Quantifying observable movement reserve in multiple sclerosis via upper diurnal activity quantiles

Background: The ability to sustain high-effort motor activity throughout the day may serve as a marker of functional capacity in individuals with multiple sclerosis (MS). We refer to this ability as observable movement reserve. Conventional assessments, including the Expanded Disability Status Scale (EDSS), often miss subtle functional changes. While accelerometry captures 24-hour motor activity, it typically overlooks peak activity and its diurnal variability. Objective: To evaluate whether upper diurnal activity quantiles from accelerometry capture observable movement reserve and are associated with disability in MS. Methods: In 248 adults with MS (mean age 54.8, 71% female, EDSS 0–6.5), wrist-worn accelerometry was recorded over 2 weeks. Scalar-on-function regression examined associations between EDSS and diurnal activity curves: mean, variability, and 50th–100th percentiles, adjusting for age, sex, body mass index (BMI), cognition, and fatigue. Cross-validated R 2 assessed model performance. Results: Upper quantiles (95th–100th) were most strongly associated with EDSS. Late afternoon and evening activity contributed most to predictive power. Conclusion: Diurnal peak activity is a sensitive, time-specific marker of observable movement reserve that correlates with EDSS. Capturing timing and intensity, this metric may improve detection of functional decline and support more precise disability tracking in MS.