Background: Many modern smart watches and activity trackers feature an optical
sensor that estimates the wearer’s heart rate. Recent studies have evaluated
the performance of these consumer devices in the laboratory. Objective: The
objective of our study was to examine the accuracy and sensitivity of a common
wrist-worn tracker device in measuring heart rates and detecting 1-min bouts
of moderate to vigorous physical activity (MVPA) under free-living conditions.
Methods: Ten healthy volunteers were recruited from a large university in
Singapore to participate in a limited field test, followed by a month of
continuous data collection. During the field test, each participant would wear
one Fitbit Charge HR activity tracker and one Polar H6 heart rate monitor.
Fitbit measures were accessed at 1-min intervals, while Polar readings were
available for 10-s intervals. We derived intraclass correlation coefficients
(ICCs) for individual participants comparing heart rate estimates. We applied
Centers for Disease Control and Prevention heart rate zone cut-offs to
ascertain the sensitivity and specificity of Fitbit in identifying 1-min
epochs falling into MVPA heart rate zone. Results: We collected paired heart
rate data for 2509 1-min epochs in 10 individuals under free-living conditions
of 3 to 6 hours. The overall ICC comparing 1-min Fitbit measures with average
10-s Polar H6 measures for the same epoch was .83 (95% CI .63-.91). On
average, the Fitbit tracker underestimated heart rate measures by −5.96 bpm
(standard error, SE=0.18). At the low intensity heart rate zone, the
underestimate was smaller at −4.22 bpm (SE=0.15). This underestimate grew to
−16.2 bpm (SE=0.74) in the MVPA heart rate zone. Fitbit devices detected 52.9%
(192/363) of MVPA heart rate zone epochs correctly. Positive and negative
predictive values were 86.1% (192/223) and 92.52% (2115/2286), respectively.
During subsequent 1 month of continuous data collection (270 person-days),
only 3.9% of 1-min epochs could be categorized as MVPA according to heart rate
zones. This measure was affected by decreasing wear time and adherence over
the period of follow-up. Conclusions: Under free-living conditions, Fitbit
trackers are affected by significant systematic errors. Improvements in
tracker accuracy and sensitivity when measuring MVPA are required before they
can be considered for use in the context of exercise prescription to promote
better health
