Whole-body walking illusions modulate spinal excitability

Whole-body walking illusions modulate spinal excitability | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Whole-body walking illusions modulate spinal excitability Teja Aryudha, Kenya Tanamachi, Khin Win Thu, Takuya Yada, Fuminari Kaneko This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8024408/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Walking is a fundamental human function, yet how the nervous system constructs the perception of locomotion remains poorly understood. Previous research has shown that kinesthetic illusions of single joints can modulate corticospinal excitability, but whether such illusions extend to whole-body walking has remained unknown. Here, we demonstrate that synchronizing participants movements with a full-body avatar in virtual reality (VR) induces a vivid whole-body walking illusion and modulates spinal reflex excitability in a phase-dependent manner. Using a simple motion-capture system, we enhanced body ownership over the avatar and showed that the toe-off phase elicited both the strongest illusory sensations and significantly increased soleus H-reflex amplitudes. Moreover, the intensity of the walking illusion correlated positively with reflex modulation, suggesting that locomotor perception relies on body ownership as its basis, just as previously shown for segmental illusions. These findings extend the concept of multisensory integration from joint-level illusions to whole-body locomotion, providing new evidence that the nervous system flexibly integrates visual predictions and proprioceptive templates to construct locomotor awareness. This paradigm not only advances fundamental understanding of human motor control but also offers a translational framework for VR-based gait rehabilitation. Biological sciences/Neuroscience Biological sciences/Psychology Social science/Psychology Avatar walking H-reflex Spinal reflex excitability Virtual reality Figures Figure 1 Figure 2 Figure 3 Figure 4 Significance Statement Through integrating a motion capture device with VR, this study provides new understanding into the multisensory integration required for enhancing walking illusions while watching avatar walking. Real time synchronizing a participant’s body with a full-body avatar using a motion capture device in a VR system could enhance the whole-body walking illusion and produce better results in inducing spinal reflex excitability than VR alone. Introduction Body ownership illusions, such as the rubber hand illusion and full-body avatar illusions, demonstrate that when the human brain integrates multisensory information, the result is the emergence of a sense of ownership over an artificial or virtual body( 1 , 2 ) .In virtual reality (VR), synchronous visual and somatosensory stimulation can generate the compelling illusion that a full-body avatar is one’s own body. Building on this framework, if individuals experience a sense of ownership over a full-body avatar, then observing that avatar walking could plausibly induce an illusory sensation of walking even while standing still. Since our previous studies demonstrated that stronger body ownership over body parts was associated with stronger kinesthetic illusions with physiological effects ( 3 – 6 ), this possibility provided the primary motivation for the present study. VR has been increasingly applied in experimental motor neuroscience, with gait training paradigms such as treadmill walking in virtual environments providing controlled settings to study locomotion( 7 ). Beyond locomotor practice, immersive VR scenarios have been shown to modulate spinal excitability even in the absence of actual movement; for example, simulated falling in VR elicits spinal responses comparable to those observed during real postural challenges( 8 ). Visual stimulation can also generate kinesthetic illusions that alter body self-consciousness and engage motor networks. Previous work demonstrated that such illusions can facilitate corticomotor excitability( 6 ) and modify neural activity in response to simple visual cues( 9 ). These findings indicate that immersive visual input is capable of modulating both cortical and spinal motor pathways. Induction of a walking illusion through full-body avatar synchronization could therefore provide a unique means to examine the neurophysiological basis of locomotor perception, an area of human motor neuroscience that has remained largely unexplored. Previous studies have primarily examined kinesthetic illusions at the segmental level, such as single-joint movements of the wrist or ankle( 6 , 10 – 14 ). Extending these phenomena to whole-body walking is of scientific value, as it enables the investigation of multimodal integration processes underlying locomotor perception and the role of body ownership in generating the sense of self-movement. If whole-body walking illusions can be reliably induced, they would provide a new experimental platform to test whether body ownership constitutes a fundamental basis not only for segmental but also for locomotor illusions. The concept of a whole-body walking illusion is novel, and to date it has not been systematically investigated. A fundamental question is whether such an illusion can in fact be reliably induced in humans. Establishing the feasibility of this phenomenon is therefore a critical first step. However, inducing a walking illusion poses unique challenges compared to previously studied illusions of body ownership or limb movement, given the complexity of locomotor control and the demands of synchronizing a full-body avatar in real time. Based on these considerations, we hypothesized that synchronizing participants’ movements with a full-body avatar in VR could enhance the experience of body ownership and generate a robust walking illusion, which in turn would modulate spinal reflex excitability in a phase-dependent manner. Assessing the Hoffmann reflex (H-reflex) provides an optimal approach to evaluate how visual stimulation influences the gain of spinal reflex pathways, thereby offering indirect insight into the modulation of central pattern generator (CPG) activity during locomotion. Demonstrating the spinal effects of this novel illusion is of particular significance, as it not only establishes the physiological basis of the phenomenon but also provides a foundation for future physiotherapy applications targeting walking impairments. Importantly, subjective reports of illusory experience can sometimes be ambiguous or inconsistent, whereas changes in H-reflex amplitude offer an objective physiological marker at the spinal level. By quantifying spinal excitability in this way, the present study aims to clarify the neurophysiological impact of whole-body walking illusions and thereby open a new line of inquiry into how the nervous system represents locomotor perception. The aim of this study was therefore to test whether a whole-body walking illusion can be realized in humans and whether such an illusion modulates spinal reflex excitability in a phase-dependent manner. Results Seventeen participants were included in the final analysis. All tolerated the procedures well, and no adverse effects were reported. Subjective illusion ratings The result of each illusion rating is illustrated in Fig. 3 A. The 7-point Likert scale ratings (Q1–Q3) did no interaction between group and phase (Q1; p = 0.830, F = 0.293, Q2; p = 0.822, F = 0.305, Q3; p = 0.913, F = 0.176). In contrast, significant phase-dependent differences were observed (Q1; p = 0.002, F = 5.272, Q2; p = 0.009, F = 4.035, Q3; p = 0.017, F = 3.544): Q1 (body ownership): toe-off vs. mid-stance (p = 0.018) and toe-off vs. mid-swing (p = 0.029). Q2 (kinesthetic illusion): toe-off vs. mid-stance (p = 0.003). Q3 (agency): toe-off vs. mid-stance (p = 0.021). These results indicate that illusory sensations varied across gait phases but were not significantly influenced by the experimental manipulation. H-reflex amplitude A repeated-measures ANOVA revealed a significant phase × condition interaction for normalized H-reflex amplitude (p = 0.031, F = 5.936). Post-hoc Bonferroni tests showed that in the experimental condition, H-reflex amplitude was significantly greater at toe-off compared with heel contact (Fig. 4 A; p = 0.014). Raw values are presented in Fig. 4 B, illustrating a clear increase in reflex amplitude at toe-off during the experimental condition. Correlation between illusion ratings and H-reflex amplitude Correlation analyses revealed positive associations between subjective illusion ratings and H-reflex amplitude in the experimental condition (Fig. 3 B): Q1 (body ownership): A significant correlation with H-reflex amplitude was observed at mid-swing (R² = 0.468, p = 0.002). Q2 (kinesthetic illusion): A significant correlation was observed at toe-off (R² = 0.294, p = 0.024) and at mid-swing (R² = 0.535, p < 0.001). By contrast, Q3 (agency) showed no significant correlations with reflex amplitudes in any phase or condition. Discussion We originally hypothesized that synchronizing a participant’s movements with a full-body avatar in VR would enhance the whole-body walking illusion and thereby modulate spinal reflex excitability more strongly than VR observation alone. The present findings partially support this hypothesis. Specifically, we observed that H-reflex amplitudes were significantly increased during the toe-off phase under the experimental condition, and that these physiological changes were positively correlated with subjective ratings of kinesthetic illusion and body ownership. These results suggest that the induction of a walking illusion through motor–visual matching can exert measurable effects on spinal excitability. Illusion vividness and phase specificity A key observation was that the illusion was phase-dependent: the toe-off phase consistently produced stronger kinesthetic illusions and greater H-reflex amplitudes. This phase is biomechanically critical for forward propulsion, and its distinctive sensory and motor signatures may facilitate multisensory integration. Our findings align with earlier studies showing that visually induced illusions of limb movement enhance corticospinal excitability in a direction- or position-specific manner( 6 , 13 ). Extending this principle to whole-body representations, it is plausible that toe-off evokes particularly strong engagement of motor association areas, leading to descending facilitation of spinal motoneuron pools. Neurophysiological mechanisms The observed correlations between illusion vividness and spinal excitability strengthen the interpretation that subjective experience is accompanied by objective physiological changes. Prior neuroimaging and neurophysiological studies indicate that kinesthetic illusions activate the posterior parietal and premotor cortices, as well as the primary motor cortex( 5 , 9 ). Such cortical activity may project via corticospinal pathways, modulating spinal reflex gain( 15 ). Although the present study did not directly measure cortical or network-level activity, the observed spinal modulation is most parsimoniously explained by descending volleys from cortical motor areas, consistent with mechanisms demonstrated in previous work( 5 , 6 , 9 , 13 ). Importantly, earlier evidence was limited to segmental or joint-level illusions, such as dorsiflexion or plantarflexion, where kinesthetic illusions facilitated corticospinal excitability in a movement-specific manner. By demonstrating that a visually induced whole-body walking illusion can similarly modulate spinal excitability, the present findings extend this framework to the domain of locomotion. This provides novel evidence that the neurophysiological mechanisms identified at the level of single-joint movement illusions generalize to complex, whole-body actions. Moreover, our results emphasize that locomotor perception, like segmental movement perception, is fundamentally grounded in body ownership. The emergence of stronger kinesthetic illusions at toe-off, accompanied by enhanced reflex excitability, suggests that ownership of the virtual body enables the nervous system to integrate visual predictions with proprioceptive templates to construct a coherent sense of walking. This interpretation is consistent with our recent work (Kaneko et al., under review), which showed that strengthening the sense of agency during kinesthetic illusions increased electromyographic activity in task-related muscles. Together, these findings highlight a common principle: both limb-specific and whole-body illusions rely on body ownership as their foundation, and when this ownership is reinforced, descending cortical signals can amplify spinal excitability. Methodological contribution The use of a simple motion capture system was essential in strengthening body ownership and facilitating the walking illusion. By aligning visual input with participants’ own movements prior to observation, the system established a proprioceptive–visual congruence that amplified the subsequent illusion. This approach provides a methodological advance for experimentally inducing robust whole-body illusions without complex or costly equipment. Limitations Several limitations should be acknowledged. First, the sample size was modest, which may limit the generalizability of the findings and the detection of smaller effects. Second, the experimenter administering the motion capture procedure was not blinded, raising the possibility of experimenter bias. Third, the control condition (observation without motor–visual matching) always preceded the experimental condition, so potential carry-over or residual effects cannot be entirely excluded. Finally, although we observed correlations between illusion vividness and spinal excitability, causality cannot be definitively established, and future studies using neuroimaging or causal perturbation methods (e.g., TMS) are warranted. Conclusion In summary, the present study demonstrates that visually induced whole-body walking illusions can modulate spinal reflex excitability in a phase-dependent manner, with the toe-off phase emerging as particularly effective. By leveraging a simple motion-capture system to strengthen body ownership, we were able to induce robust kinesthetic illusions that translated into measurable spinal changes. These findings provide compelling evidence that the nervous system flexibly integrates visual and proprioceptive information to construct locomotor perception, extending prior work on limb-based illusions to the domain of whole-body locomotion. Beyond establishing this basic mechanism, our paradigm offers a tractable model for probing multisensory integration in human motor control and may provide a foundation for future translational applications, such as VR-based rehabilitation strategies aimed at enhancing gait function in neurological populations. This preliminary study investigated whole-body walking illusions induced by full-body avatar walking which may provide insight into the influence of spinal reflex excitability. Materials and Methods Participants Eighteen healthy right-handed volunteers (8 males and 10 females; age range 20–35 years, mean ± SD: 23.9 ± 3.9 years) participated in the study. Data from one participant were excluded because Mmax could not be obtained in the H–M recruitment curve, leaving 17 participants (7 males and 10 females; 24.1 ± 4.0 years) for analysis. All participants received a detailed explanation of the experimental protocol and safety considerations before providing written informed consent. The study was approved by the Ethics Review Committee of Tokyo Metropolitan University for Research Involving Human Participants (approval no. 24012) and conducted in accordance with the Declaration of Helsinki. Experiment system The experimental setup is illustrated in Fig. 1 A. A head-mounted display (HMD; Meta Quest Pro) presented the full-body avatar. Trigger signals generated by a original control system were sent (USB-6218 D/A converter; National Instruments, Austin, TX, USA) simultaneously to (i) an electrical stimulator/EMG amplifier (Neuropack X1 MEB-2300; Nihon Kohden, Tokyo, Japan) to evoke H-reflexes and (ii) the HMD to synchronize avatar motion onset. EMG and trigger signals were digitized (CED 1401, Cambridge Electronic Design, UK) and stored for offline analysis. Overall experimental framework General procedure In both the control and experimental conditions, participants observed a full-body avatar walking through the HMD while H-reflexes were recorded (Fig. 1 B). In each condition, the avatar walked continuously for 5 minutes. Electrical stimulation of the tibial nerve was delivered repeatedly during one predefined gait phase (heel contact, mid-stance, toe-off, or mid-swing), creating four phase-specific conditions (Fig. 2 A). Stimulation was synchronized to the target phase using an original LabView program (phase-specific triggers: 250 ms for mid-stance, 860 ms for toe-off, 1300 ms for mid-swing, and 1600 ms for heel contact). Within each condition, the precise stimulus delivery was randomized across trials to prevent predictability. The walking speed of the avatar was fixed at one gait cycle every 2 seconds (1.58 m/sec), identical for all participants. The avatar’s height and sex were matched to the participant, and the avatar was positioned 2 m in front of the participant (following Lenggenhager et al., 2007( 1 )). Control condition (avatar observation without prior motor–visual matching) In both conditions, participants observed the same visual stimulus: a walking avatar presented across four gait phases (5 minutes each, total 20 minutes). In the control condition, participants simply observed the walking avatar without prior motor–visual matching, that is, without explicit induction of kinesthetic illusion. Experimental condition (pre-observation motor–visual matching; Matching) In the experimental condition, participants observed the same visual stimulus, but before observation they underwent a motor–visual matching procedure using a simple motion-capture system (Mocopi, Sony, Tokyo, Japan). Sensors were attached to the head, wrists, ankles, and trunk, and after calibration, the avatar’s movements were synchronized with participants’ real movements in real time. This pre-exposure continued until participants reported the subjective emergence of a kinesthetic illusion (“Do you feel your body wants to move?”). Only when they answered “yes” did the walking observation with H-reflex recordings commence. 7-point Likert scale To minimize bias, participants were seated in a comfortable environment, and the experimenter carefully asked them to report their experiences immediately after observing each avatar-walking phase. Participants were instructed to respond freely and without interruption. To assess illusory experiences, they rated the following statements: Q1: “I felt the avatar body on the display belonged to me.” Q2: “I felt as if my body was moving.” Q3: “I felt I was in control of the avatar’s movement.” A 7-point Likert scale was used, ranging from + 3 (strongly agree) to − 3 (strongly disagree), with 0 indicating neutrality. H-reflex examination Electrode placement and stimulation After informed consent, electrodes were prepared on the right soleus (SOL) following SENIAM guidelines ( http://seniam.org/soleus.html ). The skin was cleaned with alcohol, gently abraded with conductive paste to reduce impedance, and disposable surface EMG electrodes were placed over the muscle belly. A ground electrode was attached to the distal tibia (malleolus). H-reflexes were elicited by tibial nerve stimulation using an electrical stimulator (Neuropack X1 MEB-2300; Nihon Kohden, Tokyo, Japan). A convex cathode and rectangular anode were positioned to optimize responses, then secured with surgical tape and an elastic band. Electrode placement was confirmed by identifying the optimal stimulation site. H-M curve examination An in-house MATLAB program (R2023b, MathWorks, Natick, MA, USA) was used to generate the H–M recruitment curve (Fig. 2 B). After electrode was placement, participants were asked to stand quietly while tibial nerve stimulation was delivered to elicit soleus H-reflexes and M-waves. The stimulus intensity was gradually increased from 0 mA until the maximal M-wave plateau (Mmax) was reached. To confirm maximal activation, intensity was increased by ~ 20% beyond the plateau. The stimulation intensity for the main experiment was then set to evoke an H-reflex corresponding to 5–10% of Mmax. After that we were record H-reflex amplitude for 5-minute as the baseline values obtained during quiet standing without visual input. H-reflex measurement H-reflex amplitudes were recorded while participants observed the full-body avatar walking. The stimulation intensity was set individually to evoke an H-reflex corresponding to 5–10% of the maximal M-wave (Mmax), as determined from the recruitment curve. During each 5-minute condition, electrical stimuli were delivered repeatedly at the predefined gait phase (heel contact, mid-stance, toe-off, or mid-swing), with timing randomized across trials to prevent anticipation. For each condition, peak-to-peak H-reflex amplitudes were measured from the soleus EMG and averaged across all trials. To account for inter-individual variability, amplitudes were normalized with baseline value from H-M recruitment curve and expressed as a percentage. Statistical analysis General Linear Model (GLM): Repeated measures were used to compare the H-reflex amplitude of both conditions (control and experimental) and phase of walking (heel contact, mid-stance, toe-off, and mid-swing) Post-hoc analysis was conducted using the Bonferroni correction, and the significance level for all analyses was set at p < 0.05 (SPSS software (IBM SPSS Statistics 29 for Mac)); GLM repeated measures and Bonferroni correction were used for the 7-point Likert scale. Pearson correlation analysis was conducted to measure the strength and direction of the linear relationship between the two variables (H-reflex amplitude and 7-point Likert scale) in each phase of the conditions. The statistical power and sample size of the powered clinical trials were calculated using the computer program G*Power after data collection. Test family: F tests, statistical test: ANOVA: Repeated measures, Whitin-between interaction, Cohen’s f (medium effect size) 0.25, significant level 0.05, statistical power (1-β) 0.8, and correlation among repeated measures 0.7, total sample size 17 participants. Declarations Author Contribution TA, KT and FK conceived the study, participated in the study design and coordination, and helped to draft the manuscript. TA, KT, KWT and TY participated in the design of the study and performed the statistical analysis. All authors read and approved the final manuscript. Acknowledgement We would like to thank all the participants in the Department of Physical Therapy, Graduate School of Human Health Sciences, Tokyo Metropolitan University. This study was supported by JSPS KAKENHI (grant Numbers 23H00458) and TMU research fund for young scientists. Data Availability The datasets generated and analyzed during the current study are not publicly available due to participant privacy restrictions but are available from the corresponding author, Dr. Fuminari Kaneko, upon reasonable request. References Lenggenhager, B., Tadi, T., Metzinger, T. & Blanke, O. Video ergo sum: manipulating bodily self-consciousness. Science 317 , 1096–1099 (2007). Botvinick, M. & Cohen, J. Rubber hands 'feel' touch that eyes see. Nature 391 , 756 (1998). Aoyama, T. et al. Influence of Visual Stimulation-Induced Passive Reproduction of Motor Images in the Brain on Motor Paralysis After Stroke. Front. Hum. Neurosci. 15 , 674139 (2021). Kaneko, F., Shibata, E., Hayami, T., Nagahata, K. & Aoyama, T. The association of motor imagery and kinesthetic illusion prolongs the effect of transcranial direct current stimulation on corticospinal tract excitability. J. Neuroeng. Rehabil. 13 (2016). Kaneko, F. et al. Brain Regions Associated to a Kinesthetic Illusion Evoked by Watching a Video of One's Own Moving Hand. PLOS ONE . 10 , e0131970 (2015). Kaneko, F., Yasojima, T. & Kizuka, T. Kinesthetic illusory feeling induced by a finger movement movie effects on corticomotor excitability. Neuroscience 149 , 976–984 (2007). Kim, M., Thawisuk, C., Kaneko, F. & Kim, H. D. Effectiveness of VR Intervention Coupled with Treadmill Training on Gait Function for Stroke Patients: A Systematic Review. NeuroRehabilitation 57 , 3–13 (2025). Grosprêtre, S., Eon, P. & Marcel-Millet, P. Virtual reality does not fool the brain only: spinal excitability changes during virtually simulated falling. J. Neurophysiol. 129 , 368–379 (2023). Kaneko, F. et al. A Case Series Clinical Trial of a Novel Approach Using Augmented Reality That Inspires Self-body Cognition in Patients With Stroke: Effects on Motor Function and Resting-State Brain Functional Connectivity. Front. Syst. Neurosci. 13 , 76 (2019). Duclos, C., Roll, R., Kavounoudias, A. & Roll, J. P. Cerebral correlates of the Kohnstamm phenomenon: an fMRI study. Neuroimage 34 , 774–783 (2007). Goble, D. J. et al. Brain activity during ankle proprioceptive stimulation predicts balance performance in young and older adults. J. Neurosci. 31 , 16344–16352 (2011). Kavounoudias, A. et al. Proprio-tactile integration for kinesthetic perception: an fMRI study. Neuropsychologia 46 , 567–575 (2008). Aoyama, T., Kaneko, F., Hayami, T. & Shibata, E. The effects of kinesthetic illusory sensation induced by a visual stimulus on the corticomotor excitability of the leg muscles. Neurosci. Lett. 514 , 106–109 (2012). Thyrion, C. & Roll, J. P. Predicting any arm movement feedback to induce three-dimensional illusory movements in humans. J. Neurophysiol. 104 , 949–959 (2010). Okada, K. et al. Cognitive effect of passively induced kinesthetic perception associated with virtual body augmentation modulates spinal reflex. J. Neurophysiol. 133 , 69–77 (2025). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 25 Feb, 2026 Reviews received at journal 25 Nov, 2025 Reviewers agreed at journal 25 Nov, 2025 Reviewers agreed at journal 24 Nov, 2025 Reviewers invited by journal 24 Nov, 2025 Editor invited by journal 19 Nov, 2025 Editor assigned by journal 13 Nov, 2025 Submission checks completed at journal 13 Nov, 2025 First submitted to journal 03 Nov, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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08:19:11","extension":"html","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":63227,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8024408/v1/ecbc96184957801d23cf97a8.html"},{"id":97125571,"identity":"bbf4b6dc-69d4-47e5-9fc6-2815a96e9a34","added_by":"auto","created_at":"2025-12-01 08:19:11","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":169467,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Avatar appearance was set from the original control system, and the image output of avatar walking were shown on the HMD. When the start signal is on, the start signal from original control system (original program and D/A converter (USB-6218; National Instrument, USB-6218) Austin, TX, USA)) was sent to the Electrical Stimulation and EMG amplifier (Neuropack X1 MEB-2300; Nihon Kohden) and triggered the same timing for electrical stimulation to the body and image output of avatar walking to the HMD. Then, the EMG signal and trigger signal from the body sent to the CED 1401 (A/D converter) as signal output, which was observed on EMG monitor and signal record. (B) The intervention procedure consisted of one day and had control condition and experiment condition. We conducted a repeated measures design, in which the subject was participated in both conditions.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8024408/v1/0957a9e29b90e9818e7b80e8.png"},{"id":97125572,"identity":"317175da-3c78-43dc-9db7-6825eeae4a63","added_by":"auto","created_at":"2025-12-01 08:19:11","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":198883,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Stimulation timing was set randomly triggered in each specific phases of walking (heel contact, mid-stance, toe-off, and mid-swing) to account for the original systematic error of the avatar walking. We set stimulation timing before the actual phase. (B) The H-reflex and M-wave recruitment curves of the soleus muscle are illustrated. The horizontal axis represents stimulation intensity, and the vertical axis represents amplitude value. The M-wave value of 5-10% Mmax was identified and the stimulus intensity at that time was used as the threshold stimulation intensity.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8024408/v1/9e2115e3a1df0d5e204a17af.png"},{"id":97141861,"identity":"8951f4b9-57fe-418b-9e84-debd7ea53e12","added_by":"auto","created_at":"2025-12-01 10:07:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":272972,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Q1: whole-body ownership, Q2: walking illusion, Q3: agency, in four specific phases of walking (heel contact, mid-stance, toe-off, mid-swing). Pearson correlation tests were performed as correlation. †; Indicate p\u0026lt;0.05 (Bonferroni correction) (B) A correlation between whole-body ownership and H-reflex amplitude was observed in the mid-swing phase, and a correlation between the walking illusion and H-reflex amplitude was observed in the toe-off and mid-swing phases.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8024408/v1/c62551cdda4f11a955f17193.png"},{"id":97142923,"identity":"70412413-e514-4c62-88b3-a686d507a74f","added_by":"auto","created_at":"2025-12-01 10:08:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":202097,"visible":true,"origin":"","legend":"\u003cp\u003e(A) The normalized H-reflex amplitude for the results of the general linear model: repeated measures for all participants. The dotted lines indicate the averaged value for each phase. Significant differences were found between heel contact and toe-off. (general linear model: repeated measures, Bonferroni correction). *; Indicate p\u0026lt;0.05 (Bonferroni correction) (B) Raw data shown changes in excitability of H-reflex during observing an avatar walking in the experimental and control condition depended on the walking phase.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8024408/v1/326254022c4fa6840d34f9e5.png"},{"id":97145089,"identity":"eea40d85-bb60-4917-a8e2-18488ce235dc","added_by":"auto","created_at":"2025-12-01 10:13:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1411596,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8024408/v1/b6795457-3954-4d5a-a4b7-0945c6584a5a.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Whole-body walking illusions modulate spinal excitability","fulltext":[{"header":"Significance Statement","content":"\u003cp\u003eThrough integrating a motion capture device with VR, this study provides new understanding into the multisensory integration required for enhancing walking illusions while watching avatar walking. Real time synchronizing a participant\u0026rsquo;s body with a full-body avatar using a motion capture device in a VR system could enhance the whole-body walking illusion and produce better results in inducing spinal reflex excitability than VR alone.\u003c/p\u003e"},{"header":"Introduction","content":"\u003cp\u003eBody ownership illusions, such as the rubber hand illusion and full-body avatar illusions, demonstrate that when the human brain integrates multisensory information, the result is the emergence of a sense of ownership over an artificial or virtual body(\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e) .In virtual reality (VR), synchronous visual and somatosensory stimulation can generate the compelling illusion that a full-body avatar is one\u0026rsquo;s own body. Building on this framework, if individuals experience a sense of ownership over a full-body avatar, then observing that avatar walking could plausibly induce an illusory sensation of walking even while standing still. Since our previous studies demonstrated that stronger body ownership over body parts was associated with stronger kinesthetic illusions with physiological effects (\u003cspan additionalcitationids=\"CR4 CR5\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e), this possibility provided the primary motivation for the present study.\u003c/p\u003e\u003cp\u003eVR has been increasingly applied in experimental motor neuroscience, with gait training paradigms such as treadmill walking in virtual environments providing controlled settings to study locomotion(\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e). Beyond locomotor practice, immersive VR scenarios have been shown to modulate spinal excitability even in the absence of actual movement; for example, simulated falling in VR elicits spinal responses comparable to those observed during real postural challenges(\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e). Visual stimulation can also generate kinesthetic illusions that alter body self-consciousness and engage motor networks. Previous work demonstrated that such illusions can facilitate corticomotor excitability(\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e) and modify neural activity in response to simple visual cues(\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e). These findings indicate that immersive visual input is capable of modulating both cortical and spinal motor pathways. Induction of a walking illusion through full-body avatar synchronization could therefore provide a unique means to examine the neurophysiological basis of locomotor perception, an area of human motor neuroscience that has remained largely unexplored.\u003c/p\u003e\u003cp\u003ePrevious studies have primarily examined kinesthetic illusions at the segmental level, such as single-joint movements of the wrist or ankle(\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan additionalcitationids=\"CR11 CR12 CR13\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e). Extending these phenomena to whole-body walking is of scientific value, as it enables the investigation of multimodal integration processes underlying locomotor perception and the role of body ownership in generating the sense of self-movement. If whole-body walking illusions can be reliably induced, they would provide a new experimental platform to test whether body ownership constitutes a fundamental basis not only for segmental but also for locomotor illusions. The concept of a whole-body walking illusion is novel, and to date it has not been systematically investigated. A fundamental question is whether such an illusion can in fact be reliably induced in humans. Establishing the feasibility of this phenomenon is therefore a critical first step. However, inducing a walking illusion poses unique challenges compared to previously studied illusions of body ownership or limb movement, given the complexity of locomotor control and the demands of synchronizing a full-body avatar in real time. Based on these considerations, we hypothesized that synchronizing participants\u0026rsquo; movements with a full-body avatar in VR could enhance the experience of body ownership and generate a robust walking illusion, which in turn would modulate spinal reflex excitability in a phase-dependent manner.\u003c/p\u003e\u003cp\u003eAssessing the Hoffmann reflex (H-reflex) provides an optimal approach to evaluate how visual stimulation influences the gain of spinal reflex pathways, thereby offering indirect insight into the modulation of central pattern generator (CPG) activity during locomotion. Demonstrating the spinal effects of this novel illusion is of particular significance, as it not only establishes the physiological basis of the phenomenon but also provides a foundation for future physiotherapy applications targeting walking impairments. Importantly, subjective reports of illusory experience can sometimes be ambiguous or inconsistent, whereas changes in H-reflex amplitude offer an objective physiological marker at the spinal level. By quantifying spinal excitability in this way, the present study aims to clarify the neurophysiological impact of whole-body walking illusions and thereby open a new line of inquiry into how the nervous system represents locomotor perception.\u003c/p\u003e\u003cp\u003eThe aim of this study was therefore to test whether a whole-body walking illusion can be realized in humans and whether such an illusion modulates spinal reflex excitability in a phase-dependent manner.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eSeventeen participants were included in the final analysis. All tolerated the procedures well, and no adverse effects were reported.\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eSubjective illusion ratings\u003c/h2\u003e\u003cp\u003eThe result of each illusion rating is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3\u003c/span\u003eA. The 7-point Likert scale ratings (Q1\u0026ndash;Q3) did no interaction between group and phase (Q1; p\u0026thinsp;=\u0026thinsp;0.830, F\u0026thinsp;=\u0026thinsp;0.293, Q2; p\u0026thinsp;=\u0026thinsp;0.822, F\u0026thinsp;=\u0026thinsp;0.305, Q3; p\u0026thinsp;=\u0026thinsp;0.913, F\u0026thinsp;=\u0026thinsp;0.176). In contrast, significant phase-dependent differences were observed (Q1; p\u0026thinsp;=\u0026thinsp;0.002, F\u0026thinsp;=\u0026thinsp;5.272, Q2; p\u0026thinsp;=\u0026thinsp;0.009, F\u0026thinsp;=\u0026thinsp;4.035, Q3; p\u0026thinsp;=\u0026thinsp;0.017, F\u0026thinsp;=\u0026thinsp;3.544):\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eQ1 (body ownership): toe-off vs. mid-stance (p\u0026thinsp;=\u0026thinsp;0.018) and toe-off vs. mid-swing (p\u0026thinsp;=\u0026thinsp;0.029).\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eQ2 (kinesthetic illusion): toe-off vs. mid-stance (p\u0026thinsp;=\u0026thinsp;0.003).\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eQ3 (agency): toe-off vs. mid-stance (p\u0026thinsp;=\u0026thinsp;0.021).\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eThese results indicate that illusory sensations varied across gait phases but were not significantly influenced by the experimental manipulation.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eH-reflex amplitude\u003c/h3\u003e\n\u003cp\u003eA repeated-measures ANOVA revealed a significant phase \u0026times; condition interaction for normalized H-reflex amplitude (p\u0026thinsp;=\u0026thinsp;0.031, F\u0026thinsp;=\u0026thinsp;5.936). Post-hoc Bonferroni tests showed that in the experimental condition, H-reflex amplitude was significantly greater at toe-off compared with heel contact (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e4\u003c/span\u003eA; p\u0026thinsp;=\u0026thinsp;0.014). Raw values are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e4\u003c/span\u003eB, illustrating a clear increase in reflex amplitude at toe-off during the experimental condition.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eCorrelation between illusion ratings and H-reflex amplitude\u003c/h3\u003e\n\u003cp\u003eCorrelation analyses revealed positive associations between subjective illusion ratings and H-reflex amplitude in the experimental condition (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3\u003c/span\u003eB):\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eQ1 (body ownership): A significant correlation with H-reflex amplitude was observed at mid-swing (R\u0026sup2; = 0.468, p\u0026thinsp;=\u0026thinsp;0.002).\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eQ2 (kinesthetic illusion): A significant correlation was observed at toe-off (R\u0026sup2; = 0.294, p\u0026thinsp;=\u0026thinsp;0.024) and at mid-swing (R\u0026sup2; = 0.535, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001).\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eBy contrast, Q3 (agency) showed no significant correlations with reflex amplitudes in any phase or condition.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eWe originally hypothesized that synchronizing a participant\u0026rsquo;s movements with a full-body avatar in VR would enhance the whole-body walking illusion and thereby modulate spinal reflex excitability more strongly than VR observation alone. The present findings partially support this hypothesis. Specifically, we observed that H-reflex amplitudes were significantly increased during the toe-off phase under the experimental condition, and that these physiological changes were positively correlated with subjective ratings of kinesthetic illusion and body ownership. These results suggest that the induction of a walking illusion through motor\u0026ndash;visual matching can exert measurable effects on spinal excitability.\u003c/p\u003e\n\u003ch3\u003eIllusion vividness and phase specificity\u003c/h3\u003e\n\u003cp\u003eA key observation was that the illusion was phase-dependent: the toe-off phase consistently produced stronger kinesthetic illusions and greater H-reflex amplitudes. This phase is biomechanically critical for forward propulsion, and its distinctive sensory and motor signatures may facilitate multisensory integration. Our findings align with earlier studies showing that visually induced illusions of limb movement enhance corticospinal excitability in a direction- or position-specific manner(\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e). Extending this principle to whole-body representations, it is plausible that toe-off evokes particularly strong engagement of motor association areas, leading to descending facilitation of spinal motoneuron pools.\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eNeurophysiological mechanisms\u003c/h2\u003e\u003cp\u003eThe observed correlations between illusion vividness and spinal excitability strengthen the interpretation that subjective experience is accompanied by objective physiological changes. Prior neuroimaging and neurophysiological studies indicate that kinesthetic illusions activate the posterior parietal and premotor cortices, as well as the primary motor cortex(\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e). Such cortical activity may project via corticospinal pathways, modulating spinal reflex gain(\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e). Although the present study did not directly measure cortical or network-level activity, the observed spinal modulation is most parsimoniously explained by descending volleys from cortical motor areas, consistent with mechanisms demonstrated in previous work(\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eImportantly, earlier evidence was limited to segmental or joint-level illusions, such as dorsiflexion or plantarflexion, where kinesthetic illusions facilitated corticospinal excitability in a movement-specific manner. By demonstrating that a visually induced whole-body walking illusion can similarly modulate spinal excitability, the present findings extend this framework to the domain of locomotion. This provides novel evidence that the neurophysiological mechanisms identified at the level of single-joint movement illusions generalize to complex, whole-body actions.\u003c/p\u003e\u003cp\u003eMoreover, our results emphasize that locomotor perception, like segmental movement perception, is fundamentally grounded in body ownership. The emergence of stronger kinesthetic illusions at toe-off, accompanied by enhanced reflex excitability, suggests that ownership of the virtual body enables the nervous system to integrate visual predictions with proprioceptive templates to construct a coherent sense of walking. This interpretation is consistent with our recent work (Kaneko et al., under review), which showed that strengthening the sense of agency during kinesthetic illusions increased electromyographic activity in task-related muscles. Together, these findings highlight a common principle: both limb-specific and whole-body illusions rely on body ownership as their foundation, and when this ownership is reinforced, descending cortical signals can amplify spinal excitability.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eMethodological contribution\u003c/h3\u003e\n\u003cp\u003eThe use of a simple motion capture system was essential in strengthening body ownership and facilitating the walking illusion. By aligning visual input with participants\u0026rsquo; own movements prior to observation, the system established a proprioceptive\u0026ndash;visual congruence that amplified the subsequent illusion. This approach provides a methodological advance for experimentally inducing robust whole-body illusions without complex or costly equipment.\u003c/p\u003e\n\u003ch3\u003eLimitations\u003c/h3\u003e\n\u003cp\u003eSeveral limitations should be acknowledged. First, the sample size was modest, which may limit the generalizability of the findings and the detection of smaller effects. Second, the experimenter administering the motion capture procedure was not blinded, raising the possibility of experimenter bias. Third, the control condition (observation without motor\u0026ndash;visual matching) always preceded the experimental condition, so potential carry-over or residual effects cannot be entirely excluded. Finally, although we observed correlations between illusion vividness and spinal excitability, causality cannot be definitively established, and future studies using neuroimaging or causal perturbation methods (e.g., TMS) are warranted.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, the present study demonstrates that visually induced whole-body walking illusions can modulate spinal reflex excitability in a phase-dependent manner, with the toe-off phase emerging as particularly effective. By leveraging a simple motion-capture system to strengthen body ownership, we were able to induce robust kinesthetic illusions that translated into measurable spinal changes. These findings provide compelling evidence that the nervous system flexibly integrates visual and proprioceptive information to construct locomotor perception, extending prior work on limb-based illusions to the domain of whole-body locomotion. Beyond establishing this basic mechanism, our paradigm offers a tractable model for probing multisensory integration in human motor control and may provide a foundation for future translational applications, such as VR-based rehabilitation strategies aimed at enhancing gait function in neurological populations. This preliminary study investigated whole-body walking illusions induced by full-body avatar walking which may provide insight into the influence of spinal reflex excitability.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003eParticipants\u003c/h2\u003e\u003cp\u003eEighteen healthy right-handed volunteers (8 males and 10 females; age range 20\u0026ndash;35 years, mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD: 23.9\u0026thinsp;\u0026plusmn;\u0026thinsp;3.9 years) participated in the study. Data from one participant were excluded because Mmax could not be obtained in the H\u0026ndash;M recruitment curve, leaving 17 participants (7 males and 10 females; 24.1\u0026thinsp;\u0026plusmn;\u0026thinsp;4.0 years) for analysis. All participants received a detailed explanation of the experimental protocol and safety considerations before providing written informed consent. The study was approved by the Ethics Review Committee of Tokyo Metropolitan University for Research Involving Human Participants (approval no. 24012) and conducted in accordance with the Declaration of Helsinki.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eExperiment system\u003c/h2\u003e\u003cp\u003eThe experimental setup is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003eA. A head-mounted display (HMD; Meta Quest Pro) presented the full-body avatar. Trigger signals generated by a original control system were sent (USB-6218 D/A converter; National Instruments, Austin, TX, USA) simultaneously to (i) an electrical stimulator/EMG amplifier (Neuropack X1 MEB-2300; Nihon Kohden, Tokyo, Japan) to evoke H-reflexes and (ii) the HMD to synchronize avatar motion onset. EMG and trigger signals were digitized (CED 1401, Cambridge Electronic Design, UK) and stored for offline analysis.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003eOverall experimental framework\u003c/h2\u003e\u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\u003ch2\u003eGeneral procedure\u003c/h2\u003e\u003cp\u003eIn both the control and experimental conditions, participants observed a full-body avatar walking through the HMD while H-reflexes were recorded (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). In each condition, the avatar walked continuously for 5 minutes. Electrical stimulation of the tibial nerve was delivered repeatedly during one predefined gait phase (heel contact, mid-stance, toe-off, or mid-swing), creating four phase-specific conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). Stimulation was synchronized to the target phase using an original LabView program (phase-specific triggers: 250 ms for mid-stance, 860 ms for toe-off, 1300 ms for mid-swing, and 1600 ms for heel contact). Within each condition, the precise stimulus delivery was randomized across trials to prevent predictability. The walking speed of the avatar was fixed at one gait cycle every 2 seconds (1.58 m/sec), identical for all participants. The avatar\u0026rsquo;s height and sex were matched to the participant, and the avatar was positioned 2 m in front of the participant (following Lenggenhager et al., 2007(\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e)).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003eControl condition (avatar observation without prior motor\u0026ndash;visual matching)\u003c/h2\u003e\u003cp\u003eIn both conditions, participants observed the same visual stimulus: a walking avatar presented across four gait phases (5 minutes each, total 20 minutes). In the control condition, participants simply observed the walking avatar without prior motor\u0026ndash;visual matching, that is, without explicit induction of kinesthetic illusion.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003eExperimental condition (pre-observation motor\u0026ndash;visual matching; Matching)\u003c/h2\u003e\u003cp\u003eIn the experimental condition, participants observed the same visual stimulus, but before observation they underwent a motor\u0026ndash;visual matching procedure using a simple motion-capture system (Mocopi, Sony, Tokyo, Japan). Sensors were attached to the head, wrists, ankles, and trunk, and after calibration, the avatar\u0026rsquo;s movements were synchronized with participants\u0026rsquo; real movements in real time. This pre-exposure continued until participants reported the subjective emergence of a kinesthetic illusion (\u0026ldquo;Do you feel your body wants to move?\u0026rdquo;). Only when they answered \u0026ldquo;yes\u0026rdquo; did the walking observation with H-reflex recordings commence.\u003c/p\u003e\u003cp\u003e\u003cb\u003e7-point Likert scale\u003c/b\u003e\u003c/p\u003e\u003cp\u003eTo minimize bias, participants were seated in a comfortable environment, and the experimenter carefully asked them to report their experiences immediately after observing each avatar-walking phase. Participants were instructed to respond freely and without interruption. To assess illusory experiences, they rated the following statements:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eQ1: \u0026ldquo;I felt the avatar body on the display belonged to me.\u0026rdquo;\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eQ2: \u0026ldquo;I felt as if my body was moving.\u0026rdquo;\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eQ3: \u0026ldquo;I felt I was in control of the avatar\u0026rsquo;s movement.\u0026rdquo;\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eA 7-point Likert scale was used, ranging from +\u0026thinsp;3 (strongly agree) to \u0026minus;\u0026thinsp;3 (strongly disagree), with 0 indicating neutrality.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\u003ch2\u003eH-reflex examination\u003c/h2\u003e\u003cdiv id=\"Sec20\" class=\"Section3\"\u003e\u003ch2\u003eElectrode placement and stimulation\u003c/h2\u003e\u003cp\u003eAfter informed consent, electrodes were prepared on the right soleus (SOL) following SENIAM guidelines (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://seniam.org/soleus.html\u003c/span\u003e\u003cspan address=\"http://seniam.org/soleus.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). The skin was cleaned with alcohol, gently abraded with conductive paste to reduce impedance, and disposable surface EMG electrodes were placed over the muscle belly. A ground electrode was attached to the distal tibia (malleolus). H-reflexes were elicited by tibial nerve stimulation using an electrical stimulator (Neuropack X1 MEB-2300; Nihon Kohden, Tokyo, Japan). A convex cathode and rectangular anode were positioned to optimize responses, then secured with surgical tape and an elastic band. Electrode placement was confirmed by identifying the optimal stimulation site.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\u003ch2\u003eH-M curve examination\u003c/h2\u003e\u003cp\u003eAn in-house MATLAB program (R2023b, MathWorks, Natick, MA, USA) was used to generate the H\u0026ndash;M recruitment curve (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). After electrode was placement, participants were asked to stand quietly while tibial nerve stimulation was delivered to elicit soleus H-reflexes and M-waves. The stimulus intensity was gradually increased from 0 mA until the maximal M-wave plateau (Mmax) was reached. To confirm maximal activation, intensity was increased by ~\u0026thinsp;20% beyond the plateau. The stimulation intensity for the main experiment was then set to evoke an H-reflex corresponding to 5\u0026ndash;10% of Mmax. After that we were record H-reflex amplitude for 5-minute as the baseline values obtained during quiet standing without visual input.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\u003ch2\u003eH-reflex measurement\u003c/h2\u003e\u003cp\u003eH-reflex amplitudes were recorded while participants observed the full-body avatar walking. The stimulation intensity was set individually to evoke an H-reflex corresponding to 5\u0026ndash;10% of the maximal M-wave (Mmax), as determined from the recruitment curve. During each 5-minute condition, electrical stimuli were delivered repeatedly at the predefined gait phase (heel contact, mid-stance, toe-off, or mid-swing), with timing randomized across trials to prevent anticipation. For each condition, peak-to-peak H-reflex amplitudes were measured from the soleus EMG and averaged across all trials. To account for inter-individual variability, amplitudes were normalized with baseline value from H-M recruitment curve and expressed as a percentage.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec23\" class=\"Section2\"\u003e\u003ch2\u003eStatistical analysis\u003c/h2\u003e\u003cp\u003eGeneral Linear Model (GLM): Repeated measures were used to compare the H-reflex amplitude of both conditions (control and experimental) and phase of walking (heel contact, mid-stance, toe-off, and mid-swing) Post-hoc analysis was conducted using the Bonferroni correction, and the significance level for all analyses was set at p\u0026thinsp;\u0026lt;\u0026thinsp;0.05 (SPSS software (IBM SPSS Statistics 29 for Mac)); GLM repeated measures and Bonferroni correction were used for the 7-point Likert scale. Pearson correlation analysis was conducted to measure the strength and direction of the linear relationship between the two variables (H-reflex amplitude and 7-point Likert scale) in each phase of the conditions. The statistical power and sample size of the powered clinical trials were calculated using the computer program G*Power after data collection. Test family: F tests, statistical test: ANOVA: Repeated measures, Whitin-between interaction, Cohen\u0026rsquo;s f (medium effect size) 0.25, significant level 0.05, statistical power (1-β) 0.8, and correlation among repeated measures 0.7, total sample size 17 participants.\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eTA, KT and FK conceived the study, participated in the study design and coordination, and helped to draft the manuscript. TA, KT, KWT and TY participated in the design of the study and performed the statistical analysis. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003e We would like to thank all the participants in the Department of Physical Therapy, Graduate School of Human Health Sciences, Tokyo Metropolitan University. This study was supported by JSPS KAKENHI (grant Numbers 23H00458) and TMU research fund for young scientists.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets generated and analyzed during the current study are not publicly available due to participant privacy restrictions but are available from the corresponding author, Dr. Fuminari Kaneko, upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLenggenhager, B., Tadi, T., Metzinger, T. \u0026amp; Blanke, O. Video ergo sum: manipulating bodily self-consciousness. \u003cem\u003eScience\u003c/em\u003e \u003cb\u003e317\u003c/b\u003e, 1096\u0026ndash;1099 (2007).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBotvinick, M. \u0026amp; Cohen, J. Rubber hands 'feel' touch that eyes see. \u003cem\u003eNature\u003c/em\u003e \u003cb\u003e391\u003c/b\u003e, 756 (1998).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAoyama, T. et al. Influence of Visual Stimulation-Induced Passive Reproduction of Motor Images in the Brain on Motor Paralysis After Stroke. \u003cem\u003eFront. Hum. Neurosci.\u003c/em\u003e \u003cb\u003e15\u003c/b\u003e, 674139 (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKaneko, F., Shibata, E., Hayami, T., Nagahata, K. \u0026amp; Aoyama, T. The association of motor imagery and kinesthetic illusion prolongs the effect of transcranial direct current stimulation on corticospinal tract excitability. \u003cem\u003eJ. Neuroeng. Rehabil.\u003c/em\u003e \u003cb\u003e13\u003c/b\u003e (2016).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKaneko, F. et al. Brain Regions Associated to a Kinesthetic Illusion Evoked by Watching a Video of One's Own Moving Hand. \u003cem\u003ePLOS ONE\u003c/em\u003e. \u003cb\u003e10\u003c/b\u003e, e0131970 (2015).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKaneko, F., Yasojima, T. \u0026amp; Kizuka, T. Kinesthetic illusory feeling induced by a finger movement movie effects on corticomotor excitability. \u003cem\u003eNeuroscience\u003c/em\u003e \u003cb\u003e149\u003c/b\u003e, 976\u0026ndash;984 (2007).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKim, M., Thawisuk, C., Kaneko, F. \u0026amp; Kim, H. D. Effectiveness of VR Intervention Coupled with Treadmill Training on Gait Function for Stroke Patients: A Systematic Review. \u003cem\u003eNeuroRehabilitation\u003c/em\u003e \u003cb\u003e57\u003c/b\u003e, 3\u0026ndash;13 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGrospr\u0026ecirc;tre, S., Eon, P. \u0026amp; Marcel-Millet, P. Virtual reality does not fool the brain only: spinal excitability changes during virtually simulated falling. \u003cem\u003eJ. Neurophysiol.\u003c/em\u003e \u003cb\u003e129\u003c/b\u003e, 368\u0026ndash;379 (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKaneko, F. et al. A Case Series Clinical Trial of a Novel Approach Using Augmented Reality That Inspires Self-body Cognition in Patients With Stroke: Effects on Motor Function and Resting-State Brain Functional Connectivity. \u003cem\u003eFront. Syst. Neurosci.\u003c/em\u003e \u003cb\u003e13\u003c/b\u003e, 76 (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDuclos, C., Roll, R., Kavounoudias, A. \u0026amp; Roll, J. P. Cerebral correlates of the Kohnstamm phenomenon: an fMRI study. \u003cem\u003eNeuroimage\u003c/em\u003e \u003cb\u003e34\u003c/b\u003e, 774\u0026ndash;783 (2007).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGoble, D. J. et al. Brain activity during ankle proprioceptive stimulation predicts balance performance in young and older adults. \u003cem\u003eJ. Neurosci.\u003c/em\u003e \u003cb\u003e31\u003c/b\u003e, 16344\u0026ndash;16352 (2011).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKavounoudias, A. et al. Proprio-tactile integration for kinesthetic perception: an fMRI study. \u003cem\u003eNeuropsychologia\u003c/em\u003e \u003cb\u003e46\u003c/b\u003e, 567\u0026ndash;575 (2008).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAoyama, T., Kaneko, F., Hayami, T. \u0026amp; Shibata, E. The effects of kinesthetic illusory sensation induced by a visual stimulus on the corticomotor excitability of the leg muscles. \u003cem\u003eNeurosci. Lett.\u003c/em\u003e \u003cb\u003e514\u003c/b\u003e, 106\u0026ndash;109 (2012).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eThyrion, C. \u0026amp; Roll, J. P. Predicting any arm movement feedback to induce three-dimensional illusory movements in humans. \u003cem\u003eJ. Neurophysiol.\u003c/em\u003e \u003cb\u003e104\u003c/b\u003e, 949\u0026ndash;959 (2010).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eOkada, K. et al. Cognitive effect of passively induced kinesthetic perception associated with virtual body augmentation modulates spinal reflex. \u003cem\u003eJ. Neurophysiol.\u003c/em\u003e \u003cb\u003e133\u003c/b\u003e, 69\u0026ndash;77 (2025).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Avatar walking, H-reflex, Spinal reflex excitability, Virtual reality","lastPublishedDoi":"10.21203/rs.3.rs-8024408/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8024408/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWalking is a fundamental human function, yet how the nervous system constructs the perception of locomotion remains poorly understood. Previous research has shown that kinesthetic illusions of single joints can modulate corticospinal excitability, but whether such illusions extend to whole-body walking has remained unknown. Here, we demonstrate that synchronizing participants movements with a full-body avatar in virtual reality (VR) induces a vivid whole-body walking illusion and modulates spinal reflex excitability in a phase-dependent manner. Using a simple motion-capture system, we enhanced body ownership over the avatar and showed that the toe-off phase elicited both the strongest illusory sensations and significantly increased soleus H-reflex amplitudes. Moreover, the intensity of the walking illusion correlated positively with reflex modulation, suggesting that locomotor perception relies on body ownership as its basis, just as previously shown for segmental illusions. These findings extend the concept of multisensory integration from joint-level illusions to whole-body locomotion, providing new evidence that the nervous system flexibly integrates visual predictions and proprioceptive templates to construct locomotor awareness. This paradigm not only advances fundamental understanding of human motor control but also offers a translational framework for VR-based gait rehabilitation.\u003c/p\u003e","manuscriptTitle":"Whole-body walking illusions modulate spinal excitability","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-01 08:19:06","doi":"10.21203/rs.3.rs-8024408/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"334890518909863328110501953191398180816","date":"2026-02-25T20:28:42+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-25T18:25:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"310903889529084637850464450183478558731","date":"2025-11-25T14:07:10+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"134361941914693279750078914038552732886","date":"2025-11-24T21:19:54+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-24T21:19:07+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-11-19T12:00:09+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-14T04:08:05+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-14T04:07:08+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-11-04T04:43:19+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"545eb48d-bbf7-4584-8275-84625d90cfa8","owner":[],"postedDate":"December 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":58675950,"name":"Biological sciences/Neuroscience"},{"id":58675951,"name":"Biological sciences/Psychology"},{"id":58675952,"name":"Social science/Psychology"}],"tags":[],"updatedAt":"2025-12-01T08:19:07+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-01 08:19:06","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8024408","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8024408","identity":"rs-8024408","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}