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PurposeThe purpose of this study was to compare laterality in motor unit firing behavior between females and males.MethodsTwenty-seven subjects (14 females) were recruited for this study. The participants performed ramp up and hold isometric index finger abduction at 10, 30, and 60% of their maximum voluntary contraction (MVC). High-density surface electromyography (HD-sEMG) signals were recorded in the first dorsal interosseous (FDI) muscle and decomposed into individual motor unit (MU) firing behavior using a convolution blind source separation method.ResultsIn total, 769 MUs were detected (females, n = 318 and males, n = 451). Females had a significantly higher discharge rate than males at each relative torque level (10%: male dominant hand, 13.4 ± 2.7 pps vs. female dominant hand, 16.3 ± 3.4 pps; 30%: male dominant hand, 16.1 ± 3.9 pps vs. female dominant hand, 20.0 ± 5.0 pps; and 60%: male dominant hand, 19.3 ± 3.8 vs. female dominant hand, 25.3 ± 4.8 pps; p < 0.0001). The recruitment threshold was also significantly higher in females than in males at 30 and 60% MVC. Furthermore, males exhibited asymmetrical discharge rates at 30 and 60% MVC and recruitment thresholds at 30 and 60% MVC, whereas no asymmetry was observed in females.ConclusionIn the FDI muscle, compared to males, females exhibited different neuromuscular strategies with higher discharge rates and recruitment thresholds and no asymmetrical MU firing behavior. Notably, the findings that sex differences in neuromuscular activity also occur in healthy individuals provide important information for understanding the pathogenesis of various diseases.

Electrical muscle stimulation (EMS) has been shown to stimulate the production of myokines (i.e., brain-derived neurotrophic factor (BDNF)), but the most effective EMS parameters for myokine production have not been fully elucidated. The purpose of this study was to quantify the optimal EMS frequency for stimulating myokine production. This study included sixteen young adults (male, n  = 13, age = 27.3 ± 5.5 years). Participants underwent four EMS interventions (20 min each) with the following conditions: (1) 4 Hz, (2) 20 Hz, (3) 80 Hz, and (4) control (no intervention). Blood samples were obtained before and immediately after EMS. For the control condition, blood samples were taken before and after 20 min of quiet sitting. BDNF and cathepsin-B levels were analyzed in serum. Compared to preintervention levels, stimulation at 20 Hz resulted in significantly greater postintervention cathepsin-B and BDNF levels ( p  < 0.01). On the other hand, the control condition did not result in a significant change between pre- and posttreatment. Furthermore, stimulation at 20 Hz caused significantly larger increases in cathepsin-B and BDNF levels than stimulation at 4–80 Hz or the control condition ( p  < 0.05). In conclusion, stimulation at 20 Hz effectively causes a robust cathepsin-B and BDNF response. Based on these results, we suggest a new strategy for rehabilitation of people with neurological disorders.

PurposeElectrical muscle stimulation (EMS) is known to be effective at stimulating brain-derived neurotrophic factor (BDNF) levels, but the relationship between the volume of muscle stimulated and BDNF levels is not clear. The purpose of this study was to quantify BDNF as a function of muscle volume stimulated in young adults.MethodsTwelve young adults (male, n = 9, age = 27.3 ± 5.5 years) were enrolled in this study. Participants completed three testing conditions in randomized order: 23 min of maximum tolerated bilateral stimulation of (1) the quadriceps muscle or (2) the musculature of the entire lower limbs and (3) control testing and retesting after 23 min without an intervention. Blood samples were collected before, immediately after, 20 min after, and 40 min after the intervention when EMS was applied to the thighs or the entire lower limb conditions. Serum obtained from blood collection was used for BDNF analysis.ResultsThe delta value of BDNF for the test and retest in the control condition was − 42.1 ± 73.8 pg/mL, and there was no significant difference between the test and retest BDNF. Compared to stimulation of the quadriceps muscle, stimulation of the entire lower limbs produced significantly higher BDNF at 20 min post-treatment than those at pre-treatment or 40 min post-treatment, and BDNF was also significantly higher immediately post-treatment than those at pre-treatment. Only stimulation of the quadriceps muscle did not induce a significant change between pre- and post-treatment.ConclusionOur findings suggest that the volume of muscle stimulation is important for increased BDNF.

This systematic review and meta-analysis examined the efficacy of neuromuscular electrical stimulation (NMES) on lower limb muscle strength and health-related quality of life (HR-QOL) after thoracic and abdominal surgery. We searched the Cochrane Central Register of Controlled Trials, MEDLINE via PubMed, Excerpta Medica Database via Elsevier, Physiotherapy Evidence Database, Cumulative Index to Nursing and Allied Health Literature, World Health Organization International Clinical Trials Registry Platform via their dedicated search portal, and ClinicalTrials.gov on November 2021 and updated in April 2023 to identify randomized controlled trials that examined the effects of NMES after thoracic and abdominal surgery. The primary outcomes were lower limb muscle strength, HR-QOL, and adverse events. We used the Cochrane Risk of Bias Tool and the Grading of Recommendations, Assessment, Development, and Evaluation approach to assess the certainty of evidence. A total of 18 randomized control trials involving 915 participants, including 10 on cardiovascular surgery, two on pulmonary surgery, five on digestive system surgery, and one on other surgery, were included. NMES slightly increased lower limb muscle strength and adverse events in cardiovascular surgery. Adverse events (hypotension, pain, and muscle discomfort) occurred in seven patients. HR-QOL was measured in two studies on cardiovascular surgery, but these were not pooled due to concept heterogeneity. Overall, NMES slightly increases lower limb muscle strength after cardiovascular surgery without serious adverse events. However, higher-quality randomized control trials in thoracic and abdominal surgeries are needed.

Forward head posture (FHP) is a serious problem causing head and neck disability, but the characteristics of muscle activity during long-term postural maintenance are unclear. This study aimed to investigate a comparison of electromyography (EMG) activation properties and subjective fatigue between young adults with and without habitual FHP. In this study, we examined the changes in the spatial and temporal distribution patterns of muscle activity using high-density surface EMG (HD-SEMG) in addition to mean frequency, a conventional measure of muscle fatigue. Nineteen male participants were included in the study (FHP group (n = 9; age = 22.3 ± 1.5 years) and normal group (n = 10; age = 22.5 ± 1.4 years)). Participants held three head positions (e.g., forward, backward, and neutral positions) for a total of 30 min each, and the EMG activity of the trapezius pars descendens muscle during posture maintenance was measured by HD-SEMG. The root mean square (RMS), the modified entropy, and the correlation coefficient were calculated. Additionally, the visual analogue scale (VAS) was evaluated to assess subjective fatigue. The RMS, VAS, modified entropy, and correlation coefficients were significantly higher in the FHP group than in the normal group ( p  < 0.001). With increasing postural maintenance time, the modified entropy and correlation coefficient values significantly decreased, and the mean frequency and VAS values significantly increased ( p  < 0.001). Furthermore, the forward position had significantly higher RMS, correlation coefficient, modified entropy, and VAS values than in the neutral position ( p  < 0.001). The HD-SEMG potential distribution patterns in the FHP group showed less heterogeneity and greater muscle activity in the entire muscle and subjective fatigue than those in the normal group. Excess muscle activity even in the neutral/comfortable position in the FHP group could potentially be a mechanism of neuromuscular conditions in this population.

The plantar fascia and intrinsic foot muscles (IFM) modulate foot stiffness. However, it is unclear whether the corresponding ultrasonography findings reflect it. This study aimed to examine the effect of the plantar fascia and IFM morphologies on force attenuation during landing and reactivity when jumping in healthy adults (n = 21; age, 21–27 years). Thickness, cross-sectional area (CSA), and hardness of the plantar fascia, abductor hallucis (AbH), and flexor hallucis brevis (FHB) muscles were measured using ultrasonography. Single-leg drop landing and repetitive rebound jumping tests assessed the ground reaction force (GRF) and reactive jump index (RJI), respectively. The CSA of FHB was negatively correlated with maximum vertical GRF (r = −0.472, p = 0.031) in the single-leg drop landing test. The CSA of AbH was negatively correlated with contact time (r = −0.478, p = 0.028), and the plantar fascia thickness was positively correlated with jump height (r = 0.615, p = 0.003) and RJI (r = 0.645, p = 0.002) in the repetitive bound jump test. In multivariate regression analysis, only the plantar fascia thickness was associated with RJI (β = 0.152, 95% confidence interval: 7.219–38.743, p = 0.007). The CSA of FHB may contribute to force attenuation during landing. The thickness of the plantar fascia and CSA of AbH may facilitate jumping high with minimal contact time.

Background Sedentary behavior, particularly prevalent during the 2019 coronavirus disease (COVID-19), has been shown to lead to depression. In medical students, this could negatively affect the provision of healthcare. Thus, this study aimed to investigate the association between sedentary behavior and depression in Japanese medical students during the COVID-19 pandemic. Methods An online cross-sectional survey was conducted from July 30 to August 30, 2021, using anonymous questionnaire system to assess respondents’ sociodemographic characteristics, physical activity, sedentary behavior among 1000 Japanese university students. Depression was assessed using the Patient Health Questionnaire-2 (PHQ-2). The data of 484 respondents (48.4%) were included in a stepwise analysis, where we set the difference between medical and non-medical students as Model 1 and that between medical students with and without depression as Model 2. For group comparisons of both models, the chi-square test was used for sociodemographic characteristics, and the Mann–Whitney U-test was used for physical activity and sedentary behavior. In Model 2, factors associated with depression among medical students were analyzed by logistic regression analysis. Results In Model 1, medical students were less physically active ( p  < 0.001), had longer sedentary time ( p  < 0.001), and had higher PHQ-2 scores ( p  = 0.048) than non-medical students. In Model 2, medical students with depression had longer sedentary time ( p  = 0.004) and longer leisure screen time than those without depression ( p  = 0.007). Moreover, logistic regression analysis adjusted for potential confounders showed that sedentary time (OR = 1.001, p  = 0.048) and leisure screen time (OR = 1.003, p  = 0.003) were significantly associated with depression among medical students. Conclusions Based on these results, it is evident that reducing Japanese medical students’ sedentary time and leisure screen time can help combat depression during the COVID-19 pandemic; thus, these results can guide the development of appropriate interventions to prevent and treat depression.

This study aimed to evaluate motor unit recruitment during submaximal voluntary ramp contraction in the medial head of the gastrocnemius muscle (MG) by high-density spatial electromyography (SEMG) before and after static stretching (SS) in healthy young adults. SS for gastrocnemius was performed in 15 healthy participants for 2 min. Normalized peak torque by bodyweight of the plantar flexor, muscle activity at peak torque, and muscle activation patterns during ramp-up task were evaluated before and after SS. Motor unit recruitment during the submaximal voluntary contraction of the MG was measured using SEMG when performing submaximal ramp contractions during isometric ankle plantar flexion from 30 to 80% of the maximum voluntary contraction (MVC). To evaluate the changes in the potential distribution of SEMG, the root mean square (RMS), modified entropy, and coefficient of variation (CV) were calculated from the dense surface EMG data when 10% of the MVC force was applied. Muscle activation patterns during the 30 to 80% of MVC submaximal voluntary contraction tasks were significantly changed from 50 to 70% of MVC after SS when compared to before. The variations in motor unit recruitment after SS indicate diverse motor unit recruitments and inhomogeneous muscle activities, which may adversely affect the performance of sports activities.

Restoring knee extensor strength after anterior cruciate ligament (ACL) reconstruction using a quadriceps tendon (QT) autograft remains challenging. Whether altered quadriceps neuromuscular activity contributes to this weakness is unclear. Quadriceps strength recovery after ACL reconstruction using QT would be impaired owing to altered quadriceps neuromuscular activity. Cross-sectional study; Level of evidence, 3. A total of 31 patients (Tegner activity scale score ≥7) who had undergone ACL reconstruction 4 months earlier were enrolled into either the QT group (n = 15) or hamstring tendon (HT) group (n = 16). Knee extensor strength was assessed using maximum voluntary isometric and isokinetic (60 deg/s) force. Neuromuscular activity of the vastus lateralis (VL) and vastus medialis (VM) during isometric contraction was recorded with high-density surface electromyography to evaluate motor unit discharge rate (MU DR). A generalized linear mixed-effects model was used to test side (involved and uninvolved limbs) and task (25%, 50%, and 70% of isometric contraction) effects, and regression analysis was used to examine associations between side-to-side isometric strength and MU DR differences. Baseline characteristics did not differ significantly between the QT (9 males, 6 females; mean age, 17.1 ± 3.9 years) and HT (10 males, 6 females; mean age, 18.2 ± 2.7 years) groups. The mean time from surgery to testing was also similar (QT group: 120.8 ± 8.0 days; HT group: 121.3 ± 7.2 days). The limb symmetry index for isometric strength showed no difference (QT group: 75.0% ± 19.6%; HT group: 78.2% ± 14.3%; = .63). In contrast, isokinetic strength at 60 deg/s was significantly lower in the QT group (70.5% ± 13.2%) than in the HT group (83.9% ± 11.6%) ( < .01). In the QT group, MU DR showed a side × task interaction, with higher discharge in the involved (11.7 ± 3.2 pluse per second [pps]) versus uninvolved (10.4 ± 1.9 pps) limbs during 70% isometric contraction in VM ( < .01). Correlations were observed between isometric strength differences and MU DR in both VL ( = 0.65; = .01) and VM ( = 0.67; = .01). Neuromuscular changes were evident in the QT group, particularly in the VM, but not in the HT group. Notably, in the QT group, altered MU activity was correlated with muscle strength decline.

The aim of this study was to examine the effect of electrical muscle stimulation on muscle atrophy caused by neuromyelitis optica. Two neuromyelitis optica patients with flaccid paralysis participated. The participants underwent a general rehabilitation program (transfer, balance, and gait training) and electrical muscle stimulation on the quadriceps femoris muscle of their paralyzed side. The change in the thickness of the vastus lateralis muscle ranged from −0.6 to 1.4 mm. These changes were minimal and did not indicate muscle atrophy. These findings suggest that in addition to general physiotherapy, electrical muscle stimulation seems safe and feasible in the acute phase of neuromyelitis optica.