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

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 semistructured 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).