FitBit monitors exercise in myositis patients

The wearable device FitBit can provide accurate measures of physical activity among myositis patients, according to a recent study. Along with other fitness tracking devices, the FitBit can also help encourage exercise in this population.

“Our findings suggest that daily step counts and peak 1-min cadence measured by Fitbit were largely accurate when compared to the FDA-approved activity monitor, ActiGraph. Future research should include larger longitudinal cohorts of myositis patients to further evaluate Fitbit as a physical activity outcome measure,” the researchers said.

The prospective longitudinal observational study included 24 patients: 10 with dermatomyositis, eight with polymyositis or necrotizing myositis, and six with antisynthetase syndrome. To record physical activity, participants were asked to wear a waist-worn FitBit and ActiGraph for 7 days every month for 6 months.

Assessments were conducted at 0, 1, 3, and 6 months, during which the six myositis core measurements were evaluated, along with physical function, fatigue, and functional measures.

The FitBit device showed good 1-month test-retest reliability in patients with stable disease, with an intraclass correlation coefficient (ICC) of 0.89 for daily step counts and 0.86 for average peak 1-minute cadence. [Rheumatology 2021;doi:10.1093/rheumatology/keab700]

Moreover, device measures correlated with several core myositis measures. For example, FitBit average daily steps was associated positively with manual muscle testing (MMT) score (p=0.04) and with Short-Form 36 results (p=0.004), while it was linked inversely with physician- (p=0.05) and patient-reported (p=0.02) disease activity, pain (p=0.007), and fatigue (p=0.002). Similar trends were reported for average peak 1-minute cadence.

FitBit measurements were also responsive to changes in the disease. Deviations in average daily steps and peak 1-minute cadence had moderate–strong correlations with subsequent changes in physician-reported disease activity, MMT, and physical function parameters. For instance, those who showed a >5-percent improvement in MMT also saw a median 24-percent increase in average daily step count and a 7-percent improvement in average peak 1-minute cadence over 6 months.

Notably, FitBit agreed well with the FDA-approved ActiGraph for both average daily step count (ICC, 0.96) and average peak 1-minute cadence (ICC, 0.97). The two devices also measured different levels of physical activity comparably, showing the highest agreement for light activity (ICC, 0.96), followed by moderate activity (ICC, 0.73). Concordance for vigorous activity was only moderate (ICC, 0.59).

However, FitBit overestimated daily step counts in 60 percent of participants, with a median discrepancy of 301 steps. In the remaining patients, FitBit underestimated step counts by 10 percent, corresponding to a median difference of 481 steps. The device also overestimated average peak 1-min cadence in 95 percent of patients by a median of 7.5 mins.

“A major limitation of our study is the small sample size, especially as it relates to the longitudinal assessment of myositis patients whose disease activity varies over time. Thus, results from this pilot proof of concept study must be validated in larger cohorts of myositis patients,” the researchers said.