Biology subjects

Farina, D.

Publications and source records attributed to Farina, D..

At least 19 recordsLinked to original sources

bioRxiv2026

Lateralised Transmission of Beta Oscillations From Cortex to Muscle During Movement Cancellation Reflects Selective Motor Inhibition

Beta oscillations in the sensorimotor cortex are modulated across various stages of motor control, including movement preparation and cancellation, and are known to project to the motor neuron pool and muscles during Go/No-Go tasks. While these oscillations have been linked to motor inhibition, the specificity of their transmission to end-effector muscles during movement cancellation remains unclear. We investigated the selectivity of beta-band activity using a lateralised Go/No-Go paradigm. Participants maintained steady bimanual wrist extensor contractions and either executed a unilateral ballistic wrist extension (Go) or cancelled the planned movement (No-Go). Concurrent electroencephalographic and electromyographic recordings were measured throughout. Successful movement cancellation was characterised by contralateral cortical beta modulation accompanied by stronger and more clearly lateralised peripheral beta synchronisation in the muscle associated with the suppressed movement. Peripheral beta activity exhibited greater magnitude and clearer separability between left- and right-sided conditions than cortical signals. These findings demonstrate effector-specific brain-to-muscle beta transmission during movement cancellation and highlight the potential of peripheral measures to provide enhanced resolution of motor cortical dynamics.

bioengineering↗

bioRxiv2026

Out-of-sight partners: autonomous wearable supernumerary limbs for dynamic co-manipulation

Supernumerary robotic limbs (SLs) could extend human motor capabilities beyond the natural body, yet most demonstrations have been limited to simple, quasi-static tasks or direct teleoperation under visual control. Whether humans can safely and intuitively collaborate with autonomous SLs during complex, dynamic tasks remains unknown. We studied this question in a demanding assembly task requiring coordinated transport and handovers of a long object, which could not be completed by a single person. We developed a reconfigurable backpack platform mounting up to four high-payload robotic arms, together with safety-aware motion planning that generates human-compatible trajectories from task states without training data. Despite the SLs operating out of sight, participants adapted quickly, completed the task reliably, and reported low cognitive demand alongside high perceived safety and predictability. Kinematic and force analyses showed that control design governed both movement fluency and whole-body stability: faster limb motions yielded smoother interactions and shorter completion times, and a mirrored coordination strategy--in which one robotic limb counterbalanced the other--further shortened duration and reduced variability in left-right ground reaction force differences during co-manipulation. These results show that humans can integrate autonomous wearable limbs as out-of-sight partners in dynamic whole-body collaboration, and delineate behavioral and control principles underpinning fluency, stability, and usability in human augmentation.

bioengineering↗

bioRxiv2026

Neurofeedback Training on Motor Cancellation Enhances Peripheral but not Cortical Beta Band Oscillations

Beta oscillations (13-30 Hz) are a prominent sensorimotor neural rhythm and an important biomarker in neurorehabilitation. These oscillations propagate along the corticospinal pathway and are expressed in the discharge patterns of spinal motor neurons, enabling the assessment of corticomuscular coupling. Moreover, peripheral beta band oscillations have recently emerged as a potential control signal for motor augmentation interfaces. However, it remains unclear whether peripheral beta activity simply reflects cortical oscillations or is partly shaped by peripheral mechanisms, and to what extent it can be voluntarily controlled. To address these questions, we developed a 10-day neurofeedback protocol in which participants learned to up-regulate peripheral beta band activity. Subjects were trained to exploit movement cancellation, a behaviour naturally associated with increased cortical and muscle beta band activity, as a two-state strategy to voluntarily modulate peripheral beta band power. Each session included a guided familiarization phase based on a GO/NO-GO task, in which participants familiarized with movement cancellation through guided visual cues, followed by an asynchronous control phase in which they self-initiated the same strategy without external guidance to increase peripheral beta band activity in a target window. Participants progressively improved their ability to voluntarily modulate peripheral beta band activity. Peripheral beta band power during movement cancellation increased significantly across training days in both the familiarization and asynchronous control phases. Intramuscular coherence in the beta band also increased, indicating enhanced common synaptic input to the motor neuron pool in this band. In contrast, cortical beta power and corticomuscular coherence remained unchanged. Together, these findings demonstrate that peripheral beta band activity is a dynamic neural feature that can be voluntarily shaped through training, supporting its potential as a non-invasive control signal for future neurorehabilitation and motor augmentation technologies.

bioengineering↗

bioRxiv2026

An axonal sodium current gates motoneuron doublets and force amplification

Brief high-frequency bursts of action potentials shape neural coding. In spinal motoneurons, they appear as doublets: closely spaced spike pairs, observed for a century, that amplify muscle force nonlinearly through the catch-like property. However, the cellular and anatomical origins of doublets remain unresolved. Combining intracellular recordings, a conductance-based model, and human motor unit and force data, we show that initial and repetitive doublets arise from the same mechanism. Both spikes are initiated at the axon initial segment, but the second is driven by a persistent sodium (NaP) current at the first node of Ranvier, which returns toward the initial segment and sums with somatic NaP current, the calcium-mediated afterdepolarization, and the passive membrane response. In our model, blocking any of these active currents abolishes the doublet, and monoaminergic facilitation of NaP determines whether doublets occur. Human motor units discharged repetitive doublets whose interval lengthened over time, a time course set by inactivation of the somatic NaP current, and produced nonlinear increases in force. Because monoaminergic drive both gates the doublet and sets how its interval evolves, neuromodulation shapes not only the gain but also the timing of motoneuron output.

neuroscience↗

bioRxiv2026

Unilateral resistance training induces greater rate coding adaptations in high-threshold motor units during maximal voluntary contractions

Resistance training lasting a few weeks increases maximal force mainly through neural adaptations that enhance the drive from the nervous system to muscle. While these adaptations have been well documented at the motor unit (MU) level during submaximal force contractions, the mechanisms underlying force increases during maximal voluntary contractions are poorly understood. This is due to a classic technical limitation in tracking MUs longitudinally during maximal force tasks. Here, we solved this technical challenge, enabling the investigation of MU adaptations during MVCs in both the trained and untrained limbs following unilateral resistance training. High-density surface electromyography was recorded from the biceps brachii of both limbs before and after a 4-week unilateral resistance-training intervention, and the same MUs were longitudinally tracked across sessions during MVCs by concatenation of three MVC trials of [~]5-s each. Unilateral training increased maximal force in the trained limb (+16%) and induced strength transfer to the untrained limb (+8%). In both limbs, maximal contractions after training were characterized by greater EMG amplitude, faster muscle-fiber conduction velocity, and higher MU discharge rates, indicating enhanced neural drive to the motoneuron pool. These adaptations were strongly associated with improvements in maximal force (R2 > 0.7 for all). Importantly, longitudinal MU tracking revealed a non-uniform adaptation across the MU pool: MUs with higher baseline conduction velocity, indicative of higher recruitment threshold, exhibited the largest pre-post increases in discharge rate, whereas lower-threshold units showed smaller changes. Collectively, these findings demonstrate that gains in maximal force and their transfer to the untrained limb are primarily mediated by enhanced rate coding of higher-threshold MUs during MVCs.

physiology↗

bioRxiv2026

Validation of a multiscale Hill-type actuator against comprehensive benchmarks of motor unit and muscle force measurements

Computational Hill-type muscle models are widely used to simulate muscle force production because of their efficiency and physiological interpretability. However, their formulation relies on limiting assumptions, including debated multiscale simplifications, a simplified excitation-activation dynamics and an inability to capture slow and fast fibres. Moreover, existing Hill-type models remain insufficiently validated across physiological scales, fibre types, and contraction modes. We addressed these limitations by developing a multiscale fibre-type specific Hill-type neuromuscular actuator with mechanistic excitation-activation dynamics and systematically validated it against comprehensive experimental benchmarks. The model built upon a previously proposed motoneuron-driven actuator incorporating calcium-kinetics-based activation dynamics. The excitation-activation formulation was further refined to strengthen its physiological basis, while the contraction dynamics was extended by including an activation- and length-dependent force-velocity relationship, elastic tendon, passive elastic element, and the fibre-type-specific effects of yielding and sag. Validation was performed against four benchmark datasets spanning motor-unit and whole-muscle scales, including slow and fast fibres under both isometric and dynamic conditions. Experimental force traces were obtained from six muscles of rats and cats using a broad range of stimulation frequencies, muscle lengths, and imposed length changes, combining previous literature datasets with experiments performed ad hoc for this study. Overall, the model reproduced forces across all benchmark conditions, with mean absolute errors typically below 15% of the maximum isometric force, although larger errors were observed in specific submaximal and dynamic trials. The inclusion of physiologically based excitation-activation dynamics, together with yielding and sag, improved model performance under submaximal activation conditions. This study presents the first systematic validation of a single multiscale Hill-type neuromuscular actuator against comprehensive experimental motor unit and muscle force data, providing a benchmark framework for the development and assessment of future models. Author summarySkeletal muscles generate force through a complex sequence of events that links neural signals to muscle contraction. Because direct measurements are difficult to obtain, researchers often rely on computer models to investigate neuromuscular function and estimate muscle forces. However, most modelling approaches rely on simplifying assumptions about how force is generated across different biological scales, how muscles are activated, and how slow and fast muscle fibres behave. Moreover, they have not been validated against comprehensive experimental data. As a result, it remains unclear how accurately these models can reproduce muscle force across different physiological conditions. In this study, we established the first comprehensive set of experimental benchmarks spanning both motor-unit and whole-muscle scales, including slow and fast muscles under isometric and dynamic conditions. We used these benchmarks to validate a newly developed multiscale muscle model that explicitly represents the physiological pathway from neural stimulation to force production. The model incorporates experimentally based descriptions of calcium dynamics, activation, tendon elasticity, and fibre-type-specific contractile properties. We then compared simulated and experimental force responses across a wide range of stimulation frequencies, muscle lengths, and length-change conditions.

bioengineering↗

bioRxiv2026

Neuromodulation enables transient flexible control of motoneurons

A motoneuron pool is often regarded as a rigid controller because the largely shared synaptic input across motoneurons leads to strongly correlated activity. However, brief deviations from this correlated behavior have been observed even in some constrained tasks, raising the question of whether these results reflect limitations of the rigid view of motoneuron pool control. Here we show that they do not. We developed a biophysical model of a motoneuron pool receiving shared excitatory and inhibitory synaptic inputs that also included the motoneuron-specific effects of neuromodulation; model parameters were tuned based on large-scale motoneuron recordings in humans. Simulations showed that the intrinsic differences in how motoneurons respond to neuromodulation are both necessary and sufficient to transiently decorrelate pairs of motoneurons receiving a shared synaptic input. Crucially, such transient decorrelation is only observed when motoneurons have different sensitivity to neuromodulation, consistent with experimental observations during volitional control in humans. Our model also explains how participants can improve their ability to transiently decorrelate the activity of motoneurons innervating the same muscle by leveraging refined behavioral strategies that exploit the differential response of motoneurons to neuromodulation, rather than through physiological changes. These results identify that heterogeneous sensitivity to neuromodulation enables brief flexibility in the otherwise rigid control of motoneurons enforced by a shared synaptic input, and show how practice allows participants to exploit latent flexibility within otherwise rigid constraints.

neuroscience↗

bioRxiv2026

High-frequency common inputs entrain motoneuron subpopulations differently

Spinal motoneuron (MN) pools behave as linear systems that transmit common synaptic input to muscles. However, MNs are biophysically heterogeneous and intrinsically nonlinear. How different MN subpopulations integrate and transmit high-frequency inputs remains poorly understood, partly because conventional analyses treat the MN pool as a single functional system rather than examining subpools with different firing rates. Here, we addressed this gap using a combination of computational simulations and human MN recordings. Simulations of MNs receiving a common synaptic input at varying frequencies showed that MNs firings become phase-locked to input oscillations when the input frequency approximates the neurons firing rate or its harmonics. We refer to this frequency-dependent synchronization as entrainment. Importantly, this subpool-specific effect was masked when MN activity was analysed at the whole-pool level. Because entrained MNs effectively sample the input at their firing instants, we developed a MN-firing locked method that uses individual MN firings as endogenous triggering events for peristimulus frequencygrams across the pool. In simulations, this method revealed entrainment-driven firing rate modulations across MN subpools. We then applied this MN-firing locked method to MNs decomposed from high-density surface electromyography recordings obtained during isometric contractions in healthy individuals. We found that faster-firing MNs exhibited larger transient firing rate increases, time-locked to slower MN activity. Furthermore, these modulations correlated with common input in the alpha and beta bands implicating high frequency common input as the driving source. Together, these findings demonstrate that MN nonlinearities generate heterogeneous, frequency-dependent dynamics that remain hidden in conventional pool-level analyses.

neuroscience↗

bioRxiv2026

TMS-Evoked Corticospinal Beta Oscillations in Humans Recorded from Muscles

BackgroundTranscranial magnetic stimulation (TMS) can entrain oscillatory brain activity, offering a promising approach to study motor-related neural oscillations such as beta rhythms. However, how TMS-induced corticospinal oscillations are generated, propagated, and related to endogenous activity remains unclear, partly due to limitations in brain recording techniques. Recording muscle activity provides an alternative and physiologically grounded window into corticospinal dynamics. MethodsWe investigated whether subthreshold TMS over the motor cortex induces corticospinal oscillatory activity detectable in muscles, and whether these responses share neural generators with endogenous beta rhythms. Single-pulse subthreshold TMS was applied over the motor cortex in healthy participants while electromyography (EMG) was recorded from tonically active muscles. Stimulation intensity, coil orientation, and stimulation site were systematically varied. Concurrent electroencephalography (EEG) was used to assess cortical responses and corticomuscular transmission. In additional experiments, advanced EMG techniques were employed to track motor neuron pools and characterize how TMS-evoked oscillations are transmitted at the motor unit level. ResultsTMS elicited a robust and short-latency increase in beta-band activity in the EMG. The analysis of the elicited muscle responses and the comparison of results across different TMS configurations indicate that the beta responses resulted from activation of inhibitory interneurons in the targeted primary motor cortex. Importantly, the characteristics of cortico-muscular coherence and beta projection to the muscles indicate that the elicited beta responses with TMS have same cortical sources as endogenously generated beta activity. ConclusionsThese findings demonstrate that muscle recordings provide a sensitive and physiologically meaningful readout of TMS-induced corticospinal beta oscillations.

neuroscience↗

bioRxiv2025

Probabilistic inference of Homonymous and Heteronymous Recurrent Inhibition in Human Muscles from Large-Scale Motor Neuron Recordings

Understanding how spinal circuits shape motor neuron behavior during muscle contractions remains a major challenge. Here, we combined large-scale motor unit recordings with simulation-based inference to generate probabilistic estimates of homonymous and heteronymous recurrent inhibition, a key spinal circuit that has remained largely inaccessible during natural voluntary contractions. We constructed synchronization cross-histograms from motor neuron spike trains and extracted features representative of recurrent inhibition. Because these features are also influenced by higher-frequency components of common synaptic input, we developed a simulation-based inference framework to disentangle these effects. Following validation, we applied this framework to experimental data from six muscles at two contraction intensities, revealing previously uncharacterized muscle-and intensity-dependent patterns: recurrent inhibition decreased with contraction intensity in most muscles but increased in the vastus lateralis and medialis. The pipeline is openly available and designed for reuse on comparable datasets and for adaptation to diverse experimental contexts, including other spinal circuits. TeaserLarge-scale motor-unit recordings enable in-vivo probabilistic mapping of spinal recurrent inhibition.

neuroscience↗

bioRxiv2025

Spinal Motor Neuron Pools May be Partly Driven by Impulsive Common Inputs

Spinal motor neurons serve as the link between the nervous system and muscles. As the final common pathway of the neuromuscular system, they receive inputs from both higher-level controllers and afferent pathways. It is often assumed that spinal motor neurons are primarily driven by continuous common inputs (cCI) within different frequency bands. Within this framework, the motor neuron pool behaves as a linear amplifier of the cCI. However, this framework overlooks the possibility that motor neurons could also be driven by impulsive common inputs (iCI), which can induce synchronization among them and disrupt the linear transmission of other synaptic inputs at the pool level. To test this hypothesis, computational simulations and experimental data from human subjects were used to characterize different aspects related to motor neuron spiking synchronization at the pool level. Our findings suggest that, indeed, iCI can account for relevant features observed in experimental data such as the presence of synchronization events at the pool level. We also observed that such impulsive inputs can affect the linearity in the transmission of cCI by the motor neuron pool. This study represents pioneering indirect evidence of the existence of iCI as inputs to motor neurons. Significant StatementMotor unit pool behavior in terms of spiking synchronization and spectral content typically observed in experimental recordings cannot be reproduced in simulations that only use continuous common inputs (cCI) to motor neurons. This study shows, for the first time, evidence supporting that spinal motor neurons receive a portion of their synaptic input in the form of impulsive common inputs (iCI) that synchronize their activity. The study also shows how such iCI can affect the linear transmission of other cCI by the motor neuron pool.

neuroscience↗

bioRxiv2025

Propagation of Beta bursts from the motor cortex to the motor units of multiple upper-limb muscles

Beta band (13-30 Hz) oscillations are crucial for motor control, though their functional significance remains debated. Recent research suggests that beta activity occurs in transient bursts, which may better capture its role in movement regulation than sustained oscillations. While cortical and sub- cortical beta bursts have been extensively studied, their transmission to muscles--particularly in the upper limb--remains poorly understood and has been limited by traditional bipolar EMG techniques. In this study, we used high-density surface electromyography (HDsEMG) and electroencephalography (EEG) to investigate the cortico-peripheral dynamics of beta bursts in forearm extensor muscles during isometric contractions at the motor unit (MU) level. We show that MU activity in the upper limb exhibits discrete beta bursts that are temporally aligned with cortical beta activity. Notably, beta bursts in the periphery were time-locked to cortical bursts, suggesting strong coordination and synchronisation of bursting across the corticospinal tract. We also found stronger beta synchronisation in the extensor carpi ulnaris compared to the extensor carpi radialis, indicating muscle-specific differences in shared neural drive. These findings provide the first demonstration of beta burst propagation from cortex to upper-limb MUs and show that HDsEMG can reliability detect such events in the upper limb. This work supports the cortical origin and structure of peripheral beta activity and demonstrates its potential as a neurophysiological biomarker for targeting corticospinal dynamics in motor disorders such as Parkinsons disease. Significance StatementWe provide the first evidence of cortical beta bursting in motor unit (MU) activity of forearm extensors across varying force levels. Our results show that MU beta bursting is time-locked to cortical beta bursts, supporting the idea of direct corticospinal transmission in the upper limb. Our findings highlight the value of MU level analysis in understanding beta burst transmission and indicate that burst timing is robust to changes in contraction levels. Unlike traditional bipolar EMG, MU-level analysis offers higher temporal precision and source specificity, enabling the reliable detection of beta bursts in individual muscles. These findings demonstrate a robust cortical-peripheral beta relationship during isometric contractions and reinforces the utility of HDsEMG for investigating burst dynamics and refining neurophysiological biomarkers.

neuroscience↗

bioRxiv2025

Neural determinants of the increase in muscle strength and force steadiness of the untrained limb following a 4-week unilateral training

Enhanced untrained muscle strength and force steadiness following unilateral resistance training (i.e., cross-education) is typically attributed to neural responses. However, the mechanisms of these adaptations for spinal motoneurons remain unexplored. Therefore, we examined maximal-voluntary-force (MVF), steady-force variability (CovF), and longitudinally tracked motor unit adaptations in 10 individuals completing a 4-week unilateral strength intervention compared to 9 controls. High-density surface electromyography was recorded from the biceps brachii during steady (10%MVF) and trapezoidal (35%MVF) contractions. The relative proportion of common synaptic input (CSI) to motoneurons and its variability (CSI-V) were estimated using coherence and spectral analysis. Indirect estimates of persistent inward currents using firing rate hysteresis ({Delta}F) and motor unit recruitment thresholds (MURT) were assessed during the ramp forces (35%MVF). MVF increased in both the trained (+14%, p<0.001) and untrained limbs (+6%, p=0.004), and CovF decreased in both limbs (p<0.001). Greater CSI was observed on both sides (p<0.01), concomitant with reduced CSI-V (p<0.01). {Delta}F increased exclusively in the trained limbs (+1.61{+/-}0.71 pps; p<0.001), and both sides exhibited lower MURT (p<0.001). In trained limbs, MVF gains were strongly associated with changes in CSI, MURT, and {Delta}F (R{superscript 2}>0.70, p<0.01), while the contralateral muscle MVF increase was associated exclusively with CSI and MURT (R{superscript 2}>0.65, p<0.01). In both limbs, lower CovF was strongly associated with reduced CSI-V (R{superscript 2}>0.70, p<0.01). Our findings suggest that enhanced untrained muscle force and steadiness are mediated by increased relative strength of shared synaptic input with respect to independent noise and decreased variability of this shared input, with gains in trained muscle MVF being associated with {Delta}F. Key pointsO_LIUnilateral resistance training improves strength and force steadiness in the contralateral untrained limb, suggesting neural adaptations without directly overloading the muscle. C_LIO_LIDespite established force-related modifications, the specific neural mechanisms mediating these functional responses remain largely unknown. C_LIO_LI4-week unilateral training intervention enhanced muscle strength and force steadiness in the untrained limbs of 10 individuals, alongside a greater proportion of shared synaptic input, reduced variance in common input, and lower motor unit recruitment thresholds. C_LIO_LIWe demonstrate that the neural mechanisms underlying improved strength and force control in muscles without mechanical overloading are associated with a higher relative shared input among motoneurons and reduced variance in these common input components. C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=148 SRC="FIGDIR/small/645159v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@1a9e7feorg.highwire.dtl.DTLVardef@1bdd9a6org.highwire.dtl.DTLVardef@11c5ba0org.highwire.dtl.DTLVardef@c39a42_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOC_FLOATNO unilateral training intervention reduced the variance in common synaptic input (CSI-V), which was associated with decreased variability in force steadiness (CovF) in both the trained and contralateral untrained limbs. On the exercised side, the increase in maximal voluntary force (MVF) was accompanied by a higher proportion of common synaptic input (CSI), a lower motor unit recruitment threshold (RT), enhanced persistent inward current (PIC) amplitude, and increased neural drive. In contrast, the contralateral untrained limb exhibited higher shared synaptic input and a lower RT but with unaltered PIC amplitude and neural drive ({leftrightharpoons}). Overall, these adaptations resulted in a 14% increase in MVF in the exercised limb and a 6% increase in the untrained limb. C_FIG

physiology↗

bioRxiv2025

Personalized mapping of inhibitory spinal circuits via neural decoding of high-density electromyography and in silico modelling

Studying human motoneuron activity through electromyography (EMG) can yield insights into the operation of fundamental spinal cord microcircuits. Traditional surface and needle electromyography (EMG) methodologies have limited capacity to shed light on the diversity of motor unit (MU) control strategies that may be unique to each individual. Here, we employed high-density surface EMG (HDsEMG) to sample multiple MUs per subject to investigate the dynamics of inhibitory spinal microcircuits in both upper and lower limb control. We characterised the net inhibition as a function of individual MU firing rates, revealing subject-specific relationships. In silico modelling replicated these experimental characteristics and suggested that properties of the inhibitory currents rather than motoneuron size are responsible for net functional inhibition. Our results show that HDsEMG can highlight distinct control strategies across circuits and motor pools, revealing subject-specific properties of inhibitory spinal microcircuits. TEASERHigh-density surface EMG electrodes can reveal the functional properties of inhibitory spinal circuits

neuroscience↗

bioRxiv2025

Projection of Cortical Beta Band Oscillations to a Motor Neuron Pool Across the Full Range of Recruitment

Cortical beta band oscillations (13-30 Hz) are associated with sensorimotor control, but their precise role remains unclear. Evidence suggests that for low-threshold motor neurons, these oscillations are conveyed to muscles via the fastest corticospinal fibers. However, their transmission to motor neurons of different sizes may vary due to differences in the relative strength of corticospinal and reticulospinal projections across the motor neuron pool. Consequently, it remains uncertain whether corticospinal beta transmission follows similar pathways and maintains consistent strength across the entire motor neuron pool. To investigate this, we examined beta activity in motor neurons innervating the tibialis anterior muscle across the full range of recruitment thresholds in a study involving 12 participants of both sexes. We characterized beta activity at both the cortical and motor unit levels while participants performed contractions from mild to submaximal levels. Corticomuscular coherence remained unchanged across contraction forces after normalizing for the net motor unit spike rate, suggesting that beta oscillations are transmitted with uniform strength to motor neurons, regardless of size. To further explore beta transmission, we estimated corticospinal delays using the cumulant density function, identifying peak correlations between cortical and muscular activity. Once compensated for variable peripheral axonal propagation delay across motor neurons, the corticospinal delay remained stable, and its value (approximately 14 ms) indicated projections through the fastest corticospinal fibers for all motor neurons. These findings demonstrate that corticospinal beta band transmission is determined by the fastest pathway connecting in the corticospinal tract, projecting uniformly across the entire motor neuron pool.

bioengineering↗

bioRxiv2025

Classification of motor-related brain states using high frequency information from muscle recordings

Motor neural interfaces use recordings from the nervous system to extract control signals used to interact with the environment. Muscle signals are becoming an increasingly popular choice for motor interfaces, especially when used to estimate the neural motor commands driving muscle contractions and eventually movement. Here we study the possibility of using electromyography (EMG) to classify motor-related cortical states associated with the cancellation of movement in a GO/NO-GO task, which has been previously linked to changes in cortical beta oscillations. We show that beta (13-30Hz) and low gamma (30-45Hz) frequency bands have the most predictive power in electroencephalography (EEG) and EMG recordings. Moreover, we found comparable accuracy in cancellation state decoding from EEG (average of 74%) and EMG (average of 77%) recordings, which supports the concept of using peripheral signals to predict cortical activity associated to specific motor-related brain states.

neuroscience↗

bioRxiv2025

Rigid Control of Motor Unit Firing Rates in the Human Tibialis Anterior Muscle Persists during Neurofeedback

The conventional framework of motor-unit (MU) control assumes that MUs in a MU pool are constrained by a fixed recruitment order and a common input. This rigid-control framework has been challenged by recent findings suggesting that MU activity could be flexibly modulated, potentially mediated by descending cortical inputs. In this study, rather than evaluating flexibility from the perspective of recruitment thresholds, we investigated control flexibility by assessing if human participants can voluntarily modulate MU firing rates beyond rigid-control constraints. Specifically, we examined whether participants could voluntarily modulate the firing rates of a pair of MUs from the tibialis anterior muscle during real-time feedback. Two tasks involving target-reach with different visual feedback derived from the MUs firing rates were conducted. In both tasks, there was no evidence that participants were able to change MU firing rates in a way that would violate rigid control robustly. Our findings demonstrate limited flexibility in MU control in human tibialis anterior muscle within single-session training, even when real-time MU activity feedback was provided. The results suggest that MU flexibility is not inherently present in the human lower limb.

neuroscience↗

bioRxiv2024

Mental Tasks Induce Common Modulations of Oscillations in Cortex and Spinal Cord

We investigated whether power modulations of cortical oscillations induced by mental tasks are paralleled by the same modulations in spinal motor neurons. We recruited 15 human participants and recorded high-density electromyography signals (HD-EMG) from the tibialis anterior muscle, as well as electroencephalography (EEG) signals. The cumulative spike train (CST) was computed from the activity of spinal motor neurons decoded from HD-EMG signals. The participants performed sustained dorsiflexion concurrent with foot motor imagery, hand motor imagery, mental arithmetic, or no specific mental task. We found significant power correlations between CST and EEG across trials irrespective of the mental task and across mental tasks at the intra-muscular coherence peak (Kendalls{tau} coefficient{tau} trial = 0.08{+/-}0.10,{tau} task = 0.33{+/-}0.19, respectively; mean{+/-}std. dev.). CST power in beta and low-gamma bands could provide a novel control signal for neural interface applications, as power changes in these bands are not translated into actual force changes. To evaluate the potential of CST bands as a control signal, we classified the mental tasks from CST bandpower with a linear classifier and obtained classification accuracies slightly but significantly above chance level (30%{+/-}5%; chance level = 25%). These results show that mental tasks can modulate the power of cortical and spinal oscillations concurrently. This supports the notion that movement-unrelated oscillations can leak down from the cortex to the spinal level. Impact StatementSpike trains of spinal motor neurons exhibit frequency components above 10 Hz, which may partly reflect force-unrelated cortical oscillations and are modulated by mental tasks.

neuroscience↗