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.