Quantifying movement reserve in multiple sclerosis via diurnal activity quantiles.

Background: Conventional clinical assessments in multiple sclerosis (MS), such as the Expanded Disability Status Scale (EDSS), often miss subtle functional changes. While accelerometry provides an objective measure of real-world motor activity, most daily summaries focus on average values, neglecting both peak performance and its variability throughout the day. This diurnal peak performance variability may reflect a person's capacity to sustain high-effort activity despite fatigue, a phenomenon we term observable movement reserve. Objective: To evaluate whether upper diurnal activity quantiles derived from accelerometry data quantify observable movement reserve, and to examine their association with EDSS-measured disability in MS. Methods: In a cohort of 248 adults with MS (mean age 54.8 years, 71% female; EDSS range 0-6.5), continuous wrist accelerometry was collected over two weeks. We used novel scalar-on-function regression (SOFR) to compare several diurnal activity characteristics: mean, variability, and 50th-100th percentiles. SOFR models adjusted for age, sex, and BMI, and their cross-validated Results: The upper diurnal activity quantiles, particularly the 95th to 100th percentiles, demonstrated the strongest association with EDSS, outperforming both diurnal mean and variability-based diurnal activity curves (cross-validated Conclusion: Diurnal peak activity provides a sensitive, time-of-day-specific measure of observable movement reserve that closely aligns with EDSS-measured disability. This reserve fluctuates across the day, likely reflecting circadian patterns of energy and fatigue. By capturing both the timing and intensity of peak activity, the proposed metrics offer a clinically meaningful tool for monitoring functional change over time and may enhance the ability to track disease progression in MS.