Validation of heart rate measured by the consumer-level wearable device Fitbit: a cross-sectional observational study

[Purpose] The present study aimed to validate the heart rate of the consumer-level wearable device Fitbit Inspire 3. [Participants and Methods] Participants were 22 healthy university students (20.9 ± 0.6 years, 11 males and 11 females). They performed 5-minute resting lying, sitting, walking (3.2 km/h, 4.2 km/h, 6.0 km/h and 3.2 km/h with 3 kg backpack) and running (8.4 km/h running) on a treadmill, and stair climbing, with Fitbit- and electrocardiogram-based heart rate (Fitbit-HR and ECG-HR). The relationship between Fitbit-HR and ECG-HR was evaluated using a generalized linear mixed model. Additionally, Brandt-Altman analysis and absolute percentage error were used to examine the measurement error. [Results] Fitbit-HR was strongly associated with ECG-HR and oxygen uptake. In the Brandt-Altman analysis with Fitbit-HR and reference ECG-HR, most plots were within the limit of agreement. Fitbit-HR showed significant fixed errors, which were, on average, three beats smaller than those of ECG-HR. No significant proportional errors are observed. The mean absolute percentage error for most of the tasks was less than 10%. [Conclusion] The Fitbit Inspire 3 showed a strong correlation and clinically acceptable agreement with ECG-HR in healthy young adults, despite a small, consistent underestimation. Its validity was supported by its association with oxygen uptake.