This study evaluated the sensitivity and robustness of a motor unit (MU)–based EEG filtering approach for estimating corticomuscular coherence. The effects of three factors were examined: the number of MUs (No. MUs) used to construct the MU-based EEG filter, the EEG extension factor (FEEG) applied during preprocessing, and muscle contraction level. By applying the MU-based EEG filter to the EEG signals, we obtain an EEG component (EEGcomp) that is coupled to MU activity. Coherence between the EEGcomp and the cumulative spike train was computed and quantified using three metrics: the maximum value of coherence (COHmax), the number of frequency bins exceeding the significance threshold (COHNbin), and the Root Mean Square (RMS) of coherence (COHRMS). Analyses were performed separately in the alpha, beta, and low gamma frequency bands. Across all bands, coherence metrics increased with No. MUs, with the strongest differences observed between small and large No. MUs, and saturation occurring at approximately 10–12 MUs. FEEG had a limited influence on the alpha and beta bands, whereas in the gamma band its effect was stronger, with saturation occurring at relatively high FEEG values (FEEG ≥ 20). Contraction level significantly affected coherence and interacted with No. MUs in the alpha and gamma bands, indicating contraction-dependent modulation of MU-related coherence estimates.

Dopamine is a critical neuromodulator of motor function. In Parkinson's disease (PD), the degeneration of dopaminergic neurons not only disrupts motor function but may also alter motoneuron excitability, contributing to motor deficits. The aim of this study was to investigate motoneuron excitability in people with PD during ON and OFF medication states and to compare these responses with those of controls of similar age. Fourteen people with PD (4 females) were tested in two sessions (ON and OFF medication), and 13 controls (5 females) completed the same protocol in one session. Participants performed slow triangular elbow flexion isometric contractions to 30% of their maximum voluntary contraction (MVC). Motor unit discharge rates, recruitment threshold, discharge rate hysteresis (ΔF), brace height, acceleration and attenuation slopes, ascending and descending rate modulations, and self-sustained discharge duration were calculated. People with PD showed higher initial discharge rates (P < 0.05) and lower recruitment thresholds (P < 0.001) in both medication states compared with controls, with no significant differences between the OFF and ON conditions (P > 0.05). ΔF and acceleration slope were both elevated in the OFF state compared with controls (P = 0.04 and P = 0.05, respectively), with no significant medication effects on either measure (P > 0.05). Motoneuron excitability was higher in people with PD compared with controls, and dopaminergic medication does not fully normalize motoneuron excitability or suppress persistent inward current (PIC)-related amplification. These findings raise the possibility that changes associated with long-term monoaminergic loss may not be fully restored by medication.NEW & NOTEWORTHY This study provides new evidence that motoneuron excitability is consistently elevated in people with Parkinson's disease (PD) independent of dopaminergic medication. People with PD showed higher initial discharge rates, lower recruitment thresholds, and greater indicators of persistent inward currents than controls, pointing to lasting spinal adaptations driven by chronic monoaminergic deficits. The persistence of these abnormalities despite dopamine replacement therapy indicates that chronic monoaminergic depletion produces enduring plasticity within the spinal circuits that persists despite dopamine replacement.

This study investigated the spinal neural mechanisms underlying postactivation potentiation in 10 healthy young males (21.9 ± 4.8 yr). Participants performed a 10-s maximal isometric plantarflexion, after which we measured twitch torque and assessed spinal excitability using the soleus H-reflex, D1 presynaptic inhibition, and heteronymous Ia facilitation (HF). High-density surface EMG was decomposed to track single motor unit responses. The conditioning contraction increased twitch torque by 12.2 Nm (P < 0.001) immediately and returned to baseline within 9 min. This mechanical potentiation was accompanied by a 29% reduction in H-reflex amplitude (P < 0.001), which recovered within 3 min. Paradoxically, neurophysiological indices of presynaptic inhibition, D1, and HF were significantly increased (D1: P < 0.017; HF: P < 0.001), resulting in spinal facilitation. Single MU analysis revealed increased discharge probability, particularly in higher-threshold units, indicating overall spinal facilitation. These results demonstrate that postactivation potentiation involves a complex dissociation; H-reflex pathway inhibition along with facilitation of presynaptic spinal mechanisms. This paradox can be explained by either postactivation depression (caused by depletion of neurotransmitter at the Ia-motoneuron synapse) or muscle thixotropy, a contraction history-dependent decrease in muscle spindle sensitivity, which reduces the efficacy of the Ia afferent volley independently of spinal inhibitory mechanisms. Our findings highlight a dissociation between spinal presynaptic facilitation and the decreased H-reflex, underscoring the need for future studies to explicitly test the roles of postactivation depression and muscle thixotropy after conditioning contractions.NEW & NOTEWORTHY This study provides evidence that postactivation potentiation is accompanied by a reduction in soleus H-reflex amplitude and a concurrent facilitation of presynaptic spinal mechanisms. By combining global EMG and single motor unit analyses extracted from high-density surface EMG, we reveal a dissociation between spinal disinhibition and reflex depression. These findings suggest that acute postcontraction reflex suppression might be mediated by mechanisms other than presynaptic inhibition, potentially involving postactivation depression or changes in muscle spindle sensitivity.

Singular spectrum analysis (SSA) is a nonparametric spectral estimation method that decomposes time series signals into interpretable components. With the rise of big time series, the demand for effective and scalable SSA techniques has become increasingly urgent. In this paper, we propose a novel multiway extension of SSA, called higher-order multivariate SSA (HO-MSSA), specifically designed for multivariate and multichannel time series signal analysis via tensor decomposition. HO-MSSA utilizes time-delay embedding and tensor singular value decomposition to transform multichannel time series signals into trajectory tensors, which are then decomposed into elementary components in the Fourier domain, rather than the time domain as in traditional SSA methods. These components are grouped into disjoint subsets using spectral clustering, enabling the reconstruction of the underlying source signals. Experimental results demonstrate that HO-MSSA outperforms state-of-the-art SSA methods in various biomedical applications, including electromyography (EMG), electrocardiography (ECG), and electroencephalogram (EEG) signals.

This study examined the effect of the knee-joint angle on motor unit (MU) discharge properties of the vastii muscles and their modulation with contraction level. Twelve young adults performed unilateral isometric knee-extension contractions during three experimental sessions at either 25°, 55°, and 85° of knee flexion (full extension: 0°) in a randomized order. Each session involved maximal voluntary contractions (MVCs) followed by submaximal trapezoidal and triangular contractions at different levels relative to maximal voluntary torque (MVT). High-density surface electromyograms were recorded from vastus lateralis and medialis muscles and, subsequently, decomposed to obtain discharge timings of individual MUs. MVT was the greatest, whereas MU discharge rate (DR) during MVCs and submaximal contraction levels (≥30% MVT) was the lowest at the intermediate joint angle (55°). The highest DR during MVCs and high-level contractions (70% MVT), however, was at the most flexed knee position (85°), which was due to a greater DR increase 50%-70% MVT compared with 25° and 55°. The onset-offset DR hysteresis (ΔF), an estimate of persistent inward current contribution to motoneuron discharge, decreased with knee flexion and increased with contraction level, whereas the degree of motoneuron input-output nonlinearity (brace height) did not vary with joint angle but decreased with contraction level. At 85°, ΔF increased more and brace height decreased less with contraction level compared with 25° and 55°. These findings indicate that vastii MU DR and its modulation with contraction level vary with knee-joint angle, which could be partly explained by the modulation of motoneuron intrinsic electrical properties.NEW & NOTEWORTHY This study explored the relationship between motoneuron output to the vastii muscles at different knee-joint angles (quadriceps lengths) and isometric contraction levels. We showed that the motor unit discharge rate was lowest at the angle of the greatest absolute torque capacity, whereas the contraction-level-induced increases in discharge rate and motoneuron excitability were the greatest in the flexed position. These findings suggest that joint-angle-dependent adjustments in sensory feedback modulate motor control of the knee-extensor muscles.

Several approaches have been proposed to extend the permutation entropy (PE) by incorporating amplitude information, which is typically discarded in the original PE formulation. In this paper, we introduce a new PE variant that explicitly accounts for amplitude information using the well-established spectral properties of circulant determinants. This circulant determinant-based PE (CDPE) effectively captures the theoretical PE of sinusoids within the normalized frequency range $[0.01,0.2]$, outperforming existing amplitude-dependent PE (ADPE) methods. Since this range aligns with the spectral characteristics of surface electromyography (sEMG) signals $(5-500 \text{Hz})$ sampled at 2048 Hz, the second key contribution of our work is investigating the sensitivity of CDPE and existing ADPE measures to excitation levels. To this end, these ADPE methods were applied to simulated physiological high-density sEMG (HDsEMG). HDsEMG represent a spatially distributed acquisition of sEMG using a dense array of electrodes, enabling a refined analysis of muscle electrical activity. Our findings provide a deeper understanding of the applicability and robustness of ADPE-based methods for HDsEMG signal analysis.

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.

We investigated the agreement and accuracy of manual editing of the high-density electromyogram (hdEMG) decomposition results by seven human operators with various experience levels. All operators edited the same automatically decomposed experimental hdEMG from the first dorsal interosseous (FDI), tibialis anterior (TA), vastus lateralis (VL), and biceps brachii (BB) muscles, and synthetic hdEMG from soleus (SO) and BB muscles at 10%, 30%, 50% and 70% of maximum voluntary contraction. On average, operators kept $13.7~\pm ~7.4$ motor units (MUs) after editing and demonstrated relatively large disagreement in the calculated MU pulse trains (normalized root mean square difference) but relatively high agreement in the identified MU discharges. Inter-operator agreement positively correlated with the initial MU Pulse-to-Noise Ratio used as a quality measure of automatic MU identification, and negatively correlated with the muscle contraction level. Operators agreed more on the results of the simulated than experimental hdEMG. Among the experimental muscles tested, the greatest agreement was demonstrated for VL and the lowest for BB. We obtained similar results when comparing editing to the results of the most experienced operator and to ground truth in simulated cases: the greatest precision and sensitivity were demonstrated for VL, and the lowest for BB. The level of the operator’s experience had a significant impact on the editing of synthetic hdEMG and the detection of the first MU discharge, but not on the rate of agreement or editing time of experimental hdEMG.

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.

The interconnected nature of orofacial, neck musculature, and the neural system suggests that localized activities, such as teeth clenching, can influence remote spinal excitability. Although stretching exercises are known to have both local and remote effects, the specific impact of orofacial muscle stretching remains underexplored. This study investigates the effects of two interventions: 25 guided orofacial and neck stretching and mobility exercises (exercises), and chewing six chewing gums for six minutes (chewing), on the soleus H-reflex and D1 presynaptic inhibition. Ten volunteers (mean age: 28.75 ± 9 yr) participated, with H-reflex measurements collected using high-density electromyography (HDsEMG) before and after each intervention. Latency (HLAT), duration (HDUR), peak-to-peak (HP2P, D1P2P), and positive peak (HPOS) amplitudes were extracted from unconditioned and conditioned H-reflexes. The ratio (D1P2P/HP2P) between conditioned (D1P2P) and unconditioned (HP2P) H-reflex was calculated to study the D1 presynaptic inhibition mechanisms. In addition, 8,400 firings from 376 distinct motor units (MUs), categorized by firing threshold were analyzed for latency, firing ratio, and inhibition probability (D1PROB). HP2P, HPOS decreased and HDUR was significantly increased after the exercise intervention, whereas the chewing intervention had no effect on these parameters. The D1P2P/HP2P ratio and D1PROB remained unchanged, suggesting that the observed drop in HP2P is not mediated by presynaptic inhibition mechanisms. Single MU analysis confirmed the H-reflex findings. The results of this study suggest that stretching and mobility exercises targeting the neck and orofacial region can reduce neuromuscular excitability, offering potential for nonpharmacological management of conditions associated with motoneuron hyperexcitability and general whole body relaxation.NEW & NOTEWORTHY This study provides the first evidence that orofacial and neck mobility exercises can acutely reduce spinal excitability in remote lower-limb muscles. By combining high-density surface EMG with both global and single motor unit H-reflex analyses, we demonstrate a decrease in soleus H-reflex amplitude independent of presynaptic inhibition. These findings suggest potential nonpharmacological applications for managing motoneuron hyperexcitability and promoting whole body relaxation in individuals with cervical and orofacial constraints.

The aim of this study was to determine whether there are sex differences in motor unit firing behavior in patients with Parkinson's disease. Twenty-seven patients with Parkinson's disease (females = 14 [age = 71.1 ± 6.8], males = 13 [age = 69.2 ± 10.3], Unified Parkinson's Disease Rating Scale Part III score; females = 10.8 ± 4.8, males = 11.4 ± 1.4) performed a contraction at 30% of the maximal voluntary contraction. For each participant, motor unit spike trains were decomposed from high-density surface electromyography data recorded from bilateral vastus lateralis muscles via blind source separation algorithms. In addition to the mean discharge rates, persistent inward currents were estimated via a paired motor unit analysis. Females presented significantly greater laterality of discharge rate ( p  = 0.001) and persistent inward currents ( p  = 0.0121) than males. A significant correlation was observed between the discharge rate and the recruitment threshold on the bilateral side of males and the less-affected side of females but not on the more-affected side of females. These findings indicate that sex differences in motor unit behavior exist in Parkinson's disease patients. Motor unit behavior may be a sensitive and quantitative evaluation tool to highlight differences in disease presentation between males and females.

Alaska pollack protein (APP), has been reported as a protein source that can enhance muscle hypertrophy more than other protein sources in animal studies. This study aimed to examine the effects of APP ingestion on muscle quantity and quality in young adults. Fifty-five young college students were assigned to two groups: APP and placebo (whey protein: WP) groups, and instructed to ingest 4.5 g of each protein in addition to daily meals, and to maintain their usual daily physical activities for 3 mo. Twenty-one and 23 students completed the intervention and were analyzed in APP and WP groups, respectively. The maximum knee extension torque significantly increased in both groups during the intervention. The motor unit discharge rate, which is an indicator of activation, for a given force level significantly decreased in both groups during the intervention, but its decrease in the APP group was significantly greater than in the WP group. Echo intensity of the vastus lateralis evaluated by ultrasound images significantly decreased in both groups. The muscle thickness and skeletal muscle mass did not change. Small amount of additional APP intake induces greater effects on neural activation than WP, suggesting the greater neural economy of generation of force.

Understanding the ability of older adults to control pedal position angle and investigating whether this ability can be enhanced through practice may contribute to the prevention of traffic accidents. This study aimed to investigate repetitive effects on variability of the pedal position and neural drive during car-pedal operation in older adults. Thirteen older and 11 young adults performed 105 (21 sets x 5 repetitions) pedal angle control tasks with plantar flexor contraction. High-density surface electromyograms were recorded of triceps surae muscles. A cumulative spike train as a neural drive was calculated using continuously active motor unit activities. The coefficient of variation of the angle was higher in older (1.47 +/- 1.06 %) than young (0.41 +/- 0.21 %) adults in the first sets, and improved to 0.67 +/- 0.51 % in the final sets in older adults only. There was no significant difference in neural drive variability between older and young adults. Our results suggest that repetition improves angular steadiness in older adults. However, this effect could not be explained by neural output which is estimated from lower threshold motor units that are continuously active.

The diversity in electromyography (EMG) techniques and their reporting present significant challenges across multiple disciplines in research and clinical practice, where EMG is commonly used. To address these challenges and augment the reproducibility and interpretation of studies using EMG, the Consensus for Experimental Design in Electromyography (CEDE) project has developed a checklist (CEDE-Check) to assist researchers to thoroughly report their EMG methodologies. Development involved a multi-stage Delphi process with seventeen EMG experts from various disciplines. After two rounds, consensus was achieved. The final CEDE-Check consists of forty items that address four critical areas that demand precise reporting when EMG is employed: the task investigated, electrode placement, recording electrode characteristics, and acquisition and pre-processing of EMG signals. This checklist aims to guide researchers to accurately report and critically appraise EMG studies, thereby promoting a standardised critical evaluation, and greater scientific rigor in research that uses EMG signals. This approach not only aims to facilitate interpretation of study results and comparisons between studies, but it is also expected to contribute to advancing research quality and facilitate clinical and other practical applications of knowledge generated through the use of EMG.

Skeletal muscles power movement. Deriving the forces produced by individual muscles has applications across various fields including biomechanics, robotics, and rehabilitation. Since direct in vivo measurement of muscle force in humans is invasive and challenging, its estimation through non-invasive methods such as electromyography (EMG) holds considerable appeal. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, summarizes recommendations on the use of EMG to estimate muscle force. The matrix encompasses the use of bipolar surface EMG, high density surface EMG, and intra-muscular EMG (1) to identify the onset of muscle force during isometric contractions, (2) to identify the offset of muscle force during isometric contractions, (3) to identify force fluctuations during isometric contractions, (4) to estimate force during dynamic contractions, and (5) in combination with musculoskeletal models to estimate force during dynamic contractions. For each application, recommendations on the appropriateness of using EMG to estimate force and justification for each recommendation are provided. The achieved consensus makes clear that there are limited scenarios in which EMG can be used to accurately estimate muscle forces. In most cases, it remains important to consider the activation as well as the muscle state and other biomechanical and physiological factors— such as in the context of a formal mechanical model. This matrix is intended to encourage interdisciplinary discussions regarding the integration of EMG with other experimental techniques and to promote advances in the application of EMG towards developing muscle models and musculoskeletal simulations that can accurately predict muscle forces in healthy and clinical populations.

The aim of the present study was to investigate the acute effect of caffeine or quercetin ingestion on motor unit firing patterns and muscle contractile properties before and after resistance exercise. High-density surface electromyography (HDs-EMG) during submaximal contractions and electrically elicited torque in knee extensor muscles were measured before (PRE) and 60 min after (POST1) ingestion of caffeine, quercetin glycosides, or placebo, and after resistance exercise (POST2) in ten young males. The Convolution Kernel Compensation technique was used to identify individual motor units of the vastus lateralis muscle for the recorded HDs-EMG. Ingestion of caffeine or quercetin induced significantly greater decreases in recruitment thresholds (RTs) from PRE to POST1 compared with placebo (placebo: 94.8 ± 9.7

ABSTRACT Acute intermittent hypoxia (AIH) is an emerging technique for enhancing neuroplasticity and function in respiratory and limb musculature. Thus far, AIH-induced improvements in strength have been reported for upper and lower limb muscles after chronic incomplete cervical spinal cord injury (iSCI) but the underlying mechanisms have been elusive. We used high-density surface electromyography (HDsEMG) to determine if motor unit discharge behaviour is altered after 15 × 60 s exposures to 9% inspired oxygen interspersed with 21% inspired oxygen (AIH), compared to breathing only 21% air (SHAM). We recorded HDsEMG from the biceps and triceps brachii of seven individuals with iSCI during maximal elbow flexion and extension contractions, and motor unit spike trains were identified using convolutive blind source separation. After AIH, elbow flexion and extension torque increased by 54% and 59% from baseline (p = 0.003), respectively, whereas there was no change after SHAM. Across muscles, motor unit discharge rates increased by ∼4 pulses per second (p = 0.002) during maximal efforts, from pre to post AIH. These results suggest that excitability and/or activation of spinal motoneurons are augmented after AIH, providing a mechanism to explain AIH-induced increases in voluntary strength. Pending validation, AIH may be helpful in conjunction with other therapies to enhance rehabilitation outcomes due to these enhancements in motor unit function and strength.