Muscle Responses Research Articles

A three filament mechanistic model of musculotendon force and impedance

The force developed by actively lengthened muscle depends on different structures across different scales of lengthening. For small perturbations, the active response of muscle is well captured by a linear-time-invariant (LTI) system: a stiff spring in parallel with a light damper. The force response of muscle to longer stretches is better represented by a compliant spring that can fix its end when activated. Experimental work has shown that the stiffness and damping (impedance) of muscle in response to small perturbations is of fundamental importance to motor learning and mechanical stability, while the huge forces developed during long active stretches are critical for simulating and predicting injury. Outside of motor learning and injury, muscle is actively lengthened as a part of nearly all terrestrial locomotion. Despite the functional importance of impedance and active lengthening, no single muscle model has all these mechanical properties. In this work, we present the viscoelastic-crossbridge active-titin (VEXAT) model that can replicate the response of muscle to length changes great and small. To evaluate the VEXAT model, we compare its response to biological muscle by simulating experiments that measure the impedance of muscle, and the forces developed during long active stretches. In addition, we have also compared the responses of the VEXAT model to a popular Hill-type muscle model. The VEXAT model more accurately captures the impedance of biological muscle and its responses to long active stretches than a Hill-type model and can still reproduce the force-velocity and force-length relations of muscle. While the comparison between the VEXAT model and biological muscle is favorable, there are some phenomena that can be improved: the low frequency phase response of the model, and a mechanism to support passive force enhancement.

A three filament mechanistic model of musculotendon force and impedance.

The force developed by actively lengthened muscle depends on different structures across different scales of lengthening. For small perturbations, the active response of muscle is well captured by a linear-time-invariant (LTI) system: a stiff spring in parallel with a light damper. The force response of muscle to longer stretches is better represented by a compliant spring that can fix its end when activated. Experimental work has shown that the stiffness and damping (impedance) of muscle in response to small perturbations is of fundamental importance to motor learning and mechanical stability, while the huge forces developed during long active stretches are critical for simulating and predicting injury. Outside of motor learning and injury, muscle is actively lengthened as a part of nearly all terrestrial locomotion. Despite the functional importance of impedance and active lengthening, no single muscle model has all these mechanical properties. In this work, we present the viscoelastic-crossbridge active-titin (VEXAT) model that can replicate the response of muscle to length changes great and small. To evaluate the VEXAT model, we compare its response to biological muscle by simulating experiments that measure the impedance of muscle, and the forces developed during long active stretches. In addition, we have also compared the responses of the VEXAT model to a popular Hill-type muscle model. The VEXAT model more accurately captures the impedance of biological muscle and its responses to long active stretches than a Hill-type model and can still reproduce the force-velocity and force-length relations of muscle. While the comparison between the VEXAT model and biological muscle is favorable, there are some phenomena that can be improved: the low frequency phase response of the model, and a mechanism to support passive force enhancement.

KLF3 impacts insulin sensitivity and glucose uptake in skeletal muscle.

Skeletal muscle is one of the predominant sites involved in glucose disposal, accounting for approximately 80% of postprandial glucose uptake, and plays a critical role in maintaining glycemic homeostasis. Dysregulation of energy metabolism in skeletal muscle is involved in developing insulin resistance and type 2 diabetes (T2D). Transcriptomic responses of skeletal muscle to exercise found that the expression of Klf3 was increased in T2D Goto-Kakizaki (GK) rats and decreased after exercise with improved hyperglycemia and insulin resistance, implying that Klf3 might be associated with insulin sensitivity and glucose metabolism. We also found that knockdown of Klf3 promoted basal and insulin-stimulated glucose uptake in L6 myotubes, while overexpression of Klf3 resulted in the opposite. Through pairwise comparisons of L6 myotubes transcriptome, we identified 2256 and 1988 differentially expressed genes in Klf3 knockdown and overexpression groups, respectively. In insulin signaling, the expression of Slc2a4, Akt2, Insr and Sorbs1 was significantly increased by Klf3 knockdown and decreased with klf3 overexpression; Ptprf and Fasn were markedly downregulated in klf3 reduced group and upregulated in klf3 overexpressed group. Moreover, downregulation of Klf3 promoted the expression of GLUT4 and AKT proteins, as well as the translocation of GLUT4 to the cell membrane in the basal situation, and enhanced insulin sensitivity, characterized by increased insulin-stimulated GLUT4 translocation and AKT, TBC1D1 and TBC1D4 phosphorylation, while overexpression of Klf3 showed contrary results. These results suggest that Klf3 affects glucose uptake and insulin sensitivity via insulin signal transduction and intracellular metabolism, offering a novel potential treatment strategy for T2D.

Changes in inflammatory edema and fat fraction of thigh muscles following a half-marathon in recreational marathon runners.

It is known that microtrauma exists in the thigh muscles after long-distance running such as the half-marathon. Moreover, training characteristics of long-distance runners may influence the specificity of the distribution of muscle fiber types in the thigh and affect muscle responses to lipid metabolism. However, the specific changes in microtrauma and intramuscular lipid in thigh muscles after a half-marathon are unknown. A cohort of 20 healthy recreational marathon runners was recruited to complete a half-marathon. MRI T2 mapping and 6-echo q-Dixon sequences were employed at baseline (P1), 2-3h after running (P2), and 1day after running (P3). Inflammatory markers (the T2 values) and intramuscular fat fraction (the proton density fat fraction, PDFF) were measured in thigh muscles to detect microtrauma and intramuscular lipid changes, respectively. One-way analysis of variance showed significant time effects for T2 values and PDFF. Post hoc analysis of the 14 datasets collected at three time points revealed significantly higher T2 values in all thigh muscles after running (all p<0.05). Significant differences in T2 values persisted for all thigh muscles at P3 compared to P1 (all p<0.05). The PDFF of the vastus lateralis and vastus medialis was significantly decreased at P2 compared to P1 (p<0.05). No significant differences in PDFF were observed for the thigh muscles at P3 compared to P1. The manifestations of inflammation edema and intramuscular lipid investigated through MRI may offer valuable insights for recreational marathon runners regarding the lower limb movement characteristics during half-marathon running.

Acute Effects of Soft Tissue Modalities on Muscular Ultrasound Characteristics and Isometric Performance

Prior to training, many athletes perform different soft-tissue preparation protocols. Many of these protocols involve stretching, foam rolling, and/or percussion massage. Many of these modalities have been studied, but not as a group to observe muscle alterations and differences in males and females. In total, 40 (20 males, 20 females) participants performed five minutes of static stretching, foam rolling, and percussion massage. Pre- and post-isometric leg strength, muscle activation and ultrasound assessments (cross-sectional area, echo intensity, pennation angle, fascicle length, and muscle thickness) were taken. The results indicate that there is no significant difference among modalities, and that they do not significantly alter any muscle characteristic or improve performance. There is a significant difference in size between males and female, with males having larger muscle and greater pennation angles than females. This allows males to generate significantly more muscle force. However, they both respond similarly to each modality. In conclusion, the muscle response to static stretching, foam rolling, and percussion massage do not differ among modalities and do not contribute to an increase or decrease in maximal isometric knee extension with similar effects between males and females.

A new modular neuroprosthesis suitable for hybrid FES-robot applications and tailored assistance

BackgroundTo overcome the application limitations of functional electrical stimulation (FES), such as fatigue or nonlinear muscle response, the combination of neuroprosthetic systems with robotic devices has been evaluated, resulting in hybrid systems that have promising potential. However, current technology shows a lack of flexibility to adapt to the needs of any application, context or individual. The main objective of this study is the development of a new modular neuroprosthetic system suitable for hybrid FES-robot applications to meet these needs.MethodsIn this study, we conducted an analysis of the requirements for developing hybrid FES-robot systems and reviewed existing literature on similar systems. Building upon these insights, we developed a novel modular neuroprosthetic system tailored for hybrid applications. The system was specifically adapted for gait assistance, and a technological personalization process based on clinical criteria was devised. This process was used to generate different system configurations adjusted to four individuals with spinal cord injury or stroke. The effect of each system configuration on gait kinematic metrics was analyzed by using repeated measures ANOVA or Friedman’s test.ResultsA modular NP system has been developed that is distinguished by its flexibility, scalability and personalization capabilities. With excellent connection characteristics, it can be effectively integrated with robotic devices. Its 3D design facilitates fitting both as a stand-alone system and in combination with other robotic devices. In addition, it meets rigorous requirements for safe use by incorporating appropriate safety protocols, and features appropriate battery autonomy, weight and dimensions. Different technological configurations adapted to the needs of each patient were obtained, which demonstrated an impact on the kinematic gait pattern comparable to that of other devices reported in the literature.ConclusionsThe system met the identified technical requirements, showcasing advancements compared to systems reported in the literature. In addition, it demonstrated its versatility and capacity to be combined with robotic devices forming hybrids, adapting well to the gait application. Moreover, the personalization procedure proved to be useful in obtaining various system configurations tailored to the diverse needs of individuals.

Stimulation of the internal superior laryngeal nerve as a potential therapy for obstructive sleep apnea in a porcine model.

Impaired pharyngeal sensing of negative pressure (NP) can lead to a blunted response of the upper airway dilator muscles and contribute to the development of obstructive sleep apnea (OSA). This response is modulated by the nerve fibers in the internal branch of the superior laryngeal nerve (iSLN), mediating negative pressure sensation. Artificial excitation of these fibers could be a potential treatment target for OSA. To evaluate this, electrostimulation of the iSLN was performed in a porcine-isolated upper airway model. Artificial obstructions were induced by varying the levels of negative pressure, and the ability of the animal to resolve these obstructions was evaluated. The pressure at which the animal was still able to resolve the obstruction was quantified as "Resolvable Pressure." Thereby, the effects on pharyngeal patency (n = 35) and the duration of the therapeutic effect outlasting the stimulation (n = 6) were quantified. Electrostimulation before the introduction of an artificial obstruction improved the median resolvable pressure from -28.3 cmH2O [IQR: -45.9; -26.1] to -92.6 cmH2O [IQR: -105.1; -78.6]. The median therapeutic effect was found to outlast the last stimulation burst applied by 163 s when five stimulation bursts were applied in short succession [IQR: 58; 231], 58 s when two were applied [IQR: 7; 65], and 6 s when one was applied [IQR: 0; 51]. Stimulation of the iSLN increased electromyography (EMG) in the genioglossus (GG). The proposed treatment concept can improve pharyngeal patency in the model. Transfer of the results to clinical application could enable the development of a new neuromodulation therapy for OSA.NEW & NOTEWORTHY Electrostimulation before the introduction of an artificial obstruction to induce artificial sleep apnea in the pig model improves the response of the upper airway to negative pressure (NP). The electrostimulation creates a sustained therapeutic effect that outlasts the initial electrostimulation. The use of this therapy in clinical practice has the potential to treat obstructive sleep apnea (OSA).

Comparison of muscle response in patients treated with rigid and flexible fixed functional appliances.

The present study was undertaken to evaluate and compare muscle activity after the treatment with rigid and flexible fixed functional appliance. The study was conducted on 14 skeletal Class II malocclusion patients in the age group of 13-17 years, divided into two groups comprising 7 patients in each group. Group I was treated with a rigid fixed functional appliance (MPA IV), and Group II was treated with a flexible fixed functional appliance (Churro Jumper). Masseter and anterior temporalis muscle activities were recorded using needle electromyography (EMG) at postural rest, saliva swallowing, and clenching during five intervals (T0 to T4) during fixed functional appliance treatment. Unpaired t-test, Mann-Whitney U, and Wilcoxon sign rank test were applied for statistical analysis, and a P value of <.05 was considered statistically significant. Group I (MPA IV) showed a significant increase in EMG activity during postural rest position (P = 0.003, P = 0.001), swallowing (P = 0.013, P = 0.005), and clenching (P = 0.001, P = 0.002) in masseter and anterior temporalis muscle, respectively. Group II (Churro jumper) also showed a significant increase in EMG activity during postural rest position (P = 0.000, P = 0.000), swallowing (P = 0.001, P = 0.000), and clenching (P = 0.001, P = 0.000) in masseter and anterior temporalis muscle, respectively. Both rigid (MPA IV) and flexible (Churro Jumper) fixed functional appliances caused a significant increase in EMG activity of masseter and anterior temporalis muscle during postural rest position, swallowing, and clenching in 6 months of the observation period, but the flexible appliance (Churro Jumper) showed more significant increase.

The Influence of Body Position on the Resting Motor Threshold of Posterior Root-Muscle Reflexes Evoked via Transcutaneous Spinal Cord Stimulation.

Background: Thoracolumbar transcutaneous spinal cord stimulation (tSCS) non-invasively evokes posterior root-muscle reflexes (PRMR) with the aim of neuromodulating sensorimotor function following spinal cord injury. Research is still in its infancy regarding the effect of body position on the nature of these spinally evoked responses. Therefore, the aim of this study was to investigate the influence of body position on the nature of PRMR responses during tSCS. Methods: A total of 11 (6M, 5F) participants completed a full PRMR recruitment curve from 10 ma up to 120 ma (10 ma increments) at the T11/12 intervertebral space using a singular 3.2 cm diameter cathode. At each intensity, three paired pulses (50 ms inter-pulse interval), followed by three singular pulses with a six-second delay were applied in each body position (supine, supine 90-90, sitting and standing) in a randomised order. The PRMR responses in lower limb muscles were recorded using wireless electromyographic sensors placed on the Soleus, Tibialis Anterior, Rectus Femoris and Bicep Femoris long head. A two-way (body position × muscle) repeated measures analysis of variance was used to investigate the effect of body position on PRMR-evoked responses. Results: There was a significant main effect of body position on PRMR resting motor threshold (RMT) (p < 0.001), first response peak-to-peak amplitude (p = 0.003) and percentage post-activation depression (%PAD) (p = 0.012). Sitting had significantly higher RMT and significantly lower first response peak-to-peak amplitudes compared to all other positions, but significant differences in %PAD were only detectible between supine and standing. Conclusions: Body position influences the nature of PRMR-evoked responses during tSCS.

Analysis and study on biomarkers of local muscle fatigue caused by repetitive lifting task

BackgroundWork-related musculoskeletal disorders (WMSDs) show a rapid growth trend. It has brought a huge economic burden to the society and become a serious occupational health problem that needs to be solved urgently. This study aimed to analyze the local muscle response under continuous ergonomic load, screen sensitive fatigue-related biomarkers and provide data support for the early prevention of local muscle damage and the exploration of early warning indicators.MethodsThirteen male college student volunteers were recruited to perform simulated repetitive manual lifting tasks in the laboratory. The lifting task was designed for 4 periods which lasted for 12 min in each, and then paused for 3 min for sampling. Local muscle fatigue is assesed by the Rating of perceived exertion (RPE) and the Joint analysis of sEMG spectrum and amplitude (JASA). Elbow venous blood was collected and 14 kinds of biomarkers were analyzed, which included Metabolic markers Ammonia (AMM), Lactic acid (LAC), Creatine kinase (CK), Lactate dehydrogenase (LDH), Cartilage oligomeric matrix protein (COMP), C-telopeptide of collagen I and II (CTX-I, CTX-II) and Calcium ion (Ca2+); Oxidative stress marker Glutathione (GSH); Inflammatory markers C-reaction protein (CRP), Prostaglandin E2 (PG-E2), Interleukin-6 (IL-6) and Tumor necrosis factor α (TNF-α); Pain marker Neuropeptide Y (NPY). Repeated measures analysis of variance (Repeated ANOVA), linear regression analysis, t-test and spearman correlation analysis were used to analyze the data.ResultsBoth subjective and objective fatigue appeared at the same period. Serum AMM, LAC, CK, LDH, COMP, CTX-II, Ca2+ and NPY after fatigue were significantly higher than those before fatigue (p < 0.05). There was a certain degree of correlation between the markers with statistical differences before and after fatigue.ConclusionsMetabolic markers (serum AMM, LAC, CK, LDH, COMP, CTX-II, Ca2+) and pain markers (serum NPY) can reflect local muscle fatigue to a certain extent in repetitive manual lifting tasks. It is necessary to further expand the research on fatigue-related biomarkers in different types of subjects and jobs in the future.

Effect of Tourniquet-Related Nerve Ischemia on Response to Handheld Nerve Stimulation in Ulnar Nerve Transposition.

Tourniquet-related nerve ischemia has been well studied in several reconstructive procedures, but the time-course of impaired response to intraoperative stimulation is unclear. The present study evaluated ischemic effects on conduction during ulnar nerve transposition and examined the relationship between intra- and pre- operative diagnostics. We hypothesized that intraoperative ischemia would have minimal impact on conduction. Thirty patients scheduled for anterior transposition were enrolled after preoperative examination, electrodiagnostic testing, and ultrasound. Demographic and symptom severity data were recorded. A handheld biphasic nerve stimulator was used intraoperatively to assess minimum amplitude and pulse duration needed for muscle response. Measurements were taken at 15-minute intervals after placement. Changes in threshold amplitude and pulse duration were calculated between each 15-minute interval; no significant difference was found in the change of either value (p = 0.70 and 0.178). A weak negative correlation existed between preoperative CMAP amplitudes and average intraoperative pulse duration, which increased to a moderate correlation when compared to 45-minute pulse duration (r=-0.62, p<0.01). Preoperative ulnar nerve cross-sectional area (CSA) demonstrated no significant correlation with average pulse duration but a moderate correlation with pulse duration at 45 minutes (r=0.63, p=0.01). Tourniquet use did not prevent effective intraoperative stimulation of the ulnar nerve for at least 45 minutes. The window for meaningful stimulation with tourniquet usage appears to be greater than previously thought. Preoperative nerve CMAP amplitude and CSA does appear to influence pulse duration required after 45 minutes of ischemia, suggesting that injured nerves are more susceptible to ischemia. 1.

The Effects of the Leg Position on the Nordic Hamstring Exercise Eccentric Force: A Randomized Cross-Over Study

Given the posterior chain configuration, it is anticipated that tibial positioning influences hamstring recruitment; medial hamstrings should be more activated during medial rotation, while lateral hamstrings should be more activated during lateral rotation. However, most studies showing this pattern have focused on concentric or isometric strength, leaving the influence on eccentric strength fairly unknown. Therefore, the aim of this study was to investigate the electromyographic response of the hamstring muscles during Nordic hamstring exercises in three leg positions: internal rotation, external rotation, and neutral. This study encompassed a randomized crossover study and used surface electromyography to analyze the activity of the biceps femoris and semitendinosus muscles during the Nordic hamstring exercise, in the three positions. Results indicated no statistically significant differences in muscle activation between positions or sides (p &gt; 0.05), though small effect sizes were observed for the biceps femoris in different positions (η2 = 0.01–0.03). Furthermore, the internal rotation position generally elicited the highest muscle activations. Notably, biceps femoris muscles exhibited higher activations compared to semitendinosus muscles, with the greatest differences seen in the internal rotation position. This suggests that tibial rotation influences hamstring recruitment patterns; however, it was lower than expected.

Exploration of Clozapine-Induced Cardiomyopathy and Its Mechanism.

In this study, by pooling the clinical data of patients who died with a history of long-term clozapine use and by examining their hearts, it was found that long-term clozapine use can lead to cardiomyopathy and that its presentation resembles arrhythmogenic cardiomyopathy (ACM), i.e., it exhibits a predominantly right ventricular fatty infiltration with mild left ventricular damage. The transcriptomic data of rat cardiomyocytes after clozapine intervention were analyzed by transcriptomic approach to explore the causes of clozapine cardiomyopathy. The cause of clozapine cardiomyopathy was then explored by a transcriptomic approach, which revealed that its clozapine action on cardiomyocytes enriched cardiomyocyte-related differential genes in biological processes such as muscle development and response to hypoxia, as well as pathways such as fatty acid metabolism and cellular autophagy. Transcriptomic analysis showed that Egr1, Egr2, ler2, Jun, Mapk9, Nr1d2, Atf3, Bhlhe40, Crem, Cry1, Cry2, Dbp were hub genes for clozapine injury to the myocardium, and that these genes may play an important role in the myocardial ACM-like changes caused by clozapine. Combined with the results of pathological examination and transcriptomic analysis, it can be concluded that the long-term action of clozapine on cardiomyocytes leads to cellular autophagy and subsequent structural remodeling of the heart, and in the remodeling affects fatty acid metabolism, which eventually leads to ACM-like changes.

Infants’ psychophysiological responses to eye contact with a human and with a humanoid robot

Eye contact with a human and with a humanoid robot elicits attention- and affect-related psychophysiological responses. However, these responses have mostly been studied in adults, leaving their developmental origin poorly understood. In this study, 114 infants (6–8 months old) viewed direct and averted gaze directions of a live human and an embodied humanoid robot while their heart rate deceleration (attention orienting), skin conductance (affective arousal), and facial muscle activity (affective valence) were measured. In addition, a non-humanoid object (a vase) was used as a control stimulus. Infants’ attention orienting was stronger to averted versus direct gaze of a human and a robot, but indifferent to the averted versus direct orientation of the non-humanoid object. Moreover, infants’ attention orienting was equally intensive toward a human and a robot, but less intensive toward a non-humanoid object. Affective arousal was insensitive to gaze direction and did not differ between the human, the robot, and the non-humanoid object. Facial muscle responses showed sensitivity to the gaze direction of a human and of a robot but not to the orientation of the non-humanoid object. These results suggest that infants recognize the attentional and affective/affiliative significance not only in a human’s gaze but also in a robot’s gaze.

Oxidative response to cadmium and lead accumulations in the tissues of blue swimming crabs Portunus pelagicus from the Trang Province coastline, Southern Thailand.

Metals have been reported to alter the oxidative status of both redox-active and redox-inactive metals accompanying oxidative stress induction. In aquatic ecosystems, metal contamination is regarded as serious pollutants and bioaccumulation, especially when aquatic seafood products are involved, which results in human risk. The blue swimming crab Portunus pelagicus is a highly popular crab species for consumption as seafood in Thailand. The meat parts and the hepatopancreas (HP) together with gonad are consumed and in high demand. Therefore, the present study aimed to investigate bioaccumulation of cadmium (Cd) and lead (Pb) along with tissue oxidative responses in P. pelagicus. Sixty-seven samples of P. pelagicus were obtained from small-scale fishers along the coastline of Trang Province. Bioaccumulation of Cd and Pb and oxidative response in gill, muscle, and HP + gonad were evaluated. Cadmium and Pb accumulation levels were highest in the HP and gonad, followed by the gill and then muscle, indicating that Cd and Pb have a high affinity to be concentrated in the HP and gonad. An organ-specific oxidative response to Cd and Pb accumulation was demonstrated in which Cd significantly activated superoxide dismutase (SOD) activity in the gills and muscle tissue, while Pb significantly activated the SOD activity only in the HP and gonad. Only Cd accumulation in gill tissue represented a significant activation of lipid peroxidation, as indicated by the malondialdehyde level. This study implied that P. pelagicus exhibits an "adaptive stage" in the oxidative response of tissue due to metal accumulation. Additionally, the data presented here further indicate that the consumption of only the meat parts and removal of the HP and gonad would reduce human exposure to metal toxicity.

Responses to brief perturbations of stance: EMG, midline cortical, and subcortical changes.

We studied simultaneous EMG and midline EEG responses, including over the cerebellum, in 10 standing subjects (35 ± 15 yr; 5 females, 5 males). Recordings were made following repeated taps to the sternum, stimuli known to evoke short-latency EMG responses in leg muscles, consistent with postural reflexes. EEG power had relatively more high-frequency components (>30 Hz) when recorded from electrodes over the cerebellum (Iz and SIz) compared with other midline electrodes. We confirmed a previous report using a similar stimulus that evoked short-latency potentials over the cerebellum. We showed clear midline-evoked EEG potentials occurring at short latency over the cerebellum (P23, N31, N42, and P54) and frontally (N28 and N57) before the previously described perturbation-evoked potential (P1/N1/P2). The P23 response correlated with the subsequent EMG response in the tibialis anterior muscles (r = 0.72, P = 0.018), confirming and extending previous observations. We did not find a correlation with the N1 amplitude. We conclude that early activity occurs from electrodes over the inion in response to a brief tap to the sternum. This is likely to represent cerebellar activity and it appears to modulate short-latency postural EMG responses.NEW & NOTEWORTHY We studied the effects of a brief tap to the sternum in human subjects, known to evoke short-latency postural responses. Using an extended EEG recording system, we showed early evoked responses over the midline cerebellum, including the P23 potential, which correlated with the EMG responses in tibialis anterior, consistent with a cerebellar role in postural reflexes. The stimulus also evoked later EEG responses, including the perturbation potential.

BINGE DRINKING HAS NO EFFECT ON MUSCULAR STRENGTH AND NEUROMUSCULAR RESPONSES DURING MAXIMAL AND HIGH-INTENSITY ISOMETRIC FATIGUE PROTOCOL

We investigated the effects of binge drinking on maximal strength, time to exhaustion during a high-intensity isometric fatigue protocol, and neuromuscular responses of the elbow flexor muscles. Ten young male participants were randomized in two conditions: (1) alcohol consumption (ALC) or (2) placebo consumption (PLA). In each condition, volunteers ingested 1g of alcohol per kg of body mass of alcoholic beer (ALC) or non-alcoholic beer (PLA). Neuromuscular performance (elbow flexors peak torque and time to exhaustion – TTE - during an isometric fatigue protocol at 70% of peak torque) and EMG parameters (amplitude and median frequency) of biceps brachii (BB) and brachioradialis (BRA) were assessed before and after drink ingestion. A breath alcohol concentration of 1.1 ± 0.25 mg/L was observed in ALC condition. Torque was similar between-conditions (p = 0.76) and condition*moment interaction (p = 0.92), with a significant reduction in post in both conditions (p = 0.01; ALC: -4.4%; PLA: -4.7%). TTE and maximal EMG amplitude of BB were not affected by conditions and moments (p &gt; 0.05). Maximal EMG amplitude of BRA was reduced in post in both conditions (ALC: -16.4%; PLA: -14.6%). EMG amplitude of BB and BRA during the fatigue protocol, the behavior was similar between moments and conditions (p &gt; 0.05) and we only observed an increase during protocol (p &lt; 0.05). Median frequency of BB was higher in PLA compared to ALC (+9.8%). Decreases were observed in both muscles during protocol (p &lt; 0.001). Our results revealed that binge drinking did not alter maximal strength, time to exhaustion during high-intensity isometric fatigue protocol, and neuromuscular responses.

Critical variables regulating age-related anabolic responses to protein nutrition in skeletal muscle.

Protein nutrition is critical for the maintenance of skeletal muscle mass across the lifecourse and for the growth of muscle in response to resistance exercise - both acting via the stimulation of protein synthesis. The transient anabolic response to protein feeding may vary in magnitude and duration, depending on, e.g., timing, dose, amino acid composition and delivery mode, which are in turn influenced by physical activity and age. This review aims to: (i) summarise the fundamental metabolic responses of muscle to protein feeding, (ii) discuss key variables regulating muscle anabolic responses to protein feeding, and (iii) explore how these variables can be optimised for muscle anabolism in response to physical activity and ageing.

Impact of combined intermittent fasting and high-intensity interval training on apoptosis and atrophy signaling in rat fast- and slow-twitch muscles.

This study aimed to evaluate the influence of combined intermittent fasting (IF) and high-intensity interval training (HIIT) on morphology, caspase-independent apoptosis signaling pathway, and myostatin expression in soleus and gastrocnemius (white portion) muscles from healthy rats. Sixty-day-old male Wistar rats (n = 60) were divided into four groups: control (C), IF, high-intensity-interval training (T), and high-intensity-interval training and intermittent fasting (T-IF). The C and T groups received ad libitum chow daily; IF and T-IF received the same standard chow every other day. Animals from T and T-IF underwent a HIIT protocol five times a week for 12 weeks. IF reduced gastrocnemius mass and increased pro-apoptotic proteins apoptosis-inducing factor (AIF) and endonuclease G (EndoG) in soleus and cleaved-to-non-cleaved PARP-1 ratio and myostatin expression in gastrocnemius white portion. HIIT increased AIF and apoptosis repressor with caspase recruitment domain expression in soleus and cleaved-to-total PARP-1 ratio in gastrocnemius muscle white portion. The combination of IF and HIIT reduced fiber cross-sectional area in both muscles, increased EndoG and AIF expression, and decreased cleaved-to-non-cleaved PARP-1 ratio in gastrocnemius muscle white portion. Muscle responses to IF and HIIT are directly impacted by the muscle fiber type composition and are modulated, at least in part, by myostatin and caspase-independent apoptosis signaling.

Methylglyoxal reduces resistance exercise-induced protein synthesis and anabolic signaling in rat tibialis anterior muscle.

Resistance exercise provides significant benefits to skeletal muscle, including hypertrophy and metabolic enhancements, supporting overall health and disease management. However, skeletal muscle responsiveness to resistance exercise is significantly reduced in conditions such as aging and diabetes. Recent reports suggest that glycation stress contributes to muscle atrophy and impaired exercise-induced muscle adaptation; however, its role in the muscle response to resistance exercise remains unclear. Therefore, in this study, we investigated whether methylglyoxal (MGO), a key factor in glycation stress, affects the acute responsiveness of skeletal muscles to resistance exercise, focusing on protein synthesis and the key signaling molecules. This study included 12 8-week-old male Sprague-Dawley rats divided into two groups: one received 0.5% MGO-supplemented drinking water (MGO group) and the other received regular water (control group). After 10 weeks, the left tibialis anterior muscle of each rat was subjected to electrical stimulation (ES) to mimic resistance exercise, with the right muscle serving as a non-stimulated control. Muscle protein-synthesis rates were evaluated with SUnSET, and phosphorylation levels of key signaling molecules (p70S6K and S6rp) were quantified using western blotting. In the control group, stimulated muscles exhibited significantly increased muscle protein synthesis and phosphorylation levels of p70S6K and S6rp. In the MGO group, these increases were attenuated, indicating that MGO treatment suppresses the adaptive response to resistance exercise. MGO diminishes the skeletal muscle's adaptive response to ES-simulated resistance exercise, affecting both muscle protein synthesis and key signaling molecules. The potential influence of glycation stress on the effectiveness of resistance exercise or ES emphasizes the need for individualized interventions in conditions of elevated glycation stress, such as diabetes and aging.