Heart Rate Equivalency of the Fitbit Charge HR During Continuous Aerobic Exercise

Abstract

Purpose: Consumer-grade wrist-worn activity monitors frequently include photoplethysmography (PPG) sensors for estimating heart rate (HR). The Fitbit Charge HR is marketed specifically for tracking fitness; therefore, HR accuracy is critical, especially during exercise. This study examined HR equivalency of the Fitbit Charge HR during continuous aerobic exercise. Method: Participants (N = 19) concurrently wore a Polar H1 and Fitbit Charge HR during a measurement visit that included seated rest (5 minutes), warm-up (5 minutes), continuous treadmill exercise (30 minutes), and cool-down (5–10 minutes). Mean HR differences were examined by protocol phase, total activity (i.e., warm-up, exercise, and cool-down combined), and the first, middle, and last 5 minutes of continuous exercise. Mean absolute percent error (MAPE), Bland-Altman plots, and 95% equivalence testing explained overall and individual HR agreement between devices. Results: The Fitbit Charge HR significantly underestimated HR for all measurement phases (all p ≤ .01) except cool-down (p > .33). HR agreement was notably weaker during warm-up (r = 0.66, d = 0.57) and differences were greatest for the first 5 minutes compared to the middle and end of exercise (6.94±2.16 beats per minute [bpm] vs. 1.76±0.59 bpm, and 1.74±0.58 bpm), F = 4.87, p = .04). Mean exercise HRs were equivalent between devices (±2.69 bpm, 95% CI: 1.41–3.97 bpm); MAPE was 1.96%. Conclusion: The Fitbit Charge HR is relatively accurate for measuring HR during continuous aerobic exercise. Whereas the accuracy of PPG-based HR appears limited at exercise onset, agreement improves throughout the exercise bout and HR differences are negligible.