Taian Martins Vieira

This study aimed to examine how horizontal resistance affects spatiotemporal kinematics and the electromyographic (EMG) activity of the biceps femoris long head (BFlh) and semitendinosus (ST) in sprinting. Eleven trained male football players ( n = 10) performed 40 m sprints under three resistance conditions (0, 10, and 20% decrements in maximum sprint velocity, %vdec). Spatiotemporal variables were captured using a photoelectric system, while high-density surface EMG recorded regional muscle activity in BFlh and ST. Analyses were conducted at both maximum speed and matched running speed across conditions to isolate the effects of resistance and velocity. Regional EMG distribution (centroid analysis) was evaluated in a subset of participants with sufficient signal quality ( n = 5). At matched speed, spatiotemporal variables showed no differences across resistance levels, however ST EMG activity was higher at 0 %v _dec condition compare to 10 %v _dec condition during the swing phase (70.50–75.70% P = 0.001). At maximum speed, moderate differences were found in step length and frequency. ST EMG activity was higher at 0 %v _dec compare to 20 %v _dec during stance phase (10.22–18.02% P = 0.001) and 0 %v _dec compare to 10 %v _dec during swing phase (68.50–71.00% P = 0.004). Centroid analysis revealed individual-specific shifts in the region of the highest EMG amplitude within the muscles. Moderate horizontal resistance has no or small impact on global hamstring EMG activity or sprint mechanics at matched speed but may induce individual shifts in regional EMG activity. Future studies should examine how these acute neuromuscular responses translate into long-term neuromuscular adaptations.

Background: Sport climbing imposes high endurance demands on finger flexor muscles, which sustain near-isometric loads for prolonged periods. Although women often demonstrate greater muscle endurance than men, the neuromuscular mechanisms underlying potential sex differences in climbing remain unclear. Purpose: Determine whether sex differences in climbing endurance are explained by physiological muscle adaptations, neural strategies of motor unit (MU) control, or both. Methods: High-density surface electromyograms (HD-sEMG) were recorded from the dominant forearm of 9 female and 13 male intermediate climbers during sustained body suspension on a campus board until failure, using two grip depths (20 and 30 mm) to manipulate task demand. EMGs were decomposed into MU discharge trains. Physiological adaptation was assessed via temporal changes in MU action potential amplitude and median frequency (MDF), while neural strategies were evaluated using traditional MU discharge metrics and a mode-based analysis of MU firing patterns. Results: Despite anthropometric differences, time to failure did not differ between sexes. MDF declined more slowly in women, indicating greater resistance to muscle fatigue. Traditional MU metrics showed greater discharge variability and intermittency in women. Mode analysis revealed three common temporal MU firing modes across sexes; however, men exhibited greater reliance on sustained early-phase MU activation, particularly under higher task demands. Conclusion: Sex differences in climbing endurance are not reflected in outcomes alone but arise from distinct neuromuscular mechanisms. Greater physiological fatigue resistance in women is complemented by sex-specific neural strategies of MU rate coding, underscoring the importance of integrating physiological and neural analyses when examining endurance performance in climbing.

Voluntary force production depends on muscle–tendon unit length, due not only to mechanical factors (e.g., force-length properties and moment arms) but also neural modulation mediated by afferent feedback. Whether muscle length-dependent modulation of motor unit (MU) discharge differs across MU recruitment thresholds remains unclear. This study compared MU firing rates of the tibialis anterior during submaximal contractions with two ankle joint positions, while grouping MUs by recruitment threshold. Eighteen healthy adults performed isometric dorsiflexion at 0° (shortened position) and 20° (lengthened position) plantar flexion. High-density surface electromyography was decomposed into individual MU discharge times. Maximal voluntary contraction (MVC) torque was measured in each position. Participants then performed ramp-up contractions to 50% MVC. Firing rates were separately assessed for the same absolute target and relative MVC torque. MUs were categorized as low- (0–15% MVC), mid- (15–30% MVC), or high- (30–45% MVC) threshold. High-threshold MUs discharged faster in the shortened than lengthened position under both absolute and relative torque comparisons, whereas low-threshold MU firing rates did not differ between positions. These findings indicate that a shortened muscle position preferentially enhances the firing rates of higher-threshold MUs during submaximal contractions, in agreement with neural compensation for reduced mechanical efficiency at shorter lengths.

High-density surface electromyography has recently emerged as a tool to investigate neuromuscular function across different motor tasks. Muscle fiber conduction velocity, a biophysical marker estimated from high-density surface electromyography signals, quantifies the speed of action potential propagation along muscle fibers. Currently, little information is available on the responses of muscle fiber conduction velocity during dynamic exercise. We therefore explored how cycling within two distinct exercise-intensity domains (heavy and severe) elicited substantial changes in vastus lateralis muscle fiber conduction velocity. Furthermore, we compared muscle fiber conduction velocity with averaged rectified values obtained from the same high-density surface electromyography signals, and we tested differences in their respective intra-individual variability. In the severe-intensity domain, muscle fiber conduction velocity decreased from 4.44±0.22 to 4.03±0.22 m∙s⁻¹, whereas in the heavy-intensity domain, it increased from 4.56±0.33 to 5.02±0.42 m∙s⁻¹ (p<0.001). Although both muscle fiber conduction velocity and averaged rectified values changed significantly over time, indicating the myoelectric manifestation of fatigue in the severe domain, muscle fiber conduction velocity showed less variability within single time points. These findings suggest that muscle fiber conduction velocity is a more robust indicator of myoelectric fatigue than averaged rectified values, even in highly dynamic contractions like cycling. Received: 29 October 2025 Accepted after revision: 16 March 2026 Article published online: 23 April 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany

This paper investigated the suitability of the integrated Recursive Rehabilitation Control Network (RRC-Net)/ High-Density Electrode Array (HDE-Array) system for performing two multi-Degree of Freedom (DoF) control tasks, developed as proxies for Functional Electrical Stimulation control: (1) a cursor-based task and (2) a 3-DoF hand kinematic model control task. The goal of this study is enhancing rehabilitation independence for individuals with spinal cord injuries. The system was validated on both healthy and tetraplegic subjects. The hypotheses that users could successfully perform these tasks using the system and that there would be no significant performance differences between healthy and tetraplegic participants were assessed. The experiment involved 10 tetraplegic and 8 healthy subjects who completed a training phase followed by two testing phases. High-Density surface Electromyography (HD-sEMG) signals recorded from the neck during the training phase were used to train RRC-Net, a neural network designed to estimate multi-DoF movements. Subjects then performed the two control tasks in the testing phase, and performance metrics were analysed and compared between groups. Healthy and tetraplegic subjects achieved high performance in both control tasks. Hand position control performance between the two groups presented no statistically significant differences in Mean Global Distance (MGD) (p = 0.93) or Mean Angular Distance (MAD) (p = 0.77). Similarly, cursor control task performance showed no significant differences in Task Completion Score (TCS) (p = 0.68) or Normalised Distance (ND) (p = 0.63). The system’s simplicity, comfort, and effectiveness highlight its potential for rehabilitation, providing a non-invasive method for controlling assistive devices.

This study describes a novel high-density EMG dry electrode grid (ReGrid) based on a thin-film patch highly conformable to the skin and fully ultrasound-transparent. ReGrid consists of a 15 μm-thick polyurethane membrane housing silver electrodes and traces inkjet-printed on the skin-facing side. A waterproof medical-grade adhesive layer protects the outer surface, while a flexible PCB connector ensures the connection with the acquisition system. Bench tests were conducted to assess mechanical conformability and US transparency. Results showed that ReGrid conformed to curved surfaces, other than allowing for B-mode US imaging without artifacts: both the support and the electrodes resulted transparent to ultrasound. In-vivo tests on the tibialis anterior muscle confirmed low and stable electrode-skin impedance and noise levels comparable to conventional gel-based electrodes. HD-sEMG signals were recorded during isometric contractions at two force levels (10% and 20% MVC), with and without a US probe placed directly over the electrodes. Conduction velocity estimates and HD-sEMG decomposition outcomes were not significantly affected by the US probe, nor was the level of power-line interference. Thanks to its conformability, ultrasound transparency, and high signal quality, ReGrid enables combined HD-sEMG and ultrasound acquisitions from the same muscle region, supporting novel applications such as 3D and panoramic ultrasound imaging integrated with HD-sEMG.

Background: Reliable control of rehabilitation and assistive devices using High-Density surface Electromyography (HD-sEMG) remains limited by poor robustness to electrode shifts, changes in skin condition, and variability across users. Methods: This study evaluates the performance of the Recursive Prosthetic Control Network (RPC-Net)/High-Density Electrode Array (HDE-Array) system, defined in previous studies, under conditions that reflect real-life usage, including electrode repositioning and cross-subject generalization. The first test evaluated whether the RPC-Net/HDE-Array system maintained stable performance when trained without electrode repositioning and evaluated on data from a different session with altered electrode placement. The study further examined whether explicitly incorporating electrode repositioning during training mitigates the performance degradation typically observed when testing is performed in a separate session. Finally, the effects of inter-subject training were assessed. Results: Experimental results demonstrate that the RPC-Net/HDE-Array system is highly sensitive to electrode repositioning and skin condition variability when trained under static conditions. However, robustness improves significantly when such variability is included during training. The results indicate that performance improves with an increasing number of subjects in the training pool, provided the training set includes only data from subjects other than the one tested, suggesting a strong dependency on subject-specific patterns Conclusions: These findings demonstrate that the RPC-Net/HDE-Array system can achieve robust performance across sessions and users when trained under realistic conditions. This work represents a key step toward practical deployment of muscle-computer interfaces.