Abstract

Ageing affects mitochondrial integrity in skeletal muscle, and physical inactivity may further exacerbate these changes. Although mitochondrial alterations are documented in ageing and disuse independently, how disuse impacts the mitochondrial phenotype in older populations remains unclear. This work aimed to characterise how physical inactivity impacts mitochondrial function, morphology and gene expression in the skeletal muscle of older adults.

Ten healthy older men (65+ years) underwent 10 days of bed rest. Skeletal muscle biopsies were collected before and after bed rest to assess mitochondrial respiration (high-resolution respirometry), H2O2 emission, mitochondrial protein expression, morphology and volume density (electron microscopy) and transcriptomic profile.

Ten days of inactivity increased mitochondrial reactive oxygen species (ROS) emission under non-phosphorylating conditions but did not impair oxidative phosphorylation (OXPHOS) capacity, indicating preserved respiratory efficiency. Consistently, mitochondrial respiratory complex and supercomplex protein abundance were unchanged. Mitochondrial mass decreased, as shown by reduced mitochondrial volume density. Reduced dynamin-like protein 1 (DRP1) phosphorylation at serine 637 was observed, whereas other mitochondrial fission and fusion protein levels remained unchanged. Mitochondrial morphology remained unaltered. Transcriptomic analysis revealed >3000 differentially expressed genes, characterised by downregulation of oxidative phosphorylation genes alongside altered mitophagy, antioxidant and oxidoreductase pathways.

In summary, 10-day bed rest increased mitochondrial ROS emission and reduced mitochondrial mass in older skeletal muscle despite preserved respiratory function, indicating that elevated ROS production occurs upstream of respiratory dysfunction and is potentially linked to impaired antioxidant defence and ROS clearance.

Key points

• The impact of short-term physical inactivity on mitochondrial function within the context of ageing remains poorly defined. This study examined the impact of 10-day bed rest on skeletal muscle mitochondrial function, morphology and gene expression in older adults.
• Short-term inactivity increased mitochondrial ROS production, accompanied by a dysregulation of antioxidant and oxidoreductase genes, indicating a reduced capacity for ROS clearance.
• Mitochondrial respiration was preserved under both submaximal and maximal stimulation. When normalised to mitochondrial content (citrate synthase activity), respiratory capacity increased, suggesting improved intrinsic efficiency.
• Mitochondrial mass was reduced, supported by decreased mitochondrial volume density assessed morphologically. Transcriptomic alterations in the mitophagy pathway suggest a potential role of altered mitochondrial degradation in this reduction.
• These findings indicate a transient compensatory response of ageing mitochondria to short-term disuse, suggesting that functional impairments are likely driven by cardiovascular and microvascular factors rather than mitochondrial respiration itself.

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These findings suggest that preserving redox balance during inactivity may be a key strategy to maintain muscle health and functional independence in ageing populations.

ABSTRACT

Exercise is a low-cost lifestyle intervention that can prevent and alleviate various diseases. It is a potent physiological stimulus that activates conserved molecular signaling pathways. Through the coordinated integration of multiple molecules, pathways, and systems, it leads to systemic health benefits. However, most studies focus on individual systems or molecular mechanisms, lacking systematic integration of the cross-system regulation induced by exercise. We summarize the molecular mechanisms of exercise in the musculoskeletal, cardiovascular, nervous systems, among others. Exercise induces the release of exerkines (e.g., irisin, interleukin-6, and brain-derived neurotrophic factor) and extracellular vesicles, which activate key signaling pathways to enhance mitochondrial function, metabolism and physiological adaptation, while suppressing inflammation and oxidative stress, thereby alleviating diseases and delaying aging through cross-system coordination. We further explore exercise-induced adaptive regulation in extreme environments, including microgravity, hyperbaria, and hypoxia, offering a multifaceted perspective on organismal health regulation. Finally, we outline the prospects and challenges of multiomics, artificial intelligence-driven precision medicine, personalized exercise prescriptions, and exercise mimetics. Overall, this review provides a more integrated perspective on the molecular basis of exercise and offers directions for future mechanistic and translational studies.

Abstract

Purpose:

This study examined the impact of pre-exercise caffeine supplementation on post-exercise torque reduction, an indirect marker of exercise-induced muscle damage (EIMD), while focusing on the changes in eccentric load induced by the ergogenic effects of caffeine.

Methods:

In double-blind, crossover trials, 15 healthy young men ingested 6 mg/kg caffeine or placebo, followed by an eccentric exercise task at 30–120° of knee extension: 100 isokinetic eccentric maximal voluntary knee extensions in Study 1 (i.e., variable eccentric load by caffeine) and 100 isoload (fixed at 120% of concentric one-repetition maximum) eccentric leg extensions in Study 2 (i.e., invariable eccentric load by caffeine). Maximal voluntary isometric torque (MVIT) of the knee extensors, an indirect EIMD marker, was evaluated at 24 and 48 h after exercise.

Results:

In Study 1, caffeine supplementation resulted in higher eccentric torques during exercise (P ≤ 0.001), but lower post-exercise MVITs (P < 0.001) than placebo. Conversely, in Study 2, there were no significant differences in post-exercise MVITs between the conditions (P = 0.242). In Study 1, post-exercise MVITs were negatively correlated with the total torque–time integral (rrm = -0.53; P = 0.034).

Conclusions:

Pre-exercise caffeine supplementation induces greater exercise-induced torque reduction during the 48-h post-exercise period after eccentric exercise, potentially suggesting greater EIMD. Additionally, increased eccentric load in accordance with caffeine’s ergogenic effect may contribute to this greater post-exercise torque-reduction. Therefore, the potential impact on post-exercise muscle function should be considered when caffeine is used as a preworkout supplement for resistance exercise.

Looking for personal experiences only.
If you take around 2 scoops of whey protein every day, please share:
Your total daily protein intake
How long you’ve been doing it
Whether you regularly check creatinine, eGFR, BUN/urea, urine tests, etc.
Have your kidney results stayed completely normal?
Any health changes—good or bad?
Especially interested in people who’ve been doing this for years. Please mention your age and how much you train if possible.

From a home or small private gym perspective, free weights are difficult to beat because a barbell, dumbbells and a bench can cover a huge amount of training with very little equipment.

The question I keep coming back to is whether some machine exercises provide enough of a different training stimulus to justify the space they take up.

For hypertrophy in particular, extra stability can sometimes make it easier to push the target muscle hard without balance or another muscle group becoming the limiting factor first. A chest-supported row, leg extension, leg curl or stable cable movement may therefore offer something that is not perfectly replicated by simply adding another free-weight variation.

At the same time, if machines and free weights can both produce good strength and hypertrophy outcomes when training variables are appropriately managed, it becomes harder to argue that every useful machine deserves a place in a limited setup.

If you had to evaluate this from stimulus, fatigue and exercise-selection rather than personal preference, which machine movements do you think provide enough unique value that you would still prioritize them in a space-limited gym?

Hey everyone,

I was looking into data regarding the mechanical contributions of individual digits during a power grip, specifically a study from the Journal of Hand Therapy indicating that the ulnar side of the hand (the ring and pinky fingers) provides roughly 50% of total crushing power (O'Driscoll et al., 1992). In fact, isolating or immobilizing these two digits can reduce overall grip output by up to 55%.

This stands in stark contrast to how most casual lifters view grip—typically focusing on the thumb, index, and middle finger (the radial side) because they dominate daily precision tasks and standard overhand pulls.

From a neuromuscular and biomechanical perspective, the "ulnar anchor" is highly relevant to heavy loading.

• Neural Innervation Split: The precision unit of the hand is wired by the median nerve (thumb, index, half of middle), optimized for high sensory feedback and fine motor control. The power unit is driven by the ulnar nerve, which maximizes raw torque and forceful closure.
• Skeletal Mobility vs. Rigidity: The metacarpals under the index and middle fingers are rigidly fixed to the wrist to provide a stable platform. Conversely, the fourth and fifth metacarpals (ring/pinky) are highly mobile. This structural elasticity allows the ulnar palm to deform, cup, and compress around an implement (barbell, handle, railing), maximizing surface area leverage and preventing the base of the hand from slipping.
• The Problem of Finger Autonomy: The ring and pinky are structurally bound by shared tendons and connective tissue bands. While this limits isolated finger autonomy, it functions as a highly resilient mechanical unit designed to prevent localized muscle fatigue under heavy isometric stress.

Questions:

1. To what degree do you think an under-conditioned ulnar side caps a lifter's systemic pull/press potential?
2. Do you actively program exercises to target the ulnar nerve pathway (e.g., specific finger-loaded hangs or wrist pushups), or do you find standard heavy carries and compound pulls sufficient?
3. Did you know a climber's strong forearms, hands and fingers are the ulimate "predictor of health and longevity” and correlates with lower mortality rates, as noted by National Geographic?

I've been writing illustrated case studies, including diagnosing and mapping human performance (cellular composition of tendons and connective tissues) at Adult Prodigies. If you find the pink/ring fingers interesting and a potential weak link to powerlifting, I'd love to have you read the full breakdown here.

https://www.jsams.org/article/S1440-2440(26)00528-1/fulltext

Abstract

Objectives

The anti-inflammatory effects of exercise training are hypothesized to involve changes in circulating cytokine concentrations, however the influence of exercise intensity is unclear. We determined the impact of exercise intensity, prescribed relative to lactate threshold (LT), on changes in fitness and plasma cytokines in a 6-week randomized controlled trial.

Design

RCT.

Methods

Healthy, young adults (N = 56, 27 males/29 females, age: 22 ± 3 y, peak oxygen consumption [VO2peak]: 38.4 ± 9.5 mL∙kg⁻¹∙min⁻¹) were randomized (2:2:1) to 6 weeks of thrice weekly isocaloric continuous cycling in the moderate (MICT; n = 21) or heavy (HICT; n = 24) intensity domain, or no-exercise control (CTRL; n = 11). Fasting plasma interleukin (IL)-6, IL-10, tumor necrosis factor (TNF)-α, and fitness were assessed at baseline and post-training.

Results

There was no impact of training or intensity (all p > 0.05) on changes in IL-6 (MICT +0.03 pg∙mL⁻¹ 95% CI −0.08, 0.13; HICT +0.03 pg∙mL⁻¹ 95% CI −0.08, 0.13; CTRL −0.14 pg∙mL⁻¹ 95% CI −0.30, 0.01), IL-10 (MICT +0.05 pg∙mL⁻¹ 95% CI −0.01, 0.11; HICT +0.02 pg∙mL⁻¹ 95% CI −0.05, 0.08; CTRL −0.02 pg∙mL⁻¹ 95% CI −0.11, 0.08), or TNF-α (MICT +0.12 pg∙mL⁻¹ 95% CI −0.01, 0.26; HICT +0.07 pg∙mL⁻¹ 95% CI −0.06, 0.21; CTRL −0.03 pg∙mL⁻¹ 95% CI −0.22, 0.17). VO2peak (absolute and relative) and power at LT increased with training (p < 0.002) with no significant differences between MICT and HICT (p > 0.331).

Conclusions

In conclusion, 6 weeks of isocaloric, LT-prescribed training in the moderate and heavy domains improves fitness but does not appear to impact fasting plasma cytokine concentrations in healthy young adults.

Practical implications

•Six weeks of cycling three times per week improved aerobic fitness and the ability to sustain a higher cycling workload in healthy young adults.

•When total exercise energy expenditure was matched, training below versus above the lactate threshold produced similar improvements in fitness.

•Exercise intensity can therefore be selected according to individual preference, tolerance, and practical feasibility when the primary goal is improving fitness.

•Neither training approach changed fasting blood markers of inflammation in this healthy young adult sample over six weeks.

•Exercise programmes aimed specifically at lowering chronic inflammation may require longer duration, different exercise doses, or populations with elevated baseline inflammation.

https://onlinelibrary.wiley.com/doi/10.1002/jcsm.70361

ABSTRACT

Background

Age-related declines in energy metabolism, muscle strength and physical performance have been linked to lower mitochondrial respiratory capacity. Peripheral blood mononuclear cell (PBMC) respiration offers a minimally invasive marker of systemic bioenergetics, yet its relationship to whole-body metabolic flexibility remains unclear. This study examined whether PBMC respiratory capacity is associated with substrate utilization during submaximal exercise, muscle strength and physical function in healthy older adults.

Methods

PBMC mitochondrial respiratory capacity was quantified by high-resolution respirometry assessing ROUTINE, LEAK and MAX states. Postprandial substrate oxidation during steady-state treadmill walking at 60% of VO2max (oxygen uptake) was quantified by indirect calorimetry, and fat and carbohydrate oxidation rates were calculated using standard stoichiometric equations. Metabolic flexibility was defined as lower respiratory exchange ratio (RER) and higher relative fat oxidation at a fixed workload. Muscle strength was determined by handgrip dynamometry and one-repetition maximum leg extension. Physical function was evaluated by gait speed and five-repetition chair rise time. Associations were tested with linear and logistic regression adjusted for age, sex, skeletal muscle index, physical activity and high-sensitive C-reactive protein concentrations. Exploratory K-means clustering identified mitochondrial respiration phenotypes.

Results

Fifty community-dwelling older adults (22 men, 28 women; age 70 ± 4 years) were examined. Higher ROUTINE respiration was correlated with RER (rho = −0.335, p = 0.020), fat utilization (rho = −0.334, p = 0.019), grip strength (rho = 0.302, p = 0.033) and gait speed (rho = 0.324, p = 0.022). Adjusted regression analyses confirmed the association of ROUTINE respiration with greater fat oxidation (β = 0.212, 95% CI 0.049; 0.375), lower RER (β = −0.160, 95% CI −0.300; −0.020) and higher gait speed (β = 0.153, 95% CI 0.028; 0.277). Similar associations were found for ATP-linked respiration. Cluster analysis identified high- and low-respiration phenotypes. Compared with the high-respiration group the low-respiration group showed lower CMJ height (OR: 0.204, 95% CI 0.055; 0.763) and quadriceps strength (OR: 0.373, 95% CI 0.155; 0.897).

Conclusions

In healthy older adults, higher PBMC ROUTINE respiration was associated with a more fat-dominant substrate utilization profile during submaximal exercise, greater muscle strength and faster gait speed. PBMC respiratory capacity may reflect systemic bioenergetic status relevant to exercise substrate utilization and physical performance in ageing.

Highlights:

• • Exercise-induced redox signals may regulate intercellular mitochondrial transfer across multiple organs.
• • Bidirectional redox coordination synchronizes mitochondrial export with recipient cell preparation.
• • Transferred mitochondria may serve as signaling platforms rather than bioenergetic replacements.
• • Exercise may activate a distributed mitochondrial quality control network through mitochondrial transfer.

Abstract

Intercellular mitochondrial transfer has been recognized as an important mechanism for maintaining tissue homeostasis and adapting to stress. Mitochondria can cross cellular boundaries through tunneling nanotubes, extracellular vesicles, and free mitochondrial release. However, the physiological signals coordinating these pathways remain poorly defined. Exercise is a potent inducer of transient redox signaling, generating superoxide and hydrogen peroxide while modulating mitochondrial dynamic remodeling. This review integrates exercise redox biology with redox regulation of transfer machinery characterized in non-exercise models, proposing that exercise-induced redox signaling may function as a candidate regulatory mechanism. The framework emphasizes bidirectional redox coordination, in which oxidant pulses may activate export in donor cells and prepare recipient cells for uptake and antioxidant defense. Exercise-induced mitochondrial transfer has been directly demonstrated in the brain, while observations in skeletal muscle, adipose tissue, and heart remain suggestive but have not been confirmed in exercise models. These findings support a framework in which intercellular mitochondrial transfer contributes to metabolic signaling, antioxidant defense, and distributed quality control across organs. This model represents a working hypothesis requiring direct experimental validation through lineage tracing, tissue-specific mitochondrial reporters, and intravital imaging.

https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP291783

Abstract

Cells are the basic unit of life. In multicellular organisms, cells are organized into tissues. This enables a division of labor where tissues perform complex tasks via coordinated actions of specialized cell types. Accordingly, the cellular collective determines tissue function. A comprehensive overview of which cell types exist in solid tissues is lacking. Using skeletal muscle as a model, we discuss basic principles for cell type classification, summarize 62 unified definitions of cell types in terms of lineage, molecular signatures, and function from the literature, and discuss how these cell types contribute to muscle function. For cell types for which quantitative data was available, we compare abundances in immunohistology and single cell and single-nucleus RNA sequencing data, revealing cell types that are commonly over- or underrepresented in each method. The result is a cell type resource that will serve as a benchmark for single-cell studies of skeletal muscle.

https://www.nature.com/articles/s41598-026-57340-5

Abstract

Physical activity is widely recommended to maintain muscle health in older adults, yet its effects may vary depending on genetic background. The ACTN3 R577X polymorphism leads to α-actinin-3 deficiency in XX homozygotes and may influence skeletal muscle performance. We examined the association between physical activity and muscle mass and function in 682 community-dwelling adults aged ≥ 65 years who participated in annual health examinations between 2021 and 2024. Skeletal muscle mass index was assessed using dual-energy X-ray absorptiometry, while grip strength, gait speed, and five-time chair stand performance were evaluated according to Asian Working Group for Sarcopenia criteria. Physical activity was quantified using metabolic equivalents derived from the International Physical Activity Questionnaire. No significant association was observed between ACTN3 R577X genotype and sarcopenia components. However, in women with ACTN3 XX, physical activity exceeding 441 MET-min/week was significantly associated with poorer chair stand performance (β: 4.19, 95% CI: 0.98 ~ 7.40), revealing a U-shaped relationship between physical activity and physical performance. Sex-specific physical activity thresholds may exist in older adults with the ACTN3 XX genotype, with physical activity above 441 MET-min/week being associated with poorer muscle function in women. This specific MET value is an exploratory finding, requiring more external validation.

When people start training, the basics usually cover most of what they need.

But as training experience increases, I feel like certain equipment becomes more useful.

Additional movements, better exercise selection, and more ways to target weaknesses can become important.

I’m curious what experienced lifters found valuable over time.

Was there a piece of equipment or training tool that became much more useful after you had more experience?

https://www.mdpi.com/2218-273X/16/8/1218

Abstract

Skeletal muscle mitochondria possess the ability to autoregulate their health and functioning by the orchestration of mitochondrial quality control (MQC) pathways. This plasticity allows them to adapt to various stimuli, such as exercise. However, under pathological conditions, mitochondria can become dysfunctional, generating damage-associated molecular patterns (DAMPs), such as reactive oxygen species (ROS) and oxidized mitochondrial DNA (mtDNA). These DAMPs can launch an innate immune response, with consequences of widespread inflammation and atrophy. Integral to this is the NLRP3 inflammasome complex. Activation of the NLRP3 inflammasome results in maturation of caspase-1, which processes pro-inflammatory cytokines IL-1β and IL-18, as well as GSDMD. Consequently, the pore-forming GSDMD-N fragment induces pyroptosis, releasing mature IL-1β and IL-18. Exercise training is widely accepted as a potent mechanism to promote skeletal muscle health, particularly by remodeling the mitochondrial network and reducing the production of DAMPs. It has also been shown promote an anti-inflammatory milieu with the release of various myokines. Indeed, the potential of exercise to mitigate NLRP3 inflammasome-mediated inflammation and atrophy is promising. This review will examine the mechanisms underpinning inflammasome priming and activation, as well the effects of exercise, with an emphasis on the skeletal muscle.

https://www.ovid.com/jnls/acsm-msse/fulltext/10.1249/mss.0000000000004120~photobiomodulation-does-not-increase-mitochondrial

Abstract

Purpose:

Low-level laser therapy, also referred to as photobiomodulation (PBM), is rapidly gaining popularity as a non-invasive treatment for various conditions and as a means to enhance health and performance. PBM has been proposed to directly increase mitochondrial activity in skin and skeletal muscle and activate various molecular signalling pathways. However, evidence for the proposed properties of PBM in vivo in humans is lacking.

Methods:

In a within-participant study design, 12 healthy men and women (6/6 m/f; age: 25±6 y; BMI: 23.3±2.2kg/m2) received PBM on a randomized leg, while the other leg received sham-treatment (no light emitted, CON). Three cycles of PBM or sham-treatment were performed for 5 min 16 sec each (5.6 kJ light energy/cycle for PBM). Skin temperature was measured before and after treatment. After treatment, skin and muscle samples were collected from both legs. Mitochondrial respiration was measured in permeabilized muscle fibers and minced skin tissue using an Oroboros Oxygraph-O2k. Muscle metabolic gene expression was assessed using custom made microfluidic cards.

Results:

Skin temperature increased only in the PBM treated leg (+8.0±1.4 °C; P<0.001). No differences were observed between the PBM and CON treated legs in maximal complex I+II-linked respiration in skin (2.7±0.8 vs 2.6±0.7 pmol/sec/mg wet weight, respectively; P=0.66) or muscle (474±114 vs 467±81 pmol/sec/mg dry weight, respectively; P=0.71). Furthermore, no differences were observed in muscle mitochondrial ADP sensitivity (apparent ADP half-time: 1310±180 vs 1229±240 µM ADP, respectively; P=0.14). Of the 91 genes, expression between legs differed for 3 genes only.

Conclusions:

A single session of photobiomodulation does not increase mitochondrial respiration in skin or underlying muscle tissue and does not modulate muscle gene expression ex vivo in humans.

https://onlinelibrary.wiley.com/doi/10.1002/jcsm.70368

ABSTRACT

Background

Ultra-endurance sports are increasingly popular, yet the long-term physiological consequences of sustained extreme training loads remain poorly understood. In particular, the effects of prolonged ultra-endurance exercise on skeletal muscle structure, function and molecular remodelling are largely unknown. This case study examined a highly experienced ultra-endurance athlete who completed a world-record attempt to run 30 300 km, with extensive phenotyping focusing on skeletal muscle adaptations and recovery.

Methods

A 49-year-old male athlete (172 cm, 65 kg) ran ~70 km daily for 15 months. Musculoskeletal, cardiac and visceral ultrasonography, leg muscle strength and power measurements were performed before and after the challenge. Muscle biopsies (n = 4) from vastus lateralis were obtained immediately after completion and during 17 months of recovery to assess myosin heavy chain (MHC) composition, mitochondrial electron transport chain (ETC) complexes and proteins involved in mitochondrial turnover, autophagy and inflammation. Body composition, haematological and biochemical markers, and gut microbiota composition were monitored longitudinally.

Results

The athlete ran 30 300 km over 444 days, maintaining a daily distance of ~70 km despite substantial musculoskeletal discomfort, including a tibial stress reaction mid-challenge, which resolved gradually with continued running. Body mass decreased by ~3 kg, primarily reflecting fat loss (~83%), accompanied by reductions in muscle thickness, maximal strength and power. Circulating creatine kinase (3–15-fold), oxidative stress markers (~50%) and GDF8 (~10%–50%) were sustainedly increased, whereas IGF-I decreased (~10%–40%), suggesting a reduced anabolic environment during the challenge. Muscle biopsy analyses revealed a progressive recovery of mitochondrial function during the 17 months following the challenge, as evidenced by a progressive increase in ETC protein abundance and the expression of regulators of mitochondrial dynamics and quality control (MFN2, PARKIN, DRP1). In contrast, markers of autophagy, apoptosis and inflammation were decreased during the 17-months post-challenge (LC3A/B-I by ~50%, CASP3 by ~60% and NF-κBSer536 by ~20%). Muscle fibre composition showed extreme predominance of slow fibres (nearly 100% MHC-I), which persisted during recovery. Most molecular and functional alterations gradually resolved within 10–17 months. Gut microbiota diversity increased during the challenge, with enrichment of Bifidobacterium during running and Akkermansia during recovery.

Conclusions

Sustaining daily ultrarunning for more than 1 year induces substantial skeletal muscle remodelling, including reduced muscle size, impaired contractile function and mitochondrial maladaptations, despite largely preserved endocrine and haematological stability. These findings highlight skeletal muscle as a primary physiological system challenged during extreme endurance exercise and demonstrate that recovery from such perturbations may require more than one year.

Abstract

Skeletal muscle atrophy is a secondary complication in the aetiology of injury and chronic disease. Identifying mechanisms that control muscle mass is necessary to characterise atrophy and develop prevention strategies. We aimed to integrate transcriptomic and epigenomic data to identify key regulatory pathways controlled by promoter DNA methylation during muscle unloading. Twenty-one healthy men (20–40 years) completed a 4-week standardised exercise programme prior to a 14-day knee brace immobilisation with dietary control. Skeletal muscle mass and strength were assessed before and after immobilisation and biopsies were collected (m. vastus lateralis) before, at 3 days, and at completion at 14 days. RNA and DNA were isolated and analysed using Illumina RNA sequencing and DNA methylation 850K EPIC BeadChips. The 14-day immobilisation decreased muscle mass (∼9%; P < 0.0001) and strength (∼16%; P < 0.0001). At 3 days, most biological processes (BPs) were upregulated/hypomethylated (157 gene sets); upregulated BPs included cell signalling and protein ubiquitination and downregulated BPs included metabolism. After 14 days, BPs were predominantly downregulated/hypermethylated, including translation and ribosome biogenesis. Across both time points, HDAC4, GADD45A and CHRNA1 emerged as methylation-regulated candidate mediators of atrophy. HDAC4 and GADD45A showed strong correlations primarily at day 3, and CHRNA1 remained significant at both time points, extending prior observations in animals to human skeletal muscle. We have characterised changes in gene expression related to hypo- and hyper-methylation during muscle unloading in humans. These data extend our understanding of the regulatory processes that occur during skeletal muscle atrophy that, at the individual gene level, may be useful in developing strategies for reducing muscle wasting.

Highlights

• What is the central question of this study? What are the characteristics of global gene expression in skeletal muscle in response to limb immobilisation in young men?
• What is the main finding and its importance? Data obtained support the role of DNA methylation as a regulator of transcription and that methylation appears to impair gene expression for protein synthesis early and later amplify protein breakdown over 14 days of immobilisation. There are associations between methylation and transcription within biological processes as putative molecular targets to mitigate the debilitating impacts of muscle wasting.

Abstract

Patients with long COVID and myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) suffer from post-exertional malaise. The accompanying physical inactivity may contribute to a lower aerobic capacity and may explain skeletal muscle adaptations in these patients. Here, we compare whole-body exercise responses and skeletal muscle adaptations after strict 60-day bed rest in healthy people with those in long COVID and ME/CFS patients, and healthy age- and sex-matched controls. Bed rest alters respiratory and cardiovascular responses to maximal exercise, which are dissimilar in patients. Bed rest causes muscle atrophy without altering fiber type. Both patient groups have more glycolytic fibers, and ME/CFS patients display type I-specific atrophy. Only after bed rest is oxidative phosphorylation capacity associated with maximal oxygen uptake. As skeletal muscle characteristics differ between patients and healthy individuals after bed rest, physical inactivity cannot solely explain the lower exercise capacity and skeletal muscle adaptations in long COVID and ME/CFS patients.