IntroductionThe COVID-19 pandemic highlighted the importance of nutritional and lifestyle factors in recovery from severe illness, yet their role in post-hospitalization rehabilitation remains insufficiently understood.MethodsThis study investigated the association between adherence to the Mediterranean diet, body composition, and recovery-related outcomes in individuals following severe COVID-19 using an explanatory mixed-methods design. Thirty-nine patients were assessed at hospital discharge and after 8 weeks of unsupervised rehabilitation in a single-centre prospective cohort study conducted in Slovenia (ClinicalTrials.gov: NCT04860206, https://clinicaltrials.gov/study/NCT04860206).ResultsQuantitative analyses showed a significant increase in fat-free mass over time and positive associations between fat-free mass and adherence to the Mediterranean diet. Qualitative findings further revealed that individuals with higher adherence demonstrated structured routines, adaptive coping strategies, and greater psychosocial resilience during recovery.DiscussionTogether, these findings suggest that adherence to a Mediterranean dietary pattern, alongside related lifestyle behaviors, may support favorable body composition and enhance recovery processes following severe illness. This study provides an integrated perspective on physiological and behavioral determinants of recovery, highlighting the potential of nutrition-based approaches in post-COVID-19 rehabilitation.

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.

Bed rest (BR) studies have demonstrated the detrimental effects of microgravity and physical inactivity on cardiovascular function, including the negative consequences on microvascular function and O2 extraction. However, whether a short period of BR affects microvascular responsiveness and its molecular mediator, nitric oxide (NO), remain unclear. Nine male volunteers (23 ± 5 yr) completed 10-day horizontal BR. Microvascular postocclusive reactive hyperemia in the vastus lateralis (VL) and rectus femoris (RF) muscles were assessed by near-infrared spectroscopy during a vascular occlusion test. We calculated the rate of muscle deoxygenation during the first minute of occlusion (slope 1) and the half-time of the reperfusion kinetics ([Formula: see text]) during the 30 s postischemia. The inverse of [Formula: see text], K (velocity constant) was "normalized" for the intensity of the ischemic/hypoxic stimulus, as estimated by [Formula: see text], the highest [Formula: see text] value at the end of ischemia. Plasma nitrite ([Formula: see text]) and nitrate ([Formula: see text]) concentrations (NO bioavailability indexes) were determined by chemiluminescence. Slope 1 and [Formula: see text] were slower after BR in both VL (0.08 ± 0.03 vs. 0.06 ± 0.02 µM·s-1, P = 0.016; and 12 ± 3 vs. 17 ± 2 s, P = 0.002) and RF (0.07 ± 0.02 vs. 0.05 ± 0.02 µM·s-1, P = 0.008; and 12 ± 3 vs. 18 ± 4 s, P < 0.001). No differences were detected in K/[Formula: see text] after BR (VL: 0.15 ± 0.04 vs. 0.17 ± 0.03 µM·min-1, P = 0.166; RF: 0.20 ± 0.06 vs. 0.21 ± 0.05 µM·min-1, P = 0.220). Plasma [Formula: see text] concentration was reduced after BR (87 ± 37 vs. 67 ± 48 nM, P = 0.011), but not plasma [Formula: see text] concentration (43 ± 24 vs. 29 ± 11 µM, P = 0.130). In conclusion, 10-day horizontal BR does not impair microvascular postocclusive reactive hyperemia parameters, despite decreasing plasma [Formula: see text] concentration.NEW & NOTEWORTHY Short-term (10 days) horizontal bed rest does not affect skeletal muscle microvascular function when assessed via near-infrared spectroscopy and postocclusion reactive hyperemia. However, both skeletal muscle resting oxygen consumption and nitric oxide bioavailability decrease following 10-days bed rest. Future studies should investigate the time course of changes in skeletal muscle microvascular function following longer period of microgravity/inactivity.

Background Sarcopenia, the progressive decline in skeletal muscle mass and function, is a major public health concern linked to falls, hospitalization and loss of independence among older adults. Initially defined by reduced muscle mass, later also by reduced muscle strength and function, it is now recognized that standard diagnostic tools do not fully capture complexity of sarcopenia. Tensiomyography (TMG) is a non-invasive method that assesses skeletal muscle contractile parameters, which undergo change with aging and sarcopenia. The aims of this exploratory study are to determine whether TMG could be a method for contractile parameters assessment in sarcopenia classification and to evaluate the relationship between TMG-derived parameters and sarcopenia classification tests.Methods We included 654 older adults (70.6% women) and included demographics, sarcopenia classification (EWGSOP2, SDOC), muscle strength (handgrip strength, five sit-to-stand), TMG of three leg muscles, muscle mass (bioimpedance), and physical performance (gait speed, timed up-and-go). MANOVA was used to analyze contractile properties and due to low agreement between classifications, we used both classifications. A partial correlation for each sex was conducted to determine the associations between sarcopenia classification tests and TMG-derived parameters of delay time (Td), contraction time (Tc), radial displacement (Dm) and contraction velocity (Vc), controlling for age.Results One-way MANOVA confirmed difference between sarcopenic and non-sarcopenic participants according to EWGSOP2 and SDOC classifications in TMG-derived contractile parameters in all three muscles, Td was consistently longer and Dm consistently lower in sarcopenic individuals. Post-hoc univariate tests further demonstrated specific differences due to sarcopenia presence. Age-adjusted partial correlations were weak to moderate, ranging between -0.430 and 0.369.Conclusion Sarcopenic individuals exhibited longer Td and Tc, and smaller Dm, though not consistently across all muscles. Td was longer in all three muscles, reflecting electromechnical delays linked to aging. Dm was consistently lower, suggesting increased muscle stiffness. Correlations between TMG parameters and sarcopenia classification tests indicated that shorter Td and Tc, higher Dm, and greater Vc were associated with higher muscle volume, muscle strength and performance. The findings indicate that TMG parameters may be associated with neuromuscular degeneration and sarcopenia, supporting further exploration of muscle- and sex-specific differences.

Background Muscle fat fraction (MFF) obtained through magnetic resonance imaging (MRI) is the gold standard for assessing muscle quality, but it is expensive and time consuming. Portable methods to examine muscles such as tensiomyography (TMG) are emerging and could enable broader screening. This study aims to examine associations between TMG-derived muscle contractile parameters and MFF in older adults with and without sarcopenia. Methods A sample of 51 Slovenian older adults (53% females) were scanned with Dixon MRIs to evaluate muscle MFF and contractile parameters were assessed with TMG estimating delay time (Td), maximal displacement (Dm) and radial contraction velocity (Vc). Right leg vastus lateralis (VL) and biceps femoris (BF) were analyzed. Sarcopenia was defined using both European Working Group on Sarcopenia in Older People (EWGSOP2) and Sarcopenia Definition and Outcomes Consortium (SDOC) criteria. Regression models adjusted for age and sex were used to assess associations between TMG-derived contractile parameters and MFF. Results Age- and sex-adjusted models revealed associations between increased MFF and reduced Dm (R-2 = 0.29, p = .003) and Vc (R-2 = 0.32, p = .002) for the VL. SDOC-classified sarcopenic individuals showed increased VL MFF (27.2% vs. 22.5%, p = .019),while EWGSOP2 classified sarcopenia displayed no differences. Discussion The study reveals that increased MFF is associated with reduced muscle contractility in VL. MFF differs between sarcopenic and non-sarcopenic groups using only SDOC criteria. Since the TMG Dm increase is regularly found in atrophic muscles after bed rest, in sarcopenic muscle MFF explains lowering of the Dm, highlighting the TMG potential for early detection of changes in aging muscle.

PURPOSE:Tensiomyography detects early hallmarks of bed-rest-induced atrophy. We aimed to identify early tensiomyography alterations in six muscles during a 10-d bed rest (BR10), followed by a 30-d recovery period (R+30), and to compare these changes with those observed after the 50-d COVID-19 lockdown in 10 healthy males (22.9 ± 5.0 yr). METHODS:Tensiomyography and muscle thickness (assessed by ultrasound imaging) were assessed at baseline, during bed rest, at BR10, R+30, and after the COVID-19 lockdown. Additional assessments included vertical jump performance (force plate), body composition (bioimpedance), physical activity (Global Physical Activity Questionnaire), and lower back pain (VAS). RESULTS:By BR10, participants experienced a 1.9% body mass loss, a 3.8% increase in fat mass, and a 2.4% reduction in muscle mass. Although anthropometric parameters returned to baseline by R+30, fat mass increased again by 3.9% postlockdown. Jump height declined by 10.1% at BR10, returned to baseline at R+30, and remained stable thereafter. Muscle thickness decreased by 7.0% in the vastus lateralis at BR10 and by 10.1% postlockdown. Tensiomyographic amplitude increased in five leg muscles before BR10, recovered at R+30 and increased again after postlockdown, whereas a reduction was observed in the erector spinae within the first 48 h, coinciding with reported lower back pain. In the erector spinae, both delay and contraction time decreased during BR10 and postlockdown, whereas contraction time of the biceps femoris increased before BR10 and remained elevated postlockdown. CONCLUSIONS:The observed post COVID-19 lockdown alterations in tensiomyographic parameters followed the same trend as those induced by bed rest, albeit with smaller magnitude. These findings support the utility of TMG as a sensitive and noninvasive method for monitoring muscle responses across different disuse paradigms.

Background Bed rest (BR) is a widely used analogue of space flight that aims to replicate microgravity conditions, such as immobilization and a lack of sensorimotor stimuli. In a previous BR study, we evaluated the changes in irisin serum levels, a myokine produced mainly by skeletal muscle that exerts pleiotropic effects on several organs, including the brain. Objective Here, we evaluated whether the decline in circulating irisin concentrations during 10 days of horizontal BR correlated with the National Aeronautics and Space Administration Task Load Index (NASA-TLX). Methods NASA-TLX was obtained from young healthy participants at the end of the BR protocol. It is a self-report measure of task load that includes six subscales: mental demand, physical demand, temporal demand, performance, effort, and frustration. Each subscale score was correlated with the decline in serum irisin levels measured while participants were supine. Irisin decline was calculated as the difference between levels on the first day of BR (BR0) and the last day (BR9). Results Our results show a significant positive correlation between irisin decline and both mental demand and temporal demand, whereas correlations with the remaining subscales showed a positive association with irisin decline but did not reach statistical significance. Conclusion These findings suggest that reduced irisin levels are associated with increased perceived mental demand and time pressure (temporal demand) during BR. Because temporal demand can exacerbate high mental activity in a state of cognitive overload, maintaining

Lumbar multifidus (MF) muscle plays a key role in spinal stability, yet its adaptations to aging and disuse, which become increasingly prevalent with advancing age, remain unclear. We conducted two studies to investigate age- and inactivity-induced changes in MF size. In Study 1, we assessed 32 young adults (50% females) and 75 older adults (67% females), categorized as non-sarcopenic (NS) or probable sarcopenic (PS) based on EWGSOP2 criteria. In Study 2, we examined early MF responses to 10-day horizontal bed rest in 10 young males. MF cross-sectional area (CSA) and side-to-side asymmetry were measured using ultrasound imaging and compared with the vastus lateralis (VL) CSA. In Study 1, MF CSA was negatively associated with age (p < 0.0001) and probable sarcopenia, being lower in PS compared to NS (p = 0.012). MF showed larger effect sizes than VL [Y vs NS: MF g = 1.62, VL g = 1.35; Y vs PS: MF g = 2.48, VL g = 1.35]. MF asymmetry was greater in PS compared to Y (p = 0.003). In Study 2, bed rest induced early reductions in MF CSA (detectable by day 4, p = 0.047) without increasing asymmetry. In conclusion, the MF was smaller in older populations and decreased following short-term disuse, while greater asymmetry was observed only with aging. These findings suggest that the MF is a highly plastic muscle in response to aging and disuse and that its assessment may serve as a potential hallmark of muscle maladaptation in clinical and experimental settings.

BACKGROUND:Aging leads to progressive motor system decline, which can result in sarcopenia, defined as the age-related loss of muscle mass and function. Architectural changes, epitomized by a decrease in fascicle length (Lf) and muscle thickness (MT), provide a useful signature of sarcopenia. Their ratio (Lf/MT), named ultrasound sarcopenia index (USI), is a new parameter proposed for evaluating changes in muscle geometric proportions associated with muscle atrophy but lacks application in a sarcopenic population. This study aimed to explore vastus lateralis USI in a sarcopenic population. METHODS:In a sample of 139 older adults (54% females), recruited from Italy and Slovenia, we assessed muscle architecture using ultrasound imaging to measure Lf, MT, pennation angle (PA), and USI. We assessed handgrip strength, sit-to-stand test, and physical performance with timed up-and-go and gait speed. Appendicular lean mass was assessed with dual x-ray absorptiometry. Sarcopenia was classified using the EWGSOP2 and SDOC classifications. RESULTS:Sarcopenia prevalence was 15.1% and 30.9% when classified by the EWGSOP2 and SDOC classifications, respectively. Differences in muscle architecture were observed between sarcopenic and non-sarcopenic groups, with MT showing the largest effect size (Cohen's d EWGSOP2: 0.86; SDOC:0.77). USI was higher in sarcopenic compared to non-sarcopenic individuals classified with EWGSOP2 (5.33 ± 1.30 vs 4.59 ± 0.94, p = .011, Cohen's d:0.69), confirming its sensitivity in detecting sarcopenia. CONCLUSION:We showed that increased USI values are associated with sarcopenia. This study shows USI as a sensitive, non-invasive marker for sarcopenia classification, supporting its use in clinical screening and monitoring of muscle changes in older adults.

Background: Establishing whether sex-specific differences exist in physiological responses to environmental insults, such as microgravity, is a major priority in modern physiology. With females now comprising 40% of current NASA astronauts, we still have a limited understanding of sex differences in adaptations to microgravity despite their increasing exposure to spaceflight. In this study, we compared electrophysiological changes in motor unit (MU) properties between males and females subjected to head-down tilt bed rest, a well-established experimental microgravity analog. Methods: Nine healthy young males (YM; 18–36 years) and nine healthy young females (YF; 18–31 years) underwent 21 days of −6° head-down tilt bed rest. Before and after bed rest, knee extensor maximal voluntary contraction (MVC) was assessed by isometric dynamometry, while vastus lateralis (VL) cross-sectional area (CSA) at 50% of femur length was evaluated by panoramic ultrasound. Intramuscular electromyography (iEMG) recordings were obtained from the VL to evaluate MU properties and neuromuscular junction (NMJ) transmission during voluntary contractions at 25% MVC. Statistical analysis was performed using linear mixed models (fixed effects: time and sex; cluster variable: subject). Results: Knee extensor MVC declined similarly in YM (−21.4%, p < 0.001) and YF (−22.3%, p < 0.001) following bed rest. VL CSA showed a parallel reduction (YM: −11.9%, p < 0.001; YF: −13.19%, p < 0.001). However, iEMG analysis revealed a significant sex × time interaction (p = 0.004) for motoneuron firing rate, with YF showing a greater decrease (−9.8%, p < 0.001) compared to YM (−5.1%, p < 0.001). Conversely, only YM showed a reduction in motor unit potential (MUP) area (−7.2%, p = 0.001), suggesting decreased MU size with disuse. Finally, impaired NMJ transmission, reflected by increased near fiber jiggle (+11.1%, p=0.036), was observed exclusively in YM. Conclusion: Despite a comparable loss of muscle strength and size, young males and females exhibit distinct neuromuscular adaptations to simulated microgravity. Females demonstrated greater central vulnerability, as indicated by the larger decline in motoneuron firing rate, whereas exclusively males showed signs of a more peripheral neuromuscular remodeling, as suggested by a reduced MU size and impaired NMJ transmission. These findings may be of considerable relevance for human health during spaceflight and may help guide the development of sex-specific countermeasures. Funding: This study was carried out within the framework of the collaboration agreement n. 2025-4-HB.0 between ASI and the University of Padua for the “Neuromyo” project. 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.

A decrease in resting muscle O2 uptake ( V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ ) described during bed rest (BR) could determine a decreased whole-body resting energy expenditure (REE), potentially useful during prolonged spaceflights. Two groups of recreationally active men (young [Y], n = 8, age 23 ± 5 years; elderly [E], n = 10, age 69 ± 3 years) underwent a 21-day (Y) or a 10-day (E) horizontal BR without countermeasures. Measurements were performed before and post-BR. Fat-free mass (FFM) was measured by bioimpedance analysis; REE was measured by open-circuit spirometry; resting V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ was measured by time-domain near-infrared spectroscopy in the vastus medialis during a transient ischaemia; citrate synthase (CS) activity (estimate of mitochondrial volume) was measured on a vastus lateralis muscle biopsy sample. FFM decreased significantly both in Y (-8%, P < 0.001) and in E (-5%, P = 0.009). Both resting V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ (-45%, -2.1% day-1 in Y, P = 0.025; -29%, -2.9% day-1 in E, P = 0.001) and REE (-15%, -0.7% day-1 in Y, P = 0.012; -12%, -1.2% day-1 in E, P = 0.012) decreased during BR. CS activity decreased in E (-31%, P = 0.005), whereas the decrease in Y (-12%) did not reach statistical significance (P = 0.38). After resting, V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ data normalized for CS activity values in post- versus pre-BR were lower in Y (P = 0.021) but not in E (P = 0.99). The decreased resting V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ and REE may represent a 'recalibration' of ATP supply to a reduced ATP demand, aimed at preventing excessive reactive oxygen species production and muscle atrophy. It would mitigate biological and logistic challenges of prolonged spaceflights, but could negatively impact the health status of the subjects.

BACKGROUND:Prolonged bed rest is a well-established model for studying the effects of immobility on functional performance, especially gait and postural control. Understanding how these functions deteriorate and recover across age groups is essential for developing targeted rehabilitation strategies. RESEARCH QUESTION:How does a 10-day horizontal bed rest affect gait performance and postural control in young and older adults, and to what extent do these effects differ by age? METHODS:Following a multistage screening process from a large pool of healthy male volunteers, ten older men (68.5 ± 2.6 years) and ten young men (22.9 ± 4.7 years) completed 10 days of horizontal bed rest in a controlled laboratory setting, followed by a standardized 30-day recovery program. Postural control and gait speed assessed under self-selected and brisk walking conditions were measured before bed rest, immediately after, and following a 30-day recovery program. RESULTS:Following bed rest, self-selected walking speed did not differ significantly from baseline in either group. In contrast, brisk walking speed significantly decreased in older (-11.4%, p < 0.05) and young adults (-9.9%, p < 0.05), with both groups returning to baseline values after recovery. Postural control deteriorated significantly in older adults (-15.5%), whereas no significant difference from baseline was observed in young adults. After baseline adjustment, postural control showed a significant group effect both after bed rest (η² > 0.244) and post-recovery (η² > 0.395). SIGNIFICANCE:The findings reveal clear age-related differences in responses to inactivity and recovery. Older adults experienced greater impairments and slower restoration of balance, indicating heightened vulnerability to functional decline during periods of immobilization. These findings underscore the age-dependent impact of bed rest and the importance of tailored rehabilitation strategies.

ObjectivesThe aim of this umbrella review was to investigate the frequency of physical activity-related injuries (PARI) among adolescents. Our secondary objective was to describe the available reviews of injuries in three settings: organized sports, leisure time physical activity (PA), and school physical education (PE).MethodsWe conducted an overview of reviews consistent with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement guidelines on the frequency of physical activity related injuries in adolescents.ResultsWe identified 19 systematic reviews with at least moderate quality to include in the review. We were not able to pool data from reviews and conduct meta-analysis due to heterogeneity of injury definitions, exposure times, and sample demographics. All reviews included studies of injuries sustained in organized sports, and injury incidence was higher during matches than training. No systematic reviews were found focusing on leisure time or school-based PA injuries.ConclusionHigh-quality research is essential to understand the frequency of various types of physical activity related injuries among adolescents in organized sports, PE, and leisure time activities to develop more effective prevention strategies.

The aim of the study was to examine the effects of eccentric- and concentric-based plyometric programmes on the strength, speed, and tensiomyography (TMG) parameters of female athletes. The study included twenty junior female participants from three different sports equally divided into two experimental groups of n = 10. Two plyometric programmes with contrasting designs were conducted for a period of six weeks, with sessions held twice per week. The first plyometric programme (ECC-CON-G) was based on exercises with eccentric, and the second (CONC-CON-G) with concentric contractions. TMG was used to evaluate neuromuscular performances of six muscles of both legs - vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), semitendinosus (SM), gastrocnemius lateralis (GL), and gastrocnemius medialis (GM)) - and two strength and speed tests: countermovement jump (CMJ), and sprint test at 10 m and 20 m. The results show that both groups had significant effects between pre-post measurements in CMJ (Diff, ECC-CON-G = 9.02%, and CONC-CON-G = 5.59%, p < 0.05), at 10 m (Diff, ECC-CON-G = 9.23%, and CONC-CON-G = 9.35%, p < 0.001), and 20 m (Diff, ECC-CON-G = 6.16%, and CONC-CON-G = 5.35%, p < 0.001), and TMG parameters in ECC-CON-G (all 6 left leg muscles, and right leg-VL, BF, GL, GM, p < 0.05), in CONC-CON-G (left leg-BF, SM, GL, GM, and right leg-VL, BF, GL, GM, p < 0.05). There were significantly better effects in ECC-CON-G compared to CONC-CON-G for CMJ height and time, for only time in 20 m sprint, and TMG parameters for left leg VL and VM, and right leg BF and GM. A plyometric programme based on exercises with eccentric contractions proved more beneficial for strength, speed, and TMG parameters in young female athletes compared to a programme based on concentric contractions.

Previous studies showed that properties of higher-threshold motor units (MUs) and neuromuscular junction (NMJ) function are preserved during short-term disuse. This study aimed to test how a longer disuse period affects MU properties, NMJ transmission, and NMJ morphology remodeling. Nine young healthy males (age: 18-29 yr) underwent 21 days of horizontal bed rest. Pre- (BR0) and postbed rest (BR21), quadriceps maximal voluntary contraction (MVC), and size were assessed. We combined intramuscular electromyography (iEMG) and high-density surface electromyography (HDsEMG) recordings on the vastus lateralis to assess MU properties at 25% and 50% of MVC. Muscle biopsies and blood samples were also collected. Quadriceps MVC and size decreased at BR21. We found alterations in MU properties at both contraction intensities, including reduced discharge rate, MU potential area changes, and increased complexity. NMJ transmission was found to be reduced at BR21 at 25% MVC. This functional NMJ impairment was biochemically corroborated by an increase in serum C-terminal agrin fragment concentration, a biomarker of NMJ instability. In addition, a direct assessment of NMJ morphology revealed the presence of some denervated NMJs exclusively at BR21. In conclusion, 21-day bed rest altered MU properties across different contraction intensities and impaired NMJ transmission with initial signs of remodeling/denervation. Disuse duration appears to be a critical factor, as previous shorter studies failed to detect some of these changes. We believe these findings are clinically relevant for disuse after trauma, surgery, or illness and may support the development of effective countermeasures. NEW & NOTEWORTHY Leveraging both intramuscular and high-density surface EMG recordings in the vastus lateralis, we identified alterations in motor unit (MU) properties in young adults after 21 days of bed rest. These included reduced discharge rates and changes in MU potential size and complexity, observed at both low and moderate contraction intensities. Evidence of impaired neuromuscular junction (NMJ) function and denervation was also found. Our findings indicate that medium-term disuse elicits MU-level changes not detected in shorter-duration studies.

Background: Ageing is associated with muscle morphological and functional changes whereas in sarcopenia those changes severely affect the normal quality of life. The aim of this cross-sectional study was to evaluate relationship between architectural changes and contractile properties of the sarcopenic muscle and to validate ultrasound sarcopenia index in a diagnosed sarcopenic population. Methods: Sample consisted of 705 older adults (31.6% males) aged 73.16 ± 9.10 years from 11 Slovenian regions which were classified as non-sarcopenic and sarcopenic using a revised algorithm by European Working Group on Sarcopenia in Older People (EWGSOP2) and an algorithm by Sarcopenia Definition and Outcome Consortium (SDOC). Their proposed tests of muscle strength, muscle mass and physical performance were aquired. Muscle contractile properties were examined with tensiomyography (TMG) on vastus lateralis, biceps femoris and gastrocnemius medialis of the right leg. Additionally, on a subsample of 51 participants (47% males) aged 82.3 ± 8.2 years muscle architecture properties were assessed with ultrasonographic measurements of vastus lateralis. Muscle fascicle length (Lf) and thickness (Tm) at 35% of femur length were used to calculate Lf/Tm ratios known as ultrasound sarcopenia index (USI) as indices of muscle geometric proportion. Results: TMG-derived parameters (delay time, contraction time, radial displacement and contraction velocity) were different between non-sarcopenic and sarcopenic groups in all three assessed muscles. With EWGSOP2 classification, USI values were different (p =.008) between non-sarcopenic (n = 27; USI = 4.49 ± 1.06) and sarcopenic group (n = 12; USI = 5.41 ± 1.48). Similarly, both groups were also significantly different (p =.008) according to SDOC classification: non-sarcopenic (n = 19; USI = 4.29 ±.76) and sarcopenic group (n = 20; USI = 5.24 ± 1.47). USI was correlated with delay time (r =.388; p =.15), contraction time (r =.388; p=.015), contractile velocity (r= -.338; p=.035), and presented a trend for radial displacement (r = -.302; p =.062). Discussion: USI shows to be a reliable tool in distinguishing sarcopenic and non-sarcopenic muscles and thereforecould be used for sarcopenia diagnosis confirmation as they are non-invasive, novel and inexpensive muscle quality markers. Further, TMG-derived parameters were also correlated to USI values; however it needs to be further explored whether could be used for sarcopenia classification purposes. The study was funded by the Slovenian Research Agency (research core funding No. P5-0381 and J7-2605 – Validation of a new muscle quality marker for the diagnosis of sarcopenia). 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.

Background: Short-term disuse minimally affects higher-threshold motor units (MUs) and neuromuscular junction (NMJ) function, but the consequences of prolonged disuse on neuromuscular function remain unclear. This study examined how 21 days of bed rest impact MU properties, NMJ transmission, and NMJ morphology. We hypothesise that this disuse period impairs MU properties across different contraction intensities, and induces alterations in NMJ structure and function. Methods: Nine healthy young males (18–29 years) underwent 21 days of strict horizontal bed rest. Pre- (BR0) and post-bed rest (BR21), we assessed quadriceps maximal voluntary contraction (MVC) and muscle size. Intramuscular (iEMG) and high-density surface electromyography (HDsEMG) were used to evaluate MU properties at 25% and 50% MVC. NMJ transmission was assessed electrophysiologically via near-fibre jiggle and segment jitter, while NMJ morphology was analyzed from muscle biopsies. Serum C-terminal agrin fragment (CAF), a biomarker of NMJ remodeling, was also measured. Results: Quadriceps MVC and size significantly decreased at BR21. MU discharge rate declined, MU potential area changed, and complexity increased at both contraction intensities, overall indicating alterations in MU properties. NMJ transmission impairment was detected at 25% MVC, with increased near-fibre jiggle and segment jitter. Serum CAF levels were elevated at BR21, corroborating NMJ remodeling. Notably, NMJ morphological analysis revealed denervated NMJs exclusively at BR21. Conclusion: Medium-term disuse alters MU properties, impairs NMJ transmission, and triggers early NMJ remodeling, including signs of denervation. These findings highlight disuse duration as a key factor and reinforce the need for countermeasures in clinical, rehabilitation, and spaceflight contexts. Space It Up project funded by the Italian Space Agency, ASI, and the Ministry of University and Research, MUR, under contract n. 2024-5-E.0 - CUP n. I53D24000060005 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.