Étude de l’impact biomécanique et perceptuel d’une incarnation dans un avatar possédant une déformation asymétrique des membres inférieurs en réalité virtuelle

Abstract

Immersive virtual reality enables users to embody avatars with altered anthropometry, potentially modifying how they perceive their own bodies. Numerous studies have shown that users can experience a strong sense of embodiment even when the avatar’s morphological proportions differ from those of their real body. This plasticity highlights the adaptability of internal body representations, including body image and body schema, in response to sensory signals. Because the body schema plays a central role in the planning and execution of movement, a deformed virtual morphology could influence motor behavior. However, the current literature provides limited insight into how sensorimotor integration accommodates a localized morphological deformation of the virtual body, particularly in the lower limbs. To address this gap, this dissertation investigates the perceptual, attentional, and biomechanical consequences of altering a specific segment of the virtual body. Across three experimental studies, we developed protocols combining various kinematic and kinetic capture systems within a Unity 3D application that automated data collection and the application of virtual body deformations. The main goal of this work is to examine how aprecise morphological alteration of the self-avatar body affects internal body representations and the resulting motor responses. The first study assessed how embodying an avatar with an enlarged leg influences movement preparation and execution during gait initiation, a task that strongly relies on multisensory integration. Thirty participants embodied an avatar featuring either an enlarged swing leg, an enlarged stance leg, or two non-deformed legs. Center of mass, center of pressure, and margin of stability were measured, while embodiment was assessed using a validated questionnaire. No significant differences were found in biomechanical variables related to dynamic stability. The lack of effect may reflect high interindividual variability, possibly due to differing interpretations of the deformation. The second study focused on a different type of morphological alteration by lengthening one virtual leg. Twenty-five participants initiated gait with a lengthened swing leg, a lengthened stance leg, or two non-deformed legs. Biomechanical analyses revealed an initial shift of the center of pressure toward the non-deformed leg, which altered subsequent movement amplitudes, trajectories, dynamic stability, and lateral foot placement. Embodiment remained comparable across conditions, indicating that users can accept a substantial deformation as long as visuomotor and visuotactile cues remain congruent. This study shows that virtual leg lengthening directly influences the body schema, with biomechanical effects similar to those observed in hemiparetic patients during gait initiation, suggesting potential applications in rehabilitation. The third study examined the perceptual and attentional consequences when lengthening the leg. This study focused on proprioception, embodiment, and visual scanning, measured through state gaze entropy and gaze transition entropy. High or low embodiment levels were induced through congruent or incongruent visuotactile stimulation, respectively. Across the twenty six participants, embodiment remained high despite the deformation, whether applied to the dominant or non-dominant side. A marked proprioceptive drift, corresponding to 78% of the virtual elongation, was observed regardless of visuotactile congruence. While the leg deformation did not affect visual entropies, incongruent visuotactile stimulation reduced both embodiment and visual entropies, indicating a modulation of top-down signals involved in gaze control. Together, these results show that even substantial deformations of the virtual lower limbs can be incorporated into body representations without disrupting multisensory integration. Virtual reality thus provides a powerful means to reshape the body schema, influence the subsequent movements and alter the proprioceptive integration by manipulating avatar morphology. These f indings open promising perspectives for rehabilitation, particularly for patients exhibiting proprioceptive deficits or motor asymmetries.

Date	2 Apr 2026
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	David Labbé (Supervisor) & Rachid Aissaoui (Co-supervisor)