Validity and reliability of physical activity measures in multiple sclerosis

Purpose The purpose was to evaluate the psychometric properties of physical activity measures in persons with multiple sclerosis (PwMS). Methods Adults with multiple sclerosis were recruited, n = 30 (validation) and n = 57 (test-retest). Steps measured with PiezoRX, Yamax SW200 and ActiGraph GT9X Link (AGlink) and time in different positions measured with AGlink were validated against data from video analysis. Psychometric properties of the Physical Activity and Disability Survey - Revised Swedish version (PADS-R(Sw)) was evaluated. Results The most valid measures were AGlink using the low-frequency extension filter, and PiezoRX with median absolute percentage errors (MeAPEs) of 0.9-3.1% and 1.3-3.3%. The MeAPEs were higher for Yamax SW200 (2.9-21.0%), AGlink display (3.6-44.8%) and AGlink normal filter (8.9-48.9%), indicating low validity. AGlink was not valid in measurements of sitting (MeAPE 12.0-12.5%) or lying (MeAPE 31.0-41.7%). The correlation between PADS-R(Sw) and AGlink steps was r = 0.492 (p = .009). The relative reliability of PADS-R(Sw) was ICC2,1 0.85 (CI 0.76-0.91), and absolute reliability was SEM 0.54. Conclusion AGlink and PiezoRX were valid measures of steps in PwMS. The questionnaire PADS-R(Sw) was valid, with high relative reliability, but its absolute reliability was unsatisfactory.