could restore redox balance and improve skeletal muscle function in aged mice. Young (5 mo) and aged (26 mo) female C57BL/6Nia mice were treated for 8-weeks with 3 mg/kg/day SS-31. Mitochondrial function was assessed in vivo using ³¹P and optical spectroscopy. SS-31 reversed age-related decline in maximum mitochondrial ATP production (ATPmax) and coupling of oxidative phosphorylation (P/O). Despite the increased in vivo mitochondrial capacity, mitochondrial protein expression was either unchanged or reduced in the treated aged mice and respiration in permeabilized gastrocnemius (GAS) fibers was not different between the aged and aged+SS-31 mice. Treatment with SS-31 also restored redox homeostasis in the aged skeletal muscle. The glutathione redox status was more reduced and thiol redox proteomics indicated a robust reversal of cysteine S-glutathionylation post-translational modifications across the skeletal muscle proteome. The gastrocnemius in the age+SS-31 mice was more fatigue resistant with significantly greater mass compared to aged controls. This contributed to a significant increase in treadmill endurance compared to both pretreatment and untreated control values. These results demonstrate that the shift of redox homeostasis due to mitochondrial oxidant production in aged muscle is a key factor in energetic defects and exercise intolerance. Treatment with SS-31 restores redox homeostasis, improves mitochondrial quality, and increases exercise tolerance without an increase in mitochondrial content. Since elamipretide is currently in clinical trials these results indicate it may have direct translational value for improving exercise tolerance and quality of life in the elderly.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

Autophagy and mitophagy flux in young and aged skeletal muscle following chronic contractile activity

KEY POINTS

A healthy mitochondrial pool is dependent on the removal of dysfunctional organelles via mitophagy, but little is known about how mitophagy is altered with ageing and chronic exercise. Chronic contractile activity (CCA) is a standardized exercise model that can elicit mitochondrial adaptations in both young and aged muscle, albeit to a lesser degree in the aged group. Assessment of mitophagy flux revealed enhanced targeting of mitochondria for degradation in aged muscle, in contrast to previous theories. Mitophagy flux was significantly reduced as an adaptation to CCA suggesting that an improvement in organelle quality reduces the need for mitochondrial turnover. CCA enhances lysosomal capacity and may ameliorate lysosomal dysfunction in aged muscle.

ABSTRACT

Skeletal muscle exhibits deficits in mitochondrial quality with age. Central to the maintenance of a healthy mitochondrial pool is the removal of dysfunctional organelles via mitophagy. Little is known on how mitophagy is altered with ageing and chronic exercise. We assessed mitophagy flux using colchicine treatment in vivo following chronic contractile activity (CCA) of muscle in young and aged rats. CCA evoked mitochondrial biogenesis in young muscle, with an attenuated response in aged muscle. Mitophagy flux was higher in aged muscle and was correlated with the enhanced expression of mitophagy receptors and upstream transcriptional regulators. CCA decreased mitophagy flux in both age groups, suggesting an improvement in organelle quality. CCA also reduced the exaggerated expression of TFEB evident in aged muscle, which may be promoting the age-induced increase in lysosomal markers. Thus, aged muscle possesses an elevated drive for autophagy and mitophagy which may contribute to the decline in organelle content observed with age, but which may serve to maintain mitochondrial quality. CCA improves organelle integrity and reduces mitophagy, illustrating that chronic exercise is a modality to improve muscle quality in aged populations.

Myotendinous junction plasticity in aged ovariectomized rats submitted to aquatic training

The study aims to describe the tissue plasticity of MTJ through the morphological analysis of MTJ soleus in ovariectomized aged female Wistar rats submitted to aquatic training. Forty aged Wistar rats, 1 year and 2 months of age, were divided into four groups: sedentary (S), trained (T), ovariectomized (O), and trained/ovariectomized (OT). Employing the transmission electron microscopy, the ultrastructural and morphometric elements were revealed. In the S group, changes in morphological characteristics as a consequence of the aging process were seen, demonstrated by the conical shape of the muscle cell extremity, a large area with collagen deposit, and misalignment of sarcomeres in series. The T group presented ample adjustments when revealed the organization of MTJ, through the increase of the contact area and greater lengths of sarcoplasmatic invaginations and evaginations. The O group revealed extensive tissue disorganization with muscle atrophy, reduction of MTJ contact area, and consequently, changes in sarcoplasmatic invaginations and evaginations. The OT group demonstrated extensive remodeling with restructuring MTJ through the increase of tissue contact area, extensive organization, parallel arrangement, and increased length of sarcoplasmatic invaginations and evaginations. The distal sarcomeres presented higher lengths compared to the proximal sarcomeres in both the groups. We conclude that aquatic training was effective in the organization and structural remodeling of the myotendinous interface of ovariectomized aged rats. There was a greater area of contact, and consequently, greater resistance in the myotendinous interface promoting a lower predisposition to injuries.

Physical activity unveils the relationship between mitochondrial energetics, muscle quality, and physical function in older adults

BACKGROUND

The concept of mitochondrial dysfunction in ageing muscle is highly controversial. In addition, emerging evidence suggests that reduced muscle oxidative capacity and efficiency underlie the aetiology of mobility loss in older adults. Here, we hypothesized that studying well-phenotyped older cohorts across a wide range of physical activity would unveil a range of mitochondrial function in skeletal muscle and in turn allow us to more clearly examine the impact of age per se on mitochondrial energetics. This also enabled us to more clearly define the relationships between mitochondrial energetics and muscle lipid content with clinically relevant assessments of muscle and physical function.

METHODS

Thirty-nine volunteers were recruited to the following study groups: young active (YA, n = 2 women/8 men, age = 31.2 ± 5.4 years), older active (OA, n = 2 women/8 men, age = 67.5 ± 2.7 years), and older sedentary (OS, n = 8 women/11 men, age = 70.7 ± 4.7 years). Participants completed a graded exercise test to determine fitness (VO₂ peak), a submaximal exercise test to determine exercise efficiency, and daily physical activity was recorded using a tri-axial armband accelerometer. Mitochondrial energetics were determined by (i) ³¹ P magnetic resonance spectroscopy and (ii) respirometry of fibre bundles from vastus lateralis biopsies. Quadriceps function was assessed by isokinetic dynamometry and physical function by the short physical performance battery and stair climb test.

RESULTS

Daily physical activity energy expenditure was significantly lower in OS, compared with YA and OA groups. Despite fitness being higher in YA compared with OA and OS, mitochondrial respiration, maximum mitochondrial capacity, Maximal ATP production/Oxygen consumption (P/O) ratio, and exercise efficiency were similar in YA and OA groups and were significantly lower in OS. P/O ratio was correlated with exercise efficiency. Time to complete the stair climb and repeated chair stand tests were significantly greater for OS. Interestingly, maximum mitochondrial capacity was related to muscle contractile performance and physical function.

CONCLUSIONS

Older adults who maintain a high amount of physical activity have better mitochondrial capacity, similar to highly active younger adults, and this is related to their better muscle quality, exercise efficiency, and physical performance. This suggests that mitochondria could be an important therapeutic target for sedentary ageing associated conditions including sarcopenia, dynapenia, and loss of physical function.

Advanced quantitative methods in correlating sarcopenic muscle degeneration with lower extremity function biometrics and comorbidities

Sarcopenic muscular degeneration has been consistently identified as an independent risk factor for mortality in aging populations. Recent investigations have realized the quantitative potential of computed tomography (CT) image analysis to describe skeletal muscle volume and composition; however, the optimum approach to assessing these data remains debated. Current literature reports average Hounsfield unit (HU) values and/or segmented soft tissue cross-sectional areas to investigate muscle quality. However, standardized methods for CT analyses and their utility as a comorbidity index remain undefined, and no existing studies compare these methods to the assessment of entire radiodensitometric distributions. The primary aim of this study was to present a comparison of nonlinear trimodal regression analysis (NTRA) parameters of entire radiodensitometric muscle distributions against extant CT metrics and their correlation with lower extremity function (LEF) biometrics (normal/fast gait speed, timed up-and-go, and isometric leg strength) and biochemical and nutritional parameters, such as total solubilized cholesterol (SCHOL) and body mass index (BMI). Data were obtained from 3,162 subjects, aged 66–96 years, from the population-based AGES-Reykjavik Study. 1-D k-means clustering was employed to discretize each biometric and comorbidity dataset into twelve subpopulations, in accordance with Sturges’ Formula for Class Selection. Dataset linear regressions were performed against eleven NTRA distribution parameters and standard CT analyses (fat/muscle cross-sectional area and average HU value). Parameters from NTRA and CT standards were analogously assembled by age and sex. Analysis of specific NTRA parameters with standard CT results showed linear correlation coefficients greater than 0.85, but multiple regression analysis of correlative NTRA parameters yielded a correlation coefficient of 0.99 (P<0.005). These results highlight the specificities of each muscle quality metric to LEF biometrics, SCHOL, and BMI, and particularly highlight the value of the connective tissue regime in this regard.

Effects of Exercise and Aging on Skeletal Muscle

A substantial loss of muscle mass and strength (sarcopenia), a decreased regenerative capacity, and a compromised physical performance are hallmarks of aging skeletal muscle. These changes are typically accompanied by impaired muscle metabolism, including mitochondrial dysfunction and insulin resistance. A challenge in the field of muscle aging is to dissociate the effects of chronological aging per se on muscle characteristics from the secondary influence of lifestyle and disease processes. Remarkably, physical activity and exercise are well-established countermeasures against muscle aging, and have been shown to attenuate age-related decreases in muscle mass, strength, and regenerative capacity, and slow or prevent impairments in muscle metabolism. We posit that exercise and physical activity can influence many of the changes in muscle during aging, and thus should be emphasized as part of a lifestyle essential to healthy aging.

Near-infrared spectroscopy detects age-related differences in skeletal muscle oxidative function: promising implications for geroscience

Age is the greatest risk factor for chronic disease and is associated with a marked decline in functional capacity and quality of life. A key factor contributing to loss of function in older adults is the decline in skeletal muscle function. While the exact mechanism(s) remains incompletely understood, age-related mitochondrial dysfunction is thought to play a major role. To explore this question further, we studied 15 independently living seniors (age: 72 ± 5 years; m/f: 4/11; BMI: 27.6 ± 5.9) and 17 young volunteers (age: 25 ± 4 years; m/f: 8/9; BMI: 24.0 ± 3.3). Skeletal muscle oxidative function was measured in forearm muscle from the recovery kinetics of muscle oxygen consumption using near-infrared spectroscopy (NIRS). Muscle oxygen consumption was calculated as the slope of change in hemoglobin saturation during a series of rapid, supra-systolic arterial cuff occlusions following a brief bout of exercise. Aging was associated with a significant prolongation of the time constant of oxidative recovery following exercise (51.8 ± 5.4 sec vs. 37.1 ± 2.1 sec, P = 0.04, old vs. young, respectively). This finding suggests an overall reduction in mitochondrial function with age in nonlocomotor skeletal muscle. That these data were obtained using NIRS holds great promise in gerontology for quantitative assessment of skeletal muscle oxidative function at the bed side or clinic.

Evidence of a metabolic reserve in the skeletal muscle of elderly people

The purpose of the present study was to determine whether mitochondrial function is limited by O₂ availability or the intrinsic capacity of mitochondria to synthesize ATP in elderly individuals. To this aim, we examined, in comparison to free-flow conditions (FF), the effect of superimposing reactive hyperemia (RH), induced by a period of brief ischemia during the last min of exercise, on O₂ availability and mitochondrial function in the calf muscle. 12 healthy, untrained, elderly subjects performed dynamic plantar flexion exercise and phosphorus magnetic resonance spectroscopy (³¹P-MRS), near-infrared spectroscopy (NIRS), and Doppler ultrasound were used to assess muscle metabolism and peripheral hemodynamics. Limb blood flow [area under the curve (AUC), FF: 1.5±0.5L; RH: 3.2±1.1L, P<0.01] and convective O₂ delivery (AUC, FF: 0.30±0.13L; RH: 0.64±0.29L, P<0.01) were significantly increased in RH in comparison to FF. RH was also associated with significantly higher capillary blood flow (P<0.05) and this resulted in a 33% increase in estimated peak mitochondrial ATP synthesis rate (FF: 24±11 mM.min⁻¹; RH: 31±7 mM.min⁻¹, P<0.05). These results document a hemodynamic reserve in the contracting calf muscle of the elderly accessible by superimposing reactive hyperemia. Furthermore, this increase in O₂ availability enhanced mitochondrial function thus indicating a skeletal muscle metabolic reserve despite advancing age and low level of physical activity.

Effect of creatine supplementation during resistance training on lean tissue mass and muscular strength in older adults: a meta-analysis

The loss of muscle mass and strength with aging results in significant functional impairment. Creatine supplementation has been used in combination with resistance training as a strategy for increasing lean tissue mass and muscle strength in older adults, but results across studies are equivocal. We conducted a systematic review and meta-analysis of randomized controlled trials of creatine supplementation during resistance training in older adults with lean tissue mass, chest press strength, and leg press strength as outcomes by searching PubMed and SPORTDiscus databases. Twenty-two studies were included in our meta-analysis with 721 participants (both men and women; with a mean age of 57–70 years across studies) randomized to creatine supplementation or placebo during resistance training 2–3 days/week for 7–52 weeks. Creatine supplementation resulted in greater increases in lean tissue mass (mean difference =1.37 kg [95% CI =0.97–1.76]; p<0.00001), chest press strength (standardized mean difference [SMD] =0.35 [0.16–0.53]; p=0.0002), and leg press strength (SMD =0.24 [0.05–0.43]; p=0.01). A number of mechanisms exist by which creatine may increase lean tissue mass and muscular strength. These are included in a narrative review in the discussion section of this article. In summary, creatine supplementation increases lean tissue mass and upper and lower body muscular strength during resistance training of older adults, but potential mechanisms by which creatine exerts these positive effects have yet to be evaluated extensively.

Sex-specific impact of aging on the blood pressure response to exercise

An exaggerated blood pressure (BP) response to exercise has been linked to cardiovascular disease, but little is known about the impact of age and sex on this response. Therefore, this study examined the hemodynamic and skeletal muscle metabolic response to dynamic plantar flexion exercise, at 40% of maximum plantar flexion work rate, in 40 physical activity-matched young (23 ± 1 yr, n = 20) and old (73 ± 2 yr, n = 20), equally distributed, male and female subjects. Central hemodynamics and BP (finometer), popliteal artery blood flow (Doppler ultrasound), and skeletal muscle metabolism (³¹P-magnetic resonance spectroscopy) were measured during 5 min of plantar flexion exercise. Popliteal artery blood flow and high-energy phosphate responses to exercise were not affected by age or sex, whereas aging, independent of sex, attenuated stroke volume and cardiac output responses. Systolic BP and mean arterial pressure responses were exaggerated in old women (Δ42 ± 4 and Δ28 ± 3 mmHg, respectively), with all other groups exhibiting similar increases in systolic BP (old men: Δ27 ± 8 mmHg, young men: Δ27 ± 3 mmHg, and young women: Δ22 ± 3 mmHg) and mean arterial pressure (old men: Δ15 ± 4 mmHg, young men: Δ19 ± 2 mmHg, and young women: Δ17 ± 2 mmHg). Interestingly, the exercise-induced change in systemic vascular resistance in old women (∆0.8 ± 1.0 mmHg·l^-1·min^-1) was augmented compared with young women and young and old men (∆-2.8 ± 0.5, ∆-1.6 ± 0.6, and ∆-3.18 ± 1.4 mmHg·l^-1·min^-1, respectively, P < 0.05). Thus, in combination, advancing age and female sex results in an exaggerated BP response to exercise, likely the result of a failure to reduce systemic vascular resistance. NEW & NOTEWORTHY An exaggerated blood pressure response to exercise has been linked to cardiovascular disease; however, little is known about how age and sex impact this response in healthy individuals. During dynamic exercise, older women exhibited an exaggerated blood pressure response driven by an inability to lower systemic vascular resistance.

Heterogeneous effects of old age on human muscle oxidative capacity in vivo: a systematic review and meta-analysis

Despite intensive efforts to understand the extent to which skeletal muscle mitochondrial capacity changes in older humans, the answer to this important question remains unclear. To determine what the preponderance of evidence from in vivo studies suggests, we conducted a systematic review and meta-analysis of the effects of age on muscle oxidative capacity as measured noninvasively by magnetic resonance spectroscopy. A secondary aim was to examine potential moderators contributing to differences in results across studies, including muscle group, physical activity status, and sex. Candidate papers were identified from PubMed searches (n = 3561 papers) and the reference lists of relevant papers. Standardized effects (Hedges’ g) were calculated for age and each moderator using data from the 22 studies that met the inclusion criteria (n = 28 effects). Effects were coded as positive when older (age, ≥55 years) adults had higher muscle oxidative capacity than younger (age, 20–45 years) adults. The overall effect of age on oxidative capacity was positive (g = 0.171, p < 0.001), indicating modestly greater oxidative capacity in old. Notably, there was significant heterogeneity in this result (Q = 245.8, p < 0.001; I2 = ∼70%–90%). Muscle group, physical activity, and sex were all significant moderators of oxidative capacity (p ≤ 0.029). This analysis indicates that the current body of literature does not support a de facto decrease of in vivo muscle oxidative capacity in old age. The heterogeneity of study results and identification of significant moderators provide clarity regarding apparent discrepancies in the literature, and indicate the importance of accounting for these variables when examining purported age-related differences in muscle oxidative capacity.

Denervation drives mitochondrial dysfunction in skeletal muscle of octogenarians

KEY POINTS

Mitochondria are frequently implicated in the ageing of skeletal muscle, although the role of denervation in modulating mitochondrial function in ageing muscle is unknown. We show that increased sensitivity to apoptosis initiation occurs prior to evidence of persistent denervation and is thus a primary mitochondrial defect in ageing muscle worthy of therapeutic targeting. However, at more advanced age, mitochondrial function changes are markedly impacted by persistent sporadic myofibre denervation, suggesting the mitochondrion may be a less viable therapeutic target.

ABSTRACT

Experimental denervation modulates mitochondrial function, where changes in both reactive oxygen species (ROS) and sensitivity to permeability transition are implicated in the resultant muscle atrophy. Notably, although denervation occurs sporadically in ageing muscle, its impact on ageing muscle mitochondria is unknown. Because this information has important therapeutic implications concerning targeting the mitochondrion in ageing muscle, we examined mitochondrial function in skeletal muscle from four groups of humans, comprising two active (mean ± SD age: 23.7 ± 2.7 years and 71.2 ± 4.9 years) and two inactive groups (64.8 ± 3.1 years and 82.5 ± 4.8 years), and compared this with a murine model of sporadic denervation. We tested the hypothesis that, although some alterations of mitochondrial function in aged muscle are attributable to a primary organelle defect, mitochondrial dysfunction would be impacted by persistent denervation in advanced age. Both ageing in humans and sporadic denervation in mice increased mitochondrial sensitivity to permeability transition (humans, P = 0.004; mice, P = 0.01). To determine the contribution of sporadic denervation to mitochondrial function, we pharmacologically inhibited the denervation-induced ROS response. This reduced ROS emission by 60% (P = 0.02) in sporadically denervated mouse muscle, which is similar to that seen in humans older than 75 years (-66%, P = 0.02) but not those younger than 75 years. We conclude that an increased sensitivity to permeability transition is a primary mitochondrial defect in ageing muscle. However, at more advanced age, when muscle atrophy becomes more clinically severe, mitochondrial function changes are markedly impacted by persistent sporadic denervation, making the mitochondrion a less viable therapeutic target.

Impact of aging on mitochondrial function in cardiac and skeletal muscle

Both skeletal muscle and cardiac muscle are subject to marked structural and functional impairment with aging and these changes contribute to the reduced capacity for exercise as we age. Since mitochondria are involved in multiple aspects of cellular homeostasis including energetics, reactive oxygen species signaling, and regulation of intrinsic apoptotic pathways, defects in this organelle are frequently implicated in the deterioration of skeletal and cardiac muscle with aging. On this basis, the purpose of this review is to evaluate the evidence that aging causes dysfunction in mitochondria in striated muscle with a view towards drawing conclusions about the potential of these changes to contribute to the deterioration seen in striated muscle with aging. As will be shown, impairment in respiration and reactive oxygen species emission with aging are highly variable between studies and seem to be largely a consequence of physical inactivity. On the other hand, both skeletal and cardiac muscle mitochondria are more susceptible to permeability transition and this seems a likely cause of the increased recruitment of mitochondrial-mediated pathways of apoptosis seen in striated muscle. The review concludes by examining the role of degeneration of mitochondrial DNA versus impaired mitochondrial quality control mechanisms in the accumulation of mitochondria that are sensitized to permeability transition, whereby the latter mechanism is favored as the most likely cause.

In vivo mitochondrial function in aging skeletal muscle: capacity, flux, and patterns of use

Because of the fundamental dependence of mammalian life on adequate mitochondrial function, the question of how and why mitochondria change in old age is the target of intense study. Given the importance of skeletal muscle for the support of mobility and health, this question extends to the need to understand mitochondrial changes in the muscle of older adults, as well. We and others have focused on clarifying the age-related changes in human skeletal muscle mitochondrial function in vivo. These changes include both the maximal capacity for oxidative production of energy (ATP), as well as the relative use of mitochondrial ATP production for powering muscular activity. It has been known for nearly 50 yr that muscle mitochondrial content is highly plastic; exercise training can induce an ∼2-fold increase in mitochondrial content, while disuse has the opposite effect. Here, we suggest that a portion of the age-related changes in mitochondrial function that have been reported are likely the result of behavioral effects, as physical activity influences have not always been accounted for. Further, there is emerging evidence that various muscles may be affected differently by age-related changes in physical activity and movement patterns. In this review, we will focus on age-related changes in oxidative capacity and flux measured in vivo in human skeletal muscle.

Unravelling the mechanisms regulating muscle mitochondrial biogenesis

Skeletal muscle is a tissue with a low mitochondrial content under basal conditions, but it is responsive to acute increases in contractile activity patterns (i.e. exercise) which initiate the signalling of a compensatory response, leading to the biogenesis of mitochondria and improved organelle function. Exercise also promotes the degradation of poorly functioning mitochondria (i.e. mitophagy), thereby accelerating mitochondrial turnover, and preserving a pool of healthy organelles. In contrast, muscle disuse, as well as the aging process, are associated with reduced mitochondrial quality and quantity in muscle. This has strong negative implications for whole-body metabolic health and the preservation of muscle mass. A number of traditional, as well as novel regulatory pathways exist in muscle that control both biogenesis and mitophagy. Interestingly, although the ablation of single regulatory transcription factors within these pathways often leads to a reduction in the basal mitochondrial content of muscle, this can invariably be overcome with exercise, signifying that exercise activates a multitude of pathways which can respond to restore mitochondrial health. This knowledge, along with growing realization that pharmacological agents can also promote mitochondrial health independently of exercise, leads to an optimistic outlook in which the maintenance of mitochondrial and whole-body metabolic health can be achieved by taking advantage of the broad benefits of exercise, along with the potential specificity of drug action.

Exercise Promotes Healthy Aging of Skeletal Muscle

Primary aging is the progressive and inevitable process of bodily deterioration during adulthood. In skeletal muscle, primary aging causes defective mitochondrial energetics and reduced muscle mass. Secondary aging refers to additional deleterious structural and functional age-related changes caused by diseases and lifestyle factors. Secondary aging can exacerbate deficits in mitochondrial function and muscle mass, concomitant with the development of skeletal muscle insulin resistance. Exercise opposes deleterious effects of secondary aging by preventing the decline in mitochondrial respiration, mitigating aging-related loss of muscle mass and enhancing insulin sensitivity. This review focuses on mechanisms by which exercise promotes "healthy aging" by inducing modifications in skeletal muscle.

A Practical Approach to Fatigue Management in Colorectal Cancer

Cancer-related fatigue is serious and complex, as well as one of the most common symptoms experienced by patients with colorectal cancer, with the potential to compromise quality of life, activities of daily living, and ultimately survival. There is a lack of consensus about the definition of cancer-related fatigue; however, definitions have been put forward by the European Association for Palliative Care (EAPC) and the National Comprehensive Cancer Network (NCCN). Numerous cancer- and treatment-related factors can contribute to fatigue, including disease progression, comorbidities, medical complications such as anemia, side effects of other medications, and a number of physical and psychologic factors. This underlines the importance of tackling factors that may contribute to fatigue before reducing the dose of treatment. NCCN guidelines and the EAPC have proposed approaches to managing fatigue in cancer patients; however, relatively few therapeutic agents have been demonstrated to reduce fatigue in randomized controlled trials. It is recognized that physical activity produces many beneficial physiologic modifications to markers of physical performance that can help to counteract various causes of fatigue. In appropriately managed and monitored patients with colorectal cancer, emerging evidence indicates that exercise programs may have a favorable influence on cancer-related fatigue, quality of life, and clinical outcomes, and therefore may help patients tolerate chemotherapy. This review assesses fatigue in patients with colorectal cancer and proposes updates to a treatment algorithm that may help clinicians manage this common problem.

Creatine supplementation in the aging population: effects on skeletal muscle, bone and brain

This narrative review aims to summarize the recent findings on the adjuvant application of creatine supplementation in the management of age-related deficits in skeletal muscle, bone and brain metabolism in older individuals. Most studies suggest that creatine supplementation can improve lean mass and muscle function in older populations. Importantly, creatine in conjunction with resistance training can result in greater adaptations in skeletal muscle than training alone. The beneficial effect of creatine upon lean mass and muscle function appears to be applicable to older individuals regardless of sex, fitness or health status, although studies with very old (>90 years old) and severely frail individuals remain scarce. Furthermore, there is evidence that creatine may affect the bone remodeling process; however, the effects of creatine on bone accretion are inconsistent. Additional human clinical trials are needed using larger sample sizes, longer durations of resistance training (>52 weeks), and further evaluation of bone mineral, bone geometry and microarchitecture properties. Finally, a number of studies suggest that creatine supplementation improves cognitive processing under resting and various stressed conditions. However, few data are available on older adults, and the findings are discordant. Future studies should focus on older adults and possibly frail elders or those who have already experienced an age-associated cognitive decline.

Mitochondria, muscle health, and exercise with advancing age

Skeletal muscle health is dependent on the optimal function of its mitochondria. With advancing age, decrements in numerous mitochondrial variables are evident in muscle. Part of this decline is due to reduced physical activity, whereas the remainder appears to be attributed to age-related alterations in mitochondrial synthesis and degradation. Exercise is an important strategy to stimulate mitochondrial adaptations in older individuals to foster improvements in muscle function and quality of life.

Hypothesis on Skeletal Muscle Aging: Mitochondrial Adenine Nucleotide Translocator Decreases Reactive Oxygen Species Production While Preserving Coupling Efficiency

Mitochondrial membrane potential is the major regulator of mitochondrial functions, including coupling efficiency and production of reactive oxygen species (ROS). Both functions are crucial for cell bioenergetics. We previously presented evidences for a specific modulation of adenine nucleotide translocase (ANT) appearing during aging that results in a decrease in membrane potential - and therefore ROS production-but surprisingly increases coupling efficiency under conditions of low ATP turnover. Careful study of the bioenergetic parameters (oxidation and phosphorylation rates, membrane potential) of isolated mitochondria from skeletal muscles (gastrocnemius) of aged and young rats revealed a remodeling at the level of the phosphorylation system, in the absence of alteration of the inner mitochondrial membrane (uncoupling) or respiratory chain complexes regulation. We further observed a decrease in mitochondrial affinity for ADP in aged isolated mitochondria, and higher sensitivity of ANT to its specific inhibitor atractyloside. This age-induced modification of ANT results in an increase in the ADP concentration required to sustain the same ATP turnover as compared to young muscle, and therefore in a lower membrane potential under phosphorylating-in vivo-conditions. Thus, for equivalent ATP turnover (cellular ATP demand), coupling efficiency is even higher in aged muscle mitochondria, due to the down-regulation of inner membrane proton leak caused by the decrease in membrane potential. In the framework of the radical theory of aging, these modifications in ANT function may be the result of oxidative damage caused by intra mitochondrial ROS and may appear like a virtuous circle where ROS induce a mechanism that reduces their production, without causing uncoupling, and even leading in improved efficiency. Because of the importance of ROS as therapeutic targets, this new mechanism deserves further studies.

Skeletal muscle mitochondrial H2 O2 emission increases with immobilization and decreases after aerobic training in young and older men

Currently, it is not known whether impaired mitochondrial function contributes to human ageing or whether potential impairments in mitochondrial function with age are secondary to physical inactivity. The present study investigated mitochondrial respiratory function and reactive oxygen species emission at a predefined membrane potential in young and older men subjected to 2 weeks of one-leg immobilization followed by 6 weeks of aerobic cycle training. Immobilization increased reactive oxygen species emission and decreased ATP generating respiration. Subsequent aerobic training reversed these effects. By contrast, age had no effect on the measured variables. The results of the present study support the notion that increased mitochondrial reactive oxygen species production mediates the detrimental effects seen after physical inactivity and that ageing per se does not cause mitochondrial dysfunction. Mitochondrial dysfunction, defined as increased oxidative stress and lower capacity for energy production, may be seen with ageing and may cause frailty, or it could be that it is secondary to physical inactivity. We studied the effect of 2 weeks of one-leg immobilization followed by 6 weeks of supervised cycle training on mitochondrial function in 17 young (mean ± SEM: 23 ± 1 years) and 15 older (68 ± 1 years) healthy men. Submaximal H2 O2 emission and respiration were measured simultaneously at a predefined membrane potential in isolated mitochondria from skeletal muscle using two protocols: pyruvate + malate (PM) and succinate + rotenone (SR). This allowed measurement of leak and ATP generating respiration from which the coupling efficiency can be calculated. The protein content of the anti-oxidants manganese superoxide dismuthase (MnSOD), CuZn superoxide dismuthase, catalase and gluthathione peroxidase 1 was measured by western blotting. Immobilization decreased ATP generating respiration using PM and increased H2 O2 emission using both PM and SR similarly in young and older men. Both were restored to baseline after the training period. Furthermore, MnSOD and catalase content increased with endurance training. The young men had a higher leak respiration at inclusion using PM and a higher membrane potential in State 3 using both substrate combinations. Collectively, the findings of the present study support the notion that increased mitochondrial reactive oxygen species mediates the detrimental effects seen after physical inactivity. Age, on the other hand, was not associated with impairments in anti-oxidant protein levels, mitochondrial respiration or H2 O2 emission using either protocol.

A novel method for determining human ex vivo submaximal skeletal muscle mitochondrial function

The present study utilized a novel method aiming to investigate mitochondrial function in human skeletal muscle at submaximal levels and at a predefined membrane potential. The effect of age and training status was investigated using a cross-sectional design. Ageing was found to be related to decreased leak regardless of training status. Increased training status was associated with increased mitochondrial hydrogen peroxide emission. Despite numerous studies, there is no consensus about whether mitochondrial function is altered with increased age. The novelty of the present study is the determination of mitochondrial function at submaximal activity rates, which is more physiologically relevant than the ex vivo functionality protocols used previously. Muscle biopsies were taken from 64 old or young male subjects (aged 60-70 or 20-30 years). Aged subjects were recruited as trained or untrained. Muscle biopsies were used for the isolation of mitochondria and subsequent measurements of DNA repair, anti-oxidant capacity and mitochondrial protein levels (complexes I-V). Mitochondrial function was determined by simultaneous measurement of oxygen consumption, membrane potential and hydrogen peroxide emission using pyruvate + malate (PM) or succinate + rotenone (SR) as substrates. Proton leak was lower in aged subjects when determined at the same membrane potential and was unaffected by training status. State 3 respiration was lower in aged untrained subjects. This effect, however, was alleviated in aged trained subjects. H2 O2 emission with PM was higher in aged subjects, and was exacerbated by training, although it was not changed when using SR. However, with a higher manganese superoxide dismuthase content, the trained aged subjects may actually have lower or similar mitochondrial superoxide emission compared to the untrained subjects. We conclude that ageing and the physical activity level in aged subjects are both related to changes in the intrinsic functionality of the mitochondrion in skeletal muscle. Both of these changes could be important factors in determining the metabolic health of the aged skeletal muscle cell.

Exercise and the Regulation of Mitochondrial Turnover

Exercise is a well-known stimulus for the expansion of the mitochondrial pool within skeletal muscle. Mitochondria have a remarkable ability to remodel their networks and can respond to an array of signaling stimuli following contractile activity to adapt to the metabolic demands of the tissue, synthesizing proteins to expand the mitochondrial reticulum. In addition, when they become dysfunctional, these organelles can be recycled by a specialized intracellular system. The signals regulating this mitochondrial life cycle of synthesis and degradation during exercise are still an area of great research interest. As mitochondrial turnover has valuable consequences in physical performance, in addition to metabolic health, disease, and aging, consideration of the signals which control this cycle is vital. This review focuses on the regulation of mitochondrial turnover in skeletal muscle and summarizes our current understanding of the impact that exercise has in modulating this process.

Mitochondrial respiratory capacity and coupling control decline with age in human skeletal muscle

Mitochondrial health is critical to physiological function, particularly in tissues with high ATP turnover, such as striated muscle. It has been postulated that derangements in skeletal muscle mitochondrial function contribute to impaired physical function in older adults. Here, we determined mitochondrial respiratory capacity and coupling control in skeletal muscle biopsies obtained from young and older adults. Twenty-four young (28 ± 7 yr) and thirty-one older (62 ± 8 yr) adults were studied. Mitochondrial respiration was determined in permeabilized myofibers from the vastus lateralis after the addition of substrates oligomycin and CCCP. Thereafter, mitochondrial coupling control was calculated. Maximal coupled respiration (respiration linked to ATP production) was lower in muscle from older vs. young subjects (P < 0.01), as was maximal uncoupled respiration (P = 0.06). Coupling control in response to the ATP synthase inhibitor oligomycin was lower in older adults (P < 0.05), as was the mitochondria flux control ratio, coupled respiration normalized to maximal uncoupled respiration (P < 0.05). Calculation of respiratory function revealed lower respiration linked to ATP production (P < 0.001) and greater reserve respiration (P < 0.01); i.e., respiratory capacity not used for phosphorylation in muscle from older adults. We conclude that skeletal muscle mitochondrial respiratory capacity and coupling control decline with age. Lower respiratory capacity and coupling efficiency result in a reduced capacity for ATP production in skeletal muscle of older adults.

Reduced skeletal muscle oxidative capacity and impaired training adaptations in heart failure

Systolic heart failure (HF) is associated with exercise intolerance that has been attributed, in part, to skeletal muscle dysfunction. The purpose of this study was to compare skeletal muscle oxidative capacity and training-induced changes in oxidative capacity in participants with and without HF. Participants with HF (n = 16, 65 ± 6.6 years) were compared with control participants without HF (n = 23, 61 ± 5.0 years). A subset of participants (HF: n = 7, controls: n = 5) performed 4 weeks of wrist-flexor exercise training. Skeletal muscle oxidative capacity was determined from the recovery kinetics of muscle oxygen consumption measured by near-infrared spectroscopy (NIRS) following a brief bout of wrist-flexor exercise. Oxidative capacity, prior to exercise training, was significantly lower in the HF participants in both the dominant (1.31 ± 0.30 min⁻¹ vs. 1.59 ± 0.25 min⁻¹, P = 0.002; HF and control groups, respectively) and nondominant arms (1.29 ± 0.24 min⁻¹ vs. 1.46 ± 0.23 min⁻¹, P = 0.04; HF and control groups, respectively). Following 4 weeks of endurance training, there was a significant difference in the training response between HF and controls, as the difference in oxidative training adaptations was 0.69 ± 0.12 min⁻¹ (P < 0.001, 95% CI 0.43, 0.96). The wrist-flexor training induced a ∼50% improvement in oxidative capacity in participants without HF (mean difference from baseline = 0.66 ± 0.09 min⁻¹, P < 0.001, 95% CI 0.33, 0.98), whereas participants with HF showed no improvement in oxidative capacity (mean difference from baseline = −0.04 ± 0.08 min⁻¹, P = 0.66, 95% CI −0.24, 0.31), suggesting impairments in mitochondrial biogenesis. In conclusion, participants with HF had reduced oxidative capacity and impaired oxidative adaptations to endurance exercise compared to controls.

Physical exercise and sarcopenia in older people: position paper of the Italian Society of Orthopaedics and Medicine (OrtoMed)

Sarcopenia is the age-associated loss of skeletal muscle mass and function. It is a major clinical problem for older people and research in understanding of pathogenesis, clinical consequences, management, and socioeconomic burden of this condition is growing exponentially. The causes of sarcopenia are multifactorial, including inflammation, insulin resistance, changing endocrine function, chronic diseases, nutritional deficiencies and low levels of physical activity. Operational definition of sarcopenia combines assessment of muscle mass, muscle strength and physical performance. The diagnosis of sarcopenia should be based on having a low appendicular fat free mass in combination with low handgrip strength or poor physical functioning. Imaging techniques used for estimating lean body mass are computed tomography, magnetic resonance imaging, bioelectrical impedance analysis and dual energy X-ray absorptiometry, the latter considered as the preferred method in research and clinical use. Pharmacological interventions have shown limited efficacy in counteracting the age-related skeletal muscle wasting. Recent evidence suggests physical activity and exercise, in particular resistance training, as effective intervention strategies to slow down sarcopenia. The Italian Society of Orthopaedics and Medicine (Or-toMed) provides this position paper to present the update on the role of exercise on sarcopenia in the elderly.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.