Journal of applied physiology最新文献

Despite the critical role of persistent inward currents (PICs) in modulating motor neuron output, and thus neuromuscular performance, it remains unknown whether their contribution to motor neuron discharge behaviour varies throughout the day. This study aimed to determine whether PIC-related effects on motor neuron activity during submaximal dorsiflexion tasks differ between the early morning and late afternoon. Eighteen healthy adults (4 females; 27.4±5.6 years) performed triangular isometric contractions at two randomized time-points on separate days: early morning (7:00-8:30a.m.) and late afternoon (5:00-7:30p.m.). Two conditions were tested: (1) a relative condition, where the target force corresponded to 40% of the maximal voluntary force (MVF) measured during that session, and (2) an absolute condition, where the target force was 40%MVF recorded during the first session. High-density surface electromyography signals were recorded from the tibialis anterior and decomposed into motor unit spike trains. The prolongation effect of PICs, estimated via ΔF, was significantly greater in the late afternoon in both the relative-absolute force conditions. The amplification effect of PICs, estimated by the acceleration phase of the discharge trajectory, was higher in the late afternoon, but only in the relative force condition. Brace height did not differ between morning and afternoon, but attenuation was lower in the late afternoon during the relative force condition. Collectively, these findings suggest a time-of-day modulation of PIC contribution to motor neuron discharge behaviour, likely mediated by a change in inhibitory-excitatory balance between early morning and late afternoon rather than by changes in neuromodulatory drive.

Loss of dystrophin alters the biomechanical properties of skeletal muscle, including stiffness. Stiffness is typically assessed passively in excised muscle, but here we present the development of an in vivo rheological method to assess the mechanical properties of the tibialis anterior muscle in anaesthetized wild-type (WT; dystrophin-positive) and mdx (dystrophin-deficient) mice using a custom-designed apparatus compatible with an MCR 702 rheometer. To characterize stiffness, compressibility, and elasticity, rheological testing included compressive and shear strain protocols, along with recovery and assessments following contraction-induced strength loss. Relative to WT mice, the tibialis anterior of mdx mice was thicker, stiffer, and less compressible. These genotype differences aligned with hydroxyproline content, a marker of fibrosis. Postdeformation recovery was impaired in mdx mice under shear strain, and eccentric contraction-induced injury further increased stiffness and energy dissipation in the tibialis anterior of mdx mice. This rheological platform maintained the in vivo integrity of the tibialis anterior muscle of mice and consistently showed that storage and loss moduli can sensitively detect the detrimental impact of dystrophin deficiency on the in vivo viscoelastic properties of skeletal muscle. This rheological platform, termed myomechanical profiling, could be a viable and sensitive tool for assessing muscle quality and mechanical behavior of skeletal muscle, where viscoelastic properties are affected by disease.NEW & NOTEWORTHY Myomechanical profiling was developed using cyclic rheometry to assess the in vivo viscoelastic properties of mouse skeletal muscle-the in vivo environment is maintained alongside high measurement sensitivity and spatial resolution, and the ability to apply deformation transverse to fiber orientation. Myomechanical profiling was trialed in dystrophin-positive and dystrophin-negative (model of Duchenne muscular dystrophy) skeletal muscle, and showed that the loss of the biomechanical protein dystrophin increased stiffness and impaired elasticity after compressive and rotational shear deformation.

In this study, we aimed to determine the association between changes in estimates of neural drive and global measures of electromyographic (EMG) amplitude elicited by short-term strength training. A cohort of 13 individuals performed 4 wk of strength training, which increased the maximal voluntary force (MVF) of the ankle dorsiflexors by approximately 14%, maximal root mean square (RMS) EMG amplitude for the tibialis anterior by approximately 42%, motor unit discharge rate by approximately 11%, and decreased motor unit recruitment threshold by approximately 10%. The increase in EMG amplitude during the submaximal contractions was observed at 50 and 70% of MVF (P < 0.05) but only for the absolute (µV) and not the normalized (% of MVF) root mean square (RMS) values. At the level of individual participants, it was possible to predict with moderate strength the changes in recruitment threshold and discharge rate after training (recruitment threshold vs. RMS, r = -0.55, P = 0.041; discharge rate vs. RMS, r = 0.56, P = 0.037, repeated measures correlations). These associations were not statistically significant when the EMG amplitude was normalized by the RMS values during the MVF contractions. Moreover, modeling the EMG with only the tracked motor units produced a strong correlation between the changes after training for both the reconstructed and measured EMG (r = 0.86, P < 0.001). These results demonstrate that the adaptations in neural drive experienced by individual participants after short-term (<1 mo) training interventions can be estimated from the absolute amplitude of multichannel EMG signals.NEW & NOTEWORTHY The absolute EMG amplitude estimated with high-density electrode grids can partially capture within-participant changes in motor unit discharge rates and recruitment thresholds, provided the intervention does not alter muscle fiber membrane properties. These adaptations, however, are not detectable in EMG signals normalized to peak values during maximal contractions. Consequently, global EMG analysis can serve as an approximate indicator of neural adaptations within participants during the early stages of strength training.

Sex differences in endurance performance are well established, but the magnitude of these differences appears smaller in ultraendurance events, particularly swimming. This study examined sex differences in solo English Channel swimming performances and assessed the influence of age and water temperature. A retrospective analysis included 2,593 unique solo English Channel swims (1,724 males and 869 females) recorded between 1875 and 2025. Swims were obtained from the Channel Swimming & Piloting Federation database. To minimize variability from noncompetitive crossings, analyses were restricted to the top 100 swimmers of all time within each age and water temperature subgroup. Sex differences were assessed across all finishers, among the top 100 swimmers, within age and water temperature categories, and over consecutive 5-yr periods. Across all swims, males were 2.2% faster than females (13.54 ± 0.06 h vs. 13.84 ± 0.10 h, P = 0.009). Restricting analysis to the top 100 swimmers, the sex difference was 7.2% (9.00 ± 0.08 h vs. 9.67 ± 0.08 h, P < 0.001). Sex differences increased with age, ranging from 5.2% in the 20- to 29-yr group to 26.5% among swimmers >60 yr. Males outperformed females across all water temperature categories, and the fastest swims for both sexes were observed at 17°C to 17.9°C. Male swimmers are faster than female swimmers in English Channel crossings, with differences most pronounced among top performers and older athletes. However, sex differences are smaller than those reported in ultraendurance running, suggesting that the unique environmental and physiological demands of open-water swimming partially attenuate male performance advantages.NEW & NOTEWORTHY This study leverages an experiment of nature framework to examine sex-based differences in ultraendurance swimming using English Channel performance data. Males were faster than females, with the greatest differences observed among older swimmers and top performers. Performance was optimal in moderately cool water temperatures (17-18°C). These findings highlight how naturally occurring endurance events provide unique opportunities to study physiological limits and the combined influences of sex, age, and environment on human performance.

Exertional dyspnea is the main symptom of chronic obstructive pulmonary disease (COPD). Based on the positive relationship between the levels of dyspnea (Borg score) and the electrical activity of respiratory muscles [electromyogram (EMG)] during an incremental cardiopulmonary exercise test, it has been suggested that respiratory EMG can provide a physiological biomarker for dyspnea. This study aimed to characterize the relationship between dyspnea and EMG during exercises simulating daily activities. Surface EMG was measured at two locations on the chest of 28 patients with COPD while they were performing constant-work rate cycling tests and walking/cycling exercises that were part of their rehabilitation program. Simultaneously, the level of dyspnea was assessed using the Borg score at several timepoints throughout the exercise sessions, along with respiration rate (RR), heart rate (HR), and transcutaneous oxygen saturation ([Formula: see text]). Patients completed each up to 10 such study visits during their 8-wk stay at the rehabilitation center (CIRO, the Netherlands). In total, 1981 Borg scores with associated EMG measurements were recorded during 263 study visits. A linear-mixed model was used to assess the relation of the Borg score with EMG while controlling for RR, HR, (type of) exercise, [Formula: see text], age, and sex. Random effects for patient and visit were included to account for correlation in the measurements. EMG had a highly significant association with the Borg score (P < 0.0001). Respiratory EMG and Borg score showed consistent positive correlations, of which the magnitude varied between patients. These results indicate that respiratory EMG can provide a physiological biomarker for dyspnea during activities of daily living in patients with COPD.NEW & NOTEWORTHY This study demonstrates that, even after controlling several physiological variables, electromyography (EMG) of the respiratory muscles is significantly associated with dyspnea, as assessed by the Borg scale in patients with chronic obstructive pulmonary disease (COPD) during daily activities. This finding suggests that respiratory EMG may serve as a physiological biomarker for dyspnea. Surface EMG measured on the chest offers valuable insights for assessing dyspnea, providing an additional, objective tool to capture the intensity of dyspnea during daily living.

Pulmonary surfactant is a vital component of respiratory physiology. Surfactant homeostasis is disrupted in various pulmonary disease states, and exogenous surfactant therapy has been proposed as a treatment to improve lung function and recovery. Although this therapy has demonstrated clinically significant benefit in neonatal respiratory distress syndrome, for adult patients with acute respiratory distress syndrome (ARDS), the evidence is inconclusive. To understand the potential shortcomings of past trials and potential opportunities for more effective exogenous surfactant use in ARDS, we review in detail past trials and literature involving exogenous surfactant therapy in adult patients with ARDS. We assess various factors that may have impacted trial results and propose potential solutions and areas for future research. Advances in surfactant research suggest a potential role for exogenous surfactant therapy for adult patients with ARDS.

Although some studies report attenuated net muscle glycogenolysis with carbohydrate ingestion, others show no effect, possibly due to small sample sizes or methodological differences. Objective of this study is to determine whether carbohydrate ingestion during endurance exercise reduces net skeletal muscle glycogen use and to identify potential moderating factors. A meta-analysis was conducted using data from 31 studies, which included 48 unique effect sizes derived from crossover trials comparing carbohydrate versus placebo ingestion during prolonged endurance exercise. Standardized mean differences (SMDs) in net muscle glycogen use were calculated. A multilevel random-effects model accounted for repeated estimates within studies. Subgroup and meta-regression analyses tested potential moderators. Sensitivity analyses were conducted using a range of plausible pre-/postcorrelation values. Carbohydrate ingestion was associated with a small but statistically significant muscle glycogen-sparing effect [SMD = -0.16, 95% confidence interval (CI): -0.30 to -0.02, P = 0.021]. Subgroup and moderator analyses revealed no significant effects of exercise mode, carbohydrate type, ingestion rate, or preexercise glycogen on the observed effect. Translating the standardized effect into absolute units, carbohydrate ingestion was estimated to spare ∼24 mmol·kg^-1 dry wt (95% CI: 4-45 mmol·kg^-1) of muscle glycogen, relative to placebo, during ∼100 min of exercise. Carbohydrate ingestion during endurance exercise leads to a small but statistically significant reduction in net skeletal muscle glycogen utilization. Although no consistent moderating variables were identified, the direction of effect was consistent across studies, and the absolute magnitude of sparing may be physiologically meaningful during prolonged or repeated efforts.

This study aimed to elucidate the effects of repetition duration (contraction duration for each repetition) of resistance exercise on muscle hypertrophy and its underlying mechanisms using a rat exercise model. Male Sprague-Dawley rats were randomly assigned to three groups trained with short (S), medium (M), and long (L) repetition durations. During resistance exercise, the right gastrocnemius muscles were electrically stimulated to induce maximal tetanic contractions, each lasting for 1, 3, and 9 s in S, M, and L groups, respectively. The number of contractions in each set and the interset rest duration were the same across groups, whereas the number of sets was adjusted to match the total torque-time integral of plantar flexion. The left (untrained) and right (trained) gastrocnemius muscles were sampled 48 h after 12 exercise bouts. The average and peak torques during each exercise were consistently highest in S and lowest in L. The muscle mass and fiber cross-sectional area significantly increased in S and M but not in L. The concentrations of total RNA and 18S + 28S rRNA increased only in S and were correlated with muscle mass when the three groups were combined. We also sampled muscle tissues 6 h after a single exercise bout and found no significant difference in muscle protein synthesis, mTOR signaling activity, ribosome biogenesis, or protein degradation between the groups. These results suggest that a long repetition duration does not promote but rather diminishes the hypertrophic effects of resistance exercise, and that acute molecular responses to resistance exercise cannot predict such effects.NEW & NOTEWORTHY Using a rat model, we investigated the hypertrophic effects of resistance exercise with different repetition durations under conditions where the torque-time integrals were matched. The results suggest that exercise with short but not long repetition duration effectively induces muscle hypertrophy. The lack of hypertrophic response or ribosome biogenesis with long repetition duration may not be predicted by acute molecular responses to exercise but may be explained in part by attenuated torque production during exercise.

During spaceflight, humans are exposed to unfamiliar gravitational fields and to rapid transitions in the magnitude of these gravitational fields. Many aspects of sensorimotor neural function are altered by these transitions, and adaptation after transitions has been characterized. However, it is important to know whether human physiology has inherent limitations in hypogravity (i.e., gravity between 0 and 1.0 G) that cannot be overcome by adaptation. To address this critical gap, we studied manual control performance using a laboratory-based centrifuge that was configured and used to mimic hypogravity. Ten healthy human subjects performed a manual control task using a joystick to control the tilt of the motorized chair upon which they were sitting. Manual control performance worsened immediately after transition from 1.0 G_c to 0.5 G_c (69%), partly adapted over 18 min in 0.5 G_c, and remained significantly worse despite adaptation (42%) (1.0 G_c = 9.81 m/s² of centripetal acceleration). We propose that in hypogravity, any particular body tilt will result in diminished shear force on sensory graviceptors relative to 1 G, reducing signal relative to intrinsic neural noise. This necessitates larger tilt angles before manual control inputs can be determined, thus worsening performance. These results add to prior studies providing evidence supporting the hypothesis that closed-loop sensorimotor performance may be fundamentally limited by signal-to-noise ratio, including in hypogravity. This may contribute to risk during lunar piloting and ambulation. We also studied underlying mechanisms using a computational model of closed-loop control and found that adaptation was associated with increasing control gain (K_P).NEW & NOTEWORTHY Prior studies suggest that humans can adapt to hypogravity (i.e., 0-1.0 Earth G). We examined human manual control performance during the transition from an Earth-gravity to a hypogravity condition. We found that performance worsened significantly after the transition. With practice in the hypogravity condition, performance partially improved with practice over repeated trials, but remained impaired in the hypogravity condition relative to the Earth-gravity condition.