Non-equivalent, but still valid: Establishing the construct validity of a consumer fitness tracker in persons with multiple sclerosis

Abstract

Tools for monitoring daily physical activity (PA) are desired by persons with multiple sclerosis (MS). However, current research-grade options are not suitable for longitudinal, independent use due to their cost and user experience. Our objective was to assess the validity of step counts and PA intensity metrics derived from the Fitbit Inspire HR, a consumer-grade PA tracker, in 45 persons with MS (Median age: 46, IQR: 40–51) undergoing inpatient rehabilitation. The population had moderate mobility impairment (Median EDSS 4.0, Range 2.0–6.5). We assessed the validity of Fitbit-derived PA metrics (Step count, total time in PA, time in moderate to vigorous PA (MVPA)) during scripted tasks and free-living activity at three levels of data aggregation (minute, daily, and average PA). Criterion validity was assessed though agreement with manual counts and multiple methods for deriving PA metrics via the Actigraph GT3X. Convergent and known-groups validity were assessed via relationships with reference standards and related clinical measures. Fitbit-derived step count and time in PA, but not time in MVPA, exhibited excellent agreement with reference measures during scripted tasks. During free-living activity, step count and time in PA correlated moderately to strongly with reference measures, but agreement varied across metrics, data aggregation levels, and disease severity strata. Time in MVPA weakly agreed with reference measures. However, Fitbit-derived metrics were often as different from reference measures as reference measures were from each other. Fitbit-derived metrics consistently exhibited similar or stronger evidence of construct validity than reference standards. Fitbit-derived PA metrics are not equivalent to existing reference standards. However, they exhibit evidence of construct validity. Consumer-grade fitness trackers such as the Fitbit Inspire HR may therefore be suitable as a PA tracking tool for persons with mild or moderate MS.

Feasibility and scalability of a fitness tracker study: Results from a longitudinal analysis of persons with multiple sclerosis

“…Sufficient device wear time, defined as at least 10 h of wear time between 6:00 a.m. and 11:00 p.m., was computed and compared before and after rehabilitation stay discharge (Supplementary Appendix Table AS2). Details on the further processing of the PA tracker data in the BarKA-MS study can be found elsewhere (26).…”

“…In addition, the study participant wore an Actigraph GT3X (Manufacturing Technology, Inc., FL, USA), a threedimensional accelerometer validated for PwMS (28,29), on their non-dominant hip during their last week of rehabilitation and the first week back home. These data were published elsewhere (26,30).…”

“…In light of these scalability challenges, we developed the Barriers to physical activity in people with MS (BarKA-MS; https://clinicaltrials.gov/ct2/show/study/NCT04746807) fitness tracker study to explore barriers to PA among people with MS (PwMS) who returned home after an inpatient rehabilitation stay. The primary and secondary endpoints of the BarKA-MS study (analyzed elsewhere) explored common barriers to PA among PwMS and investigated the quality, reliability, internal consistency, and validity of PA metrics derived from a consumer-grade wearable device (26). A further analysis concerns the evaluation of the impact of inpatient rehabilitation on walking ability, PA and the perception of obstacles to PA, selfefficacy, fatigue, depression, pain, and health-related quality of life (Sieber et al, unpublished data, 2022).…”

“…Barriers to Health Promoting Activities for Disabled Persons Scale (score range 18-72), median (IQR)(26)(27)(28)(29)(30)(31)(32) International Physical Activity Questionnaire-Short Form, median (IQR) of total active minutes per day in the last seven days 155 (90-240)…”

Exploring the Major Barriers to Physical Activity in Persons With Multiple Sclerosis: Observational Longitudinal Study

“…Activity sensors and Fitbit devices in particular have seen increasing adoption in MS research over the past years [ 11 ]; for example, such devices have been used to reduce sedentary behavior in persons with MS [ 12 ] or for remote monitoring of MS disability [ 13 ]. Despite the lower accuracy of Fitbit sensors at lower activity intensity [ 14 ] and slower walking speed [ 15 - 18 ], particularly relevant in the case of persons with MS, earlier studies have demonstrated the validity of Fitbit sensors in measuring step count [ 19 - 21 ]. These sensors enable individualized, passive, and inconspicuous monitoring of various metrics, including PA intensity and step counts, over an extended period of time [ 22 , 23 ].…”