Multiple sclerosis (MS) is a demyelinating and neurodegenerative disease of the central nervous system, with common symptoms including muscle weakness, balance impairments, gait difficulties, and fatigue. These symptoms significantly affect individuals' functional mobility, independence, and quality of life. In individuals with MS, reduced lower extremity muscle strength leads to decreased walking speed and loss of balance, resulting in substantial limitations in activities of daily living.

Conventional clinical assessment tools, such as the Timed Up and Go (TUG), Five-Repetition Sit-to-Stand Test (5xSTS), and the Timed 25-Foot Walk (T25FW), have been widely used for many years to evaluate functional mobility in individuals with MS. However, these tests typically rely on unidimensional measurement parameters and may demonstrate subjective variability depending on the testing environment and the examiner's experience. Advances in measurement technologies now allow these clinical tests to be transformed into more objective, standardized, and interactive assessment tools.

Virtual reality (VR) provides a new perspective for the assessment of motor performance by enabling three-dimensional and interactive simulations of real-world scenarios. VR-based applications allow for the simultaneous evaluation of both motor and cognitive components while also increasing individuals' motivation and engagement. Previous studies have demonstrated that VR technology is effective in neurological rehabilitation, particularly in improving balance, walking speed, and functional mobility in the MS population. In contrast to VR, mixed reality (MR) is a technology that allows individuals to maintain interaction with the real environment while integrating virtual objects into the physical space.

The existing literature reveals a notable gap in research focusing on the validity and reliability of VR or MR-based functional tests in individuals with multiple sclerosis. However, no studies to date have investigated the adaptation of lower extremity functional tests (TUG, 5xSTS, and T25FW) to a mixed reality environment. Therefore, the primary rationale of this study is to evaluate the digital applicability of commonly used functional tests in individuals with MS on the Meta Quest 3 virtual reality platform and to determine the validity and reliability of these tests. Additionally, this study aims to examine the relationships between data obtained from the virtual tests and fatigue (Modified Fatigue Impact Scale, MFIS), walking performance (12-Item Multiple Sclerosis Walking Scale: MSWS-12), and knee extensor muscle strength in order to establish the convergent validity of this novel approach.

The findings of this study are expected to contribute to the development of an objective, standardized, safe, and innovative digital assessment tool for functional evaluation in individuals with multiple sclerosis. Furthermore, the results will strengthen the scientific foundation for remote monitoring of the rehabilitation process and for the development of personalized treatment programs.

Inclusion Criteria:

• A confirmed diagnosis of multiple sclerosis (MS) established by a neurologist,
• Not being in a relapse phase and having no MS relapse within the past 6 months,
• Aged between 18 and 60 years,
• Disease severity within an Expanded Disability Status Scale (EDSS) range of 1-5 (mild to moderate functional impairment),
• Visual, auditory, and vestibular functions sufficient to tolerate mixed reality-based applications,
• Adequate cognitive capacity to actively participate in the study (e.g., Mini-Mental State Examination score ≥ 24),
• Ability to read, understand, and follow instructions in Turkish,
• Willingness to participate voluntarily in the study and provision of written informed consent.

Exclusion Criteria:

• Having experienced a multiple sclerosis relapse or undergone a change in MS-related treatment within the past 6 months,
• Presence of severe musculoskeletal disorders (e.g., fracture, arthritis, advanced osteoarthritis, muscle tear) that may interfere with test performance,
• Presence of severe cardiopulmonary disease (e.g., uncontrolled hypertension, heart failure, chronic obstructive pulmonary disease) or any medical condition that may pose a risk during exercise testing,
• Risk of loss of balance or spatial disorientation in the mixed reality environment due to visual, vestibular, or auditory impairments,
• Inability to perform functional tests due to severe spasticity, contractures, or orthopedic deformities,
• History of epilepsy, migraine, or other neurological conditions that contraindicate the use of mixed reality or virtual reality devices,
• Inability to understand test instructions due to cognitive impairment or severe mental health disorders (e.g., dementia, major depression, psychotic disorders),
• Experiencing dizziness, nausea, or virtual reality-related symptoms (VR sickness) during testing,
• Insufficient motivation or poor compliance that may hinder participation in the study.

Individuals with MS who applied to the Selçuk University Faculty of Medicine Hospital MS Living Center