# Title: Phantom sensations as an intrinsic mechanism for restoring lower-limb somatosensory feedback during prosthetic gait: Study protocol

Emmanuel Jamin, Amélie Touillet, Pierrick Herbe, Jonathan Pierret, Jozina B de Graaf

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Emmanuel Jamin, Amélie Touillet, Pierrick Herbe, Jonathan Pierret, Jozina B de Graaf. Title: Phantom sensations as an intrinsic mechanism for restoring lower-limb somatosensory feedback during prosthetic gait: Study protocol. 2026. hal-05528257

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Subject: Submission of Study Protocol – "Phantom sensations as an intrinsic mechanism for restoring lower-limb somatosensory feedback during prosthetic gait"

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<table><tr><td>Manuscript Number:</td><td></td></tr><tr><td>Article Type:</td><td>Study Protocol</td></tr><tr><td>Full Title:</td><td>Subject: Submission of Study Protocol – "Phantom sensations as an intrinsic mechanism for restoring lower-limb somatosensory feedback during prosthetic gait"</td></tr><tr><td>Short Title:</td><td>Short title: Phantom sensations to restore somatosensory feedback during prosthetic gait</td></tr><tr><td>Corresponding Author:</td><td>Emmanuel Jamin, M.D.<br>AMU: Aix-Marseille Universite<br>Marseille, FRANCE</td></tr><tr><td>Keywords:</td><td>Phantom sensation; Gait; Amputee; Somatosensory feedback; Rehabilitation</td></tr><tr><td>Abstract:</td><td>The loss of somatosensory inputs following lower limb amputation significantly alters gait and increases cognitive demand. While several technological solutions have been developed to compensate for this deficit, most rely on invasive approaches or require complex training. Our study takes a different perspective by focusing on a natural phenomenon frequently experienced by amputees: non-painful phantom sensations (PS). We aim to determine whether these sensations can be characterized, modulated, and potentially harnessed to improve gait. The objectives are: (i) to assess the prevalence and characteristics of PS during walking (ii) to investigate their interaction with gait at biomechanical and cognitive levels within a large cohort; and (iii) to evaluate the feasibility of inducing, or enhancing PS through targeted adjustments of the prosthetic socket. This prospective multicenter study integrates observational assessments with an interventional protocol. Two hundred lower limb amputees will be enrolled, stratified into four subgroups according to the presence -and if so subjective perception- of PS during gait (neutral, helpful, disturbing or no PS), together with a control group of non-amputee. All amputated participants will complete a semi-structured interview exploring the characteristics of their PS. Gait analysis will be conducted both in laboratory and ecological conditions. Participants for whom PS can be enhanced by tactile stimulation will undergo mapping of residual limb stimulation areas. Based on these results, 10 transfemoral amputees will benefit from personalized prosthetic socket modifications to modify their PS and comparative gait assessments. Cognitive load and perceived effort will be systematically evaluated. Primary outcomes are the prevalence and phenomenological features of PS and their association with gait parameters. Secondary outcomes include the feasibility of modulating these sensations through socket adjustments. Findings are expected to deepen our understanding of PS, their influence on gait, and their potential clinical applications. This trial is registered at ClinicalTrials.gov (NCT07191795).</td></tr><tr><td>Order of Authors:</td><td>Emmanuel Jamin, PhD student<br>Amélie Touillet, PhD<br>Pierrick Herbe<br>Jonathan Pierret, PhD<br>Jozina B. De Graaf, Professor, PhD</td></tr><tr><td>Opposed Reviewers:</td><td></td></tr><tr><td>Additional Information:</td><td></td></tr><tr><td>Question</td><td>Response</td></tr><tr><td>Financial Disclosure</td><td>Yes</td></tr><tr><td>Enter a financial disclosure statement that</td><td></td></tr></table>
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E.J. is a PhD student funded (34 000€/year) by the French National Research Agency (ANR), which also funded the project (816 112€) associated with this paper. J.P. is employed by the study's sponsor, UGECAM NORD-EST, and serves as the co-supervisor for E.J.'s doctoral research.

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February 20th, 2026

**Subject: Submission of Study Protocol – "Phantom sensations as an intrinsic mechanism for restoring lower-limb somatosensory feedback during prosthetic gait"**

Dear Editor,

This manuscript outlines the protocol for a prospective multicenter trial, funded by the French national agency for research (ANR), designed to address the significant impairment of locomotor function because of loss of somatosensory information in lower limb amputees. Our study proposes a novel perspective by focusing on **non-painful phantom sensations (PS)**—a natural phenomenon frequently experienced by amputees—as an intrinsic mechanism to restore, at least partly, somatosensory feedback.

Our study contributes to the scientific literature by detailing a comprehensive methodology to assess (1) the prevalence of PS in a large cohort of 200 lower limb amputees, (2) to evaluate the impact of non-painful PS on gait, and (3) to evaluate the feasibility of voluntary modulating these sensations through targeted prosthetic socket modifications. We aim to demonstrate these "induced phantom sensations" (iPS) can be harnessed to improve gait biomechanics, reduce cognitive load, and decrease perceived effort during walking. If successful, this approach could significantly simplify rehabilitation processes and enhance prosthesis acceptance by transforming the phantom limb from a perceived symptom into a functional rehabilitation tool.

This work directly builds upon and extends previous research, specifically the findings of Bachini et al. (2022, 2024), demonstrating the interaction between gait and PS as perceived by the amputees themselves, as well as the improvement of gait by non-invasively eliciting specific iPS in a single case. Our study expands this concept to a large population. Furthermore, our protocol addresses the shortcomings of prior sensory feedback systems (e.g., vibration or electrical stimulation) that stimulate the residual limb rather than the phantom limb, thereby requiring complex cognitive interpretation. By using natural PS, we aim to provide intuitive somatosensory feedback that integrates seamlessly with the dynamic interaction between the prosthesis and the environment.

Thank you for your consideration of this protocol, which aims to deepen the clinical understanding of non-painful phantom sensations and their potential for prosthetic gait rehabilitation.

Sincerely,

On behalf of the authors

Emmanuel Jamin
<figure><img src="image_2.png" /></figure>

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STUDY PROTOCOL

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Title: Phantom sensations as an intrinsic mechanism
for restoring lower-limb somatosensory feedback
during prosthetic gait: Study protocol

Short title: Phantom sensations to restore
somatosensory feedback during prosthetic gait

Emmanuel Jamin<fnref n="1" />,<fnref n="2" />*, Amélie Touillet <fnref n="2" />,<fnref n="3" />, Pierrick Herbe <fnref n="2" />, Jonathan
Pierret<fnref n="2" />,<fnref n="3" />¶, Jozina B. De Graaf <fnref n="1" />¶

<footnote n="1">¹ Aix-Marseille Univ, CNRS, ISM, Marseille, France</footnote>

<footnote n="2">² Institut Régional de Réadaptation, UGECAM-NE, Nancy, France</footnote>

<footnote n="3">³ Université de Lorraine, DevAH, F-54000, Nancy, France</footnote>

<footnote>*Corresponding author: emmanuel.jamin@etu.univ-amu.fr (EJ)</footnote>
Abstract

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deepen our understanding of PS, their influence on gait, and their potential clinical
applications. This trial is registered at ClinicalTrials.gov (NCT07191795).

**Keywords** : Phantom sensation; Gait; Amputee; Somatosensory feedback;
Rehabilitation.

**Data Availability Statement:** No data sets were generated or
analyzed during the current study. All relevant data from this study will be made
available upon study completion.

**Funding:** The study has been funded by the French National Research Agency
(n° ANR-23-CE17-0067-02).

**Competing interests:** The authors have declared that no competing
interests exist.

**Trial sponsor:** "UGECAM NORD-EST" sponsors the trial (contact:
JONATHAN.PIERRET@ugecam.assurance-maladie.fr)
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63 **Abbreviations**

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65 ANOVA: Analysis Of Variance

66 CTRL: Control Group

67 EEG: Electroencephalographic

68 EGVI: Enhanced Gait Variability Index

69 FAPS: Functional Ambulation Performance Score

70 IMU: Inertial Measurement Unit

71 iPS: Induced Phantom Sensations

72 LLA: Lower Limb Amputations

73 PL: Phantom Limb

74 PS: Phantom Sensations

75 QMA: Quantified Motion Analysis

76 RL: Residual Limb

77 SPIRIT: Standard Protocol Items: Recommendations for interventional Trials

78 sPS: Spontaneous Phantom Sensations

79 TFA: Transfemoral Amputees

80 VAS: Visual Analog Scale

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# 1. Introduction

## 1.1 General

The incidence of major lower limb amputations (LLA) varies globally, ranging from approximately 5 per 100,000 inhabitants in France to over 22 per 100,000 in Canada, attributable to diverse etiologies including vascular diseases, trauma, congenital conditions, cancer, and diabetes [1]. Major LLA profoundly and permanently impairs locomotor function, substantially reducing individual autonomy [2]. Indeed, individuals with LLA exhibit marked asymmetry in both posture and gait, relying predominantly on the intact limb during walking [3-6]. Their locomotion is also slower and requires greater energy expenditure [7,8]. This impairment is partly due to the loss of somatosensory information—particularly from the ankle joint and plantar skin— which are essential for interacting with the environment [9,10]. After amputation, the primary source of somatosensory feedback regarding prosthesis-environment interaction originates from the residual limb via the socket interface. This differs from the original sensory input used for gait regulation, which was localized in the ankle and the foot. Therefore, the somatosensory information is less rich and precise, and its interpretation increases cognitive load and perceived effort [11-13].

Recent advances in invasive and non-invasive technologies aim to substitute lost somatosensory inputs. These interventions have demonstrated improvements in gait, including reduced cognitive load, increased walking speed, and enhanced balance on irregular terrain [13-15]. However, such somatosensory feedback systems are typically indirect and non-intuitive, requiring extensive training and limiting accessibility for most amputees. Instead of relying on simulated or invasive sensory substitution, our focus is on phantom sensations (PS), a natural somatosensory phenomenon occurring frequently after amputation.
## 1.2 Phantom sensations

Most amputees experience perception of their absent limb, referred to as "phantom limb" (PL), through the phenomenon of phantom sensations (PS) [16]. Contrary to common misconceptions, PS are predominantly non-painful [17]. These PS vary from one individual to another, and can vary over time for the same individual in terms of both location and nature of sensations (tingling, pressure, temperature, simple presence, muscular sensations, electricity, itching, pricking...) [17,18]. Although PS can diminish or disappear over time, a significant proportion of individuals continue to perceive their PL even decades post-amputation [16,19,20]. While many studies are limited by small sample sizes or restricted to case reports, Bachini and colleagues [21] conducted a large-scale study on a cohort following LLA. They found that a significant proportion of participants (96%) reported experiencing PS wearing their prosthesis, with a large minority describing these sensations as exclusively painful (5%). Fifty-nine percent of participants perceived their PS during walking. They perceived them either as helpful (46%), neutral (46%)—mainly because they had always experienced PS and were therefore unable to compare—or as disturbing (8%) for gait. Interestingly, most helpful sensations were described as cyclic variations coherent with the gait cycle, with the majority in the foot, particularly the toes. All these mentioned findings suggest that non-painful PS during gait are not an isolated phenomenon.

PS can be classified as spontaneous (sPS) or induced (iPS). Spontaneous PS are sensations that occur without any stimulation of the residual limb (RL). In contrast, induced PS occur when global or local stimulation of the RL triggers the appearance, disappearance, or modulation of PS in distinct areas of the PL (e.g., [10,17,21,22]). sPS may be perceived while wearing a prosthesis, but in that case their presence is
not directly influenced by prosthetic contact or use. In the contrary, iPS occur when global stimulation of the RL or focal stimulation of certain regions on the RL induce appearance, disappearance or modulation of PS in distinct zones of the PL [10,17,21,22]. iPS zones has been little studied for lower limb amputees [16]. The underlying physiological mechanism is hypothesized to involve peripheral nerve sprouting. Sensory nerves that originally innervated the distal part of the limb (skin and other tissues) and were axotomized by amputation may randomly reinnervate adjacent skin areas, ligaments, tendons and other tissues on the residual limb, thereby activating cortical regions corresponding to the missing limb [22]. This might explain the variability of PS between individual amputees.

These non-painful PS, while present in many lower limb amputees, are indeed highly variable and not always very intense, making the perception of these PS sometimes difficult. Recent technological and surgical advances enabled the use of *invasive* methods to induce PS (thus iPS) in upper limb amputees, providing intuitive somatosensory feedback for prosthesis users [23-25]. However, a recent pilot study demonstrated that it is possible to *non-invasively* elicit iPS in lower limb amputees through RL stimulation without surgical intervention, and that such iPS are exploitable to improve prosthetic gait [10]. Indeed, the study of Bachini et al. [10] showed that inducing helpful PS is associated with changes in the spatiotemporal gait parameters, with in particular a reduction in double support time. This was achieved through focal pressure applied at a specific iPS zone previously determined by a mapping on the RL and activated within the socket during walking. The identified stimulation zones appeared to be rather unpredictably distributed on the RL, which might suggest that the iPS zones are extremely heterogeneous between individual amputees.
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This study of Bachini et al. [10] demonstrates the feasibility of providing somatosensory feedback from the PL through iPS by modifying the socket design. While pressure was the stimulus used in their study to evoke iPS, other types of stimulation such as light touch [18], vibration [26] or electrical stimulation [26,27] have also been shown to deliver feedback on gait cycle phases [13,28]. Yet, these stimulations were applied without consideration of individual iPS zones, meaning that their studies induced sensations on the RL instead of in the phantom limb. Consequently, this non-natural feedback requires cognitive interpretation to associate the stimulation with its meaning (i.e., the gait cycle phase) rather than a direct perception of the limb in the moving prosthetic limb. The present study aims to leverage the natural phenomenon of PS—whether spontaneous or induced—to restore the perception of the phantom limb in a manner coherent with the dynamic interaction between the prosthesis and the environment.

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## 1.3 Objectives and Hypothesis

The objectives of this study, running over 5 years, are multiple. **First**, we want to assess, during walking, the prevalence and characteristics (i.e., localization, type and coherence with the gait cycles) of PS and iPS in LLA as well as their by-the-participant perceived use. **Second**, we aim to evaluate, in a large cohort of individuals with LLA, the objective biomechanical and cognitive load impacts of the interaction between PS and prosthetic gait as a function of these characteristics. Because very little or no attention is usually paid to these non-painful PS during rehabilitation, we will explore whether directing the participant's attention to these sensations increases their perceived intensity during walking, and consequently their objective impact. **Finally**, we aim to test whether iPS coherent with walking impact the biomechanical parameters as well as the cognitive load associated with prosthetic gait. This will be done by slight
modifications at specific localizations within the socket, individualized for several
participants.

Based on previous obtained results in our team and in the literature, we
formulated the following hypotheses.

**Hypothesis 1:** The prevalence of PS during walking in our large population after LLA
is above 60%.

**Hypothesis 2:** The characteristics and perceived helpfulness of PS during walking are
variable between participants.

**Hypothesis 3:** Participants who reported PS to be helpful for walking—whether
spontaneous or induced—demonstrated biomechanical profiles consistent with their
subjective assessments of helpfulness.

**Hypothesis 4:** Participants experiencing PS perceived as helpful for walking (whether
spontaneous or induced) exhibit a locomotor pattern more closely resembling that of a
healthy control group compared to participants without PS or those who perceive their
PS as neutral or disturbing for walking.

**Hypothesis 5:** Participants experiencing PS perceived as helpful for walking (whether
spontaneous or induced) show a reduced cognitive load as well as perceived effort
during gait, relative to participants without PS or those who perceive their PS as neutral
or disturbing for walking.

**Hypothesis 6:** Participants with or without PS during walking with their usual
prosthesis show improved locomotor patterns and reduced cognitive load when helpful
PS are locally induced through socket modification.
**2. Materials and methods**

**2.1 Study context**

This trial contains prospective observational phases and an interventional phase (illustrated in S1 Fig 1). Recruitment began in July 2025. It follows the SPIRIT statement and guidelines (Standard Protocol Items: Recommendations for interventional Trials, 2013) (S1 Fig 1).

**S1 Fig 1. SPIRIT Table.** For the registration, intervention and evaluation schedule. * : Reassessment of the group after completion of the gait analysis. T0 correspond to the inclusion, t1 to t4 correspond to phase 1 to 4. Abbreviations are explained page 3. X*: means that the participants can be reassigned in a different group depending on how they feel during the gait evaluation.

The study is multicentric: Regional Institute of Physical Medicine and Rehabilitation (IRR UGECAM), Louis Pierquin Center Nancy; Chantecler Clinic Marseille; Saint Hélier Foundation Rennes. The analysis of the gait (phase 2) as well as the modification of the socket (phase 5) are carried out only within the IRR at the Louis Pierquin Center in Nancy (Fig 2).

**Fig 2. Flowchart of the study.** * = Nancy Regional Institute of Physical Medicine and Rehabilitation (IRR), Chantecler Clinic Marseille, Saint Hélier Foundation Rennes. ** = groups: (1) without phantom sensations, (2) with phantom sensations judged helpful for walking, (3) with phantom sensations judged disturbing for walking, (4) with phantom sensations judged neutral (neither helpful nor disturbing for walking). *** = Transfemoral Amputee. Phases 2 and 3 are carried out in parallel.

**2.2 Ethical approval and registration**

The principal sponsor for ethical approval is the French union of health insurance funds - North East. This category 2 minimal-risk-and-constraint-research has obtained approval of the Committee for the Protection of Persons Est II (n° SI 24.03144.000274) the 20/11/2024. Participation in the study is voluntary and requires the written informed consent of each participant obtained by the principal investigator or an approved member of the team for each different phase.
## 2.3 Recruitment criteria and eligibility

A total of 200 participants will be recruited as part of their routine care at the three rehabilitation centers involved in the study concerning at least Phase 1. This sample size is based on the centers' previous recruitment experience and anticipated participants availability. Additionally, a control group (CTRL), composed of non-amputated individuals, will be recruited exclusively for Phase 2.

A medical investigator at each center will inform eligible patients, currently receiving care, about the study. Participants with major LLA from outside institutions as well as non-amputated participants will also be recruited via informational posters displayed within the centers. Interested individuals will be invited to attend an inclusion interview during which they will be provided with an information notice and a written consent form; completion of the latter is required for inclusion. Upon receipt of the signed consent, a date for Phase 1 will be scheduled, corresponding to the participant's ongoing care or at a convenient time for external participants. Scheduling of subsequent phases will follow a similar procedure.

Global inclusion criteria for amputated participants are as follows: major LLA, proficiency in French language comprehension and expression, and affiliated to a social security scheme. Inclusion for Phases 2 and 4 requires the ability to walk 500 meters without technical aids. Phase 3 inclusion requires the presence of iPS detected in phase 1. Unilateral TFA with stable iPS will be selected for Phase 4. For control participants, inclusion criteria are the absence of major lower limb surgery within the past three years, no minor injuries affecting the trunk, lower or upper limbs within the past year that could influence gait, and affiliated to a social security scheme.

Exclusion criteria for amputee participants include history of evolutive psychiatric disorders, pregnancy or breastfeeding, age under 18 years, legal protection
status such as guardianship or trusteeship, and judicial protection. The exclusion
criterion for Phases 2 and 4 is pain impacting gait (involving the trunk, the residual and
phantom limb, and the contralateral limb).

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## 2.4 Study procedure

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### 2.4.1 Phase 1: Semi-structured interview

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The participants (n=200) will undergo a semi-structured interview with the aim
of knowing the characteristics of each participant's PS: the presence or absence of
PS, the location of the sensations on the PL, the nature of the sensations. For
participants who are able to walk at the time of the interview, they will be asked about
the intensity of PS, stability or variation of sensations during walking cycles and
subjective evaluation of whether these sensations are helpful, neutral or disturbing for
walking using a rating scale from -5 (very disturbing) to +5 (very helpful). Data on the
type of prosthesis used at the time of the interview will be systematically collected. An
initial screening for the presence of iPS will be carried out in order to select participants
for Phase 3. 3D prints of feet and lower limbs will be made available to participants to
simplify the exchange with the investigators (Fig 3). As a function of the subjective
evaluations of PS, participants who were able to walk will be then assigned to one of
the following four groups: (1) without PS during walking, (2) with PS that are perceived
as helpful during walking, (3) with PS that are perceived as disturbing, and (4) with
neutral PS, i.e., perceived as neither helpful nor disturbing during walking. A precise
description of what helps or disturbs the participant is requested in order to personalize
the global analysis of the kinematic and kinetic data. The aim is to have an equal
number of participants in these four groups for Phase 2.

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To mitigate interviewer bias during the semi-structured interviews, an interview
guide was utilized to standardize the process for both the interviewer and the

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284 Fig 3. 3D print of feet and lower limbs.

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## 2.4.2 Phase 2: Quantified motion analysis and cognitive load assessment during gait

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A control group (n=48) and 84 of the walking amputated participants who underwent the interview during Phase 1 will participate in this phase. Phase 2 includes a quantified movement analysis (QMA) and an assessment of cognitive load and perceived effort in a laboratory setting as well as in an ecological environment. Details will now be given about the QMA assessment in the (1) controlled, and (2) ecological environments, and (3) for the assessments of cognitive load and perceived effort. To reliably assess the impact of PS on walking, we regularly quantified the evolution of these sensations during this phase.

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### 2.4.2.1 Gait in a controlled environment

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All participants will walk back-and-forth in a 10 m corridor in the movement analysis laboratory of the Centre Louis Pierquin (Fig 4A.), at a spontaneous speed without other particular instructions [29-32], as well as on a 10° wooden slope (Fig 4B.) [33,34]. A rating of the intensity of their PS and their perceived impact on walking will be requested at the end of each condition (slope or flat). Then the participants who have declared the presence of PS during walking will repeat this protocol, yet with the instruction to pay *specific attention* to their PS during walking, and the intensity of their PS and their perceived impact on walking will be requested again.

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**Fig 4. Experimental setting in the motion analysis lab.** (A) The 10-m walkway and the configuration of the force plates: two in series with 10 cm between them, and one aligned longitudinally to the left off the two others. (B) The 10° wooden slope placed on the 3 force platforms
308 Kinematic and kinetic data will be collected using a VICON optoelectronic
309 system (Vicon Motion Systems® Ltd., Oxford, UK) with 10 infrared cameras (100 Hz),
310 coupled to 3 AMTI force platforms (Advanced Mechanical Technology, Inc.,
311 Watertown, MA, USA) with an acquisition frequency of 1000 Hz [35,36]. We will use
312 the conventional biomechanical gait model [37] that is implemented via the "Plug-In-
313 Gait" module in the Vicon Nexus software (version 2.12, Vicon Motion Systems Ltd.,
314 Oxford, UK). This model involves the placement of 39 markers on precise anatomical
315 landmarks (Fig 5.) and allows obtaining data related to body kinematics and joint
316 kinetics, and thus of spatiotemporal parameters of walking.
317

317 **Fig 5. Application of the reflective markers.** Placement of the 39 markers for the Plug-In-
318 Gait model. Plug-In-Gait reference guide, Vicon motion Systems Limited, 2016.

319 In order to obtain at least one complete gait cycle for each participant, two
320 platforms are aligned in series (one behind the other) and the third is positioned
321 laterally with respect to the first two (Fig 4A.). The platforms are used during the
322 overground walking as well as during the slope-walking. In this condition, the wooden
323 slope was positioned across all three force platforms in a custom-built configuration
324 designed to ensure accurate force transmission from the participant to the platforms.
325 Moreover, to measure the perpendicular force for each gait cycle in all walking laps,
326 pressure-sensing insoles incorporating three-point pressure sensors (one on the heel,
327 one on the medial and one on the lateral side of the forefoot (Loadsol®, Novel GmbH,
328 Munich, Germany, 200 Hz)) will be integrated into the participants' usual shoes [38,39].
329

**2.4.2.2 Gait in an ecological environment**

330 Participants will complete a walking course encompassing interior and exterior
331 zones of the center, visualized in Fig 6. Each trial will include walking on horizontal
332 even ground (1042.5 m), upslope (90.5 m), downslope (84 m) and side-slope (52.5 m),
333 as well as staircases (8 steps ascent and 46 steps descent). Participants will be asked
334 to walk at their self-selected, comfortable speed while prioritizing safety to prevent 335 stumbling or falls [40,41]. To further ensure participant safety, an investigator will 336 accompany each participant throughout the entire trial. Participants will be 337 instrumented with pressure-sensing insoles identical to those employed in the 338 controlled environment, here above. Additionally, motion data are captured using four 339 mHealth inertial measurement units (IMUs) (mTest3 system mHealth) affixed to the L3 340 vertebra, the right thigh (proximal to the lateral tibiofemoral joint line), and bilaterally 341 on the anterior tibial crest (midway between the medial malleolus and the tibial 342 tuberosity) [42].

343 **Fig 6. Ecological path.** Representation of the different stages of the ecological path.

344 345 **2.4.2.3 Assessment of cognitive load, perceived effort and perceived 346 interaction between PS and gait**

347 The assessment of cognitive load associated with the walking tasks will employ 348 psychometric and neurophysiological approaches. Central to this evaluation is the use 349 of the French version of the NASA Task Load Index (NASA-TLX) self-report 350 questionnaire [43-45]. This established instrument systematically quantifies the 351 perceived demands of the task across six dimensions: Mental Demand, Physical 352 Demand, Temporal Demand, Frustration, Effort, and Performance. Each dimension is 353 rated on a 0–10 scale, with 0 indicating the lowest and 10 the highest perceived 354 demand, except for the Performance scale, which uses inverse anchoring. The 355 classical NASA-TLX was completed with a seventh dimension, i.e., the perceived 356 embodiment of the prosthesis that will be specifically studied as a function of the 357 characteristics of the population group [46,47]. From the six original dimensions, we 358 will determine the RAW-TLX score (RTLX, a subdimension-averaged score strongly 359 correlated to the weighted score (TLX) [43,48,45]. Furthermore, to quantify the 360 subjective experience of physical exertion, participants will complete the Borg scale
360

(1–10) following the walking task for each experimental condition [49]. In the ecological condition, the Borg scale will be administered after traversing each different type of ground to effectively assess how perceived effort is modulated by the environmental characteristics.

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A Visual Analog Scale (VAS), spanning from -5 to 5, will be administered to participants with PS to precisely quantify their perceived interaction between their PS and walking performance [50-52]. This scale is a continuous 10 cm line, anchored by the qualitative descriptors “Very annoying” at -5 and “Very helpful” at 5, with 0 being neutral [51]. It should be noted that participants in Group 1 and the Control group will not complete this specific scale. The scales evaluating the PS-gait interaction will be completed following each sub-condition (spontaneous and phantom sensation-focused).

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To objectively assess cognitive load during task execution—independent of memory or self-evaluation—electroencephalographic (EEG) activity will be continuously recorded under both controlled laboratory and ecological conditions. The data acquisition will be performed with a sixteen channels ANT-Neuro® EegoSports system, operating at a sampling frequency of 1 kHz.

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377

### 2.4.3 Phase 3: Exploration of iPS

378

A subset of participants (n = 40), identified through the initial semi-structured interviews as likely being able to experience iPS, will undergo a systematic and detailed exploration of these phenomena, drawing inspiration from the methodology described by Bachini et al. (2022) [10]. This procedure is designed to explore the quality, the intensity, characteristics and localization of iPS as a function of the type and the localization on the RL of the stimulation. This exploration involves stimulating various regions of the RL using specialized tools, including a soft brush for light tactile

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384
385 stroking, a round rubber tip for the application of pressure, a pointed tip for highly
386 localized stimulation, a surface vibrating stimulator adapted for use on the RL, and
387 thermosensory stimulation.

388 During each stimulation, participants will report the presence or absence of iPS.
389 If iPS are reported, the participant will specify its nature, the precise location on the PL,
390 and rate the sensation's intensity in real time using a three-level ordinal scale: weak,
391 medium, or strong (all illustrated in Fig 7). To facilitate clear communication between
392 the participant and the investigator, 3D prints of feet and lower limbs will be available
393 as visual aids for indicating the precise localization on the phantom limb (see Fig. 2).

394 This protocol will culminate in the construction of an individualized "iPS map" for
395 each participant and for each distinct type of mechanical stimulation. The mapping data
396 will be systematically documented on a grid that is organized according to the
397 stimulated zone (as illustrated in Fig 7.). To ensure accurate localization and
398 subsequent documentation, the experimenter will mark each stimulation site on the
399 residual limb using a dermo-graphic marker, with numbered crosses corresponding to
400 the chronological order of the exploration. Upon the completion of the assessment,
401 standardized photographs will be taken of sides of the residual limb (anterior, posterior,
402 medial, and lateral) and added on the corresponding data sheets.

403 **Fig 7. Documenting the results of exploration of iPS.** Separate sheets are used for each
404 exploration zone to characterize the nature and localization of iPS on the RL and the PL.
405 Different types of stimulation are indicated by different colors. Here is shown a data sheet
406 completed for the exploration on one particular zone of the RL.

407 A reassessment will be carried out 6 months after the first for some participants
408 to assess stability.
## 2.4.4 Phase 4: Assessing impact of socket modifications

### 2.4.4.1 Socket's modifications

Sensory feedback impairment in transfemoral amputees (TFA) represents a critical challenge that profoundly impacts postural and locomotor control. Unlike able-bodied individuals or transtibial amputees, those with transfemoral amputations face a dual deficit: the loss of both the biological ankle/foot complex and knee joint [40,53]. Moreover, the larger soft tissue mass at the residual limb-socket interface in TFAs provides improved comfort compared to their transtibial counterparts. In this regard, TFAs exhibiting consistent and functionally interesting iPS and being able of ambulating 500 meters without assistive devices will be selected to examine the potential modulation of iPS through socket modifications and to evaluate the subsequent effects on both perceived and objective gait quality during phase 4. The optimal material and technique for inducing iPS without compromising both short and long-term comfort will be addressed through a collaborative investigation involving the multidisciplinary team at the Louis Pierquin Center of the IRR in Nancy and the project partner company Proteor®. To define the characteristics of the required modification, material properties will be combined with geometric considerations to identify the most effective pairing for eliciting iPS without causing pain or discomfort in the residual or phantom limb. Once the material will be selected, a localized pressure modification will be implemented by adding a localized thickness within the socket at zones identified from the stable iPS zones. These interventions will generate two experimental conditions: walking with their usual prosthesis and walking with the addition of focal thickness.

This phase will commence with a non-invasive, painless assessment of pressure distribution within the participant's habitual socket using a thin pressure sensor array (Mescan®), specifically adapted for this application. Based on pressure
data and the participant's iPS stimulation map, socket modifications will be tailored to induce iPS, or enhance existing iPS, and to assess whether these sensations are perceived during walking as helpful, neutral or disturbing. Particular attention will be paid to maintaining participant comfort and to avoiding the induction or exacerbation of phantom sensations perceived as unpleasant or painful during ambulation.

#### 2.4.4.2 Impact assessment

Gait analysis will be conducted in both laboratory and ecological environments following the previously described protocol. Evaluations will be performed under two experimental conditions: (1) usual socket without modification, (2) usual socket with focal thickness addition. For each condition, participants will provide detailed descriptions of their PS and their interactions with gait, using structured interviews and VAS. Cognitive load will be assessed using the NASA-TLX questionnaire and EEG, while perceived exertion will be measured via the Borg scale. Finally, each participant will be asked to identify the condition they perceive as most facilitating for walking. Moreover, a precise description of what helps or disturbs the participant gait will be requested in order to personalize the global analysis of the kinematic and kinetic data.

To ensure participant safety and socket comfort throughout the study, all procedures will be conducted in collaboration with an experienced prosthetist.

## 2.5 Undesirable event

Every session is conducted under the supervision of a medical doctor and a physical therapist. The experimental condition is immediately terminated in the event of pain exceeding a score of 3/10 (where 0 indicates no pain and 10 indicates unbearable pain), exacerbation of phantom limb pain, risk of falling during the ecological condition, intense exertion (score greater than 6 on the 0-10 Borg scale),
459

or any undesirable event. A report is subsequently submitted to the Ethics Committee (CPP) as required by regulations.

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461

## 3. Trials Monitoring

462

All study investigators will meet every six months to present progress updates for each site and report any issues encountered. These meetings intend to establish objectives for the upcoming six-month period. Furthermore, they will facilitate the centralization of all data collected to date.

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## 4. Data analysis

467

For all types of obtained data, data processing and statistical analysis will be respectively conducted using Matlab software (version R2023b) and R (version 4.5.0). The normality of distributions will be checked by the Shapiro-Wilk test and the homogeneity of variances (if necessary) by the Levene test. The thresholds of significance for these two tests will be set at p = 0.05. In case of multiple comparisons of data, a Bonferroni correction will be applied.

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473

### 4.1 Data from semi-structured interviews

474

The data from semi-structured interviews will be analyzed in a descriptive way. The prevalence of PS (both sPS and iPS) will be calculated for the entire population. The characteristics of PS will then be described by their location, nature and intensity, frequencies of appearance and their perceived link with walking (following Bachini et al 2024 [21]).

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Statistical comparisons between the demographic and clinical characteristics and the fitting data of the four groups of amputees will be made using analysis of variance (ANOVA) for continuous variables and contingency tables with the Chi² test for categorical variables. Correlations (Pearson or Spearman coefficient depending on

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the normality or not of data) will be performed to evaluate the links between the intensity of PS and their perceived usefulness for walking, evaluated by a VAS.

## 4.2 Data from gait analysis

Marker trajectories and kinetic data from the Loadsol Force/Footings platforms will be low-pass filtered using a 4th-order Butterworth filter with a cutoff frequency of 10 Hz. Then, the signals are down sampled to 100 data points to match the percentage of the gait cycle [36,54,55]. The first and last gait cycle of each passage will be excluded from analysis. The start and end of each gait cycle will be determined from kinetic data of the force platforms and Loadsol soles, allowing identifying heel strike and toes detachment moments. For each gait cycle, all the variables studied are described in Table 1 and will be detailed here-under.

<table><caption>**Table 1. Complete list of measured variables**</caption><thead><tr><th>Phases</th><th>Measured/calculated variables</th></tr></thead><tbody><tr><td rowspan="5">QMA</td><td>Hip, knee and ankle joint moments, vertical ground reaction force</td></tr><tr><td>Hip, knee and ankle joint angles, spatiotemporal parameters, asymmetry</td></tr><tr><td>Trunk acceleration</td></tr><tr><td>Position of the center of mass and symmetry index</td></tr><tr><td>Composite scores (FAPS and EGVI)</td></tr><tr><td>Cognitive load</td><td>NASA-TLX (score/dimension and RTLX), EEG</td></tr><tr><td>Perceived effort</td><td>Borg scale (Score 0 – 10)</td></tr><tr><td>Perceived SF/walking interaction</td><td>VAS (Score -5 - 5)</td></tr></tbody></table>

FAPS : Functional Ambulation Performance Score ; EGVI : enhanced Gait Variability Index ;

NASA-TLX : Nasa Task Load Index ; RTLX : Raw Task Load Index ; EEG :

Electroencephalographic activity ; VAS : Visual Analog Scale.
Joint angles will be calculated by the Vicon Nexus software (version 2.12, Vicon 498
Motion Systems Ltd., Oxford, UK) from the trajectory of the lower limb markers. The 499
range of motion of the joints will be defined as the difference between the minimum 500
and the maximum amplitude of the articular angles during the gait cycle [56]. The 501
moments of joint forces will be calculated from the kinetic data of the Loadsol 502
force/footings platforms and kinematic data by the Vicon Nexus software. The position 503
of the centre of mass is determined from data from force platforms and Loadsol 504
footings [35,36,54, 57]. Composite scores [58-61] and spatial-temporal parameters 505
(walking speed, step length, cadence, phase duration) are calculated from kinematic 506
data collected by the Vicon system [62-65]. 507

EEG data will be preprocessed by applying a bandpass filter (1–300 Hz) to 508
attenuate power line noise and other irrelevant frequency components. Subsequently, 509
independent component analysis will be performed to identify and remove common 510
artifacts, such as eye blinks and muscle activity, and to isolate components of neural 511
origin [66,67]. The cleaned EEG traces will then be band-pass filtered at different 512
frequency ranges in order to extract cognitive load (see [46] for an overview of different 513
EEG methods). 514

Spatial-temporal parameters as well as the Borg scale scores will be compared 515
between the four participants groups and the control group via a one-factor ANOVA. 516
Intra-individual comparisons will also be conducted during Phases 2 and 4. In Phase 517
2, this analysis will be restricted to participants exhibiting PS during walking, comparing 518
the spontaneous walking condition to the walking condition with specific attention to 519
PS. Phase 4 will involve a comparison between the habitual prosthesis condition and 520
the modified prosthesis condition. The choice of statistical test will be either a 521
dependent t-test or a Wilcoxon signed-rank test, contingent upon data distribution. Two 522
composite scores (Functional Ambulation Performance Score (FAPS) and Enhanced Gait Variability Index (EGVI)), which give an idea of the functional quality (FAPS) and the variability (EGVI) of walking, will complete the analysis of the spatial-temporal parameters. The FAPS is based on the mean and standard deviation of five PST, yielding a score ranging from 30 ("highly pathological" gait) to 100 ("healthy" gait) [59]. The EGVI is calculated from the absolute difference between five PST values of consecutive steps; it varies around 100, and a score between 95 and 105 is considered indicative of "healthy" variability [60]. A one-factor ANOVA will compare the RTLX and the scores for the sub-dimensions of NASA-TLX including the "embodiment". A correlation will also be sought between the FAPS/EGVI scores and indices and the score obtained on the VAS. This analysis will utilize either Pearson's or Spearman's correlation coefficient, depending on the data distribution (i.e., normality).

A personalized analysis of kinematic and kinetic data is carried out based on descriptions of the elements that helped or disturbed walking for each individual participant. For instance, participants mentioning phantom sensations in the toes during take-off, might show a higher propulsion peak then those not perceiving this. Descriptions of the impact of phantom sensations on walking being varied, this phase will enable a personalized analysis adapted to the nature of the improvement perceived by the person.

To analyze kinematic and kinetic data, we will use the Statistical Parametric Mapping or Non-Parametric Mapping approach depending on the distribution of the data. This methodology, already used in studies on people with lower limb amputations [68] allows analyzing whole time curves during the gait cycle rather than isolated points or averaged parameters.
## 4.3 Data management and availability

The study will be conducted in accordance with the provisions of law n°78-17 of 6 January 1978 relating to data processing, files and freedoms amended by law n° 2018-493 of 20 June 2018 on the protection of personal data, and European regulation n 2016/679 (GDPR). The reference methodology for the processing of personal data carried out in the context of biomedical research used for this study is the MR-003 methodology. Personal data processing will be implemented to allow the analysis of results, under conditions ensuring their confidentiality.

The participants will be identified during the study by a code to ensure their anonymity during data processing and interpretation. Data is locally securely stored on a password-protected computer database, in a dedicated secured online platform managed by Aix-Marseille University. Any significant changes to the protocol during this study will be reported to the trial registry. The anonymous research data will be made available to the public (online repository) when the study is completed and published. Results will be disseminated through academic products (e.g., national and international conferences and publications), as well as continuing professional development workshops and at least one general public meeting. We estimate the end of recruitment and data collection for early 2028. All results are expected by the end of 2028.

## 4.4 Data attrition

Data collected across the various phases (1, 2, 3, and 4) will be analyzed provided that participants complete the respective phase in full. Participants reserve the right to discontinue the study at any time. Should they choose to withdraw, they may either grant permission to use the data already collected or decline. Based on this
decision, only data from fully completed phases will be included in the analysis,
whereas data from partially completed phases will be discarded.

## 5. Discussion

Our approach aims to enhance the understanding of the characteristics of the PL, its interaction with locomotion, and the potential to modulate PS through iPS zones in a large population and thereby influence gait. This work both builds on and extends the body of research established by Bachini et al. (2022, 2024) [10,21].

Specifically, our method proposes the natural restoration of limb perception during dynamic activities via iPS, representing a significant advancement over current sensory substitution techniques or invasive solutions for two reasons. First, in contrast to these substitution approaches, which suffer from limited spatial and/or temporal resolution in their sensory feedback [13,40]. Our strategy applies localized stimulation based on each participant's iPS mapping, providing individualized feedback. Second, and more importantly, the intuitive nature of the phantom sensation and its localization into the phantom limb, generated by our method, eliminates the need for extensive learning to associate stimulus type with its localization on the PL. This natural integration is expected to substantially reduce the post-amputation rehabilitation duration, as patients will not require a cognitively demanding adaptation phase. Even during early rehabilitation stages, this approach may decrease cognitive load during walking compared to traditional methods.
low back pain. Concurrently, disuse-related osteopenia or osteoporosis in the residual limb increases the risk of pathological fractures [4,69]. The natural somatosensory feedback obtained by the use of phantom sensations will mitigate these medium- and long-term complications associated with gait asymmetry. Furthermore, our approach could enhance current understanding of gait analysis in amputees by integrating perception-based assessments.

Beyond biomechanical and cognitive benefits, this work may contribute to changing societal and clinical perceptions of the non-painful phantom limb. Historically regarded as a taboo or simply as a symptom to be eliminated, phantom sensations will benefit from a deeper understanding of their mechanisms, causes, and potential utility—ultimately encouraging more open discussions between healthcare professionals and patients during rehabilitation. Altogether, the present study is expected to simplify rehabilitation processes while significantly enhancing patients’ quality of life, prosthesis acceptance, and long-term functional independence.

610
# 6. Supporting Information

**S1 Fig 1. Spirit Table.** (TIFF) For the registration, intervention and evaluation schedule. * : Reassessment of the group after completion of the gait analysis. T0 correspond to the inclusion, t1 to t4 correspond to phase 1 to 4. Abbreviations are explained page 3. X* : means that the participants can be reassigned in a different group depending on how they feel during the gait evaluation.

**S2 File. SPIRIT checklist.** (DOCX)

**S3 File. Protocol validated by the ethics commission** (DOCX)

**S4 File. Interview guide** (DOCX)

# 7. Acknowledgements

The study include three medical centers: Regional Institute of Physical Medicine and Rehabilitation (IRR UGECAM), Louis Pierquin Center Nancy, Clinique Chantecler Marseille and Fondation Saint Hélier Rennes. We thank Oryane Charbonnier for her work on the iPS mapping.

# 8. Authors' contributions

**Conceptualization:** Jozina De Graaf, Amélie Touillet

**Data curation:** Jozina De Graaf, Emmanuel Jamin, Jonathan Pierret, Amélie Touillet

**Funding acquisition:** Jozina De Graaf

**Methodology:** Jozina De Graaf, Jonathan Pierret, Emmanuel Jamin, Amélie Touillet

**Project Administration:** Jozina De Graaf, Jonathan Pierret

**Resources:** Funding by the French National Agency of Research [ANR-23-CE17-0067-01].

**Supervision:** Jozina De Graaf, Amélie Touillet

**Writing – original draft:** Emmanuel Jamin

**Writing – review & editing:** Jonathan Pierret, Jozina De Graaf, Amélie Touillet, Pierrick Herbé.
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<table><thead><tr><th rowspan="3">Phases of the study</th><th colspan="6">Study procedure</th></tr><tr><th>Recruitment</th><th>Allocation</th><th colspan="4">Post-attribution</th></tr><tr><th>t0</th><th>t1</th><th>t2</th><th>t3</th><th>t3.2</th><th>t4</th></tr></thead><tbody><tr><td><strong>RECRUITEMENT :</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Inclusion/Exclusion criteria</td><td>✓</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Selection t1 - t3</td><td>✓</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Selection t4</td><td></td><td></td><td></td><td>✓</td><td>✓</td><td></td></tr><tr><td>Informed consent t1 and t2</td><td>✓</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Informed consent t3</td><td></td><td>✓</td><td></td><td></td><td></td><td></td></tr><tr><td>Informed consent t4</td><td></td><td></td><td></td><td>✓</td><td>✓</td><td></td></tr><tr><td>Group allocation (1 - 5)</td><td></td><td>✓</td><td>✓*</td><td></td><td></td><td></td></tr><tr><td><strong>INTERVENTIONS :</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Group (1)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Group (2)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Group (3)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Group (4)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Group (CTRL)</td><td></td><td></td><td>✓</td><td></td><td></td><td></td></tr><tr><td><strong>EVALUATIONS</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Epidemiological data</td><td></td><td>✓</td><td></td><td></td><td></td><td></td></tr><tr><td>Spatio-temporal parameters</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>Tri-dimensional gait analysis</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>FAPS</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>eGVI</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>NASA-TLX</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>VAS of perceived interaction PS/walk</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>Borg</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>EEG</td><td></td><td></td><td>✓</td><td></td><td></td><td>✓</td></tr><tr><td>Exploration of iPS areas</td><td></td><td></td><td></td><td>✓</td><td>✓</td><td></td></tr><tr><td>Modification of the socket</td><td></td><td></td><td></td><td></td><td></td><td>✓</td></tr></tbody></table>
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**Supporting Information**

S2 SPIRIT checklist.docx
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**Supporting Information**

S3 Protocol validated by the ethics commision.docx
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**Supporting Information**
S4 Interview guide.docx