Introduction: There is great interest about sex differences in neuromuscular function and overall performance but sex-specific responses to bedrest are very limited. Young males showed larger reductions in muscle force after short (10 days) bedrest compared to females, despite a similar loss of muscle mass. Conversely, young females lost greater muscle volume than males after longer (more than 30 days) bedrest. The aim of the present study was to compare sex-specific responses in muscle function after 21 days of head-down bedrest. Methods: Nine young (18–36 years) males (9 YM) and eight young females (8 YF) underwent 21 days of head-down (-6°) bedrest. Before (BDC) and after (BR21), the torque-velocity-power relationship was determined from isokinetic (50-150-250-350°/sec) and unloaded knee extension maximal contractions. Maximum power (Pmax) and maximum unloaded velocity (V0) were calculated. Moreover, optimal torque (Topt) and velocity (Vopt) were identified at the apex of the power-velocity relationship. Maximal voluntary isometric toque (MVIT) and resting single twitch (St) evoked by femoral nerve stimulation were also performed. Vastus lateralis (VL) cross-sectional area (CSA) at 50% of femur length was evaluated by panoramic ultrasound. Statistical differences (p < 0.05) were tested by two-way ANOVA with post hoc analysis (main effects: time or sex; interaction: time x sex). Results: At BDC, Pmax was higher in YM (559±173 W) then YF (319±58 W; p< 0.001). At BR21, Pmax and V0 decreased in both YM (-18±9 % for Pmax) and YF (-17±16 % for Pmax; p=0.0018). At BDC, Topt was higher in YM (92±24 Nm) than in YF (54±9 Nm; p< 0.001) but it decreased only in YM at BR21 (-20±12 %; p=0.047). No time or sex effects were observed for Vopt. At BDC, MVIT was higher in YM (251±63Nm) compared to YF (172±36Nm; p=0.001), showing similar decrement at BR21 (-23±10 % and -27±10 %, in YM and YF respectively; p< 0.001). St showed time effect and time x sex interaction, decreasing only in YM at BR21 (-9 %; p=0.007). Interestingly, St/MCIT ratio showed a time x sex interaction (p=0.01). At BR21, VL CSA was lower in both YM (-12 %; p< 0.001) and YF (-13 %; p< 0.001). Conclusion: Overall, knee extension muscle force, power and unloaded velocity decreased similarly in young males and young females after 21 days of head-down bedrest. This reduction was partially due to loss of muscle size. The sex-specific changes in torque at the apex of the power-velocity relationship, resting twitch torque, and resting twitch – MVIT ratio seem to suggest a slightly greater impairment at skeletal muscle level in young males compared to young females.To establish whether disparities in fibre type proportions or intrinsic mechanical proprieties of single muscle fibres could explain such sex-specific differences, immunohistochemical analysis alongside force and unloaded velocity skinned muscle fibres experiments are in progress. Funding: The study belongs to the “STOP MUSCLE ATROPHY” project which was supported by ASI (Contract n. 2025-6-HB.0). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

BACKGROUND:Heart failure with reduced ejection fraction (HFrEF) affects millions worldwide and is characterized by chronic cardiac dysfunction, impaired perfusion, altered skeletal muscle energetics, and, thus, exercise intolerance. Efficient therapeutic strategies reducing the burden of the impaired muscle metabolism in HFrEF are currently lacking. Hence, in the present study, we sought to determine whether myosin dynamics and its important role in ATP consumption can constitute a potent biochemical target to optimize skeletal muscle energy usage in HFrEF. METHODS:We used skeletal muscle tissue from 11 human patients with HFrEF and 10 controls with comparable age, sex, and body mass index. We isolated individual myofibres and incubated them ex vivo with varying concentrations of a myosin inhibitor, mavacamten. We then performed 2'-(or-3')-O-(N-Methylanthraniloyl) adenosine 5'-triphosphate chase experiments, together with LC/MS-based proteomics profiling. RESULTS:We observed a distinct regulation of acetyl-lysine sites and higher myosin energy consumption in resting muscle fibers from patients with HFrEF than in controls. When exposed to mavacamten, we found a dose-dependent reduction in myosin ATP consumption in myofibres of patients with HFrEF, reversing the pathological over-consumption. CONCLUSIONS:Skeletal muscle myosin becomes inefficient in HFrEF. Pharmacological inhibition of myosin ATPase activity offers an inventive strategy to lower muscle energy demand and potentially address metabolic disturbances in HFrEF.

A 91-yr-old female set the W90+ world record in the 200-m sprint in 2024, surpassing her previous record by 1.13 s. This study characterized her cardiorespiratory fitness, skeletal muscle oxidative capacity, fiber-type distribution, capillarization, and satellite cells and compared these outcomes, where possible, to published reference data. Cardiorespiratory responses were assessed during a ramp cycling test to exhaustion, and muscle oxidative capacity (mV̇o2) was determined using near-infrared spectroscopy during repetitive arterial occlusions. Fiber-type distribution, cross-sectional area, capillarization, satellite cell content and localization, and mitochondrial respiratory capacity were analyzed in a vastus lateralis biopsy. Peak oxygen uptake (V̇o2peak) was 23 mL·kg-1·min-1 at 98 W, with a maximal heart rate of 141 beats·min-1 and cardiac output of 13.6 L·min-1. The mV̇o2 recovery rate constant (k) was 1.83 min-1. Fiber composition was 57% fast MyHC II fibers (50% IIa, 5% IIa-IIx, and 2% IIx), 38% slow MyHC I fibers, and 5% hybrid I-IIa fibers. MyHC I fibers were larger, more vascularized, and had satellite cells located closer to capillaries [4,267 ± 2,181 μm2; 1.67 individual capillary-to-fiber ratio (C/Fi), 5.58 capillaries × 1,000 μm-1 capillary-to-fiber perimeter exchange index (CFPE), 1.8-µm satellite cell-to-capillary distance] than MyHC II fibers (2,752 ± 1,608 μm2; 1.03 C/Fi, 4.00 capillaries × 1,000 μm-1 CFPE and 10.4 µm, respectively). Mitochondrial O2 flux was 58 and 68 pmol·(s·mg)-1 during coupled and uncoupled respiration, respectively. The athlete's cardiorespiratory and oxidative capacity resembled those of females in their 50s or younger. Collectively, her large, well-vascularized slow fibers, high proportion of fast fibers, and preserved muscle oxidative capacity likely contributed to her world record performance, illustrating the remarkable plasticity of skeletal muscle even in very advanced age.NEW & NOTEWORTHY We report an in vivo and ex vivo characterization of cardiorespiratory fitness and muscle oxidative capacity in a 91-yr-old female who holds the W90+ 200-m world record. She exhibited a high muscle oxidative capacity and a muscle fiber profile with large, well-vascularized slow fibers and a relatively high proportion of fast fibers. These findings suggest that components of the oxygen transport and utilization system can be well-preserved into very advanced age in a lifelong sprinter.

A 91-yr-old female set the W90+ world record in the 200 m sprint in 2024. We characterized her neuromuscular function, fatigability, denervation markers, and single-fiber contractile properties, and, where possible, compared these outcomes with published reference data. Knee extensor muscle architecture, force, power, fatigability, and motor unit (MU) behavior were assessed in vivo, and a vastus lateralis biopsy was analyzed for single-fiber contractile properties and markers of denervation. Quadriceps cross-sectional area was 36.5 cm2, with fascicle length, pennation angle, and muscle thickness of 6.7 cm, 11.6°, and 1.4 cm, respectively. Knee extensor maximal voluntary isometric contraction (MVIC) torque was 79 Nm, power was 28 W, and power declined by 43% during a 4-min fatiguing task. MU recruitment threshold was 12 ± 11% of MVIC. Single-fiber absolute and specific force were 0.88 ± 0.19 mN and 114 ± 10 kN/m2 for myosin heavy chain (MyHC) I and 0.74 ± 0.23 mN and 171 ± 23 kN/m2 for MyHC IIa fibers. Less than 1% of fibers were positive for the denervation marker neural cell adhesion molecule (NCAM), no fibers expressed embryonic MyHC, and ∼1% of MyHC II fibers expressed neonatal MyHC. In vivo muscle size, force, power, and fatigability were largely within the range for individuals one decade younger. MU behavior was comparable with individuals 20 yr younger. MyHC I fibers produced greater absolute force than reported in untrained young females and females in their eighth decade, whereas MyHC II fiber force was lower because of smaller fiber size. Denervation markers were less prevalent than typically reported at this age. Collectively, these findings illustrate the heterogeneous effects of aging on different features of neuromuscular function, even in an elite older female sprint athlete.NEW & NOTEWORTHY We report in vivo neuromuscular function, single-fiber mechanics, and denervation markers in a 91-yr-old female who holds the W90+ 200 m world record. Despite reduced muscle size, force, and power, as well as pronounced fast-fiber atrophy, markers of denervation were similar to those in young females and single-fiber force in slow fibers was preserved. Together, these data highlight the differential effects of lifelong sprint training and aging on neural versus muscular components of the neuromuscular system.

Introduction: A91-year-old female set the W90+ world record (WRW) in the 200m sprint in May 2024, with a time of 51.47s, which was 1.48s faster than the previous record. Purpose: The aim of this study was to examine the in vivo and ex vivo physiological profile underpinning the elite performance of the 91-year-old sprinter and compare, when possible, her data to the literature. Methods: Cardiorespiratory parameters were measured during a ramp cycling test until exhaustion, and muscle oxidative capacity (mVO 2 ) measured with a repetitive intermittent arterial occlusion test via NIRS. The cross-sectional area (CSA) of the knee extensors was evaluated with panoramic ultrasound. Fatigability of the knee extensors was quantified as the reduction in power during an exercise lifting a 20% maximal voluntary isometric contraction (MVIC) load as fast as possible once every 3s for 4-min. Fiber type distribution, fiber CSA and mitochondrial function were evaluated in a vastus lateralis (VL) muscle biopsy. Results: VO 2peak was 23 ml·kg −1 ·min −1 (1.332 L·min −1 ) achieved at 98 W peak power, heart rate of 141 bpm, cardiac output of 13.6 L/min. Herk-value from the mVO 2 test was 1.83 %·s −1 . CSA at 50% femur length was 36.5 cm 2 . Knee extensor MVIC was 79 Nm, and the power reduction during the exercise was 43%. VL fiber type distribution was 38% myosin heavy chain (MyHC) I, 49% IIa, 5% I-IIa, 5% IIa-IIx and 2% IIx. Interestingly, the slow fibers (4267 ± 2181 μm 2 ) were markedly larger than the fast MyHC II fibers (2752 ± 1608 μm 2 , ratio II/I = 0.59). Mitochondrial respiratory capacity (O 2 flux) was 58 and 68 pmol·(s·mg) −1 when complex I and complex II coupled together were in state of oxidative phosphorylation and in the uncoupled condition, respectively. Conclusion: The WRW’s cardiorespiratory parameters (central components of physical fitness) were comparable to the 90 th percentile of women aged 50-59 yrs (1), while her mitochondrial function (a peripheral component of physical fitness) was indistinguishable from younger women, supporting limited changes in mitochondrial function with aging (2). Her whole muscle CSA was consistent with individuals of the same age, despite her exceptionally large type I fibers and high percentage of Type II fibers. The neuromuscular function was comparable to women in their 80s (3). We speculate that the WRW’s exceptional performance was driven, in part, by her remarkably large slow fibers, a high proportion of fast fibers, and mitochondrial function equivalent to young adults. This highlights the potential for sprint training to promote health and youthfulness, even in advanced age. References: 1. Kaminsky LA, et al. Mayo Clin Proc.2022 2. Lanza IR et al. J Physiol.2024 3. Sundberg CW et al. J Appl Physiol.2018. PRIN Trajector-AGE (2020477RW5) and ReActiveAGE (P2022FNCPR) grants to MVF and SP, and a NIH-R01 grant (AG048262) to CWS. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

In ageing, denervation and neuromuscular junction (NMJ) instability occur alongside mitochondrial alterations and redox unbalance, potentially playing a significant role in the process. Moreover, the synthetic pathway was shown to be critical for proper innervation and NMJ stability. Nitric oxide (NO) modulates redox status, mitochondrial function and the synthetic pathway. Its bioavailability declines with age. We hypothesize that nitrate supplementation could counteract age-related neuromuscular alterations. We compared young (Y) (7 months old), old (O) (24 months old) and old mice supplemented daily with 1.5 mm inorganic NaNO3 dissolved in drinking water for 8 weeks (ON) (24 months old). Compared to Y, O mice displayed impaired NO signalling and transport (lower phosphorylated-neuronal NO synthase and sialin content); greater nitrosative and oxidative stress (higher 3-nitrotyrosine levels and protein carbonylation); lower glutathione peroxidase (GPX antioxidant enzyme); smaller muscle fibres; and larger muscle fibrosis. NMJ integrity was impaired, exhibiting age-related alterations such as larger fragmentation, lower overlap, larger endplate areas and lower compactness. Consistently, greater expression of denervation-associated markers (Gadd45α, MyoG, RUNX1, AChRγ and NCAM1) and higher NCAM1+ fibres percentage suggested denervation. Importantly, mitochondrial content, dynamics and function were unchanged. Compared to O, ON mice showed improved NO bioavailability in muscle (higher nitrate-nitrite concentration); lower fibrosis and improved muscle fibre size; higher phosphorylation of P70S6K and S6, downstream factors of Akt/mammalian target of rapamycin synthetic pathway; lower oxidative stress (lower carbonylated proteins and mitochondrial hydrogen peroxide production, higher GPX protein levels); reverted age-related alterations of NMJ morphology; and lower percentage of NCAM1+ fibres. Nitrate supplementation could be a therapeutic strategy to counteract muscle decline with ageing. KEY POINTS: Ageing leads to instability at the neuromuscular junction (NMJ), which is crucial for muscle size and function, ultimately giving rise to denervation and muscle fibres loss. Mitochondrial function, redox status and activation of synthetic pathway are critical processes for proper muscle innervation and stability of the NMJ. Nitric oxide was shown to modulate intracellular processes involved in NMJ stability such as balance of reactive oxygen species, mitochondrial function and protein synthesis. Its bioavailability decreases with ageing. Our study shows that nitrate supplementation in old mice improved redox balance, enhanced the anabolic pathway and stabilized nerve-muscle interactions, suggesting a potential strategy to mitigate the neuromuscular decline associated with ageing.

PURPOSE:Dietary nitrate (NO 3- ) supplementation has been shown to improve skeletal muscle contractile function and reduce fatigue, potentially due to alterations in skeletal muscle Ca 2+ handling/sensitivity. Because aging muscle can have impaired Ca 2+ handling, the aim of the study was to evaluate the effects of dietary NO 3- supplementation on muscle contractile properties in young and older adults. METHODS:Eleven older (69 ± 4 yr, O) and 11 young (26 ± 2 yr, YG) adults consumed either NO 3- -rich beetroot juice (BR) or placebo (PLA), for 7 d. After supplementations, plantar flexors of dominant leg were evaluated as follow: a) maximal voluntary isometric contraction (MVIC), b) potentiated single twitches (Tw pot ) and double twitches electrical stimulations at the frequency of 100 Hz (Db 100 ) on the tibial posterior nerve, c) a fatigue isometric (70% of MVIC) test until exhaustion. The force-frequency relationship was assessed with trains of electrical pulses across a wide range of frequencies on the muscle belly of the nondominant leg. RESULTS:BR supplementation increased plasma [NO 3- ] and nitrite [NO 2- ] in both O and YG compared with PLA (more than sevenfold; all P ≤ 0.02). No changes were observed in MVC, Tw pot , and Db 100 force after BR compared with PLA in both YG and O. Only in O, Db 100 area under the curve (-7 ± 6 N·s change from PLA) and half relaxation time (-0.05 ± 0.06 s change from PLA) were significantly reduced, and time to exhaustion (+32 ± 43 s change from PLA) was significantly longer (all P < 0.02) after BR. In O, BR also significantly increased submaximal force produced by trains of electrical pulses ( P < 0.001). CONCLUSIONS:NO 3- supplementation positively affects muscle contractile proprieties, submaximal electrically evoked force production, and fatigue resistance in older adults, whereas these positive results were not found in young.

Short-term unloading experienced following injury or hospitalisation induces muscle atrophy and weakness. The effects of exercise following unloading have been scarcely investigated. We investigated the functional and molecular adaptations to a resistance training (RT) programme following short-term unloading. Eleven males (22.09 ± 2.91 years) underwent 10 days of unilateral lower limb suspension (ULLS) followed by 21 days of knee extensor RT (three times/week). Data collection occurred at Baseline (LS0), after ULLS (LS10) and at active recovery (AR21). Knee extensor maximum voluntary contraction (MVC) was evaluated. Quadriceps volume was estimated by ultrasonography. Muscle fibre cross-sectional area, fibre type distribution, glycogen content and succinate dehydrogenase (SDH) activity were measured from vastus lateralis biopsies. Mitochondrial-related proteins were quantified by western blot and transcriptional responses were assessed by RNA sequencing. Following ULLS, quadriceps volume and MVC decreased significantly (3.7%, P < 0.05; 29.3%, P < 0.001). At AR21 (vs. LS10), MVC was fully restored (42%) and quadriceps volume increased markedly (18.6%, P < 0.001). Glycogen content and whole-body water increased at AR21 (14%, P < 0.001; 3.1%, P < 0.05). We observed a marked increase in fibre type I at AR21 (38%, P < 0.05). SDH immunoreactivity increased significantly after exercise (20%, P < 0.001). Mitochondrial fusion (MFN1, MFN2 and OPA1) and fission (DRP1) proteins were markedly increased by RT, and the most differentially expressed genes belonged to oxidative phosphorylation pathways. In contrast with what is usually observed after RT, oxidative metabolism, slow fibre type and mitochondrial dynamics were enhanced beyond expected. We propose that prior exposure to short-term muscle unloading may drive the nature of molecular adaptations to subsequent RT. KEY POINTS: Short-term unloading is often experienced during recovery from injuries and hospitalisation, leading to loss of muscle mass and strength. Although exercise can be beneficial in mitigating/reversing such alterations during disuse, only a few studies have focused on the effects of exercise following muscle unloading. With an integrative physiological approach, we aimed to elucidate the basic mechanisms of muscle function recovery in response to 21 days of resistance exercise that followed 10 days of unilateral lower limb suspension (ULLS), assessing whether the mechanisms underlying recovery are defined by a specific reversal of those that occurred during disuse. Resistance training was successful in recovering functional and structural muscle properties after 10 days of ULLS, but in contrast with what is usually observed in response to this training modality, oxidative metabolism and slow fibre type were mostly enhanced. We propose that prior exposure to short-term muscle unloading may drive the adaptations to subsequent exercise.

BACKGROUND: Breast cancer is the most frequently diagnosed malignancy amongst females. Physical activity acts as an excellent preventative therapy and plays a role in maintaining a good quality of life by reducing the incidences of comorbidity and tumor reoccurrences, as well as alleviating the side effects of pharmacological and radiation therapies. This study aims to evaluate the safety and efficacy of physical activity in improving quality of life and aerobic capacity in patients treated for breast cancer. METHODS: From May 2012 to May 2017, 140 patients treated for breast cancer were recruited on a voluntary basis. After the first consultation, 74 patients were evaluated at three months and 41 at nine months. A non-monitored individualized exercise protocol was prescribed according to the American College of Sport Medicine's guidelines (ACSM). Anthropometric analysis, VO2 peak test with ECG monitoring and the presence of medically unexplained symptoms (MUS) were assessed. RESULTS: In the 41 subjects followed for 9 months, no statistical differences were found in BMI and body composition. The VO2 peak value was noted to increase significantly in 24 subjects between the ages of 36 and 58, while in 17 patients over 58 years old the increase was not significant. MUS showed a downward trend over 9 months. CONCLUSIONS: Our study suggests that even a non-monitored physical exercise has positive effects on cardiopulmonary system and psychophysical well-being. A regular physical exercise maintained for at least 3 months is able to improve cardiovascular function. ( Cite this article as: Bidoglio F, Canepari M, De Simone A, Longa E, Poerio CS, Ceccaroli M, et al . Exercise therapy in breast cancer patients: effects on cardiorespiratory fitness and quality of life. Med Sport 2024;77:390-402. DOI: 10.23736/S0025-7826.24.04464-8)

Skeletal muscle undergoes atrophy and loss of force during long space missions, when astronauts are persistently exposed to altered gravity and increased ionizing radiation. We previously carried out mass spectrometry-based proteomics from skeletal muscle biopsies of two astronauts, taken before and after a mission on the International Space Station. The experiments were part of an effort to find similarities between spaceflight and bed rest, a ground-based model of unloading, focused on proteins located at the costameres. We here extend the data analysis of the astronaut dataset and show compartment-resolved changes in the mitochondrial proteome, remodeling of the extracellular matrix and of the antioxidant response. The astronauts differed in their level of onboard physical exercise, which correlated with their respective preservation of muscle mass and force at landing in previous analyses. We show that the mitochondrial proteome downregulation during spaceflight, particularly the inner membrane and matrix, was dramatic for both astronauts. The expression of autophagy regulators and reactive oxygen species scavengers, however, showed partially opposite expression trends in the two subjects, possibly correlating with their level of onboard exercise. As mitochondria are primarily affected in many different tissues during spaceflight, we hypothesize that reactive oxygen species (ROS) rather than mechanical unloading per se could be the primary cause of skeletal muscle mitochondrial damage in space. Onboard physical exercise might have a strong direct effect on the prevention of muscle atrophy through mechanotransduction and a subsidiary effect on mitochondrial quality control, possibly through upregulation of autophagy and anti-oxidant responses.

Nemaline myopathies are the most common form of congenital myopathies. Variants in ACTA1 (NEM3) comprise 15-25% of all nemaline myopathy cases. Patients harboring variants in ACTA1 present with a heterogeneous disease course characterized by stable or progressive muscle weakness and, in severe cases, respiratory failure and death. To date, no specific treatments are available. Since NEM3 is an actin-based thin filament disease, we tested the ability of tirasemtiv, a fast skeletal muscle troponin activator, to improve skeletal muscle function in a mouse model of NEM3, harboring the patient-based p.Asp286Gly variant in Acta1. Acute and long-term tirasemtiv treatment significantly increased muscle contractile capacity at submaximal stimulation frequencies in both fast-twitch extensor digitorum longus and gastrocnemius muscle, and intermediate-twitch diaphragm muscle in vitro and in vivo. Additionally, long-term tirasemtiv treatment in NEM3 mice resulted in a decreased respiratory rate with preserved minute volume, suggesting more efficient respiration. Altogether, our data support the therapeutic potential of fast skeletal muscle troponin activators in alleviating skeletal muscle weakness in a mouse model of NEM3 caused by the Acta1:p.Asp286Gly variant.

AbstractBackgroundDegeneration of the motoneuron and neuromuscular junction (NMJ) and loss of motor units (MUs) contribute to age‐related muscle wasting and weakness associated with sarcopenia. However, these features have not been comprehensively investigated in humans. This study aimed to compare neuromuscular system integrity and function at different stages of sarcopenia, with a particular focus on NMJ stability and MU properties.MethodsWe recruited 42 young individuals (Y) (aged 25.98 ± 4.6 years; 57% females) and 88 older individuals (aged 75.9 ± 4.7 years; 55% females). The older group underwent a sarcopenia screening according to the revised guidelines of the European Working Group on Sarcopenia in Older People 2. In all groups, knee extensor muscle force was evaluated by isometric dynamometry, muscle morphology by ultrasound and MU potential properties by intramuscular electromyography (iEMG). MU number estimate (iMUNE) and blood samples were obtained. Muscle biopsies were collected in a subgroup of 16 Y and 52 older participants.ResultsThirty‐nine older individuals were non‐sarcopenic (NS), 31 pre‐sarcopenic (PS) and 18 sarcopenic (S). A gradual decrease in quadriceps force, cross‐sectional area and appendicular lean mass was observed across the different stages of sarcopenia (for all P < 0.0001). Handgrip force and the Short Physical Performance Battery score also showed a diminishing trend. iEMG analyses revealed elevated near fibre segment jitter in NS, PS and S compared with Y (Y vs. NS and S: P < 0.0001; Y vs. PS: P = 0.0169), suggestive of age‐related impaired NMJ transmission. Increased C‐terminal agrin fragment (P < 0.0001) and altered caveolin 3 protein expression were consistent with age‐related NMJ instability in all the older groups. The iMUNE was lower in all older groups (P < 0.0001), confirming age‐related loss of MUs. An age‐related increase in MU potential complexity was also observed. These observations were accompanied by increased muscle denervation and axonal damage, evinced by the increase in neural cell adhesion molecule‐positive fibres (Y vs. NS: P < 0.0001; Y vs. S: P = 0.02) and the increase in serum concentration of neurofilament light chain (P < 0.0001), respectively. Notably, most of these MU and NMJ parameters did not differ when comparing older individuals with or without sarcopenia.ConclusionsAlterations in MU properties, axonal damage, an altered innervation profile and NMJ instability are prominent features of the ageing of the neuromuscular system. These neuromuscular alterations are accompanied by muscle wasting and weakness; however, they appear to precede clinically diagnosed sarcopenia, as they are already detectable in older NS individuals.

An exposure to hypertrophic stimuli has been shown to lead to faster and larger growth of skeletal muscle when subsequently repeated. Aside from acquired myonuclei permanence, epigenetic modification appears to be involved in the hypertrophic memory at skeletal muscle level. Endurance training interventions increase mitochondrial content and induce mitochondria biogenesis. We aimed to explore whether repeated endurance stimuli can induce changes in mitochondrial function and dynamics due to epigenetic memory. We hypothesized mitochondrial adaptations in response to high-intensity interval training might be influenced by muscle memory at epigenetic level. METHODS: Sixteen subjects (25±5years) underwent to two repeated aerobic training periods (training and retraining) separated by 12 weeks where they were invited to return to their habitual life (detraining). Each training lasted 8 weeks and consisted of a combination between high-intensity interval and sprint interval cycling exercises. At baseline and after training, detraining and retraining peak oxygen consumption (V̇O 2peak ) and peak power output (W peak ) were measured. Vastus lateralis muscle samples were collected and mitochondrial respiration (O 2 flux by high-resolution respirometry), mitochondrial dynamics, DNA methylation, and gene expression analysis were performed. RESULTS: V̇O 2peak and W peak improved during both training and retraining (all p<0.001) without differences between the two interventions (p>0.58). Mitochondrial respiration improved in both training and retraining (both p<0.05), but O 2 flux changes observed after retraining were greater than those after training (p<0.05). Mitochondrial dynamics were differently affected by training and retraining. Training induced hypomethylation in a large number of differentially methylated positions (DMPs) (14,516). Epigenetic memory profiles were identified in 3,190 DMPs characterized by a hypomethylation state maintained elevated even during detraining, in which mitochondrial respiration returned to baseline levels, and subsequently across retraining. We identified six genes related to muscle function ( ADAM19, INPP5a, MTHFD1L, PDGFB, CAPN2, and SLC16A3) as genes with methylation signature memory that also showed increased expression after retraining, indicating a memory of transcription activity following earlier training. CONCLUSIONS: Physiological adaptions at whole body level seem not to benefit from repeated interventions. However, enhanced responses to the second training were evident at cellular level since mitochondrial function and dynamics were different after retraining. Across repeated interventions, memory profiles were highlighted at epigenetic level characterized by retention of hypomethylation during long-term detraining and retraining period. Methylation and transcriptional memory profiles in genes involved in skeletal muscle metabolic pathways may represent the mechanistic basis of mitochondrial memory. Simone Porcelli was supported by a grant from Sports Medicine Italian Federation (FMSI01092021). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Letter to the EditorLong-Term Recovery from SARS-CoV-2 (COVID-19)Reply to Finsterer and Scorza: “Exercise intolerance in post-COVID syndrome cannot only be due to skeletal muscle impairment”Marta Colosio, Maria Antonietta Pellegrino, Simone Porcelli, and Roberto BottinelliMarta ColosioDepartment of Molecular Medicine, University of Pavia, Pavia, ItalyDepartment of Biomedical Sciences for Health, University of Milan, Milan, Italy, Maria Antonietta PellegrinoDepartment of Molecular Medicine, University of Pavia, Pavia, ItalyInterdepartmental Centre of Biology and Sport Medicine, University of Pavia, Pavia, Italy, Simone PorcelliDepartment of Molecular Medicine, University of Pavia, Pavia, ItalyIRCCS Fondazione Policlinico San Matteo, Pavia, Italy, andRoberto BottinelliDepartment of Molecular Medicine, University of Pavia, Pavia, ItalyIRCCS Mondino Foundation, Pavia, ItalyPublished Online:05 Dec 2023https://doi.org/10.1152/japplphysiol.00780.2023MoreSectionsPDF (250 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat to the editor: We read with interest the letter to the editor (1) from Dr. Finsterer and Dr. Scorza about our recent work (2) investigating the mechanistic bases of limited exercise tolerance in a cohort of patients with postacute sequelae of COVID-19 (PASC) syndrome and we would like to thank the journal for giving us the opportunity to reply to their letter.Finsterer and Scorza highlighted some potential pitfalls in our study and hypothesized that some weaknesses may have confounded the results. In the following sections, we will try to address the points raised by Finsterer and Scorza, hoping this will help to clarify their concerns about our conclusions.In our study, we recruited eleven (n = 11) nonhospitalized patients with PASC and twelve (n = 12) patients without long-term symptoms (CTRL). All participants were tested several months (8 ± 2) after acute infection. Pulmonary and cardiac functions were within normal range in all patients. In PASC, the cardiopulmonary exercise test combined with some functional biomarkers related to muscle oxidative metabolism showed a limited exercise tolerance mainly due to “peripheral” determinants, and a muscle biopsy suggested a major impairment at the skeletal muscle level, showing impaired mitochondrial function and reduced markers of mitochondrial biogenesis.Finsterer and Scorza suggest caution in the interpretation of our results because patients were not well characterized from a medical perspective. More specifically, they state that central nervous system dysfunctions, mood disorders, thrombosis, multiple sclerosis, and other conditions were not considered. In addition, they criticize our conclusions and recommend avoiding generalization because the number of recruited patients was limited and they were enrolled in a single center.First, we would like to clarify that we do not mean to suggest that skeletal muscle is the primary determinant of exercise intolerance in all patients with PASC. As highlighted in the limitations section, we are aware that PASC syndrome is a highly heterogeneous condition, and other organs’ dysfunction may contribute to limited exercise tolerance. In our study, inclusion and exclusion criteria were purposely highly selective to reduce the influence of confounding factors on our results. Many patients who underwent moderate/severe consequences in the acute phase of the SARS-CoV-2 infection, as well as patients with a history of anemia; relevant preexisting cardiac, respiratory, or musculoskeletal comorbidities; use of respiratory assist device; hospitalization; and glucocorticoid therapy, were excluded. No patients had a history of central nervous system diseases, clinical manifestations of peripheral nervous system alterations, or other potential causes of exercise intolerance, as highlighted in Table 1 of our study (2), reporting comorbidities, concomitant medication, and laboratory results of PASC and CTRL. In addition, mood disorders were not present in our patients with PASC because an average value of 1.5 on a score of 5 was present in the anxiety/depression dimension of the 5Q-5D-5L questionnaire (3). Thus, in our cohort of patients, exercise intolerance was mainly associated with impairments in skeletal muscle function. In our opinion, the finding that skeletal muscle impairment per se can be the major determinant of PASC syndrome in a homogeneous cohort of patients selected to rule out potential confounding factors is a strength of our study, although we know that muscle alterations alone cannot explain such a complex phenomenon in all cohorts of patients with PASC.Dr. Finsterer and Dr. Scorza also asked for information about the presence of signs of myositis in patients with PASC. This is a good point, and we thank the authors for raising this issue. Our patients had no signs of persistent, low-grade inflammation in the blood at the time of study. Moreover, no infiltrations of inflammatory cells were present. However, we cannot exclude that persistent elevated levels of circulating cytokines may have modulated mitochondrial biogenesis, negatively impacting exercise tolerance.Thus, we thank Dr. Finsterer and Dr. Scorza for extending the conversation, and appreciate the attention given to our work. However, we believe that impaired skeletal muscle function and alterations in mitochondrial function and mitochondrial dynamics were major determinants of limited exercise capacity in the cohort of recruited patients with PASC . In the future, larger and multicenter studies will have to confirm our results while elucidating the mechanistic bases of mitochondrial dysfunction after SARS-CoV-2 and other virus infections.GRANTSThis work was funded by Ministero dell'Università e della Ricerca (MUR) Grant 20228M8BNM (to S. Porcelli and M.A. Pellegrino).DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the authors.AUTHOR CONTRIBUTIONSS.P. drafted manuscript; M.C., M.A.P., S.P., and R.B. edited and revised manuscript; M.C., M.A.P., S.P., and R.B. approved final version of manuscript.REFERENCES1. Finsterer J, Scorza FA. Exercise intolerance in post-COVID syndrome cannot only be due to skeletal muscle impairment. J Appl Physiol (1985). doi:10.1152/japplphysiol.00734.2023.Link | Google Scholar2. Colosio M, Brocca L, Gatti MF, Neri M, Crea E, Cadile F, Canepari M, Pellegrino MA, Polla B, Porcelli S, Bottinelli R. Structural and functional impairments of skeletal muscle in patients with postacute sequelae of SARS-CoV-2 infection. J Appl Physiol (1985) 135: 902–917, 2023. doi:10.1152/japplphysiol.00158.2023. Link | Google Scholar3. Devlin NJ, Shah KK, Feng Y, Mulhern B, van Hout B. Valuing health-related quality of life: an EQ-5D-5L value set for England. Health Econ 27: 7–22, 2018. doi:10.1002/hec.3564.Crossref | PubMed | ISI | Google ScholarAUTHOR NOTESCorrespondence: S. Porcelli (simone.porcelli@unipv.it). Download PDF Previous Back to Top FiguresReferencesRelatedInformation Related ArticlesExercise intolerance in post-COVID syndrome cannot only be due to skeletal muscle impairment 05 Dec 2023Journal of Applied Physiology More from this issue > Volume 135Issue 6December 2023Pages 1386-1387 Crossmark Copyright & PermissionsCopyright © 2023 the American Physiological Society.https://doi.org/10.1152/japplphysiol.00780.2023PubMed38051270History Received 2 November 2023 Accepted 2 November 2023 Published online 5 December 2023 Published in print 1 December 2023 Metrics

Physical inactivity represents a heavy burden for modern societies and is spreading worldwide, it is a recognised pandemic and is the fourth cause of global mortality. Not surprisingly, there is an increasing interest in longitudinal studies on the impact of reduced physical activity on different physiological systems. This narrative review focuses on the pathophysiological mechanisms of step reduction (SR), an experimental paradigm that involves a sudden decrease in participants' habitual daily steps to a lower level, mimicking the effects of a sedentary lifestyle. Analogous animal models of reduced physical activity, namely, the "wheel-lock" and the "cage reduction" models, which can provide the foundation for human studies, are also discussed. The empirical evidence obtained thus far shows that even brief periods of reduced physical activity can lead to substantial alterations in skeletal muscle health and metabolic function. In particular, decrements in lean/muscle mass, muscle function, muscle protein synthesis, cardiorespiratory fitness, endothelial function and insulin sensitivity, together with an increased fat mass and inflammation, have been observed. Exercise interventions seem particularly effective for counteracting these pathophysiological alterations induced by periods of reduced physical activity. A direct comparison of SR with other human models of unloading, such as bed rest and lower limb suspension/immobilisation, is presented. In addition, we propose a conceptual framework aiming to unravel the mechanisms of muscle atrophy and insulin resistance in the specific context of reduced ambulatory activity. Finally, methodological considerations, knowledge gaps and future directions for both animal and human models are also discussed in the review.

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Several months after mild acute SARS-CoV-2 infection, a substantial proportion of patients present persisting, and often debilitating, symptoms and sequelae. These patients show reduced quality of life due to exercise intolerance, muscle weakness, and fatigue. The present study supports the hypothesis that “peripheral” impairments at skeletal muscle level, namely, reduced mitochondrial function and markers of mitochondrial biogenesis, are major determinants of exercise intolerance and fatigue, “central” phenomena at respiratory, and cardiac level being less relevant.