Development of a Mixed-Reality Application for Volitional Step Training in People With Multiple Sclerosis: Cross-Sectional Qualitative User-Centered Co-Design Study

Background People with multiple sclerosis (MS) are at high risk of falls, with approximately half experiencing at least 1 fall within 6 months. Multidirectional step training has shown potential for fall prevention, but evidence in people with MS remains inconsistent. Mixed reality (MR) offers adaptive and home-based rehabilitation opportunities. However, successful implementation of MR-based serious games requires structured co-design processes integrating patient, clinician, and technical perspectives. Objective This qualitative study describes the co-design and prototyping of a home-based MR serious game called Head Your Limits for volitional step training in people with MS, aiming to identify patient-, clinician-, and technology-driven design requirements and translate them into an MR rehabilitation application. Methods We implemented a cross-sectional qualitative user-centered co-design process. Phase 1 involved 2 online focus groups with people with MS to identify experiential and design preferences related to MR-based training. Phase 2 included a focus group with 4 clinicians and 1 engineer to define clinical and technical requirements. Phase 3 consisted of in-person hands-on testing of 2 MR prototypes (“clock” and “grapes”), developed in Unity and deployed on Meta Quest 3. Twenty people with MS were purposively screened from among individuals referred for motor rehabilitation; 14 participated in the online focus groups and 9 in in-person prototype testing (mean age 56.2, SD 7.0 years; mean Expanded Disability Status Scale score 5.1, SD 1.3; 6/9, 66.7% female). Iterative refinement followed each phase, translating stakeholder-derived requirements into architectural modifications of the MR application. Focus group recordings and moderator notes were reviewed to identify themes and design requirements guiding iterative prototype refinement. Results Five primary patient-derived requirements emerged: multisensory feedback, gamification, progress tracking, visual accessibility, and ergonomic comfort. Clinicians emphasized adaptive difficulty, cognitive-motor integration, biomechanical validity, and caregiver compatibility. Prototype comparison showed that floor-level targets induced cervical discomfort and postural compensation, whereas auditory guidance reduced head flexion. The final application repositioned targets at eye level and implemented head-mounted display tracking to ensure appropriate center-of-mass displacement, preserve proprioceptive engagement, enable caregiver proximity, and integrate gamified feedback and performance reporting. Conclusions Iterative stakeholder engagement resolved key design tensions related to ergonomics, sensory reliance, biomechanical validity, and motivation. The transition to eye-level, head-mediated interaction emerged as a core solution integrating therapeutic and user-experience goals. Through a rigorous user-centered co-design process, aligned with contemporary human-centered development models, we developed an MR serious game for volitional step training in people with MS. This study contributes to the field by providing a transparent methodological framework integrating biomechanical validity, usability, and engagement. Although clinical effectiveness remains to be tested, these findings may inform the design of accessible, personalized home-based rehabilitation for people with MS and other neurological populations.