Exercise efficiency relates with mitochondrial content and function in older adults

Leg cycling efficiency is unaltered in healthy aging regardless of sex or training status

Muscular efficiency during exercise has been used to interrogate aspects of human muscle energetics, including mitochondrial coupling and biomechanical efficiencies. Typically, assessments of muscular efficiency have involved graded exercises. Results of previous studies have been interpreted to indicate a decline in exercise efficiency with aging owing to decreased mitochondrial function. However, discrepancies in variables such as exercise stage duration, cycling cadence, and treadmill walking mechanics may have affected interpretations of results. Furthermore, recent data from our lab examining the ATP to oxygen ratio (P:O) in mitochondrial preparations isolated from NIA mouse skeletal muscle showed no change with aging. Thus, we hypothesized that delta efficiency (Δ€) during steady-rate cycling exercise would not be altered in older healthy subjects compared with young counterparts regardless of biological sex or training status. Young (21-35 yr) and older (60-80 yr) men (n = 21) and women (n = 20) underwent continual, progressive leg cycle ergometer tests pedaling at 60 RPM for three stages (35, 60, 85 W) lasting 4 min. Δ€was calculated as: (Δ work accomplished/Δ energy expended). Overall, cycling efficiencies were not significantly different in older compared with young subjects. Similarly, trained subjects did not exhibit significantly different exercise efficiencies compared to untrained. Moreover, there were no differences between men and women. Hence, our results obtained on healthy young and older subjects are interpreted to mean that previous reports of decreased efficiency in older individuals were attributable to metabolic or biomechanical comorbidities, not aging per se.NEW & NOTEWORTHY Muscular power is reduced, but the efficiency of movement is unaltered in healthy aging.

Mitochondria as Nutritional Targets to Maintain Muscle Health and Physical Function During Ageing

The age-related loss of skeletal muscle mass and physical function leads to a loss of independence and an increased reliance on health-care. Mitochondria are crucial in the aetiology of sarcopenia and have been identified as key targets for interventions that can attenuate declines in physical capacity. Exercise training is a primary intervention that reduces many of the deleterious effects of ageing in skeletal muscle quality and function. However, habitual levels of physical activity decline with age, making it necessary to implement adjunct treatments to maintain skeletal muscle mitochondrial health and physical function. This review provides an overview of the effects of ageing and exercise training on human skeletal muscle mitochondria and considers several supplements that have plausible mechanistic underpinning to improve physical function in ageing through their interactions with mitochondria. Several supplements, including MitoQ, urolithin A, omega-3 polyunsaturated fatty acids (n3-PUFAs), and a combination of glycine and N-acetylcysteine (GlyNAC) can improve physical function in older individuals through a variety of inter-dependent mechanisms including increases in mitochondrial biogenesis and energetics, decreases in mitochondrial reactive oxygen species emission and oxidative damage, and improvements in mitochondrial quality control. While there is evidence that some nicotinamide adenine dinucleotide precursors can improve physical function in older individuals, such an outcome seems unrelated to and independent of changes in skeletal muscle mitochondrial function. Future research should investigate the safety and efficacy of compounds that can improve skeletal muscle health in preclinical models through mechanisms involving mitochondria, such as mitochondrial-derived peptides and mitochondrial uncouplers, with a view to extending the human health-span.

Exercise-induced BDNF promotes PPARδ-dependent reprogramming of lipid metabolism in skeletal muscle during exercise recovery

Post-exercise recovery is essential to resolve metabolic perturbations and promote long-term cellular remodeling in response to exercise. Here, we report that muscle-generated brain-derived neurotrophic factor (BDNF) elicits post-exercise recovery and metabolic reprogramming in skeletal muscle. BDNF increased the post-exercise expression of the gene encoding PPARδ (peroxisome proliferator-activated receptor δ), a transcription factor that is a master regulator of lipid metabolism. After exercise, mice with muscle-specific Bdnf knockout (MBKO) exhibited impairments in PPARδ-regulated metabolic gene expression, decreased intramuscular lipid content, reduced β-oxidation, and dysregulated mitochondrial dynamics. Moreover, MBKO mice required a longer period to recover from a bout of exercise and did not show increases in exercise-induced endurance capacity. Feeding naïve mice with the bioavailable BDNF mimetic 7,8-dihydroxyflavone resulted in effects that mimicked exercise-induced adaptations, including improved exercise capacity. Together, our findings reveal that BDNF is an essential myokine for exercise-induced metabolic recovery and remodeling in skeletal muscle.

Physiological determinants of mechanical efficiency during advanced ageing and disuse

This study aimed to determine which physiological factors impact net efficiency (ηnet) in oldest-old individuals at different stages of skeletal muscle disuse. To this aim, we examined ηnet, central haemodynamics, peripheral circulation, and peripheral factors (skeletal muscle fibre type, capillarization and concentration of mitochondrial DNA [mtDNA]). Twelve young (YG; 25 ± 2 years), 12 oldest-old mobile (OM; 87 ± 3 years), and 12 oldest-old immobile (OI; 88 ± 4 years) subjects performed dynamic knee extensor (KE) and elbow flexors (EF) exercise. Pulmonary oxygen uptake, photoplethysmography, Doppler ultrasound and muscle biopsies of the vastus lateralis and biceps brachii were used to assess central and peripheral adaptations to advanced ageing and disuse. Compared to the YG (12.1 ± 2.4%), the ηnet of lower-limb muscle was higher in the OM (17.6 ± 3.5%, P < 0.001), and lower in the OI (8.9 ± 1.9%, P < 0.001). These changes in ηnet during KE were coupled with significant peripheral adaptations, revealing strong correlations between ηnet and the proportion of type I muscle fibres (r = 0.82), as well as [mtDNA] (r = 0.77). No differences in ηnet were evident in the upper-limb muscles between YG, OM and OI. In view of the differences in limb-specific activity across the lifespan, these findings suggest that ηnet is reduced by skeletal muscle inactivity and not by chronological age, per se. Likewise, this study revealed that the age-related changes in ηnet are not a consequence of central or peripheral haemodynamic adaptations, but are likely a product of peripheral changes related to skeletal muscle fibre type and mitochondrial density. KEY POINTS: Although the effects of ageing and muscle disuse deeply impact the cardiovascular and skeletal muscle function, the combination of these factors on the mechanical efficiency are still a matter of debate. By measuring both upper- and lower-limb muscle function, which experience differing levels of disuse, we examined the influence of central and peripheral haemodynamics, and skeletal muscle factors linked to mechanical efficiency. Across the ages and degree of disuse, upper-limb muscles exhibited a preserved work economy. In the legs the oldest-old without mobility limitations exhibited an augmented mechanical efficiency, which was reduced in those with an impairment in ambulation. These changes in mechanical efficiency were associated with the proportion of type I muscle fibres. Recognition that the mechanical efficiency is not simply age-dependent, but the consequence of inactivity and subsequent skeletal muscle changes, highlights the importance of maintaining physical activity across the lifespan.

Lower mitochondrial respiration does not lead to decreased fat oxidation in young African American women without obesity

OBJECTIVE

The prevalence of type 2 diabetes in African American women (AAW) is nearly twice that of White women. Lower insulin sensitivity and decreased mitochondrial function may be contributing factors. The purpose of this study was to compare fat oxidation in AAW and White women.

METHODS

Participants were 22 AAW and 22 White women, matched for age (18.7-38.3 years) and BMI (< 28 kg/m2). Participants completed two submaximal (50% VO2max) exercise tests with indirect calorimetry and stable isotope tracers to assess total, plasma, and intramyocellular triglyceride fat oxidation.

RESULTS

The respiratory quotient during the exercise test was nearly identical in AAW and White women (0.813 ± 0.008 vs. 0.810 ± 0.008, p = 0.83). Although absolute total and plasma fat oxidation was lower in AAW, adjusting for the lower workload in AAW eliminated these racial differences. There was no racial difference in plasma and intramyocellular triglyceride source of fat for oxidation. No racial differences were observed in rates of ex vivo fat oxidation. Exercise efficiency was lower in AAW when adjusted to leg fat free mass.

CONCLUSIONS

The data suggest that fat oxidation is not lower in AAW compared with White women, but additional studies are needed across exercise intensity, body weight, and age to confirm these results.

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."

High-Intensity Interval Training is Safe, Feasible and Efficacious in Nonalcoholic Steatohepatitis: A Randomized Controlled Trial

BACKGROUND

High-Intensity Interval Training (HIIT) involves bursts of high-intensity exercise interspersed with lower-intensity exercise recovery. HIIT may benefit cardiometabolic health in people with nonalcoholic steatohepatitis (NASH).

AIMS

We aimed to examine the safety, feasibility, and efficacy of 12-weeks of supervised HIIT compared with a sham-exercise control (CON) for improving aerobic fitness and peripheral insulin sensitivity in biopsy-proven NASH.

METHODS

Participants based in the community [(n = 14, 56 ± 10 years, BMI 39.2 ± 6.7 kg/m², 64% male), NAFLD Activity Score 5 (range 3-7)] were randomized to 12-weeks of supervised HIIT (n = 8, 4 × 4 min at 85-95% maximal heart rate, interspersed with 3 min active recovery; 3 days/week) or CON (n = 6, stretching; 3 days/week). Safety (adverse events) and feasibility determined as ≥ 70% program completion and ≥ 70% global adherence (including session attendance, interval intensity adherence, and duration adherence) were assessed. Changes in cardiorespiratory fitness (V̇O₂peak), exercise capacity (time-on-test) and peripheral insulin sensitivity (euglycemic hyperinsulinemic clamp) were assessed. Data were analysed using ANCOVA with baseline value as the covariate.

RESULTS

There were no HIIT-related adverse events and HIIT was globally feasible [program completion 75%, global adherence 100% (including adherence to session 95.4 ± 7.3%, interval intensity 95.3 ± 6.0% and duration 96.8 ± 2.4%)]. A large between-group effect was observed for exercise capacity [mean difference 134.2 s (95% CI 19.8, 248.6 s), ƞ² 0.44, p = 0.03], improving in HIIT (106.2 ± 97.5 s) but not CON (- 33.4 ± 43.3 s), and for peripheral insulin sensitivity [mean difference 3.4 mg/KgLegFFM/min (95% CI 0.9,6.8 mg/KgLegFFM/min), ƞ² 0.32, p = 0.046], improving in HIIT (1.0 ± 0.8 mg/KgLegFFM/min) but not CON (- 3.1 ± 1.2 mg/KgLegFFM/min).

CONCLUSIONS

HIIT is safe, feasible and efficacious for improving exercise capacity and peripheral insulin sensitivity in people with NASH.

CLINICAL TRIAL REGISTRATION NUMBER

Australian New Zealand Clinical Trial Registry (anzctr.org.au) identifier ACTRN12616000305426 (09/03/2016).

Aging-associated accumulation of mitochondrial DNA mutations in tumor origin

The majority of cancer patients are among aged population, suggesting an urgent need to advance our knowledge on complicated relationship between aging and cancer. It has been hypothesized that metabolic changes during aging could act as a driver for tumorigenesis. Given the fact that mitochondrial DNA (mtDNA) mutations are common in both tumors and aged tissues, it is interesting to contemplate possible role of age-related mtDNA mutations in tumorigenesis. MtDNA encodes genes essential for mitochondrial metabolism, and mtDNA mutates at a much higher rate than nuclear genome. Random drifting of somatic mtDNA mutations, as a result of cell division or mitochondrial turnover during aging, may lead to more and more cells harboring high-frequency pathogenic mtDNA mutations, albeit at different loci, in single-cells. Such mutations can induce metabolic reprogramming, nuclear genome instability and immune response, which might increase the likelihood of tumorigenesis. In this review, we summarize current understanding of how mtDNA mutations accumulate with aging and how these mutations could mechanistically contribute to tumor origin. We also discuss potential prevention strategies for mtDNA mutation-induced tumorigenesis, and future works needed in this direction.

Effects of high-intensity interval training with hyperbaric oxygen

Hyperbaric Oxygen (HBO₂) has been proposed as a pre-conditioning method to enhance exercise performance. Most prior studies testing this effect have been limited by inadequate methodologies. Its potential efficacy and mechanism of action remain unknown. We hypothesized that HBO₂ could enhance aerobic capacity by inducing mitochondrial biogenesis via redox signaling in skeletal muscle. HBO₂ was administered in combination with high-intensity interval training (HIIT), a potent redox stimulus known to induce mitochondrial biogenesis. Aerobic capacity was tested during acute hypobaric hypoxia seeking to shift the limiting site of whole body V̇O2 from convection to diffusion, more closely isolating any effect of improved oxidative capacity. Healthy volunteers were screened with sea-level (SL) V̇O₂peak testing. Seventeen subjects were enrolled (10 men, 7 women, ages 26.5±1.3 years, BMI 24.6±0.6 kg m⁻², V̇O₂peak SL = 43.4±2.1). Each completed 6 HIIT sessions over 2 weeks randomized to breathing normobaric air, “HIIT+Air” (PiO₂ = 0.21 ATM) or HBO₂ (PiO₂ = 1.4 ATM) during training, “HIIT+HBO₂” group. Training workloads were individualized based on V̇O₂peak SL test. Vastus Lateralis (VL) muscle biopsies were performed before and after HIIT in both groups. Baseline and post-training V̇O₂peak tests were conducted in a hypobaric chamber at PiO2 = 0.12 ATM. HIIT significantly increased V̇O₂peak in both groups: HIIT+HBO₂ 31.4±1.5 to 35.2±1.2 ml kg⁻¹·min⁻¹ and HIIT+Air 29.0±3.1 to 33.2±2.5 ml kg⁻¹·min⁻¹ (p = 0.005) without an additional effect of HBO₂ (p = 0.9 for interaction of HIIT x HBO₂). Subjects randomized to HIIT+HBO₂ displayed higher skeletal muscle mRNA levels of PPARGC1A, a regulator of mitochondrial biogenesis, and HK2 and SLC2A4, regulators of glucose utilization and storage. All other tested markers of mitochondrial biogenesis showed no additional effect of HBO₂ to HIIT. When combined with HIIT, short-term modest HBO₂ (1.4 ATA) has does not increase whole-body V̇O₂peak during acute hypobaric hypoxia. (ClinicalTrials.gov Identifier: NCT02356900; https://clinicaltrials.gov/ct2/show/NCT02356900).

Metabolic changes in aging humans: current evidence and therapeutic strategies

Aging and metabolism are inextricably linked, and many age-related changes in body composition, including increased central adiposity and sarcopenia, have underpinnings in fundamental aging processes. These age-related changes are further exacerbated by a sedentary lifestyle and can be in part prevented by maintenance of activity with aging. Here we explore the age-related changes seen in individual metabolic tissues - adipose, muscle, and liver - as well as globally in older adults. We also discuss the available evidence for therapeutic interventions such as caloric restriction, resistance training, and senolytic and senomorphic drugs to maintain healthy metabolism with aging, focusing on data from human studies.

The superior beneficial effects of exercise training versus hormone replacement therapy on skeletal muscle of ovariectomized rats

Previous studies have highlighted the positive effects of Estradiol (E2) replacement therapy and physical exercise on skeletal muscle during menopause. However, the comparison effects of exercise training (ET) and estradiol replacement therapy during menopause on skeletal muscle have not been investigated to date. This study aimed to compare the effects of endurance exercise training versus E2 replacement therapy on mitochondrial density, redox status, and inflammatory biomarkers in the skeletal muscle of ovariectomized rats. Thirty female Wistar rats (12-week-old) were randomly assigned into three groups: Untrained ovariectomized rats (UN-OVX, n = 10); untrained ovariectomized rats treated with estradiol replacement therapy (E2-OVX); and, trained ovariectomized rats (TR-OVX). After ovariectomy, the E2-OVX rats were treated subcutaneously with E2 (implanted Silastic® capsule containing 360 μg of 17β-estradiol/mL) while the TR-OVX group performed an exercise training protocol (50–70% of maximal running speed on a treadmill, 60 min/day, 5 days/week for 8 weeks). After euthanasia, the soleus muscle was processed for histological and biochemical evaluations. Only exercise prevented the reduction of maximal oxygen consumption and increased mechanical efficiency (ME). While mitochondrial muscle density, total antioxidant capacity (FRAP), catalase (CAT) activity, and interleukin 10 levels were higher in TR-OVX, only OVX-E2 presented higher CAT activity and lower interleukin 6 levels. Endurance exercise training compared with E2 replacement therapy maintains the aerobic capacity improving the ME of OVX rats. In addition, only endurance exercise training raises the skeletal muscle mitochondrial content and tends to balance the redox and inflammatory status in the skeletal muscle of OVX rats.

Metabolic Flexibility and Mechanical Efficiency in Women Over-60

Purpose: Aging deteriorates metabolic flexibility (MF). Moreover, recent studies show that glycolysis is barely increased despite impoverished lipid metabolism, in addition to increased relevance of muscle power in older adults. This study aims to analyze MF, i.e., fat and carbohydrates oxidation rates (FATox and CHOox), and the point of maximal fat oxidation (MFO), in a group of active women over-60. It also aims to delve into the role of power production and mechanical efficiency regarding MF. This will help to decipher their metabolic behavior in response to increasing intensity. Methods: Twenty-nine women (66.13 ± 5.62 years) performed a submaximal graded cycling test, increasing 10 W each 3-min15-s, from 30 W to the second ventilatory threshold (VT2). Muscle power was adjusted with a Saris-H3 roller, together with a continuous gas analysis by indirect calorimetry (Cosmed K4b2). Pre and post-test blood lactate (BLa) samples were included. Frayn's equations, MFO and CHOoxpeak (mg/min/kg FFM) were considered for MF analysis (accounting for average VO2 and VCO2 in each last 60-s), whilst delta and gross efficiencies (DE%, GE%), and exercise economy (EC), were added for Mechanical Efficiency. Mean comparisons regarding intensities 60, 80 and 100% at VT2, completed the study together with correlation analysis among the main variables. Results: MFO and CHOoxpeak were small (6.35 ± 3.59 and 72.79 ± 34.76 g/min/kgFFM respectively) for a reduced muscle power (78.21 ± 15.84 W). Notwithstanding, GE% and EC increased significantly (p < 0.01) with exercise intensity. Importantly, coefficients of variation were very large confirming heterogeneity. Whilst muscle power outcomes correlated significantly (p < 0.01) with MFO (r = 0.66) and age (r = -0.62), these latter failed to be associated. Only GE% correlated to CHOoxpeak (r = -0.61, p < 0.01) regarding mechanical efficiency. Conclusions: Despite being active, women over-60 confirmed impaired substrates switching in response to exercise, from both FAT and CHO pathways. This limits their power production affecting exercise capacity. Our data suggest that decreased power with age has a key role above age per se in this metabolic inflexibility. Vice versa, increasing power seems to protect from mitochondrial dysfunction with aging. New studies will confirm if this higher efficiency when coming close to VT2, where GE is the more informative variable, might be a protective compensatory mechanism.

Intermittent fasting & performance: The iFast clinical trial protocol

There is increasing evidence from animal and human studies suggesting that fasting can play a role in disease prevention, weight control and longevity. However, few studies have compared exercise performances in individuals adhering to an intermittent fasting (IF) in comparison to individuals who are not. Given the rising popularity of IF we aim to investigate whether this type eating pattern will improve cardiovascular performance over a period of 12 weeks through VO₂ max measurements in participants from a Lebanese community. Additionally, we will study the variation of different health parameters, physical performance and biomarkers potentially affected by IF. Participants will be recruited from a large university community and randomized into 4 arms. Baseline information will be collected from all participants, which includes biological, physical, nutritional, medical and psychological data. Two arms will follow a time-restricted fasting diet with and without physical exercise, one arm will exercise without fasting, and one will act as a control group. Throughout the study, measurements will be repeated, and data analysis will follow to evaluate results.

Impact of aging and exercise on skeletal muscle mitochondrial capacity, energy metabolism, and physical function

The relationship between the age-associated decline in mitochondrial function and its effect on skeletal muscle physiology and function remain unclear. In the current study, we examined to what extent physical activity contributes to the decline in mitochondrial function and muscle health during aging and compared mitochondrial function in young and older adults, with similar habitual physical activity levels. We also studied exercise-trained older adults and physically impaired older adults. Aging was associated with a decline in mitochondrial capacity, exercise capacity and efficiency, gait stability, muscle function, and insulin sensitivity, even when maintaining an adequate daily physical activity level. Our data also suggest that a further increase in physical activity level, achieved through regular exercise training, can largely negate the effects of aging. Finally, mitochondrial capacity correlated with exercise efficiency and insulin sensitivity. Together, our data support a link between mitochondrial function and age-associated deterioration of skeletal muscle.

Aging is associated with a progressive loss of muscle function. Here the authors characterize mitochondrial capacity and muscle function in young and older adults with similar habitual physical activity and also compared to older adults with exercise training or with physical impairment.

The physiological impact of high-intensity interval training in octogenarians with comorbidities

BACKGROUND

Declines in cardiorespiratory fitness (CRF) and fat-free mass (FFM) with age are linked to mortality, morbidity and poor quality of life. High-intensity interval training (HIIT) has been shown to improve CRF and FFM in many groups, but its efficacy in the very old, in whom comorbidities are present is undefined. We aimed to assess the efficacy of and physiological/metabolic responses to HIIT, in a cohort of octogenarians with comorbidities (e.g. hypertension and osteoarthritis).

METHODS

Twenty-eight volunteers (18 men, 10 women, 81.2 ± 0.6 years, 27.1 ± 0.6 kg·m^-2 ) with American Society of Anaesthesiology (ASA) Grade 2-3 status each completed 4 weeks (12 sessions) HIIT after a control period of equal duration. Before and after each 4 week period, subjects underwent body composition assessments and cardiopulmonary exercise testing. Quadriceps muscle biopsies (m. vastus lateralis) were taken to quantify anabolic signalling, mitochondrial oxidative phosphorylation, and cumulative muscle protein synthesis (MPS) over 4-weeks.

RESULTS

In comorbid octogenarians, HIIT elicited improvements in CRF (anaerobic threshold: +1.2 ± 0.4 ml·kg^-1 ·min^-1 , P = 0.001). HIIT also augmented total FFM (47.2 ± 1.4 to 47.6 ± 1.3 kg, P = 0.04), while decreasing total fat mass (24.8 ± 1.3 to 24 ± 1.2 kg, P = 0.0002) and body fat percentage (33.1 ± 1.5 to 32.1 ± 1.4%, P = 0.0008). Mechanistically, mitochondrial oxidative phosphorylation capacity increased after HIIT (i.e. citrate synthase activity: 52.4 ± 4 to 67.9 ± 5.1 nmol·min^-1 ·mg^-1 , P = 0.005; membrane protein complexes (C): C-II, 1.4-fold increase, P = 0.002; C-III, 1.2-fold increase, P = 0.03), as did rates of MPS (1.3 ± 0.1 to 1.5 ± 0.1%·day^-1 , P = 0.03). The increase in MPS was supported by up-regulated phosphorylation of anabolic signalling proteins (e.g. AKT, p70S6K, and 4E-BP1; all P < 0.05). There were no changes in any of these parameters during the control period. No adverse events were reported throughout the study.

CONCLUSIONS

The HIIT enhances skeletal muscle mass and CRF in octogenarians with disease, with up-regulation of MPS and mitochondrial capacity likely underlying these improvements. HIIT can be safely delivered to octogenarians with disease and is an effective, time-efficient intervention to improve muscle mass and physical function in a short time frame.

Astaxanthin supplementation enhances metabolic adaptation with aerobic training in the elderly

Endurance training (ET) is recommended for the elderly to improve metabolic health and aerobic capacity. However, ET-induced adaptations may be suboptimal due to oxidative stress and exaggerated inflammatory response to ET. The natural antioxidant and anti-inflammatory dietary supplement astaxanthin (AX) has been found to increase endurance performance among young athletes, but limited investigations have focused on the elderly. We tested a formulation of AX in combination with ET in healthy older adults (65-82 years) to determine if AX improves metabolic adaptations with ET, and if AX effects are sex-dependent. Forty-two subjects were randomized to either placebo (PL) or AX during 3 months of ET. Specific muscle endurance was measured in ankle dorsiflexors. Whole body exercise endurance and fat oxidation (FATox) was assessed with a graded exercise test (GXT) in conjunction with indirect calorimetry. Results: ET led to improved specific muscle endurance only in the AX group (Pre 353 ± 26 vs. Post 472 ± 41 contractions), and submaximal GXT duration improved in both groups (PL 40.8 ± 9.1% and AX 41.1 ± 6.3%). The increase in FATox at lower intensity after ET was greater in AX (PL 0.23 ± 0.15 g vs. AX 0.76 ± 0.18 g) and was associated with reduced carbohydrate oxidation and increased exercise efficiency in males but not in females.

Physiology of Energy Expenditure in the Weight-Reduced State

Although many individuals achieve weight loss of 10% or more, the ability to maintain a reduced body mass over months and years is much rarer. Unfortunately, our understanding of the adverse consequences of having overweight and obesity argues that long-term maintenance of a reduced weight provides the greatest health benefit. However, to achieve long-term weight reduction requires overcoming neuroendocrine systems that favor restoration of one's initial weight. Identifying and characterizing the components of these systems will be important if we are to develop therapies and strategies to reduce the rates of obesity and its complications in our modern society. During this session, Eric Ravussin and Steven R. Smith, respectively, discussed the physiology of the weight-reduced state that favors weight regain and a molecular component that contributes to this response.

Sub 3-Hour Marathon Runners for Five Consecutive Decades Demonstrate a Reduced Age-Related Decline in Performance

Estimation of the age-related decline in athletic performance by analyzing age-group world record performances presents an inherent limitation because the records generally belong to different individuals. Longitudinal studies describing the changes in performance with advancing age for the same individuals with a consistent training regimen are more appropriate to determine age-related changes in performance. The aim of this longitudinal study was to examine the age-related decline in running performance of sub 3-h marathoners for five consecutive calendar decades. The best marathon performances for each decade from the 1970s to the 2010s were analyzed for 40 sub 3-h runners (39 males and 1 female). The cohort mean personal best performance was 2 h 23 min ± 9 min at an age of 28.6 ± 4.7 years. The mean difference in age between the first and the last sub 3-h marathon races was 32.9 ± 1.6 years. The time difference in marathon performance between the personal best and the worst performance during the 5th decade was 26 ± 9 min, corresponding to a mean increase of 1 min 4 s per year, i.e., a decrease in running speed of 0.67 ± 0.29% per year. These results suggest that with consistent training and racing regimens, it is possible to limit the age-related decline in marathon performance to less than 7% per decade at least until 60 years of age. Further studies are required to verify if such a low rate of age-related decline in endurance performance could be maintained after 60 years of age.

Poor Work Efficiency is Associated with Poor Exercise Capacity and Health-Related Quality of Life in Patients with Chronic Obstructive Pulmonary Disease

Introduction

Chronic obstructive pulmonary disease (COPD) is a progressive disease with deteriorating cardiopulmonary function that decreases the health-related quality of life (HRQL) and exercise capacity. Patients with COPD often have cardiovascular and muscular problems that hinder oxygen uptake by peripheral tissues, resulting in poor oxygen consumption efficiency. It is important to develop new physiological parameters to evaluate oxygen consumption efficiency during activities and to evaluate its association with exercise capacity and HRQL. Work efficiency (WE) measures oxygen consumption efficiency during exercise. We hypothesize that patients with poor WE should have exercise intolerance and poor HRQL. Therefore, we aimed to evaluate the association between WE and exercise capacity, HRQL and other cardiopulmonary parameters.

Patients and Methods

Seventy-eight patients with COPD were evaluated with spirometry, cardiopulmonary exercise testing, and assessment of dyspnea score and HRQL (using the St. George’s Respiratory Questionnaire [SGRQ]). Cardiopulmonary exercise testing was performed using a cycle ergometer with an incremental protocol and exhaled breath analysis to assess oxygen consumption. WE was defined as the relationship between oxygen consumption and workload.

Results

There were 31 patients with normal WE (group I) and 47 patients (group II) with poor WE. Patients with poor WE had lower exercise capacity (maximal oxygen consumption, group I vs II as 1050±53 vs 845 ±34 mL/min, p=0.0011), poorer HRQL (SGRQ score 41.1±3.0 vs 55±2.2, p=0.0002), higher exertional dyspnea score (5.1±0.2 vs 6.1±0.2, p= 0.0034) and early anaerobic metabolism during exercise (anaerobic threshold, 672±27 vs 583 ±18 mL/min, p=0.0052).

Conclusion

WE is associated with exercise capacity and HRQL. Here, patients with poor WE also had exercise intolerance, poorer HRQL, and more exertional dyspnea.

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

BACKGROUND

Physical frailty in older individuals is characterized by subjective symptoms of fatigue and exercise intolerance (EI). Objective abnormalities in skeletal muscle (SM) mitochondrial high-energy phosphate (HEP) metabolism contribute to EI in inherited myopathies; however, their presence or link to EI in the frail older adult is unknown.

METHODS

Here, we studied 3 groups of ambulatory, community-dwelling adults with no history of significant coronary disease: frail older (FO) individuals (81 ± 2.7 years, mean ± SEM), nonfrail older (NFO) individuals (79 ± 2.0 years), and healthy middle-aged individuals, who served as controls (CONT, 51 ± 2.1 years). Lower extremity SM HEP levels and mitochondrial function were measured with ³¹P magnetic resonance (MR) techniques during graded multistage plantar flexion exercise (PFE). EI was quantified by a 6-minute walk (6MW) and peak oxygen consumption during cardiopulmonary testing (peak VO₂).

RESULTS

During graded exercise, FO, NFO, and CONT individuals all fatigued at similar SM HEP levels, as measured by ³¹P-MR. However, FO individuals fatigued fastest, with several-fold higher rates of PFE-induced HEP decline that correlated closely with shorter exercise duration in the MR scanner and with 6MW distance and lower peak oxygen consumption on cardiopulmonary testing (P < 0.001 for all). SM mitochondrial oxidative capacity was lower in older individuals and correlated with rapid HEP decline but less closely with EI.

CONCLUSION

Several-fold faster SM energetic decline during exercise occurs in FO individuals and correlates closely with multiple measures of EI. Rapid energetic decline represents an objective, functional measure of SM metabolic changes and a potential new target for mitigating frailty-associated physical limitations.

FUNDING

This work was supported by NIH R21 AG045634, R01 AG063661, R01 HL61912, the Johns Hopkins University Claude D. Pepper Older Americans Independence Center P30AG021334, and the Clarence Doodeman Endowment in Cardiology at Johns Hopkins.

Rapid exercise-induced skeletal muscle high-energy phosphate decline occurs in frail, older individuals and is closely linked to exercise intolerance and fatigue.

Bicycling Exercise Helps Maintain a Youthful Metabolic Cost of Walking in Older Adults

The decline of walking performance is a key determinant of morbidity among older adults. Healthy older adults have been shown to have a 15-20% lower walking economy compared with young adults. However, older adults who run for exercise have a higher walking economy compared with older adults who walk for exercise. Yet, it remains unclear if other aerobic exercises yield similar improvements on walking economy. The purpose of this study was to determine if regular bicycling exercise affects walking economy in older adults. We measured metabolic rate while 33 older adult "bicyclists" or "walkers" and 16 young adults walked on a level treadmill at four speeds between (0.75-1.75 m/s). Across the range of speeds, older bicyclists had a 9-17% greater walking economy compared with older walkers (p = .009). In conclusion, bicycling exercise mitigates the age-related deterioration of walking economy, whereas walking for exercise has a minimal effect on improving walking economy.

High Levels of Physical Activity in Later Life Are Associated With Enhanced Markers of Mitochondrial Metabolism

The age-associated reduction in muscle mass is well characterized; however, less is known regarding the mechanisms responsible for the decline in oxidative capacity also observed with advancing age. The purpose of the current study was therefore to compare mitochondrial gene expression and protein content between young and old recreationally active, and older highly active individuals. Muscle biopsies were obtained from the vastus lateralis of young males (YG: 22 ± 3 years) and older (OG: 67 ± 2 years) males not previously engaged in formal exercise and older male master cyclists (OT: 65 ± 5 years) who had undertaken cycling exercise for 32 ± 17 years. Comparison of gene expression between YG, OG, and OT groups revealed greater expression of mitochondrial-related genes, namely, electron transport chain (ETC) complexes II, III, and IV (p < .05) in OT compared with YG and OG. Gene expression of mitofusion (MFN)-1/2, mitochondrial fusion genes, was greater in OT compared with OG (p < .05). Similarly, protein content of ETC complexes I, II, and IV was significantly greater in OT compared with both YG and OG (p < .001). Protein content of peroxisome proliferator-activated receptor gamma, coactivator 1 α (PGC-1α), was greater in OT compared with YG and OG (p < .001). Our results suggest that the aging process per se is not associated with a decline in gene expression and protein content of ETC complexes. Mitochondrial-related gene expression and protein content are substantially greater in OT, suggesting that exercise-mediated increases in mitochondrial content can be maintained into later life.

Hybrid fiber alterations in exercising seniors suggest contribution to fast-to-slow muscle fiber shift

BACKGROUND

Human skeletal muscle is composed of a functional and metabolic continuum of slow (Type I) and fast fibers (IIa and IIx). Hybrid fibers co-expressing different myosin heavy chains are also present and seem to be more prominent in aging muscle. Their role is debated; hybrid fibers were reported either in a transitional state, between slow and fast fibers, or as fixed individual entities. This study examined the fate of hybrid fibers with an endurance exercise intervention in an elderly sedentary population.

METHODS

Twenty-two sedentary healthy elderly men and women underwent a 16-week supervised endurance exercise intervention. Eighteen endurance-trained age- and gender-matched volunteers served as controls. Fiber type distribution was determined by immunohistochemistry on vastus lateralis muscle biopsies pre-intervention and post-intervention.

RESULTS

A total of 13840 fibers were analyzed. At baseline, a Type II dominant fiber profile was observed compared with the control group, with more Type IIa (P = 0.0301) and Type IIx fibers (P = 0.0328). Hybrid fibers represented almost 5% of total muscle fibers in both groups. There was no significant difference between groups (I-IIa, P = 0.6719 and IIa-IIx, P = 0.0998). Intervention triggered qualitative dynamics towards an increase in Type I, and decrease in Type II fibers, paralleled by an increase in I-IIa hybrids (P = 0.0301).

CONCLUSIONS

The present study is, to our knowledge, the first to examine hybrid muscle fiber type adaptations to an endurance exercise intervention in the elderly. Hybrid fiber proportions did not differ between chronic sedentary state and chronic endurance-trained state. Exercise intervention increased Type I-IIa hybrid fibers along with shift dynamics in other fiber types suggesting the contribution of hybrid fiber to a fast-to-slow fiber type transition, eventually serving as intermediate reservoir from one monomorphic myosin heavy chain expressing fiber type to another. This finding favours the transitional theory regarding hybrid muscle fibers and exercise, crucial to understanding reversible mechanisms of sarcopenia and development of prevention measures.

Distinct patterns of skeletal muscle mitochondria fusion, fission and mitophagy upon duration of exercise training

AIM

Healthy ageing interventions encompass regular exercise to prevent mitochondrial dysfunction, key player in sarcopenia pathogenesis. Mitochondrial biogenesis has been well documented, but mitochondrial remodelling in response to exercise training is poorly understood. Here we investigated fusion, fission and mitophagy before and after an exercise intervention in older adults.

METHODS

Skeletal muscle biopsies were collected from 22 healthy sedentary men and women before and after 4 months of supervised training. Eight lifelong trained age- and gender-matched volunteers served as positive controls. Transmission electron microscopy was used to estimate mitochondrial content. Western blotting and qRT-PCR were used to detect changes in specific proteins and transcripts.

RESULTS

After intervention, mitochondrial content increased to levels of controls. While enhancement of fusion was prevalent after intervention, inhibition of fission and increased mitophagy were dominant in controls. Similarly to PARKIN, BCL2L13 content was higher in controls. The observed molecular adaptations paralleled long-term effects of training on physical fitness, exercise efficiency and oxidative capacity.

CONCLUSIONS

This study describes distinct patterns of molecular adaptations in human skeletal muscle under chronic exercise training. After 16 weeks of exercise, the pattern was dominated by fusion to increase mitochondrial content to the metabolic demands of exercise. In lifelong exercise, the pattern was dominated by mitophagy synchronized with increased fusion and decreased fission, indicating an increased mitochondrial turnover. In addition to these temporally distinct adaptive mechanisms, this study suggests for the first time a specific role of BCL2L13 in chronic exercise that requires constant maintenance of mitochondrial quality.

Mitochondrial function is impaired in the skeletal muscle of pre-frail elderly

Aging is accompanied by a gradual decline in both muscle mass and strength over time, which can eventually lead to pathologies, such as frailty and sarcopenia. While these two conditions are well characterized, further investigation of the early biological signs present in pre-frail elderly is still needed to help identify strategies for preventative therapeutic intervention. The goal of the present clinical study was to evaluate the level of mitochondrial (dys)function in a well-defined population of pre-frail elderly (>60 years of age). Pre-frail elderly were compared with an age-matched population of active elderly. Muscle mitochondrial function was assessed in vivo using phosphorus magnetic resonance spectroscopy (³¹P-MRS) and a comprehensive set of biological biomarkers were measured ex vivo in vastus lateralis muscle biopsies. In pre-frail subjects, phosphocreatine recovery was impaired and mitochondrial respiratory complex protein and activity levels were significantly lower when compared with active elderly. Analysis of microarray data showed that mitochondrial genes were also significantly down-regulated in muscle of pre-frail compared to active elderly. These results show that mitochondrial impairment is a hallmark of pre-frailty development and the onset of decline in muscle function in the elderly.

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.

Skeletal muscle bioenergetics in aging and heart failure

Changes in mitochondrial capacity and quality play a critical role in skeletal and cardiac muscle dysfunction. In vivo measurements of mitochondrial capacity provide a clear link between physical activity and mitochondrial function in aging and heart failure, although the cause and effect relationship remains unclear. Age-related decline in mitochondrial quality leads to mitochondrial defects that affect redox, calcium, and energy-sensitive signaling by altering the cellular environment that can result in skeletal muscle dysfunction independent of reduced mitochondrial capacity. This reduced mitochondrial quality with age is also likely to sensitize skeletal muscle mitochondria to elevated angiotensin or beta-adrenergic signaling associated with heart failure. This synergy between aging and heart failure could further disrupt cell energy and redox homeostasis and contribute to exercise intolerance in this patient population. Therefore, the interaction between aging and heart failure, particularly with respect to mitochondrial dysfunction, should be a consideration when developing strategies to improve quality of life in heart failure patients. Given the central role of the mitochondria in skeletal and cardiac muscle dysfunction, mitochondrial quality may provide a common link for targeted interventions in these populations.

Salivary Mitochondrial DNA Copy Number Is Associated With Exercise Ventilatory Efficiency

Chen, Y, Hill, HZ, Lange, G, and Falvo, MJ. Salivary mitochondrial DNA copy number is associated with exercise ventilatory efficiency. J Strength Cond Res 31(7): 2000-2004, 2017-Mitochondrial DNA copy number (mtDNAcn) is an index of mitochondrial content and is responsive to changes in exercise training volume. Therefore, assessment of mtDNAcn may help to optimize exercise prescription and aid in athlete monitoring. Although previous work has assessed mtDNAcn derived from skeletal muscle and blood using invasive approaches, no study has examined salivary mtDNAcn and its relationship with sport performance. Fifteen adults (32.2 ± 7.1 years) volunteered to participate in this study. Each participant provided a saliva sample for the analysis of mtDNAcn via real-time polymerase reaction. In addition, participants completed an exercise challenge test to assess oxygen consumption relative to body weight (V[Combining Dot Above]O2·kg) and ventilatory efficiency (VE/V[Combining Dot Above]CO2). Using multiple linear regression, we examined the association of V[Combining Dot Above]O2·kg and VE/V[Combining Dot Above]CO2 with salivary mtDNAcn, adjusting for self-reported physical activity (min·wk). Greater mtDNAcn was associated with lower VE/V[Combining Dot Above]CO2 (p < 0.01) and higher V[Combining Dot Above]O2·kg (p < 0.05). In our model adjusted for physical activity, greater mtDNAcn remained associated with lower VE/V[Combining Dot Above]CO2 (β = -0.186; 95% confidence interval [CI], -0.348 to -0.025; p < 0.05), but not with V[Combining Dot Above]O2·kg (β = -0.022; 95% CI, -0.113 to 0.063). Our findings suggest that salivary mtDNAcn is associated with ventilatory efficiency, which may reflect enhanced exercise efficiency as a consequence of greater total mitochondrial content. As saliva collection is noninvasive, stable at room temperature, and less costly in comparison to skeletal muscle and blood, future studies may consider using saliva for the evaluation of mitochondrial content for the purposes of monitoring exercise training as well as optimizing exercise prescription.

Enhanced Respiratory Chain Supercomplex Formation in Response to Exercise in Human Skeletal Muscle

Mitochondrial dysfunction is a hallmark of multiple metabolic complications. Physical activity is known to increase mitochondrial content in skeletal muscle, counteracting age-related decline in muscle function and protecting against metabolic and cardiovascular complications. Here, we investigated the effect of 4 months of exercise training on skeletal muscle mitochondria electron transport chain complexes and supercomplexes in 26 healthy, sedentary older adults. Exercise differentially modulated respiratory complexes. Complex I was the most upregulated complex and not stoichiometrically associated to the other complexes. In contrast to the other complexes, complex I was almost exclusively found assembled in supercomplexes in muscle mitochondria. Overall, supercomplex content was increased after exercise. In particular, complexes I, III, and IV were redistributed to supercomplexes in the form of I+III₂+IV. Taken together, our results provide the first evidence that exercise affects the stoichiometry of supercomplex formation in humans and thus reveal a novel adaptive mechanism for increased energy demand.

Older Runners Retain Youthful Running Economy despite Biomechanical Differences

PURPOSE

Sixty-five years of age typically marks the onset of impaired walking economy. However, running economy has not been assessed beyond the age of 65 yr. Furthermore, a critical determinant of running economy is the spring-like storage and return of elastic energy from the leg during stance, which is related to leg stiffness. Therefore, we investigated whether runners older than 65 yr retain youthful running economy and/or leg stiffness across running speeds.

METHODS

Fifteen young and 15 older runners ran on a force-instrumented treadmill at 2.01, 2.46, and 2.91 m·s(-1). We measured their rates of metabolic energy consumption (i.e., metabolic power), ground reaction forces, and stride kinematics.

RESULTS

There were only small differences in running economy between young and older runners across the range of speeds. Statistically, the older runners consumed 2% to 9% less metabolic energy than the young runners across speeds (P = 0.012). Also, the leg stiffness of older runners was 10% to 20% lower than that of young runners across the range of speeds (P = 0.002), and in contrast to the younger runners, the leg stiffness of older runners decreased with speed (P < 0.001).

CONCLUSIONS

Runners beyond 65 yr of age maintain youthful running economy despite biomechanical differences. It may be that vigorous exercise, such as running, prevents the age related deterioration of muscular efficiency and, therefore, may make everyday activities easier.

Mitochondrial Epigenetic Changes Link to Increased Diabetes Risk and Early-Stage Prediabetes Indicator

Type 2 diabetes (T2D) is characterized by mitochondrial derangement and oxidative stress. With no known cure for T2D, it is critical to identify mitochondrial biomarkers for early diagnosis of prediabetes and disease prevention. Here we examined 87 participants on the diagnosis power of fasting glucose (FG) and hemoglobin A1c levels and investigated their interactions with mitochondrial DNA methylation. FG and A1c led to discordant diagnostic results irrespective of increased body mass index (BMI), underscoring the need of new biomarkers for prediabetes diagnosis. Mitochondrial DNA methylation levels were not correlated with late-stage (impaired FG or A1c) but significantly with early-stage (impaired insulin sensitivity) events. Quartiles of BMI suggested that mitochondrial DNA methylation increased drastically from Q1 (20 < BMI < 24.9, lean) to Q2 (30 < BMI < 34.9, obese), but marginally from Q2 to Q3 (35 < BMI < 39.9, severely obese) and from Q3 to Q4 (BMI > 40, morbidly obese). A significant change was also observed from Q1 to Q2 in HOMA insulin sensitivity but not in A1c or FG. Thus, mitochondrial epigenetic changes link to increased diabetes risk and the indicator of early-stage prediabetes. Further larger-scale studies to examine the potential of mitochondrial epigenetic marker in prediabetes diagnosis will be of critical importance for T2D prevention.

Evaluation of in vivo mitochondrial bioenergetics in skeletal muscle using NMR and optical methods

It is now clear that mitochondria are involved as either a cause or consequence of many chronic diseases. This central role of the mitochondria is due to their position in the cell as important integrators of cellular energetics and signaling. Mitochondrial function affects many aspects of the cellular environment such as redox homeostasis and calcium signaling, which then also exert control over mitochondrial function. This complex dynamic between mitochondrial function and the cellular environment highlights the value of examining mitochondria in vivo in the intact physiological environment. This review discusses NMR and optical approaches used to measure mitochondria ATP and oxygen fluxes that provide in vivo measures of mitochondrial capacity and quality in animal and human models. Combining these in vivo measurements with more traditional ex vivo analyses can lead to new insights into the importance of the cellular environment in controlling mitochondrial function under pathological conditions. Interpretation and underlying assumptions for each technique are discussed with the goal of providing an overview of some of the most common approaches used to measure in vivo mitochondrial function encountered in the literature.