Journal of Neurophysiology

The aim of this study was to improve our understanding of muscle contractions in the arm as a function of hand orientation for grasp. While there have been several reports on arm kinematics for reach and grasp movements, little has been done at the muscular level. To this end, we analyzed the modulation of shoulder, elbow and hand muscles for a reach and grasp task involving a target in either horizontal or vertical orientation. We hypothesized that unlike what has been observed for kinematics, at the muscular level we would see less correlation between the three muscle groups. A decoding approach with Machine Learning revealed adaptation patterns that were not visible using classical methods. Reach-and-grasp has traditionally been treated as being made of two components - the reach and the grasp components. Our dynamic decoding approach revealed a more complex picture with very different dynamics in the shoulder and elbow muscle groups during reach. All muscle groups showed peak capacity for predicting hand orientation before the start of grasp and followed the ubiquitous proximo-distal organization. The patterns of muscular modulation for hand orientation were strongly perturbed by the eyes closed and slow movement conditions, potentially decreasing the available degrees of freedom for adaptation.

Humans can flexibly acquire entirely new sensorimotor mappings, a process known as de novo motor learning. A central challenge in de novo motor learning is that the learner must discover a viable solution from scratch within a highly redundant control space, without predefined task constraints. Understanding what types of sensorimotor information contribute to the formation of accurate motor behavior in such situations is therefore critical for explaining how novel sensorimotor skills are acquired. While previous studies have suggested that novel visuomotor mappings can be formed based on movement direction and target position, it remains unclear how these two types of information contribute to the learning process. To address this question, we trained 25 human participants to learn arbitrary joystick-to-cursor mapping. We then employed a generalization paradigm to selectively restrict learning experience to either movement direction or target position. Three distinct target conditions were designed: one emphasized target position (P), another emphasized movement direction (D), and a third (P&D) encouraged learning of both components separately. As a result, direction experience improved movement initiation, whereas position experience enhanced movement termination. However, in the P&D condition, combining these experiences did not yield additive generalization. Instead, endpoint accuracy was positively correlated with the degree of alignment between direction- and position-based joystick outputs within the control space. These results suggest that accurate formation of a novel sensorimotor map depends on the coordinated use of directional and positional experiences. Significant StatementHow do humans build entirely new sensorimotor relationships from scratch? This study examined how distinct sensorimotor experiences (movement direction and target position) contribute to the acquisition of a novel joystick-to-cursor mapping. By isolating these experiences, we found that direction experience improved movement initiation, while position experience enhanced movement termination. However, combining these experiences did not lead to more accurate movements as a whole. Instead, the accuracy was related to how well directional and positional joystick outputs were aligned in a control space. These findings suggest that de novo motor learning requires the coordinated use of directional and positional information.

Psychological pressure is thought to relate to performance in an inverted-U pattern, yet evidence is mixed, possibly because manipulations rarely produce high pressure. We induced scalable pressure using a streak goal that resets after a failure in a force-control task. Participants pursued ten consecutive successes (streak goal) or 100 successes irrespective of sequence (total goal). Under the streak goal, heart rate, pupil size, and perceived pressure rose as participants approached their maximum streak; under the total goal, heart rate and pupil size showed little modulation. Performance followed an inverted-U under the streak goal--improving then declining at the maximum streak--whereas the total goal showed no late-stage drop. This dissociation suggests the late-stage decline reflects pressure, not the streak itself. Despite this clear performance decrement, analyses of movement vigor, feedforward/feedback kinematics, and individual differences in pressure responses revealed no consistent systematic signatures at the group level. Fragile streak goals thus provide a multimodal pressure manipulation and a platform for testing mechanisms underlying choking in human motor control.

When reaching to a foveated target, peripheral vision of the hand can be used to make rapid, automatic adjustments to the ongoing reach movement, with the feedback gain being sensitive to features of the task and environment. These rapid corrective responses are also observed when gaze is directed to a stationary gaze target located away from the reach target. In everyday contexts, reaching often occurs concurrently with other visual or visuomotor tasks, such as tracking a moving target. Yet it remains unclear whether engaging in such tasks affects the use of peripheral vision for hand guidance. Here, we compare rapid visuomotor corrective responses to visual perturbations during fixation and smooth pursuit, and test whether pursuit-related and reach-related visuomotor processes operate independently or compete for shared visual resources. Participants either fixated a stationary target or tracked a moving target while reaching toward a spatially dissociated reach target. During the reach, the visual representation of the hand was perturbed, requiring rapid corrective responses. We found that neither the onset nor the gain of reach corrections was modulated by gaze-task demands. Moreover, response gains were strongly correlated across tasks, indicating consistent individual response profiles that were independent of the gaze condition. Despite modest increases in position error and decreases in gain, participants largely sustained engagement with the visual tasks during target reaching. These findings demonstrate that smooth pursuit and reach-related visuomotor processing can operate in parallel without mutual interference, suggesting a functional independence between them. NEW & NOTEWORTHYIn everyday life, reaching to an object can occur while the eyes are engaged in competing visual tasks. We show that engaging in smooth pursuit eye movements does not disrupt rapid visuomotor corrections during reaching. The onset and gain of corrective responses following perturbation were unchanged by gaze-task demands and were consistent across individuals. These findings demonstrate that pursuit and reach-related visuomotor processes can operate in parallel, supporting functional independence between these systems.

The capacity to cancel or adapt planned actions in response to changing environmental demands is essential for navigating our complex world. While past research has shown that an individuals expectations of upcoming movement demands influence the speed of action initiation, the effect this has on subsequent cancellation or adaption of that movement remains unknown. 25 healthy adults completed stop signal tasks and stop change tasks in which biasing cues (e.g., "70% left") accurately indicated the probability that a left, or right button press would be required. As expected, responses that were congruent with the cue were faster than incongruent responses; however, biasing cues had no effect on behavioural or physiological (electromyographical) indices of stopping speed. Stopping latencies were found to be faster in the stop change task than the stop signal task, corroborating other recent work. However, a second experiment (25 healthy adults) which used the same stimuli for both tasks (varying only the instructions), revealed no difference - highlighting the sensitivity of the stop process to stimulus effects, and a common confound in the literature. We also observed that physiological indices of action reprogramming (following a stop) were faster in congruent than incongruent trials. Collectively, these results suggests that preparatory changes that accompany expected movements influence the enaction of movement both prior to, and after stopping, but the stop mechanism itself, remains independent of these preparations. These results inform how action cancellation and adaption are applied in real world environments, where expectations continually interface with our motor plans. HighlightsO_LI* Anticipating a movement increases the speed of its enaction but not subsequent cancellation C_LIO_LI* Expected movements can be reprogrammed more quickly than unexpected movements C_LIO_LI* The latency of action cancellation is highly sensitive to stimulus effects C_LI

From a behavioral and neuronal perspective, observational and physical practice conditions have been theorized to be equivalent during motor task learning. However, some paradigms can challenge such a functional equivalence hypothesis. The perception of difficulties experienced by others may play a role in observational learning by allowing learners to partially distance themselves from these episodes, thereby limiting their impact on learning. In contrast, during physical practice, performance difficulties are directly experienced, which may constrain such distancing mechanisms. Indeed, an observer watching a model that uses a wrong physical strategy can ignore erroneous trials in order to preserve action encoding. The main goal of the present study was to prevent such observer avoidance and to test the cognitive and neuronal functional equivalence between the physical and observational practice groups. During this experiment, both groups learned two motor sequences. Only one sequence was repeatedly interrupted to perturb encoding. Behavioral results revealed that both groups were equally negatively impacted by such interruptions. Together, these findings suggest that while physical and observational practice can lead to comparable behavioral outcomes under strong disruption, they rely on partially distinct neural strategies. Physical practice predominantly engages motor and striato-cerebellar feedback loops, whereas observational learning relies more strongly on fronto-cerebellar and episodic memory networks, highlighting a context-dependent functional equivalence between learning modalities.

Tactile sensitivity is reduced when the limb is in motion, a phenomenon known as somatosensory gating. In a previous study, we demonstrated that discrimination precision but not perceived intensity differed between active and passive movements. Here, we asked whether and how spatial attention modulates tactile precision in active and passive movements. Participants judged the relative intensity of two vibrations while the arm was still, actively moved, or passively transported by a movable platform. Visual attention was directed either to the movement start or goal position. Perceptual bias was reduced during both active and passive movement, independent of attentional allocation. In contrast, precision remained stable during active movement but declined during passive movement when attention was directed to the movement start. However, when attention was focused on the movement goals, precision was also high when doing passive movements. These findings indicate that during active movements, predictions based, likely on an efference copy, ensure tactile precision, whereas passive movements require spatial attention directed to the movement goal.

Practice enhances motor acuity, enabling movement execution with greater speed and accuracy. However, the learning principles underlying improvements in speed, accuracy, and efficiency remain less understood than those supporting motor skill acquisition and adaptation. Here, we examined motor execution in a skill-based practice task to characterize learning, retention, and generalization of motor acuity. Using a gamified two-dimensional racing task, right-handed participants controlled a stylus-driven car along a curved track as quickly and accurately as possible. Across two studies (N = 83 total, 54 females), participants completed 300 training laps on Session 1 and returned for Session 2 to assess retention and generalization to novel track configurations: one with altered spatial configuration (rotated track) and one requiring movement in the opposite direction of training (reverse track). Movement speed improved rapidly and showed robust, though incomplete, retention across sessions. Speed improvements generalized substantially to both novel tracks. Accuracy was high at training onset and showed strong retention. However, we do not observe offline gains between sessions. Notably, accuracy declined transiently for the novel track configurations, suggesting interference from prior training. Movement efficiency, indexed by path length, was retained and generalized to the rotated track. However, reversing movement direction impaired efficiency, revealing a movement direction effect. This effect persisted when training direction was reversed in a second study, with counterclockwise movements remaining slower and less efficient than clockwise movements. These findings show that practice produces durable and broadly transferable motor execution improvements, while inherent movement direction biases constrain how improvements generalize across contexts. New & NoteworthyThe learning principles underlying improvements in motor acuity remain less well understood than those governing other forms of motor learning. Prior work suggests that motor execution improvements show limited generalization. In contrast, the present findings demonstrate that execution-based practice can produce robust, transferable gains, while also revealing a key constraint: inherent movement direction biases that limit generalization. By characterizing learning, retention, and generalization, this work provides new insight into how motor acuity improvements compare with skill acquisition and adaptation.

Eye movement-related eardrum oscillations (EMREOs) appear to consist of a pulse of oscillation occurring in conjunction with saccades. However, this apparent pulse could occur either because there is an increase in energy at that frequency at the time of saccades (a true pulse), or because there is saccade-related phase resetting of ongoing energy at that frequency band, thus appearing like a pulse when averaged in the time domain across many trials. Here we conducted a spectral analysis at the individual trial level in humans performing a visually guided saccade task to determine whether the power at the EMREO frequency (30-45 Hz) is higher during saccades than during steady fixation. We found both an increase in sound power in the EMREO frequency band associated with saccades, i.e. sound pulses at the individual trial level, as well as, phase resetting at saccade onset/offset. While both factors contribute to the apparently pulse-like EMREO signal, phase resetting appears to be more prevalent across participants. The prevalence of phase resetting has implications for the underlying mechanism(s) producing EMREOs as well as functional consequences for how the ear might respond to incoming sound in an eye-position dependent fashion.

A recent theoretical model of action stopping posits that the reactive cancellation of movement is underpinned by two dissociable processes: a rapid, involuntary "pause" that transiently suppresses motor output, and a slower, voluntary, suppression/retuning of motor output. Notably, the pause process has been posited to generalise broadly to infrequent and salient stimuli (irrespective of whether they bear an imperative to stop) and to be observable as suppression in electromyographical (EMG) recordings in the responding muscles. Over two experiments (N = 24 in each), participants completed standard stop signal and flanker tasks, and novel flanker task variants, where flanking arrows occurred infrequently (33% of trials), with or without a delay relative to the central imperative stimulus, or coincident with a stop signal. Presenting flankers infrequently specifically increased slowing to incongruent trials, with no effect on congruent or neutral trials (relative to a condition with flankers on every trial), and only after at least three preceding trials with no flanking stimuli. Critically, this was observed while carefully controlling for trial sequence effects. When flanker stimuli were presented infrequently, and after a delay, they did not reliably elicit suppression of EMG. These results highlight the contextual specificity with salient infrequent stimuli elicit behavioural slowing and EMG suppression, challenging the notion of a broadly generalisable pause process. Trial-level assessment of stopping speed using EMG revealed an effect of stimulus salience, whereby stop signals that occurred synchronously with Flanker arrows resulted in faster stopping than stop signals without Flanker arrows. Interestingly, this effect was specific to the faster end of stopping time distributions. Collectively, these results challenge interpretations which attribute electromyographic partial responses to specific neural pathways or mechanisms.

Despite a long history of studying presynaptic inhibition of the Ia afferent synapse that produces the monosynaptic EPSP on motoneurons, recent evidence has upset the conventional idea that GABAA receptors mediate this inhibition and instead suggests that there are mainly GABAB receptors at this synapse. However, without targeted access to the GABAergic neurons that activate these receptors, quantifying their functional contribution to presynaptic inhibition has proven difficult. We demonstrate here that focal optogenetic activation of terminals of a subpopulation of GAD2+ GABAergic neurons that exclusively project ventrally to Ia afferent synapses produce long-lasting presynaptic inhibition that is entirely mediated by GABAB receptors and simultaneously produces a characteristic brief GABAA receptor-mediated IPSP on the motoneurons. These ventral GAD2 neurons are recurrently activated by Ia afferents, contributing to post-activation depression with repeated afferent reflex testing, with a similar long time-course to post-activation depression of the H-reflex induced in humans from either repetitive activation of the same Ia afferents or from antagonist nerve conditioning. In contrast, focal activation of dorsally projecting GAD2 neurons does not directly cause presynaptic inhibition or postsynaptic IPSPs but does produce primary afferent depolarization. Following chronic spinal cord injury (SCI), the expression of GABAB receptors on the Ia terminal is halved, and in mice and humans, is associated with a similar decrease of GABAB receptor-mediated post-activation depression of Ia-EPSPs transmission, which is reversed by the GABAB receptor agonist baclofen. In summary, GABAB receptors mediate presynaptic inhibition, but are down regulated with SCI, contributing to reflex hyperexcitability associated with spasticity. Key Points SummaryO_LIPresynaptic inhibition of Ia afferents is mediated by the recurrent activation of terminal GABAB receptors by a subpopulation of ventrally projecting GAD2+ interneurons. C_LIO_LIIn contrast, dorsally projecting GAD2+ interneurons activate GABAA receptors on Ia afferent nodes to facilitate action potential conduction through branchpoints. C_LIO_LIRepetitive activation of Ia afferents at rates of every 10 s or faster produces post-activation depression via neurotransmitter depletion and from activation of terminal GABAB receptors. C_LIO_LIThese ventrally projecting GAD2+ interneurons can also be activated by other afferents that then produce PAD-evoked spikes to produce post-activation depression from conditioning nerve stimulation. C_LIO_LIThe reduction of GABAB receptors on the Ia terminal in spinal cord injury results in reduced presynaptic inhibition and post-activation depression, contributing to reflex hyperexcitability. C_LI O_FIG O_LINKSMALLFIG WIDTH=189 HEIGHT=200 SRC="FIGDIR/small/700955v2_ufig1.gif" ALT="Figure 1"> View larger version (39K): org.highwire.dtl.DTLVardef@17abd51org.highwire.dtl.DTLVardef@12316baorg.highwire.dtl.DTLVardef@a92168org.highwire.dtl.DTLVardef@1d06ca0_HPS_FORMAT_FIGEXP M_FIG Abstract legend: Schematic of GABAergic circuit producing presynaptic inhibition and primary afferent depolarization (PAD) in proprioceptive Ia afferents. We propose two populations of GAD2+ GABAergic interneurons, one with dorsal projections (purple) that activate GABAA receptors on the nodes of Ia afferents to produce PAD and subsequent facilitation of Ia afferent conduction, and another ventrally projecting population (pink) that activates GABAB receptors on the Ia afferent terminal to produce presynaptic inhibition via inhibition of VCa2+ channels and reduction of neurotransmitter release and replenishment. Both are activated by first order interneurons (grey). Repetitive activation of Ia afferents (green extensor) recurrently activates twhe ventrally projecting GAD2+ neurons to activate terminal GABAB receptors and long-lasting post-activation depression of Ia EPSPs and reflexes as measured from ventral root recordings. Strong conditioning stimulation of other afferents (blue flexor) activates dorsal GAD2+ neurons that can produce PAD-evoked spikes in extensor afferents that orthodromically activate motoneurons to set up post-activation depression of subsequent extensor reflexes. Here, PAD is also evoked in other afferents (flexor) by dorsally projecting GAD2+ neurons (light pink branch) but without activation of the ventrally projecting GAD2+ neurons or presynaptic inhibition. C_FIG

A subset of bilateral tasks requires one arm to perform a stabilizing role while the other completes a movement, such as slicing a loaf of bread. Visual attention during bilateral tasks has previously been examined with bilateral reaching tasks, demonstrating that visual attention switches between the two target locations. The goal of this study was to characterize visual attention during a cooperative mechanically coupled bilateral "stabilizing and reach" task to determine how visual attention is divided between the two limbs when one limb plays a stabilizing role. Twenty-six healthy young adults completed a robotic task in which the hands were coupled with a haptic spring. Participants were instructed to keep one hand stationary in a target while they reached for a target with the other hand, thus stretching the spring and applying a force to both arms. We found that individuals primarily fixated their gaze on the reaching target, only fixating on the stabilizing target for approximately 10% of the reaching time. Longer fixations on the reaching target were associated with faster reaching times, while longer fixations on the stabilizing target were associated with slower reaching times. While the performance of the stabilizing hand differed between the dominant and non-dominant limbs, visual strategies did not vary based on which hand was stabilizing. These results demonstrate that unlike bilateral reaching tasks in which the eyes frequently saccade between the two targets, visual guidance is primarily used for the reaching hand while minimal overt visual attention is directed to the stabilizing hand.

ObjectiveComplexity-based metrics have been applied to surface electromyography (sEMG) to characterize fatigue-related changes in the temporal structure of myoelectric signals beyond amplitude and spectral features. Optically pumped magnetometers (OPM) are sensors that enable non-invasive recordings of magnetomyographic (MMG) signals from skeletal muscle and are increasingly used to complement surface electromyography; however, it remains unclear whether complexity measures derived from magnetic recordings are comparable to those obtained from sEMG. Here, we directly compared fatigue-related dynamics of conventional and complexity-based signal features of sEMG and OPM-MMG measured from the biceps brachii during sustained elbow flexion. ApproachHealthy participants performed isometric contractions at 20% maximal voluntary contraction (MVC; 20 min) or 60% MVC (3 min). sEMG and OPM-based MMG were recorded simultaneously, and signal median frequency, root mean square (RMS), and Lempel-Ziv (LZ) complexity were calculated over time. Main resultsAcross contraction intensities, sEMG and MMG showed consistent fatigue-related changes, characterized by increasing RMS, decreasing median frequency, and a progressive decline in LZ over time. In addition, multiple regression analyses indicated that the decrease in LZ was not fully accounted for by concurrent amplitude or spectral changes, suggesting that complexity captures aspects of signal organization that are not fully explained by established features. Finally, while sEMG showed higher LZ complexity and median frequency at 60% compared to 20% MVC, corresponding intensity-dependent effects were not observed in OPM-based MMG. SignificanceThese findings suggest that complexity-based metrics capture fatigue-related changes in neuromuscular signal organization beyond conventional measures, and that sEMG and OPM-based MMG provide similar, though modality-specific, information. Together, the results support the use of complexity metrics in multimodal electrophysiological and biomagnetic assessments of neuromuscular fatigue.

Mental fatigue is known to impair cognitive and motor performance, but its impact on motor learning remains unclear. This study examined how mental fatigue affects skill acquisition in a sequential finger-tapping task. Twenty-eight participants were assigned to either a mental fatigue group, which completed a thirty-minute Stroop task, or a control group, which watched a documentary of equivalent duration. Both groups then trained on the finger-tapping task across multiple practice blocks with brief rest periods. Overall motor skill improved similarly in both groups. However, mental fatigue altered the pattern of acquisition: participants in the fatigue group showed decreased performance during practice blocks, which was compensated by larger gains during inter-block rest periods. A strong negative correlation was observed between online decrements and offline improvements, indicating that greater declines during practice were associated with larger gains during rest. This study highlights the critical role of rest periods in maintaining learning under cognitively demanding conditions and provides insight into how internal states, such as mental fatigue, can selectively influence the expression of performance without compromising overall learning.

Despite several age-related processes impacting motor performance, older adults often retain the ability to implicitly adapt to sensory prediction errors. Here, we leverage the fact that implicit adaptation is not attenuated by aging to study the impact of aging on responses to motor errors. In other domains, such as reinforcement learning, aging has been shown to influence how task outcomes or rewards are processed and used to guide subsequent actions, with some studies emphasizing that older adults react more strongly to a miss than to a hit. We aimed to extend these reinforcement learning findings to the motor domain with two preregistered experiments testing whether missing the target leads to larger implicit adaptation in young and older adults to the same extent. In addition, we compared these results to one reinforcement learning task in the motor domain (Boolean feedback after reaching in the absence of visual feedback) and one in the cognitive domain (reward-based decision making). While we found age-related effects in the cognitive domain, we did not observe a consistent effect of age on the modulation of reaching direction or motor adaptation by task outcomes. These results suggest a domain-specific nature of age-related changes in sensitivity to task outcomes.

The motor cortex is involved not only in movement execution but also in motor imagery, a process leveraged by decoding algorithms for brain-computer interface (BCI) applications in individuals with severe motor impairments. Previous work has shown that population activity during execution and imagery occupies partially overlapping regions of neural state space while also engaging distinct subspaces unique to each motor state, suggesting that decoders trained in one condition may not generalize to the other. Moreover, movement execution likely includes neural representations of both motor output and proprioceptive feedback, which themselves may occupy distinct or overlapping regions of neural state space. To explore these distinctions, we studied two individuals with incomplete spinal-cord injuries and partial residual proximal arm function performing a center-out reaching task in three conditions: motor imagery, active execution, and passive movement. We found that decoders trained on neural activity from motor imagery failed to generalize to either active or passive movements. In contrast, decoders trained on active or passive movement activity generalized reciprocally. Population analysis revealed distinct dynamics depending on limb state and proprioceptive feedback, which could explain this lack of generalization. These results suggest that motor imagery engages motor cortical representations distinct from those recruited during actual movements, either actively or passively generated, with important implications for the design of BCI decoders.

Co-contraction--the simultaneous activation of opposing muscles--influences movement efficiency, joint stability, and motor learning. While consensus exists for EMG acquisition and processing, comparable guidance for quantifying co-contraction is lacking. This study evaluated the behavior and interrelationships of six commonly used co-contraction indices (CCIs) to develop practical recommendations for their selection, calculation, and interpretation. Synthetic EMG-like signals were generated and used to evaluate CCI behavior across a range of conditions that would be difficult to achieve experimentally. Based on their formulas and observed behavior, CCIs were sorted into three categories: shape-based, amplitude-driven, and temporal indices. Shape-based CCIs increase when two EMG signals have similar shapes, regardless of amplitudes. Amplitude-driven CCIs increase when activation is high in both muscles. Temporal CCIs increase as the duration of EMG overlap increases, regardless of signal shape or amplitude. Correlation analyses showed stronger associations within-category than between-category, supporting the proposed classification scheme. CCI behavior yielded three principal findings, each paired with a practical recommendation. First, EMG amplitude normalization techniques altered co-contraction estimates, and the effect varied by index. Researchers should therefore test whether their conclusions hold across normalization methods. Second, because CCIs differ in scale and theoretical maxima, their values are not directly comparable across indices. Comparisons should instead focus on relative trends interpreted within each indexs bounds. Third, each CCI category was sensitive to different EMG features (e.g., amplitude versus shape). The choice of CCI should therefore align with hypothesized differences in EMG signals - use shape-based CCIs when waveform similarity is of interest, and amplitude-driven CCIs when differences in activation magnitude are expected. These results provide initial guidance for selecting, calculating, and interpretating CCIs, and they establish a framework for testing the robustness of these theoretical findings using experimental EMG from diverse tasks, muscle pairs, and populations.

This study investigated the spinal neural mechanisms underlying post-activation potentiation in ten healthy young males (21.9 {+/-} 4.8 years). Participants performed a 10-second 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 returning to baseline within nine minutes. This mechanical potentiation was accompanied by a 29% reduction in H-reflex amplitude (p < 0.001), which recovered within three minutes. 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 post-activation potentiation involves a complex dissociation: H-reflex pathway inhibition along with facilitation of presynaptic spinal mechanisms. This paradox can be explained by either post-activation 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 post-activation depression and muscle thixotropy during post-activation potentiation. New & NoteworthyThis study provides evidence that post-activation potentiation reduces the soleus H-reflex amplitude while concurrently facilitating 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 post-contraction reflex suppression might be mediated by mechanisms other than presynaptic inhibition, potentially involving post-activation depression spinal mechanisms or changes in muscle spindle sensitivity.

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.

The influence of explicit strategies on implicit recalibration during visuomotor adaptation has become a central question in motor learning. Because the two systems operate in tandem, explicit strategies could indirectly influence implicit recalibration. However, explicit strategies are not unitary: they may rely on algorithmic-based computations or memory-based retrieval of cached solutions. This raises the possibility that different strategy implementations interact with the cerebellar-based implicit recalibration system in qualitatively distinct ways, especially given that these strategies likely rely on different frontoparietal networks. Here, we tested whether the type of explicit strategy modulates implicit recalibration. Across a set of experiments, we observed subtle differences in the spatial profile of implicit generalization: the algorithmic strategy produced a broader generalization pattern than the retrieval strategy, even after controlling for intertrial decay, generalization structure, and between-target interactions. While this pattern is suggestive of greater flexibility afforded by algorithmic strategy use compared to memory-based retrieval, it could instead arise from increased variability in explicit aiming, which constitutes the input data driving implicit recalibration. Indeed, when we isolated the direct contribution of each strategy to implicit recalibration by rigorously controlling for reach variability and using error-clamp feedback to ensure uniform implicit learning conditions, we found no difference in implicit recalibration across strategies. Together, these findings suggest that while algorithmic and retrieval strategies differ in their behavioral signatures and influence the movement plan, the implicit recalibration process itself remains rigid with respect to the strategy employed. Significance StatementWhether explicit strategies influence implicit recalibration remains debated - a question complicated by the fact that different training conditions give rise to distinct forms of strategy. Because algorithmic and retrieval-based strategies engage partially dissociable frontoparietal networks, they may differentially interface with cerebellar subregions, yielding distinct effects on implicit recalibration, potentially reconciling prior mixed findings. Our results indicate that, once the first order spatial and temporal statistics that typically covary with these different strategies are matched, the strategies themselves do not directly influence implicit recalibration. Instead, the cerebellar-based implicit recalibration system faithfully updates the sensorimotor mapping simply based on the training data it is provided. These findings support prior work suggesting strong independence and stereotyped behavior of implicit recalibration.