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.

SARS-CoV-2 infections have led to a surge in long COVID, a post-infectious syndrome in which autoantibodies are proposed to play a pathogenic role, analogous to fibromyalgia. Here, we test this hypothesis by transferring total IgG from long COVID patients into mice. We stratified patients into three subgroups using plasma levels of glial fibrillary acidic protein (GFAP), neurofilament light chain (NFL), and interferon-β, with subgroup-specific pathways supported by plasma proteomics. Transfer of pooled total IgG induces pronounced and persistent mechanical hypersensitivity. Notably, IgG collected 2 years later from the same long COVID patients who remained symptomatic reproduced mechanical allodynia in mice, demonstrating longitudinal stability of pathogenic activity. Proteome-wide autoantibody profiling identifies elevated, subgroup-linked autoreactivities that persist over time and are validated by independent assays. Together, these findings demonstrate that long COVID IgG can induce mechanical hypersensitivity in mice, support a causal role for autoantibodies in long COVID pathogenesis, and may establish a murine model for therapeutic development.

The assessment of skeletal muscle volume is valuable for fundamental research and clinical practice, but remains limited in larger cohorts due to its time-consuming nature. Here, we developed a method to accurately estimate vastus lateralis (VL) muscle volume based on a single measurement of anatomical cross-sectional area (ACSA) or tissue thickness. Sixty-nine healthy participants (20–91 years) volunteered. In a subgroup (n = 34) we measured VL volume and ACSAs at 10% intervals along the muscle length to derive a VL muscle shape factor. We subsequently estimated VL volume by multiplying this muscle shape factor with muscle length and a single measure of ACSA at 50% muscle length (ACSAVL50%) or an estimated ACSAVL50% from a single ultrasound scan of tissue thickness in an independent cohort (n = 35). VL muscle shape factor was determined by integrating a fourth-order polynomial of muscle length and ACSA, and was dependent on muscle size. Estimating muscle volume had a high accuracy (R²=0.976, CCC = 0.987), low bias and error (< 8.5%) in both the main cohort and an independent validation group. Estimating muscle volume from stitching 2D images at 50% muscle length or estimating ACSA with a geometric model explained 91–95% of variance in measured volumes, with high accuracy and concordance correlation coefficients. VL muscle volume can be estimated by multiplying a muscle shape factor with muscle length and ACSAVL50% from a single ultrasound image. We present a novel, cost-effective, rapid, yet accurate assessment of VL muscle mass for (large-scale) studies and clinical practice.

Previous studies reporting lower skeletal muscle mitochondrial function in type 1 diabetes did not account for cardiorespiratory fitness, a key confounder when assessing mitochondrial function. We hypothesised that, compared with healthy individuals, muscle mitochondrial phenotypic differences would be abolished in individuals with type 1 diabetes when matched for age, sex, BMI and maximal oxygen uptake ( V̇O_2max ). Seventeen individuals with type 1 diabetes and seventeen healthy control individuals matched for age, sex, BMI and V̇O_2max participated and underwent a muscle biopsy from the vastus lateralis. Mitochondrial respiration was assessed by high-resolution respirometry, and mitochondrial density and morphology were assessed by transmission electron microscopy. V̇O_2max (individuals with type 1 diabetes 40±10 kg−1 min−1; control individuals 41±8 ml kg−1 min−1; p=0.51) and mitochondrial oxidative phosphorylation capacity (individuals with type 1 diabetes 101±35 [pmol O2] s−1 mg−1; control individuals 99±23 [pmol O2] s−1 mg−1, p=0.82) did not differ between groups. Both intermyofibrillar (individuals with type 1 diabetes 6.07±2.16 V̇O_2max , maximal muscle mitochondrial respiration and morphology in people with type 1 diabetes are not impaired. These findings highlight the importance of habitual exercise, optimal glucose management and a healthy BMI in maintaining mitochondrial health in individuals with type 1 diabetes.

Despite exercise countermeasures to sustain health and performance in spaceflight, complete maintenance of muscle mass and functions in microgravity is still not possible for most astronauts. The principal cause of the limited effectiveness of existing exercise countermeasures is the difficulty in achieving full loading forces in space. The implementation of countermeasures which require small devices and simulate Earth‐like loading forces to maintain muscle mass, strength and endurance is therefore highly desirable. At present, the cellular mechanisms that induce muscle atrophy in weightlessness are not yet fully known; a better understanding of how skeletal muscle cells adapt to microgravity will help in designing more effective countermeasures to sustain the health and operational capacity of the crew during long‐ and short‐duration missions. The 6° head‐down‐tilt bed rest is a powerful ground‐based analogue platform to simulate and study the physiological effects of spaceflight on the human body, and test the effectiveness of countermeasures before they are potentially applied in space. The aims of this narrative review are therefore to provide an overview of (i) the main mechanisms underlining muscle atrophy learnt from space and bed rest studies, (ii) the currently available countermeasures, and (iii) potential suitable countermeasures – such as neuromuscular electrical stimulation that is delivered with light and small portable units – to attenuate muscle wasting in astronauts during spaceflight.

Ageing substantially impairs skeletal muscle metabolic and physical function. Skeletal muscle mitochondrial health is also impaired with ageing, but the role of skeletal muscle mitochondrial fragmentation in age-related functional decline remains imprecisely characterized. Here, using a cross-sectional study design, we performed a detailed comparison of skeletal muscle mitochondrial characteristics in relation to in vivo markers of exercise capacity between young and middle-aged individuals. Despite similar overall oxidative phosphorylation capacity (young: 99 ± 17 vs. middle-aged: 99 ± 27 pmol O2.s-1.mg-1, p = 0.95) and intermyofibrillar mitochondrial density (young: 5.86 ± 0.57 vs. middle-aged: 5.68 ± 1.48%, p = 0.25), older participants displayed a more fragmented intermyofibrillar mitochondrial network (young: 1.15 ± 0.17 vs. middle-aged: 1.55 ± 0.15 A.U., p < 0.0001), a lower mitochondrial cristae density (young: 23.40 ± 7.12 vs. middle-aged: 13.55 ± 4.10%, p = 0.002) and a reduced subsarcolemmal mitochondrial density (young: 22.39 ± 6.50 vs. middle-aged: 13.92 ± 4.95%, p = 0.005). Linear regression analysis showed that 87% of the variance associated with maximal oxygen uptake could be explained by skeletal muscle mitochondrial fragmentation and cristae density alone, whereas subsarcolemmal mitochondrial density was positively associated with the capacity for oxygen extraction during exercise. Intramuscular lipid accumulation was positively associated with mitochondrial fragmentation and negatively associated with cristae density. Collectively, our work highlights the critical role of skeletal muscle mitochondria in age-associated declines in physical function.

When acute SARS-CoV-2 infections cause symptoms that persist longer than 3 months, this condition is termed long COVID. Symptoms experienced by patients often include myalgia, fatigue, brain fog, cognitive impairments, and post-exertional malaise (PEM), which is the worsening of symptoms following mental or physical exertion. There is little consensus on the pathophysiology of exercise-induced PEM and skeletal-muscle-related symptoms. In this opinion article we highlight intrinsic mitochondrial dysfunction, endothelial abnormalities, and a muscle fiber type shift towards a more glycolytic phenotype as main contributors to the reduced exercise capacity in long COVID. The mechanistic trigger for physical exercise to induce PEM is unknown, but rapid skeletal muscle tissue damage and intramuscular infiltration of immune cells contribute to PEM-related symptoms.

AIMS:Active cigarette smoking is a major risk factor for chronic obstructive pulmonary disease that remains elevated after cessation. Skeletal muscle dysfunction has been well documented after smoking, but little is known about cardiac adaptations to cigarette smoking. The underlying cellular and molecular cardiac adaptations, independent of confounding lifestyle factors, and time course of reversibility by smoking cessation remain unclear. We hypothesized that smoking negatively affects cardiac metabolism and induces local inflammation in mice, which do not readily reverse upon 2-week smoking cessation. METHODS:Mice were exposed to air or cigarette smoke for 14 weeks with or without 1- or 2-week smoke cessation. We measured cardiac mitochondrial respiration by high-resolution respirometry, cardiac mitochondrial density, abundance of mitochondrial supercomplexes by electrophoresis, and capillarization, fibrosis, and macrophage infiltration by immunohistology, and performed cardiac metabolome and lipidome analysis by mass spectrometry. RESULTS:Mitochondrial protein, supercomplex content, and respiration (all p < 0.03) were lower after smoking, which were largely reversed within 2-week smoking cessation. Metabolome and lipidome analyses revealed alterations in mitochondrial metabolism, a shift from fatty acid to glucose metabolism, which did not revert to control upon smoking cessation. Capillary density was not different after smoking but increased after smoking cessation (p = 0.02). Macrophage infiltration and fibrosis (p < 0.04) were higher after smoking but did not revert to control upon smoking cessation. CONCLUSIONS:While cigarette-impaired smoking-induced cardiac mitochondrial function was reversed by smoking cessation, the remaining fibrosis and macrophage infiltration may contribute to the increased risk of cardiovascular events after smoking cessation.

Insulin sensitivity and metabolic flexibility decrease in response to bed rest, but the temporal and causal adaptations in human skeletal muscle metabolism are not fully defined. Here, we use an integrative approach to assess human skeletal muscle metabolism during bed rest and provide a multi-system analysis of how skeletal muscle and the circulatory system adapt to short- and long-term bed rest (German Clinical Trials: DRKS00015677). We uncover that intracellular glycogen accumulation after short-term bed rest accompanies a rapid reduction in systemic insulin sensitivity and less GLUT4 localization at the muscle cell membrane, preventing further intracellular glycogen deposition after long-term bed rest. We provide evidence of a temporal link between the accumulation of intracellular triglycerides, lipotoxic ceramides, and sphingomyelins and an altered skeletal muscle mitochondrial structure and function after long-term bed rest. An intracellular nutrient overload therefore represents a crucial determinant for rapid skeletal muscle insulin insensitivity and mitochondrial alterations after prolonged bed rest.

A subgroup of patients infected with SARS-CoV-2 remain symptomatic over three months after infection. A distinctive symptom of patients with long COVID is post-exertional malaise, which is associated with a worsening of fatigue- and pain-related symptoms after acute mental or physical exercise, but its underlying pathophysiology is unclear. With this longitudinal case-control study (NCT05225688), we provide new insights into the pathophysiology of post-exertional malaise in patients with long COVID. We show that skeletal muscle structure is associated with a lower exercise capacity in patients, and local and systemic metabolic disturbances, severe exercise-induced myopathy and tissue infiltration of amyloid-containing deposits in skeletal muscles of patients with long COVID worsen after induction of post-exertional malaise. This study highlights novel pathways that help to understand the pathophysiology of post-exertional malaise in patients suffering from long COVID and other post-infectious diseases.

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Dutch CardioVascular Alliance (an initiative with support of the Dutch Heart Foundation) Grant 2020B008 RECONNEXT, and the German Center for Cardiovascular Research (DZHK; 81Z0600207) Heart failure with preserved ejection fraction lacks targeted therapies, due to insufficient understanding of pathogenesis. A multiple comorbidity swine model (MCS) was developed by exposure to three risk factors for six months including diabetes (streptozotocin), chronic kidney disease (renal embolization), and an unhealthy diet (high-fat, high salt diet). An additional risk factor ovariectomy was added on top of the MCS model (MCS+). MCS and MCS+ presented with left ventricular diastolic dysfunction, oxidative stress, coronary microvascular dysfunction and inflammation. Furthermore, myocardial oxygen consumption was increased given the same level of cardiac work (Fig. A), suggesting an impairment in myocardial mitochondrial function. Using a combined-omics approach, the purpose of this study is to unravel alterations in gene expression and proteome abundance regarding mitochondrial function in MCS swine. 15 MCS, 4 MCS+ and 10 healthy control female swine were included in the study. Proteome analysis and single nuclei RNA sequencing, were performed on frozen MCS left ventricle myocardial samples. Cardiac mitochondrial function was measured by an O2k-FlouRespirometer ex vivo in MCS+ swine versus control. Proteome analysis showed a reduced abundance of proteins involved in branched-chain amino acid (BCAA) catabolism (Fig. B). Single nuclei RNA sequencing of a subgroup of the same animals demonstrated a downregulation of these genes specifically in cardiomyocyte subpopulations. BCAA can act as a source of ATP via oxidative phosphorylation and can modulate mitochondrial substrate utilisation, and impair mitochondrial function, resulting in increased mitochondrial production of reactive oxygen species (ROS). Mitochondrial function was measured in fresh myocardial tissue, using pyruvate/malate/glutamate as substrate. Low mitochondrial respiratory sensitivity to ADP was observed 26±2.5 in MCS+ vs 43.6±1.5 pmol/(s*mg tissue) in healthy control, reflecting low oxidative capacity. Moreover, the respiratory rate after uncoupling by FCCP was reduced in MCS+ versus healthy control (73.6±11.7 vs 118.4±15.6 pmol/(s*mg tissue). These findings are consistent with our data in the MCS swine and indicate impaired BCAA in the myocardium of animals with comorbidities, which is associated with increased ROS levels (8-isoprostane 12.9±0.8 pg/mg protein in MCS vs 10.3±0.5 in healthy animals) and impaired myocardial efficiency during exercise (Fig. A). A combined omics approach suggested an alteration in cardiac mitochondria. Respiratory analyses showed impaired mitochondrial bioenergetics and increased oxidative stress, which can contribute to diastolic dysfunction.

Intramuscular injection of botulinum neurotoxin type A (BoNT-A) is commonly used to improve or maintain the joint range of motion in young children with spasticity. However, the effectiveness of BoNT-A treatment is variable and movement limitations are recurrent. Here we show long-term effects of a single, bilateral abobotulinumtoxinA (aboBoNT-A) injection in the gastrocnemius medialis and soleus muscles of wild-type and spastic (B6.Cg-Glrbspa/J with a mutation in the glycine receptor) mice at a young age (6-7 days). Specifically, we evaluated the impact of aboBoNT-A-A on gait, physical performance, and spontaneous physical behavior, as well as on contractile force characteristics, morphology, and histological phenotype of soleus and gastrocnemius muscles by comparing their results to those of saline-injected controls up to 9 weeks after the injection. The detailed time course of the study specifies the timing of the aboBoNT-A injection at 1 week, the period of behavioral studies from 4-9 weeks, and the age of the mice (10 weeks) at the time of contractile force characteristics and histology assessments. In spastic mice, aboBoNT-A injection had a minor and very specific effect on physical performance, by only modestly increasing stride length as a function of age. aboBoNT-A injection caused a reduction in the force-generating capacity and a slightly smaller physiological cross-sectional area in gastrocnemius medialis, but not in soleus. Reduced physiological cross-sectional area in aboBoNT-A-injected muscles was due to a lower number of muscle fibers, rather than reduced muscle fiber cross-sectional area. The percentage of slow-type muscle fibers and mitochondrial succinate dehydrogenase activity were increased, which was associated with an improved muscle endurance capacity. In conclusion, aboBoNT-A injection reduced the number of muscle fibers, causing muscle hypertrophy in remaining fibers and a shift towards more oxidative fibers, resulting in an improved endurance capacity and gait. This study proposed potential cellular mechanisms for the therapeutic efficacy of aboBoNT-A in spasticity.

Patients with post- COVID condition suffer from fatigue, limited exercise capacity, and post-exertional malaise. Post-exertional malaise is the worsening of symptoms after physical or mental exertion, which reduces the efficacy of most forms of rehabilitation. This article presents the current understanding in the pathophysiology of post-COVID condition, particularly the underlying causes of post-exertional malaise.

Many sports require maximal strength and endurance performance. Concurrent strength and endurance training can lead to suboptimal training adaptations. However, how adaptations differ between males and females is currently unknown. Additionally, current training status may affect training adaptations. We aimed to assess sex-specific differences in adaptations in strength, power, muscle hypertrophy, and maximal oxygen consumption ( V̇ O2max) to concurrent strength and endurance training in healthy adults. Second, we investigated how training adaptations are influenced by strength and endurance training status. A systematic review and meta-analysis was conducted according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, and a Cochrane risk of bias was evaluated. ISI Web of science, PubMed/MEDLINE, and SPORTDiscus databases were searched using the following inclusion criteria: healthy adults aged 18–50 years, intervention period of ≥ 4 weeks, and outcome measures were defined as upper- and lower-body strength, power, hypertrophy, and/or V̇ O2max. A meta-analysis was performed using a random-effects model and reported in standardized mean differences. In total, 59 studies with 1346 participants were included. Concurrent training showed blunted lower-body strength adaptations in males, but not in females (male: − 0.43, 95 V̇ O2max (P = 0.13). Data on muscle hypertrophy were insufficient to draw any conclusions. For training status, untrained but not trained or highly trained endurance athletes displayed lower V̇ O2max gains with concurrent training (P = 0.04). For other outcomes, no differences were found between untrained and trained individuals, both for strength and endurance training status. Concurrent training results in small interference for lower-body strength adaptations in males, but not in females. Untrained, but not trained or highly trained endurance athletes demonstrated impaired improvements in V̇ O2max following concurrent training. More studies on females and highly strength-trained and endurance-trained athletes are warranted. PROSPERO: CRD42022370894.

Background Mobility is a key determinant and outcome of healthy ageing but its definition, conceptual framework and underlying constructs within the physical domain may need clarification for data comparison and sharing in ageing research. This study aimed to (1) review definitions and conceptual frameworks of mobility, (2) explore agreement on the definition of mobility, conceptual frameworks, constructs and measures of mobility, and (3) define, classify and identify constructs. Methods A three-step approach was adopted: a literature review and two rounds of expert questionnaires (n = 64, n = 31, respectively). Agreement on statements was assessed using a five-point Likert scale; the answer options ‘strongly agree’ or ‘agree’ were combined. The percentage of respondents was subsequently used to classify agreements for each statement as: strong (≥ 80%), moderate (≥ 70% and < 80%) and low (< 70%). Results A variety of definitions of mobility, conceptual frameworks and constructs were found in the literature and among respondents. Strong agreement was found on defining mobility as the ability to move, including the use of assistive devices. Multiple constructs and measures were identified, but low agreements and variability were found on definitions, classifications and identification of constructs. Strong agreements were found on defining physical capacity (what a person is maximally capable of, ‘can do’) and performance (what a person actually does in their daily life, ‘do’) as key constructs of mobility. Conclusion Agreements on definitions of mobility, physical capacity and performance were found, but constructs of mobility need to be further identified, defined and classified appropriately. Clear terminology and definitions are essential to facilitate communication and interpretation in operationalising the physical domain of mobility as a prerequisite for standardisation of mobility measures.

The observation that prior heavy or severe-intensity exercise speeds overall oxygen uptake ( V̇ O2) kinetics, termed the “priming effect”, has garnered significant research attention and its underpinning mechanisms have been hotly debated. In the first part of this review, the evidence for and against (1) lactic acidosis, (2) increased muscle temperature, (3) O2 delivery, (4) altered motor unit recruitment patterns and (5) enhanced intracellular O2 utilisation in underpinning the priming effect is discussed. Lactic acidosis and increased muscle temperature are most likely not key determinants of the priming effect. Whilst priming increases muscle O2 delivery, many studies have demonstrated that an increased muscle O2 delivery is not a prerequisite for the priming effect. Motor unit recruitment patterns are altered by prior exercise, and these alterations are consistent with some of the observed changes in V̇ O2 kinetics in humans. Enhancements in intracellular O2 utilisation likely play a central role in mediating the priming effect, probably related to elevated mitochondrial calcium levels and parallel activation of mitochondrial enzymes at the onset of the second bout. In the latter portion of the review, the implications of priming on the parameters of the power–duration relationship are discussed. The effect of priming on subsequent endurance performance depends critically upon which phases of the V̇ O2 response are altered. A reduced V̇ O2 slow component or increased fundamental phase amplitude tend to increase the work performable above critical power (i.e. W´), whereas a reduction in the fundamental phase time constant following priming results in an increased critical power.