Biology subjects

Negro, F.

Publications and source records attributed to Negro, F..

At least 19 recordsLinked to original sources

bioRxiv2026

Bandpass corticospinal transmission during repetitive TMS revealed by motor unit recordings

We employed a noninvasive high-density surface electromyography (HDsEMG) framework to track spinal motor neuron responses during repetitive transcranial magnetic stimulation (rTMS) and characterize corticospinal transmission of different stimulation frequencies and intensities to the alpha motor neuron pool. Eleven healthy individuals performed isometric thumb flexion at 10% of maximal voluntary contraction while rTMS was delivered over the motor cortex at five frequencies (5, 10, 20, 30, and 50 Hz) and three subthreshold intensities (50%, 60%, and 70% of resting motor threshold). Motor units were decomposed from HDsEMG signals before stimulation and tracked during rTMS. Input-output coupling was quantified using coherence between the rTMS train and individual motor unit spike trains or the cumulative spike train (CST), with shuffled spike trains used as surrogate controls. rTMS inputs were robustly transmitted to spinal motor neurons for all frequencies except 5 Hz, indicating widespread corticospinal coupling. Transmission behaved linearly, with CST output spectra reproducing input frequencies and scaling proportionally with stimulation intensity. The estimated transfer function revealed a bandpass-like profile, with maximal transmission between 10 and 60 Hz. Transmitted inputs also induced oscillatory components in the common synaptic input to motor neurons at stimulation frequency. Simulations indicated that this frequency selectivity emerges from balanced excitatory and inhibitory inputs to the motor neuron pool, with specific synaptic dynamics. These findings demonstrate that corticospinal transmission during rTMS acts as a frequency-selective linear system and provide a framework for assessing and modulating corticospinal pathways, with potential application as tool for tracking disease progression and neurorehabilitation. Highlights- HDsEMG decomposition tracks spinal motor neuron activity during rTMS. - Corticospinal transmission scales with rTMS stimulation intensity. - Corticospinal pathways act as a frequency-selective system. - rTMS transfer function shows maximal transmission at 10-60 Hz. - EPSP-IPSP interactions explain bandpass corticospinal transmission during rTMS.

neuroscience↗

bioRxiv2026

Neural coupling between spinal motor neurons of the first dorsal interosseous muscle during individual index finger flexion and pinch tasks

ObjectivePrecision grip tasks require complex coordination of intrinsic hand muscles, yet how common synaptic inputs to motor neurons are modulated during functionally different tasks remain unclear. This study investigated whether neural coupling between motor unit spike trains in the first dorsal interosseous (FDI) muscle differs between isolated index finger flexion and precision pinch tasks. ApproachSixteen healthy participants performed isolated index finger flexion and pinch tasks at 10% and 20% of maximal voluntary contraction while high-density surface electromyography was recorded from the FDI. Motor unit spike trains were decomposed and tracked across tasks. Neural coupling was assessed using complementary methods: coherence analysis and Proportion of Common Input (PCI) index to quantify linear common oscillations in delta (1-5 Hz), alpha (5-15 Hz), and beta (15-35 Hz) frequency bands, and mutual information-based network analysis to capture nonlinear interactions. Main results.Coherence analysis and PCI revealed no significant differences between tasks across all frequency bands. In contrast, network density derived from mutual information analysis showed significantly stronger nonlinear motor unit coupling during pinch compared to isolated finger flexion (p = 0.013), independent of force level. Significance.These findings demonstrate a dissociation between linear and nonlinear measures of motor unit coupling. In particular, precision pinch tasks appear to rely on stronger higher-order common inputs and distinct neural control strategies that are not fully captured by traditional linear coherence measures. This highlights that functionally relevant precision behaviors engage additional layers of nonlinear neural coupling, offering new insight into how the nervous system adaptively modulates motor unit coordination to meet complex task demands.

neuroscience↗

bioRxiv2026

The variability of reflex amplitude estimates in motor unit pools depends on the phenotype distribution and discharge statistics

Motor unit (MU) activity during electrically or mechanically evoked reflexes is used to examine the relationship between neural inputs and MU properties. However, variations in single-MU reflex amplitudes are not fully understood and limit their reliability in determining the input-output relation of motor neurons (MNs). Using experiments and computer simulations, we investigated (i) whether MN discharge statistics and muscle activation explain the variability of reflex amplitude estimates and (ii) whether these variations are reflected differently across distinct reflex amplitude estimation methods. We analyzed MU spike trains extracted from isometric contractions of the tibialis anterior muscle at 10 % and 20 % MVC (maximum voluntary contraction). Estimating reflex amplitudes based on the peristimulus frequencygram (PSF) at 10 % MVC, the linear regression between discharge rate (DR) and reflex amplitude was always positive, with p < 0.05 in 3 out of 6 subjects; however, the linear correlation was inconsistent at 20 % MVC. We thereby observed that inter-subject variability was associated with the coefficient of variation of the interspike intervals. Furthermore, the linear correlation between DR and peristimulus time histogram (PSTH) based reflex amplitudes was inconsistent for both contraction forces. To obtain further insights into the influence of MN properties, we simulated reflexes in a heterogeneous MN population using electrical circuit models and varied MN inputs. The simulations indicate that, besides mean input current and membrane noise, MN properties also contribute to the variability of reflex amplitude estimates. The MN heterogeneity is well captured by PSF-based reflex estimates but not by PSTH-based ones. These results show that variations in amplitude estimates of individual MU reflexes are due to complex interactions between intrinsic and extrinsic factors. As PSF-based reflex amplitude estimates reflect the MN size distribution, tracking PSF-based reflex amplitudes at fixed MVC levels across individual subjects could serve as a marker for investigating spinal adaptations under (patho)physiological conditions. Author summaryMotor neurons are specialized nerve cells that control human movement. Each motor neuron activates a specific set of muscle fibers, and the functional unit consisting of a motor neuron and muscle fibers is called a motor unit. We can observe the activity of motor neurons in humans by decomposing the electrical activity of muscles (the electromyogram) into contributions from individual motor units. Reflex responses of motor units are often used to study the input-output relation of motor neurons in humans. We used a combination of experiments and computer simulations to study the factors that influence the reflex amplitude of motor units during an excitatory reflex. We found that the reflex amplitude is non-linearly influenced by a number of intrinsic and extrinsic factors, e. g., motor neuron size, but also the muscle force. Additionally, we found that these factors have different effects on the results of the two common methods used to calculate the reflex amplitude. These results provide guidance on choosing a suitable evaluation method and on interpreting reflex experiments.

neuroscience↗

bioRxiv2026

Neural and mechanical properties of vastus lateralis and vastus medialis at different rectus femoris muscle lengths

This study investigated how altering the length of one muscle influences motor unit discharge behavior of its synergists. Eighteen participants performed isometric knee extensions at 10% and 30% of maximal voluntary contraction (MVC) with the hip joint at 90{degrees} (shortened rectus femoris, RF) and 180{degrees} (lengthened RF). High-density surface electromyograms were recorded from vastus medialis, RF and vastus lateralis, and decomposed into motor unit spike trains. Mean discharge rate and coefficient of variation of interspike interval were analyzed for tracked units in the vasti and non-tracked in the RF. While no changes were observed in RF motor units, lengthening the RF led to increased discharge rates of vasti motor units at 10% MVC, but not 30% MVC. To further explore these force-dependent changes, two further experiments were conducted. The first showed that the discharge rate at recruitment during ramp-up contractions increased with RF lengthening, but only for low-threshold vasti units. In the second, electrically evoked twitch contractions in the vasti revealed significantly reduced twitches at 180{degrees} during low-frequency, but not high-frequency stimulation. These findings collectively suggest that the force-dependent changes in the vasti motor unit discharge rates are likely driven by RF-length dependent changes in the vasti muscles contractile properties.

physiology↗

bioRxiv2025

Motor unit rate coding in intrinsic hand muscles during isolated finger contractions and pinch task

PurposeMuscle force output is modulated via motor unit recruitment and rate coding, yet how rate coding in intrinsic hand muscles differs between isolated and synergistic hand tasks remains unclear. This study examined motor unit discharge behaviour in the first dorsal interosseous (FDI) and thenar during isolated index finger flexion, isolated thumb flexion, and a tip pinch task. MethodsSeventeen participants performed each task at 10%, 20%, and 30% of their maximal voluntary contraction (MVC) while high-density surface electromyograms (HDsEMG) were recorded from both muscles. Motor unit spike trains were completely decomposed from the HDsEMG recordings, tracked across force levels, and their mean discharge rates and recruitment thresholds were calculated. ResultsFor both FDI and thenar muscles, the mean discharge rate increased with force, but the FDI exhibited steeper slopes than the thenar and at 10-20% MVC than 20%-30% MVC. In addition, lower recruitment thresholds and higher mean discharge rates were observed in the FDI compared to the thenar. Task-dependent differences were also observed in the FDI, with the pinch task yielding higher discharge rates than isolated contractions. In the thenar, differences between tasks were limited to higher forces. ConclusionThese findings demonstrate muscle- and task-specific motor unit modulation across forces in the intrinsic hand muscles, where the FDI relies more on rate coding, while the thenar likely prioritizes recruitment to cope with increased force demands.

physiology↗

bioRxiv2025

A methodological framework for assessing neural control of shoulder muscles during multi-directional isometric tasks using high-density surface electromyography.

The complex function of the shoulder relies on the coordinated activation of small and large muscles, including the deltoid, pectoralis major, trapezius, and latissimus dorsi. However, detailed knowledge of their neuromuscular control remains limited. This study aimed to develop a methodological framework to investigate the neural control of the larger superficial shoulder muscles by combining a six-degree-of-freedom load cell attached to a robotic arm with high-density surface electromyograms (HDsEMG). Six healthy participants performed isometric contractions (abduction, adduction, flexion, and extension) at 30% of maximal voluntary contraction with the shoulder positioned at 30{degrees} and 65{degrees} of lateral abduction. HDsEMGs were recorded from the four muscles and analysed at the global activation, spatial distribution of activation and motor unit levels. Global activation was quantified using averaged normalized root-mean-square (RMS) amplitude and spatial distribution using coefficient of variation of the topographic maps. Moreover, HDsEMGs were decomposed into individual motor unit spike trains using convolutive blind source separation, and motor unit behaviour was characterized by mean discharge rate and spatial distribution of motor unit action potentials (MUAPs). RMS maps revealed action-specific activation within and between muscles, with the upper trapezius active across all tasks, while the anterior, middle, and posterior deltoid, clavicular pectoralis major, and latissimus dorsi were predominantly activated during abduction, flexion, and extension. Motor unit discharge rate also showed task-dependent activity. MUAP spatial distributions further showed distinct motor unit territories within arrays, suggesting region-specific recruitment strategies across actions. In conclusion, this framework demonstrates that individual motor unit activity can be reliably measured non-invasively in the superficial shoulder muscles. The approach provides a methodological basis for novel incorporation of neural control information into biomechanical models of shoulder function.

neuroscience↗

bioRxiv2025

Sensitivity of Non-Invasive Motor-Unit-Based Gesture Recognition to Signal Degradation

The information encoded by motor units has been successfully harnessed to establish high-fidelity human-machine interfaces (HMI). However, the sensitivity of these interfaces using high-density surface electromyography decomposition, a prevalent method for observing motor unit behaviour, to signal degradations commonly encountered in practical settings remains unexplored. Here, we investigated the effects of additive white Gaussian noise (WGN), channel loss, and electrode shift on pseudo-real-time MU-driven motion classification. Across six wrist movements by 13 participants, we evaluated the performance of two classifiers: linear discriminant analysis (LDA) and deep neural networks (DNN), under different noise conditions. The results indicate that spatial perturbations, including channel loss and electrode shift, significantly affected classification accuracy, with LDA being more susceptible than DNN. Conversely, under intense signal noise (WGN with 5 dB SNR), LDA outperformed DNN, and its simplicity potentially provides greater robustness in a challenging environment. Hence, application-specific signal processing considerations are required depending on the target HMI application. IMPACT STATEMENTPerturbation severely impairs non-invasive motor-unit-based gesture recognition. High signal fidelity and robust system design are essential for practical human-machine interaction.

bioengineering↗

bioRxiv2025

Effects of Spatial and Signal-Imposed Noises on Motor Unit Decomposition

High-density surface electromyography (HD-sEMG) decomposition offers insights into the neural drive through observation of individual motor units (MUs). However, ensuring that this method remains reliable under real-world signal degradations is crucial for its broader application. Therefore, we investigated the impact of three commonly modeled signal degradations on convolutive blind-source-separation (BSS) MU decomposition. 192 HD-sEMG channels were recorded from the forearm muscles of thirteen healthy participants during six wrist movements. Three broad categories of perturbation were introduced, including additive white Gaussian noise (WGN), channel loss, and electrode shift. These perturbations were chosen to mimic challenges encountered in practice, such as ambient electrical noise, electrode failures, and sensor displacement in order to test the MU decomposition algorithms sensitivity. Then, the effects of perturbations on the quantity and quality of extracted MUs and neural drive estimation were assessed. Under non-perturbed conditions, an average of 179 {+/-} 40 MUs were extracted. Severe global WGN significantly reduced extracted MUs by approximately 81%. In contrast, more localized WGN, or channel loss as high as 15%, and electrode shift had minimal impact, with reductions in the number of MU decomposed being less than 6%. Reconstruction of neural drive through smoothed cumulative spike trains was significantly impaired by global WGN, thus leading to increased root mean square error when compared to conditions, while localized perturbations had negligible effects. Therefore, the BSS-based MU decomposition methods seem to be robust against localized noise, channel loss, and minor electrode shifts but are vulnerable to global additive noise. These results highlight the importance of carefully applying MU decomposition approaches in practical settings, maintaining high SNRs in EMG recordings, and preprocessing noise treatment to specific MU-decomposition needs.

bioengineering↗

bioRxiv2025

Impact of Noise on Deep Learning-Based Pseudo-Online Gesture Recognition with High-Density EMG

Deep neural network (DNN)-based approaches have demonstrated high accuracy in surface electromyography (sEMG)-driven gesture recognition under controlled conditions. However, estimation performance is known to degrade substantially when exposed to perturbations commonly encountered in the real-world. Here, we investigate the impact of typical noises on an autoencoder augmented recurrent neural network gesture estimator driven by engineered features and feature sets. High-density sEMG (HD-sEMG) signals offering rich information particularly suited to DNN-based algorithms were collected from thirteen participants performing wrist movements. Three types of synthetic disturbances were introduced: additive white Gaussian noise (WGN), channel loss, and electrode shift. Results indicate that when using amplitude-based features (specifically, the root mean square value and the mean absolute value), the estimator maintains robust performance under increasing WGN and channel loss, whereas its performance deteriorates markedly with features reflecting signal dynamics and fluctuations, like slope sign changes and zero crossings. Under electrode shift conditions, employing a combined feature set enhances the classifiers resilience. Importantly, the degree of performance degradation depends on both the type and intensity of the noise. These findings confirm the need for noise-resilient architectures in order to achieve practical, everyday sEMG-driven human-machine interfaces. Clinical RelevanceQuantifying the sensitivity of sEMG-based gesture classifiers to noise can help clinicians tailor electrode placement and training and ultimately decrease user frustration and improve the acceptance of assistive devices.

bioengineering↗

bioRxiv2025

Adaptations in common synaptic inputs to spinal motor neurons during grasping versus a less functional hand task

Previous evidence suggests that shared synaptic inputs across spinal motor neurons play a key role in coordinating multiple muscles during hand movements, reducing control complexity. In this study, we investigated how the nervous system modulates these common synaptic inputs during a functionally relevant grip (grasping) compared to less functionally relevant hand tasks. Seventeen participants performed three different tasks: simultaneous four-finger flexion without thumb involvement (four-finger flexion), thumb flexion, and simultaneous flexion of both fingers and thumb (grasping). For each task, subjects sustained isometric contractions at 5% and 15% of maximal voluntary contraction, while high-density surface electromyograms (HDsEMG) were recorded from the superficial extrinsic flexor muscles of the hand. Motor unit spike trains were decomposed from HDsEMG and tracked across tasks, and their mean discharge rate was calculated. Coherence between motor units was quantified within the delta, alpha, and beta bands to estimate common synaptic oscillations. At both force levels, the mean discharge rate decreased during grasping compared to four-finger flexion but increased during grasping compared to thumb flexion. Additionally, the area under the curve of coherence within the alpha band decreased by [~]20% during grasping compared to the four-finger flexion task, with no significant delta or beta bands changes. These reductions in alpha band coherence were reflected in force oscillations, showing decreased force-neural drive coupling within the alpha band and increased force steadiness during grasping compared to four-finger flexion. Our findings suggest that a functionally relevant and frequently used grip involves distinct neural control mechanisms that ultimately enhance force control.

neuroscience↗

bioRxiv2025

Differential changes in the effective neural drive following new motor skill acquisition between vastus lateralis and medialis

PurposeTo investigate whether short-term learning of a new motor task is mediated by changes in common synaptic inputs to motor neurons within and between synergistic muscles. MethodsSeventeen healthy individuals performed 15 repetitions of a complex force-matching task at 10% of a maximal voluntary contraction. Two trials were selected for analysis, the one with the highest force-target error (pre-learning) and the one with the lowest (post-learning). High-density surface electromyograms recorded from vastus medialis (VM) and vastus lateralis (VL) were decomposed into their constituent motor unit spike trains, with individual motor units being tracked between trials. Motor unit discharge behavior and common synaptic oscillations across the delta, alpha, and beta bands were calculated and compared between pre- and post-learning. ResultsForce-target matching improved across trials, accompanied by a significant decrease in the coefficient of variation of the inter-spike interval (p < 0.01), while the mean discharge rate remained similar (p > 0.85). The area under the curve within delta (p < 0.003) and alpha (p < 0.004) bands decreased between trials, with no significant changes in the beta band (p > 0.05). Notably, reductions in the alpha band correlated significantly with performance improvements in VL (R = 0.81) but not in VM (R = 0.12). ConclusionThe acquisition of a new motor task is mediated by modulations in common synaptic inputs to motor units, leading to improved force control. Our findings further suggest that these changes in common synaptic inputs, particularly in the alpha band, differ between VM and VL.

neuroscience↗

bioRxiv2025

Changes in motor unit conduction velocity after unilateral lower limb suspension and active recovery correlate with muscle ion channel gene expression

The effects of muscle disuse on the propagation of action potentials along muscle units, a key process for effective muscle activation and force production, remain poorly understood. This study aimed to investigate changes in action potential propagation and to identify biological factors influencing these changes following unilateral lower limb suspension (ULLS) and active recovery (AR). Eleven young male participants underwent 10 days of ULLS followed by 21 days of AR based on resistance exercise. Maximal force of the knee extensor muscles (MVC), High-Density surface EMG recordings and muscle biopsies of the vastus lateralis muscle were collected before ULLS, after ULLS and after AR. EMG recordings collected during submaximal isometric contractions were decomposed to estimate single motor unit conduction velocity (MU CV). Muscle biopsies were used to measure muscle fibre diameters via histochemical analysis and ion channel transcriptomic profiles via mRNA-sequencing. MVC decreased after ULLS by 29% and fully recovered after AR. MU CV decreased after ULLS and fully recovered, up to exceeding baseline values after AR. Muscle fibre diameters did not change across the interventions and showed no correlation with MU CV. Conversely, a feature importance analysis revealed that mRNA expression levels of specific ion channel genes, particularly those involved in K+ transport, correlate with MU CV at baseline and across the interventions. This study highlights the crucial role of K+ ion channels in influencing MU CV in humans, offering new insights into MU CV modulation and the mechanisms of muscle force changes after disuse and active recovery. Key pointsO_LIMuscle disuse, such as in unilateral lower limb suspension, leads to a decrease in motor unit conduction velocity (MU CV), a critical factor for muscle activation and force production. C_LIO_LIActive recovery through resistance exercise results in the full recovery of MU CV, even exceeding baseline levels. C_LIO_LIMuscle fibre diameters do not change significantly after limb suspension or active recovery and show no correlation with MU CV. C_LIO_LIConversely, ion channel mRNA expression, particularly of those related to K+ transport, correlates with MU CV and its changes following disuse and recovery. C_LIO_LIThese findings highlight K+ ion channels as a key factor in regulating MU CV in humans and provide new molecular determinants of the changes in muscle force after disuse and recovery. C_LI

physiology↗

bioRxiv2025

A single low-dimensional neural component of motor unit activity explains force generation across repetitive isometric tasks

Previous studies suggest that low-dimensional control underlies motor unit activity, with low-frequency oscillations in common synaptic inputs serving as the primary determinant of muscle force production. In this study, we used principal component analysis (PCA) and factor analysis (FA) to investigate the relationship between low-dimensional motor unit components and force oscillations during repetitive isometric tasks with similar force profiles. We assessed the consistency of these components across trials in both individual (tibialis anterior; first dorsal interosseous) and synergistic muscles (vastus medialis, VM; vastus lateralis, VL). Participants performed 15 trials of a force-matching learning task. Three post-skill acquisition trials were selected for analysis to ensure high similarity in force profiles. Motor units were decomposed from high-density surface electromyograms, tracked across trials, and their smoothed discharge rates were decomposed into low-dimensional components using PCA and FA. Parallel analysis indicated that a single component could explain the smoothed discharge rates for the individual muscles and two components for VM-VL. Importantly, the first component explained most of the variance ([~]70%) in smoothed discharge rates across all muscles. The first motor unit component also showed significantly higher correlations with force oscillations than the second component and remained highly consistent across trials. These findings were further supported by a non-linear framework combining network- and information-theoretic tools, which revealed high motor unit network density in the first component of all muscles. Collectively, these results suggest that, during isometric contractions, motor unit activity is primarily controlled by a single dominant shared synaptic input that closely mirrors force oscillations.

neuroscience↗

bioRxiv2024

E-bike riding: A metabolic evaluation in the context of exercise intensity domains

IntroductionE-bikes are being promoted as a mode of transportation that can aid with meeting current physical activity guidelines. This study evaluated exertional intensity associated with E-Biking within the exercise intensity domain framework. We hypothesized that exertional intensity of E-bikes is insufficient to evoke a metabolic demand associated with the minimum intensity needed to improve cardiorespiratory fitness. MethodsForty-four participants (22 females) of varying activity levels completed two experimental sessions. The first session involved a lab-based ramp-incremental exercise test to determine [V]O2max, gas exchange threshold (GET), and respiratory compensation point (RCP). The second session involved the completion of two outdoor rides on the same bike equipped with an electrical motor E-bike. The first ride was completed without electrical assistance, while the second was. The power output (PO) and speed were set at an intensity corresponding to [~]10% above GET. ResultsAs expected, using E-bike assistance resulted in a lower metabolic demand, falling well below GET. While the absolute power output (PO) was different between sexes, relative heart rate (HR) and relative PO were similar between the rides without and with assistance. This suggests that when riding an E-bike, the internal load is similar between sexes. ConclusionDespite E-bikes facilitate a more active lifestyle and help to reduce the emission of pollutants, when interpreted within the context of the exercise intensity domain schema, their associated exertional intensity is likely insufficient to confidently elicit cardiorespiratory benefits.

physiology↗

bioRxiv2024

Greater distal activation of the biceps femoris long head during knee flexion and hip extension tasks reflects differences in peripheral, not central, motor unit properties.

PurposeRecent research has explored region-specific responses within the biceps femoris long head. However, evidence on regional muscle activation remains controversial, primarily because information derived solely from surface electromyograms (sEMG) amplitude does not necessarily provide an accurate estimate of neural drive to the muscle. To address this limitation, this study investigated whether there are proximodistal differences in motor unit properties of the biceps femoris long head during isometric hip extension and knee flexion tasks. MethodsSeventeen resistance-trained males performed isometric knee flexion and hip extension tasks at 20% and 40% of maximal voluntary contraction. High-density sEMG were recorded from proximal and distal regions of the biceps femoris long head and decomposed into individual motor units. Central motor unit properties (mean discharge rate, discharge rate variability, recruitment and de-recruitment thresholds) and action potential properties (amplitude and conduction velocity) were analyzed. Bipolar sEMG amplitude was also calculated for each region to simulate traditional sEMG measurements. ResultsBipolar sEMG amplitude, motor unit action potential amplitude and conduction velocity were significantly greater in the distal region during both tasks. In contrast, no proximodistal differences were observed in central motor unit properties. ConclusionThese findings suggest that increased bipolar sEMG amplitude in the distal region of the biceps femoris long head is driven by motor unit action potential properties rather than differences in central modulation, likely influenced by intra-muscular variations in muscle mechanics and geometry. This emphasizes limitations of relying solely on sEMG amplitude to infer neural control strategies in the biceps femoris long head.

physiology↗

bioRxiv2024

Voluntary co-contraction of ankle muscles alters motor unit discharge characteristics and reduces estimates of persistent inward currents

Motoneuronal persistent inward currents (PICs) are both facilitated by neuromodulatory inputs and highly sensitive to local inhibitory circuits (e.g., Ia reciprocal inhibition). Methods aimed to increase group Ia reciprocal inhibition from the antagonistic muscle have been successful in decreasing PICs, and the diffuse actions of neuromodulators released during activation of remote muscles have increased PICs. However, it remains unknown how motoneurons function in the presence of simultaneous excitatory and inhibitory commands. To probe this topic, we investigated motor unit (MU) discharge patterns and estimated PICs during voluntary co-contraction of ankle muscles, which simultaneously demands the contraction of agonist-antagonist pairs. Twenty young adults randomly performed triangular ramps (10s up and down) of both co-contraction (simultaneous dorsiflexion and plantarflexion) and isometric dorsiflexion to a peak of 30% of their maximum muscle activity from a maximal voluntary contraction. Motor unit spike trains were decomposed from high-density surface electromyography recorded over the tibialis anterior (TA) using blind source separation algorithms. Voluntary co-contraction altered motor unit discharge rate characteristics, decreasing estimates of PICs by 20% (4.47 pulses per second (pps) vs 5.57 pps during isometric dorsiflexion). These findings suggest that, during voluntary co-contraction, the inhibitory input from the antagonist muscle overcomes the additional excitatory and neuromodulatory drive that may occur due to the co-contraction of the antagonist muscle, which constrains PIC behavior. KEY POINTSVoluntary co-contraction is a unique motor behavior that concurrently provides increases in excitatory and inhibitory inputs to motoneurons. During co-contraction of agonist-antagonist pairs, agonist motor unit discharge characteristics are altered, consistent with reductions in persistent inward current magnitude. Reciprocal inhibition from the antagonist likely becomes proportional to the increase in neural drive to the agonist, dampening the magnitude of persistent inward currents.

physiology↗

bioRxiv2023

Muscle contractile properties directly influence shared synaptic inputs to spinal motor neurons

Alpha band oscillations in shared synaptic inputs to the alpha motor neuron pool can be considered an involuntary source of noise that hinders precise voluntary force production. This study investigated the impact of altering muscle length on the shared synaptic oscillations to spinal motor neurons, particularly in the physiological tremor band. Fourteen healthy individuals performed low-level dorsiflexion contractions at ankle joint angles of 90{degrees} and 130{degrees}, while high-density surface electromyography (HD-sEMG) was recorded from the tibialis anterior (TA). We decomposed the HDsEMG into motor units spike trains and calculated the motor units coherence within the delta (1-5 Hz), alpha (5-15 Hz) and beta (15-35 Hz) bands. Additionally, torque steadiness and torque spectral power within the tremor band was quantified. Results showed no significant differences in torque steadiness between 90{degrees} and 130{degrees}. In contrast, alpha band oscillations in both synaptic inputs and force output decreased as the length of the TA was moved from shorter (90{degrees}) to longer (130{degrees}), with no changes in delta and beta bands. In a second set of experiments, evoked twitches were recorded with the ankle joint at 70{degrees} and 130{degrees}, revealing longer twitch durations in the longer muscle lengthen condition compared to the shorter. These experimental results, supported by a simple computational simulation, suggest that increasing muscle length enhances the muscles low-pass filtering properties, influencing the oscillations generated by the Ia afferent feedback loop. Therefore, this study provides valuable insights into the interplay between muscle biomechanics and neural oscillations.

neuroscience↗

bioRxiv2023

Intrinsic properties of spinal motoneurons degrade ankle torque control in humans

Motoneurons are the final common pathway for all motor commands and possess intrinsic electrical properties that must be tuned to control muscle across the full range of motor behaviors. Neuromodulatory input from the brainstem is likely essential for adapting motoneuron properties to match this diversity of motor tasks. A primary mechanism of this adaptation, control of dendritic persistent inward currents (PICs) in motoneurons by brainstem monoaminergic systems, generates both amplification and prolongation of synaptic inputs. While essential, there is an inherent tension between this amplification and prolongation. Although amplification by PICs allows for quick recruitment and acceleration of motoneuron discharge during discrete motor tasks, PICs must be deactivated to de-recruit motoneurons upon movement cessation. In contrast, during stabilizing or postural tasks, PIC-induced prolongation of synaptic inputs is likely critical for sustained motoneuron discharge. Here, we designed two motor tasks that show PIC amplification and prolongation may conflict and generate errors that degrade the precision of motor output in humans. This included a paradigm comprised of a discrete motor task superimposed atop a stabilizing task and a paradigm with muscle length-induced changes to the balance of excitatory and inhibitory inputs available for controlling PICs. We show that prolongation from PICs introduces deficits in ankle torque control and that these deficits are further degraded at shorter muscle lengths when PIC prolongation is greatest. These results highlight the necessity for inhibitory control of PICs and showcase issues that are introduced when inhibitory control is perturbed or constrained. Our findings suggest that, like sensory systems, errors are inherent in motor systems. These errors are not due to problems in the perception of movement-related sensory input but are embedded in the final stage of motor output.

physiology↗