Fitbit as an activity monitor in idiopathic inflammatory myopathy: results from a real-world cohort

Idiopathic inflammatory myopathies are systemic autoimmune diseases that can be associated with debilitating muscle weakness and significant limitations in daily activities. Physical activity monitors (PAM) are recognised measures of the frequency and intensity of physical activities. The aim of this study is to investigate the psychometric properties and performance of Fitbit® as a wearable activity tracker in IIM.

Myositis Patient Centred Tele-Research (MyPACER) is a multi-centre observational prospective study conducted over 6 months recruiting patients with dermatomyositis (DM), necrotising myopathy (NM) and polymyositis (PM). The study had two cohorts, a Tele-Research Cohort (TRC, remote enrolment) and a Centre-Based Cohort (CBC, in person enrolment). Functional and patient-reported outcome measures were completed monthly. Participants were asked to use their wrist-worn Fitbit® for 7 consecutive days per month. Average daily steps per minute and average peak 1-minute cadence were evaluated as PAM measures.

A total of 120 IIM patients (mean age 55.5±13.43; 75% females; 81% White) were enrolled; 82 in the TRC group and 38 in the CBC group. There were 51% DM, 39% PM, and 9% NM. The TRC and CBC cohorts were similar in demographics and disease subtypes. Gender, race/ethnicity, or disease subtypes were not associated with PAM measures. Compliance with Fitbit® wear protocol was very high, with similar results for remote or local recruitment. Average steps/min and average peak 1-minute cadence showed strong test-retest reliability [r=0.89 (p<0.0001 and r=0.86 (p= 0.0001)]. A longitudinal significant positive correlation was found between physical activity metrics and patient-reported and functional measures.

In a large IIM cohort, Fitbit® PAM variables demonstrate favourable compliance and psychometric properties with strong test-retest reliability and validity. PAMs can complement current measurements to remotely track patient performance, quality of life, and disease activity.