Experimental Brain Research

Learning to reach with a visuomotor distortion has been shown to influence subsequent reaches with the same distortion and with a new distortion. Here, we examined whether learning to reach with a small 20{degrees} mirror reversed distortion leads to faster re-learning of the same distortion (i.e., demonstrates savings) and whether learning to reach with a mirror reversed distortion influences subsequent reaches with a 20{degrees} visuomotor rotation distortion. Thirty participants first learned to reach with the mirror reversed distortion. Following washout trials with aligned cursor feedback, 15 participants reached again with the mirror reversed distortion (MR-MR group), while the 15 other participants reached with a visuomotor rotation distortion (MR-VMR group). An additional twenty participants only reached with the visuomotor rotation distortion (VMR-only group). Implicit (unconscious) and explicit (conscious strategy) contributions to learning were assessed using the process dissociation procedure. Evidence of savings was evident in the MR-MR group, such that participants demonstrated reduced hand angles when re-introduced to the mirror reversed distortion. This savings was driven by explicit processes, consistent with the rapid retrieval of previously acquired task solutions. Additionally, learning to reach with the mirror reversed distortion interfered with learning to reach with the visuomotor rotation distortion, such that the MR-VMR group demonstrated increased reach variability and longer reaction times when reaching with the visuomotor rotation distortion compared to the VMR-only group. Reduced implicit contributions were also evident in the MR-VMR group compared to the VMR-only group. Together, results indicate that learning to reach with a mirror reversed distortion promotes savings and influences learning to reach with a visuomotor rotation distortion through engagement of explicit processes.

Cybersickness in virtual reality (VR) arises from conflicts between sensory signals, yet susceptibility varies considerably across individuals. Previous work in this cohort demonstrated that vestibulomotor responses during postural control are associated with cybersickness susceptibility. Here, we examined whether perceptual weighting of gravity, visual, and body cues used to estimate upright orientation is similarly associated with cybersickness and related to previously reported vestibulomotor measures. Thirty-eight healthy young adults (21 females, 17 males) completed a standing VR rollercoaster task while receiving continuous stochastic electrical vestibular stimulation (0-25 Hz, {+/-}4.5 mA). In the current analysis, perceptual cue weights were quantified before and after VR using the Oriented Character Recognition Task. Cybersickness was assessed using the Fast Motion Sickness Scale (FMS), and participants were classified as non-sick (FMS < 5), medium-sick (FMS [&ge;] 5), or high-sick (terminated the VR exposure early due to intolerance). Before VR, non-sick participants exhibited greater gravity weighting (36% vs. 25%) than high-sick participants, whereas high-sick participants showed a non-significant trend toward greater visual weighting (30% vs. 18%). Perceptual cue weights changed minimally following VR, and the magnitude of perceptual reweighting was not associated with sickness severity. Vestibulomotor measures were not correlated with perceptual gravity weighting, and postural sway during VR was not associated with visual weighting. These findings suggest that greater baseline perceptual gravity weighting, rather than short-term perceptual reweighting, is associated with reduced cybersickness susceptibility. The dissociation between perceptual and vestibulomotor measures suggests that orientation perception and postural control reflect partially distinct multisensory integration processes. New and NoteworthyThis study demonstrates that baseline perceptual gravity weighting is associated with susceptibility to cybersickness during virtual reality exposure with concurrent electrical vestibular stimulation. Greater reliance on gravity cues was associated with reduced susceptibility, whereas visual weighting showed a similar but non-significant trend and perceptual reweighting changed minimally following exposure. Perceptual measures were dissociable from vestibulomotor responses, suggesting that orientation perception and postural control reflect partially distinct multisensory integration processes.

In the current experiment, we compared reported perceptual awareness of the visuomotor rotation to motor awareness of changes in reaches established using the process dissociation procedure and drawing task following visuomotor adaptation to a large (50 degrees; R50 group) or a small (30 degrees; R30 group) cursor rotation. Results revealed that perceptual and motor awareness did not differ in magnitude for the R50 group and were significantly correlated. In contrast, while the R30 group perceptually reported being aware of the visuomotor rotation, motor awareness was significantly less and responses were not significantly correlated across tasks. Overall, results suggest that perceptual and motor tasks assess different processes underlying visuomotor adaptation to a small cursor rotation, such that perceptual awareness of the visuomotor rotation is not reflected in reaching performance on tasks assessing motor awareness.

In mirror reversed (MR) learning, the magnitude and direction of the visuomotor distortion varies with target location. To date, implicit (i.e., unconscious) processes have not been implicated in learning to reach with an MR distortion, even when the distortion is small in magnitude. Across two experiments, we examined whether explicit processes (i.e., motor and perceptual awareness of reaching strategies) are engaged when learning to reach with a small (20{degrees}) MR distortion and whether this learning generalizes to novel targets. Learning to reach with an MR distortion was compared to learning to reach with a small visuomotor rotation (VR), in which cursor feedback was rotated 20{degrees} relative to hand motion at each target. Participants in the MR group engaged both motor and perceptual awareness and learning to reach with the MR distortion generalized to novel targets. Participants in the VR group also learned to reach with the VR distortion but they did not engage either motor or perceptual awareness and there was no evidence of generalization. Reaction times were longer for the MR group compared to the VR group, consistent with engagement of explicit processes. Together, these findings suggest that learning to reach with an MR distortion is supported by motor and perceptual awareness that generalize to novel targets.

Pain demands attention and can disrupt task-related goals. Attention allocation is a key cognitive process supporting motor learning and disruption of internal schemas associated with attentional control during motor learning can result in interference in improvements in performance. Movement-contingent pain is an important characteristic of persistent musculoskeletal pain. Despite this, research exploring pain interference with motor learning and attention has exclusively utilised tonic pain paradigms. Understanding the impacts of movement-contingent pain on motor learning and attention may provide important insights into the interaction between pain and motor learning. The aim of this study was to; 1) explore the robustness of a movement-contingent pain paradigm across an extended period of training, 2) explore the impact of movement-contingent pain on improvements in performance and attentional allocation during motor learning. Three groups (healthy non-pain, healthy experimental-pain and persistent pain experimental-pain) completed ten trials of a motor sequence learning task while experiencing a movement-contingent electrical stimulation. Three task performance measures and five gaze indices, previously associated with attentional control, were collected. Results showed that; 1) low frequency electro-cutaneous stimulation could produce a valid and consistent pain experience across a sustained period of training, 2) attentional allocation becomes more efficient across learning, accompanied by improvements in task performance, 3) changes in task performance and attentional measures across training were similar in all groups despite the presence of pain, 4) movement-contingent experimental pain enhanced spatial performance at all time points in healthy participants but was not accompanied by a different pattern of attentional allocation. This study demonstrates that the impact of movement-contingent pain on motor learning is comparable to the impacts of tonic experimental pain and provides interesting insights into patterns of attentional allocation across time but little evidence that these attentional allocations are impacted by the presence of pain or a past history of pain.

Abstract Background: Reward can influence both the selection and execution of goal-directed actions. Healthy ageing is associated with changes in reward processing, raising the possibility that reward effects on motor control may be reduced in older adults. Objective: This study examined how monetary reward affects action execution and action selection during reaching movements and whether these effects differ between younger and older adults. Methods: 28 younger adults and 28 older adults performed a reward-based reaching task. Behaviourally non-distracted trials were used to assess action execution, whereas distractor-containing trials were used to assess action selection. Outcomes included maximum velocity, movement time, endpoint error, reaction time, and selection accuracy. Results: Reward increased maximum velocity and reduced movement time in both age groups without increasing error. These reward-related changes in movement vigour were larger in younger adults. During action selection, reward shortened reaction time but reduced selection accuracy in both groups, indicating faster but less accurate responses. The reward-related changes in reaction time and selection accuracy did not differ significantly between age groups. Conclusion: Ageing did not produce a uniform reduction in reward responsiveness. Instead, ageing attenuated reward-driven movement invigoration, while reward-related changes in action-selection behaviour were similar across age groups. These findings may inform the design of reward-based interventions that promote movement vigour without encouraging speed at the expense of accurate action selection.

Long-latency stretch reflex and corticospinal excitability in the tibialis anterior muscle (TA) are facilitated when balance is threatened, even without background TA activity, suggesting supraspinal modulation as preparatory tuning for ankle stabilization. However, it remains unclear whether such tuning is evident at the spinal level and specific to the TA among lower-limb muscles. We therefore examined the effects of height-induced postural threat on multi-segmental monosynaptic spinal reflexes (MMR) in lower-limb muscles during quiet standing. Seventeen healthy young males performed 90-s standing tasks under three postural threat conditions, created by combining real and virtual reality (VR) heights: (1) Low-threat (real ground & VR ground), (2) Medium-threat (real table & VR ground), and (3) High-threat (real table & VR bridge). During each condition, transcutaneous spinal cord stimulation (tSCS) was applied to the lumbar spine to elicit MMR in lower-limb muscles. Electromyograms (EMG) were recorded from six muscles of the right leg: vastus medialis (VM), biceps femoris (BF), TA, soleus (SOL), medial (MG), and lateral gastrocnemius (LG). MMR excitability was quantified as peak-to-peak EMG amplitude. Fear ratings and electrodermal activity were higher in High-threat than Low-threat (all p < 0.05), confirming successful threat induction. Peak-to-peak EMG amplitude in the TA was significantly higher in High-threat than Low-threat (17.1% increase, p = 0.0393), whereas background TA activity remained absent across conditions. These results indicate that TA has unique function to facilitate spinal excitability as a preparatory tuning for ankle stabilization. Key pointsO_LIPrevious studies have shown the supraspinal facilitation of the tibialis anterior muscle without background muscle activation as a preparatory tuning for ankle stabilization. C_LIO_LITo test the hypothesis that such tuning is also evident at the spinal level and specific to the tibialis anterior muscle, this study examined whether height-induced postural threat modulates multi-segmental monosynaptic reflex excitability in lower-limb muscles using transcutaneous spinal cord stimulation. C_LIO_LIElectrodermal activity and fear ratings increased under height-induced postural threat, confirming the successful induction of postural threat. C_LIO_LIUnder height-induced postural threat, the multi-segmental monosynaptic reflex was selectively facilitated in the tibialis anterior muscle, while its background activity remained absent. C_LIO_LIOur findings demonstrate selective facilitation of spinal excitability in the tibialis anterior muscle, which may serve as preparatory tuning for ankle stabilization under threat to balance. C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=147 SRC="FIGDIR/small/742625v1_ufig1.gif" ALT="Figure 1"> View larger version (50K): org.highwire.dtl.DTLVardef@ca21f3org.highwire.dtl.DTLVardef@7b1679org.highwire.dtl.DTLVardef@1007191org.highwire.dtl.DTLVardef@1ff88a_HPS_FORMAT_FIGEXP M_FIG C_FIG Abstract figure legendWhen balance is threatened, corticospinal excitability and long-latency stretch reflex in the tibialis anterior muscle (TA) are facilitated even in the absence of background TA activity, suggesting supraspinal preparatory tuning for ankle stabilization. This study tested the hypothesis that such facilitation is also expressed at the spinal level and is specific to the TA. Participants completed 90-s quiet standing trials under three different height-induced postural threat conditions. During each trial, transcutaneous spinal cord stimulation was delivered over the lumbar spine to elicit multi-segmental monosynaptic reflexes (MMR) in multiple lower-limb muscles. High-threat condition increased fear ratings and electrodermal activity, indicating successful threat induction. Moreover, MMR excitability was selectively increased in the TA under High-threat condition despite the absence of background TA activity. These findings suggest that spinal facilitation is selectively expressed in the TA and may reflect preparatory tuning for ankle stabilization under threat to balance.

Upright postural control during movement relies on multisensory integration. Yet, the frequency-specific contribution of vision remains poorly characterized in virtual reality (VR). This study investigated how multi-sine visual stimulation amplitude delivered in VR influences standing postural responses. Fifteen healthy adults stood on a force plate wearing a VR headset. Visuo-postural coupling was assessed through coherence and gain analyses between a multi-sine signal (10 sinusoids, 0.12 to 1 Hz) oscillating a virtual environment in one of four amplitudes (0.5, 1, 2, 4 degrees peak-to-peak) and the anteroposterior whole-body angle. All amplitudes elicited measurable postural responses. Increasing amplitude significantly increased postural oscillation and tended to increase coherence, while gain significantly decreased. These results are consistent with a nonlinear control system. The 2 degrees amplitude elicited the largest gain with significant coherence across all stimulated frequencies, suggesting it is suitable for studying visual contributions to postural control during movement execution.

The pelvic floor muscles (PFM) are critical for maintaining continence and are a primary target of physiotherapy training to manage urinary incontinence. PFM training relies on voluntarily activating this muscle group, limiting its translation to neurological populations where recovery of bladder function is a priority. Indirect evidence suggests that sensory feedback from the lower limb can modulate PFM activity, which may provide alternative strategies for PFM training. Cutaneous reflexes have been used as a proxy to study how sensory inputs from the skin influence motoneuron excitability. To explore the feasibility of eliciting cutaneous reflexes in the PFM and their role in controlling PFM activity, this study examined: 1) the input-output relationship and 2) the nerve-specificity of PFM cutaneous reflex responses from tibial and superficial peroneal nerve stimulation. Twenty-one neurologically intact adults participated in this study. We recorded PFM and lower leg muscle electromyography while participants received cutaneous stimulation to the right distal tibial nerve, bilateral distal tibial nerve, or right superficial peroneal nerve in a standing position. We delivered stimulation at the intensity below motor threshold (MT), 1.2 x MT and 1.5 x MT and quantified tibial-PFM reflex amplitude over a 50-150 ms window after stimulation. PFM reflex responses were evoked from both nerves stimulation. Reflex amplitude increased with stimulus intensity with tibial nerve stimulation but not with superficial peroneal nerve stimulation. Bilateral tibial nerve stimulation evoked larger responses compared to unilateral stimulation. These findings support the existence of neural connections between lower limb afferents and the PFM, and open up possibilities for designing rehabilitation strategies to manage pelvic health conditions in people with neurological disorders. New & NoteworthyO_LICutaneous sensory feedback from the foot, specifically that related to limb loading, can evoke reflex responses in the pelvic floor muscles C_LIO_LINerve-specific modulation was observed. Reflex amplitudes in the pelvic floor muscles increased with tibial nerve stimulation intensity but not with superficial peroneal nerve stimulation. C_LI

Tactile imagery has attracted growing fundamental and clinical interest. Previous studies often investigated neural and functional similarities between imagined and actual touch. Several functional aspects of touch, such as differences between active and passive touch, between different haptic features during active touch or sensitivity of different body parts for passive touch, have also been explored in tactile imagery. Furthermore, considerable individual differences in the ability to engage in tactile imagery have been observed. However, several important aspects, involving different imagery components and a wide variety of touch qualities remain to be explored within a single comprehensive study. The current study therefore aims to provide a wide-ranging assessment of tactile imagery in terms of imagery processing components (vividness, maintenance, transformation), type of touch (active versus passive) and touch qualities (object properties for active touch, different tactile sensations across body sites for passive touch). We developed a comprehensive questionnaire containing 72 items to assess tactile imagery ability. 136 healthy participants were asked to imagine different touch types and rate imagery vividness and their ability to maintain and transform each sensation on 5-point Likert-scales. Active touch varied by object (plastic bottle, modeling clay, sponge) and property (temperature, weight, texture, resistance). Passive touch varied by body site (lip, shin, sole of the foot, lower back) and sensation (stroking, vibration, pinching). Overall, participants were able to perform tactile imagery: the vast majority reported at least some imagery across touch types. Individual variability was substantial: scores bridged both ends of the scale. Active tactile imagery differed significantly between objects, depending on tactile property. Object-property pairs with particularly strong imagery were bottle-temperature, bottle-weight and sponge-texture, whereas bottle-resistance elicited weaker imagery, as did temperature and weight for both sponge and clay. Passive tactile imagery was significantly stronger for body sites with higher receptor density (i.e. lip and foot). Imagery of stroking was significantly weaker than vibration and pinching. Active and passive imagery showed a strong, positive correlation, though some participants had relatively strong active imagery, but weaker passive imagery, or vice versa. Our findings confirm that tactile imagery ability varies across individuals and touch types, underlining the importance of a comprehensive imagery ability assessment tool specific to the tactile domain.

Past studies using transcranial magnetic stimulation have shown larger motor-evoked potentials when people observe someone lifting a heavy object than when they observe someone lifting a light one. This means that observers may engage their own motor system in proportion to the perceived effort. However, the different responses during the observation of light and heavy objects may have been influenced by predictable trial sequences within blocked presentation, making it unclear whether corticospinal excitability reflects online processing of kinematics or is affected by top-down expectations. In this Registered Report, 57 right-handed participants passively observed videos of a precision grip and lift of heavy and light objects while receiving a single-pulse TMS to the left primary motor cortex during the lift phase of the movement. Motor-evoked potentials were recorded from the right first dorsal interosseous muscle. The study compared two main observation contexts: a predictable trial sequence in which repeated videos of the same lifts were presented in a blocked order, and an unpredictable one in which videos were presented semi-randomly and participants could rely only on kinematic cues to perceive the weight of the lifted object. In both conditions, the same videos of lifts of equivalent-looking heavy and light objects were used and only the order of presentation differed. Contrary to our predictions, in the blocked (predictable) condition, there was no significant difference in MEPs elicited by light and heavy lifts. In the unpredictable condition, participants showed greater corticospinal excitability during the observation of the light lifts compared to the heavy lifts. This suggests that in the absence of predictable information, the corticospinal system was sensitive to the observed kinematics, but contrary to previous findings, its excitability varied inversely with the object weight.

Pupillometry is commonly used to assess workload and fatigue, but its interpretation in applied, continuous-control tasks remains unclear because pupil responses are strongly shaped by task context and often lack a theory-guided interpretive basis. The Compensatory Control Model and Locus Coeruleus-Norepinephrine system offer complementary frameworks for interpreting pupil dynamics in relation to regulatory effort and engagement under sustained cognitive demand. We aimed to examine how tonic and phasic pupil dynamics reflect compensatory regulation following sustained cognitive demand, and how these dynamics relate to subsequent performance during continuous motor control. Participants completed two sessions consisting of a low- versus high- demand working-memory task followed by a joystick-based continuous motor-control task. Linear mixed-effects models tested demand effects on pupil dynamics and behavior and evaluated trial- level pupil-performance associations. Prior high demand produced small and specific changes in motor-control performance. Task-evoked pupil response showed effects of engagement dependent on prior demand and small associations with movement timing and accuracy. Our results show that pupil dynamics sensitively reflect sustained cognitive demand, but their trial-level predictive value for subsequent continuous motor performance is limited.

Background. Pain influences motor function and has been proposed to reduce corticospinal and intracortical excitability. At the same time, performance can be maintained during pain, at the cost of increased perceived effort, a centrally generated signal reflecting resource engagement. Here, we tested whether contralateral thermal heat pain-related changes in corticospinal and intracortical excitability contribute to increased effort perception. Methods. In this preregistered transcranial magnetic stimulation (TMS) study, twenty-one healthy participants received single and paired pulse TMS at rest and during submaximal isometric right wrist flexions performed at 20% maximal peak force. Trials were conducted under a control condition or during contralateral thermal stimulation (painful or non-painful warm) applied to the left forearm. After each contraction, participants rated the intensity of their perceived effort. Corticospinal and intracortical excitability of the right wrist flexor was assessed at rest and during submaximal contractions. Results. Contralateral heat pain significantly increased perceived effort compared with the control and warm conditions. Contralateral heat pain did not reduce corticospinal or intracortical excitability. Conversely, contralateral heat pain increased corticospinal excitability, reflected primarily in decreased cortical silent period duration. Perceived effort was associated with the subjective experience of pain rather than with TMS-derived variables. Conclusions. These findings suggest that increased effort during contralateral heat pain cannot be attributed to inhibition of the primary motor cortex or the corticospinal pathway. The higher perceived effort in the presence of contralateral heat pain likely reflects the cognitive cost of pain rather than alterations in the transmission of the motor command.

IntroductionAction observation modulates corticospinal excitability, with most previous studies indicating an increase in excitability in the muscles involved in the observed movement. In addition, previous work suggests that modulation of corticospinal excitability could be specific to the timing of the stimulation, muscle, and direction of movement. Here we examined the influence of these factors on corticospinal excitability. MethodParticipants observed stimuli presenting a static hand, followed by an image of the endpoint of an index/little finger abduction movement. Transcranial magnetic stimulation was delivered at time points from 100-800ms after movement onset. Stimuli were presented in various orientations to study possible effects of anatomical positioning and movement direction, compared relative to the control condition of a static hand. ResultsCorticospinal excitability was lower at early timings (100-400ms), before rising to a plateau at later timings (500-800ms) which did not differ from the static hand condition. This facilitation was muscle-specific, with higher excitability for the muscle involved in the observed movement. By contrast, the relative direction of movement did not influence corticospinal excitability. DiscussionThese results replicate the time-dependent modulation of corticospinal excitability induced by action observation; however, we argue that simply interpreting such effects as an increase in excitability may be overly simplistic. In line with previous studies, we argue that the choice of control condition used during action observation studies may be critical to the overall direction of effects.

The ability to accurately perceive arm position is essential for motor control and depends on the integration of proprioceptive and visual information. However, how lifelong visual impairment (VI) affects position sense and its neural correlates remains unclear. We quantified arm position sense and associated cognitive-motor load in right-handed visually impaired (n = 7) and normally sighted (NS; n = 7) individuals using three bilateral arm position matching tasks: joint angle matching (JAM; internal coordinates), hand direction-distance matching (DDM; external coordinates), and mirror direction-distance matching (MDDM; external coordinates kinematically identical to JAM). Cognitive load was assessed using the contingent negative variation (CNV) from EEG recordings. VI participants exhibited reduced accuracy and precision of arm position sense in most conditions, and greater CNV magnitude, particularly in the left parietal cortex. Across both groups, performance was worse and CNV magnitude was greater in the DDM task compared with JAM, whereas JAM and MDDM yielded similar behavioral and neural outcomes. These findings indicate that (i) visual experience enhances arm position sense, and (ii) representing limb position in external coordinates imposes greater cognitive demands than encoding joint-based posture. The similarity between JAM and MDDM suggests that participants preferentially rely on internal representations when task kinematics permit.

Despite extensive research on vestibular function in microgravity, particularly during orbital and parabolic flight exposure, several gaps remain regarding the spontaneous behavior of vestibular organs under non-terrestrial gravitoinertial conditions. In particular, semicircular canal function, typically assessed through vestibulo-ocular reflex (VOR) recordings, has yielded inconsistent findings, with reports describing either no effect or reduced performance in microgravity. Moreover, many of these studies are limited by methodological constraints that reduce the interpretability of their conclusions. To clarify these discrepancies, we evaluated horizontal and vertical VOR responses during parabolic flights to assess semicircular canal function under transient weightlessness. Participants were passively rotated at a constant frequency and amplitude during normogravity and microgravity phases, centered along the head vertical or inter-aural axis. Eye movements were recorded binocularly using infrared eye-tracking in darkness to eliminate visual influences, while participants were tightly restrained to minimize proprioceptive variability. Results show a reduction in VOR gain during microgravity in both axes, despite consistent rotational stimulation across gravity conditions. In addition, VOR gain remained reduced after parabolas in the horizontal plane, whereas vertical VOR performance was preserved. These are the first results to demonstrate an immediate alteration of semicircular canal function in weightlessness. Possible sources of the reduction in VOR performance in 0g are discussed. We also propose that the observed post-flight effects reflect a down-weighting of semicircular canal inputs during multisensory integration.

Limb-trunk coordination plays an essential role in daily actions. The reticulospinal excitability of limb muscles has been suggested to be modulated by long-term motor experience, such as strength training. However, it remains unclear whether reticulospinal contributions in the limb and trunk muscles during limb-trunk coordinated movements are modulated by strength training. This study aimed to determine whether reticulospinal contributions to limb-trunk coordination differ between strength-trained and untrained individuals. Fifteen long-term ([&ge;]3 yrs) strength-trained and 15 untrained healthy men participated in this study. Participants performed a rapid bilateral arm-curl task while standing, using a load corresponding to 35% of their one-repetition maximum, in response to visual, visual-auditory (80 dB), or visual-startling (115 dB) stimuli. Electromyography (EMG) was recorded from the right biceps brachii (BB) and erector spinae (ES) muscles during the task. In the trained group, EMG onset of the ES was closer to that of the BB than in the untrained group, indicating tighter temporal coordination between the limb prime mover and trunk postural muscles in strength-trained individuals. In both groups, visual-startling stimuli shortened the EMG onset of both the BB and ES, suggesting reticulospinal contributions to both muscles. Notably, the reduction in EMG onset of the BB induced by the startling stimulus was smaller in the trained group than in the untrained group, whereas no difference between groups was observed for the ES. These findings suggest that long-term strength training may modify limb-trunk muscle coordination during standing arm curls and may be associated with muscle-specific adaptations in reticulospinal contributions.

Impaired gait performance and stability is a key symptom often defining disease outcome in people with Multiple Sclerosis. Step-by-step foot placement control in response to variations in the center-of-mass kinematic state is a crucial gait stability mechanism, especially in the mediolateral direction. Even though it is known that people with Multiple Sclerosis are at an increased risk of falling, step-by-step foot placement control remains to be characterized in this population. Here, we explored characteristic foot placement control in ten people with early stage Multiple Sclerosis, compared to 21 controls walking at a similar average gait speed, during 1-minute steady-state treadmill walking. Kinematic data were analyzed using a linear feedback model that correlated foot placement with the center-of-mass kinematic state during the preceding swing phase. People with Multiple Sclerosis demonstrated step-by-step foot placement control in both the mediolateral and anteroposterior directions. No differences were found in foot placement precision between groups. However, foot placement responses to variations in center-of-mass velocity proved stronger in people with Multiple Sclerosis. Moreover, the contribution of mediolateral center-of-mass velocity feedback to the control mechanism was higher in people with Multiple Sclerosis as compared to neurologically healthy controls. Our results suggest that foot placement control is still retained in early clinically evident stages of Multiple Sclerosis, but is realized through differently weighted sensory feedback control.

Age-related decline in postural control is an important factor that increases the fall risk of older adults. Fingertip vibrotactile stimulation has been developed to provide light touch-like somatosensory input. However, evidence regarding differences among older age groups is limited. This study examined the effects of fingertip vibrotactile stimulation on postural control in 348 community-dwelling older adults classified as young-old (age 65-74 years), old-old (age 75-84 years), and oldest-old (age 85 years or older). Participants stood with eyes closed and feet together under stimulation and no stimulation conditions. The center of pressure (COP) velocity and COP area were measured using a force plate. The natural log-transformed COP area was used for the analysis. Linear mixed models were used to examine the effects of age group, stimulation conditions, and measurement segments. The COP velocity under the stimulation condition was significantly lower than that under the no stimulation condition; however, the COP area did not change significantly. Significant main effects of age group were observed for both COP indices, but no interaction between age group and stimulation condition was observed. Fingertip vibrotactile stimulation may reduce the COP velocity across older age groups, thus reflecting the effects on postural adjustment frequency.

It is well documented that both motoneuron output and the effectiveness of activated Ia afferents to discharge soleus -motoneurons decrease during eccentric (muscle lengthening) contractions. Evidence suggests that these modulations can be explained by recurrent inhibition and greater presynaptic inhibition of Ia afferents. However, the influence of angular velocity on the modulation of the effectiveness of activated Ia afferents to discharge -motoneurons observed during eccentric contractions remains unclear. We investigated the influence of angular velocity on spinal mechanisms involved in the effectiveness of activated Ia afferents to discharge -motoneurons during eccentric plantar flexor contractions using 16 healthy adults. We used both simple and conditioned Hoffmann reflex with different conditioning techniques to assess presynaptic inhibition, heteronymous Ia facilitation and heteronymous recurrent inhibition coupled with electromyography during eccentric contractions of the plantar flexors at three angular velocities. Our results showed that during eccentric contractions, the effectiveness of Ia afferents to discharge -motoneurons was lower at 90{degrees}{middle dot}s-{superscript 1} than 60{degrees}{middle dot}s-{superscript 1} and 20{degrees}{middle dot}s-{superscript 1} angular velocities. A similar velocity-dependent pattern was observed for heteronymous recurrent inhibition, decreasing at 90{degrees}{middle dot}s-{superscript 1} when compared with 60{degrees}{middle dot}s-{superscript 1} and 20{degrees}{middle dot}s-{superscript 1}. In contrast, presynaptic inhibition of Ia afferents was not different between the velocities. These demonstrate a differential influence of angular velocity on spinal recurrent inhibitory mechanisms during eccentric contractions and support distinct functional roles of recurrent and presynaptic inhibition in modulating -motoneurons discharge with increasing movement velocity. The findings provide new insights into the velocity-dependent and mechanism-specific modulation of spinal inhibitory circuits during eccentric contractions. KEY POINTSO_LIDuring eccentric contractions in soleus muscle, the effectiveness of activated Ia afferents to discharge -motoneurones decreases with increasing angular velocity, indicating a velocity-dependent modulation. C_LIO_LIPresynaptic inhibition of Ia afferents does not differ between angular velocities, suggesting that it does not contribute to the observed changes. C_LIO_LIHeteronymous recurrent inhibition from the quadriceps to the soleus increases with angular velocity, indicating that increasing movement velocity promotes a functional reorganization of intermuscular recurrent inhibition. C_LIO_LIThese findings suggest a differential functional role of the two spinal inhibitory mechanisms, indicating that increasing angular velocity primarily influences recurrent postsynaptic inhibition rather than presynaptic inhibition. C_LI