Table 2.
Overall and stratified comparisons between Fitbit and Polar of minute-by-minute epochs.
| 1-min epochs | n | Within-epoch comparison of 1-min Fitbit and average 10-s Polar heart rates (in bpm) | |||
| Intraclass correlation coefficienta
(95% CI) |
Average differenceb
(95% CI) |
P valueb | |||
| Overall | 2509 | .83 (0.63-0.91) | −5.96 (−6.33 to −5.60) | <.001 | |
| By Centers for Disease Control and Prevention heart rate zonec | |||||
| Low (<50%) | 2146 | .77 (0.55-0.87) | −4.24 (−4.53 to −3.96) | <.001 | |
| Moderate to vigorous physical activity (≥50%) | 363 | .56 (0-0.79) | −16.2 (−17.6 to −14.7) | <.001 | |
| By participant | |||||
| A | 348 | .87 (0.69-0.94) | −2.58 (−2.99 to −2.18) | <.001 | |
| B | 358 | .75 (0.50-0.86) | −3.92 (−4.55 to −3.29) | <.001 | |
| C | 169 | .70 (0.28-0.85) | −5.08 (−6.01 to −4.14) | <.001 | |
| D | 198 | .97 (0.96-0.98) | 0.25 (−1.08 to 1.58) | .70 | |
| E | 359 | .62 (0.14-0.81) | −6.40 (−7.13 to −5.67) | <.001 | |
| F | 208 | .47 (0-0.74) | −17.4 (−19.5 to −15.4) | <.001 | |
| G | 320 | .72 (0.19-0.88) | −5.05 (−5.62 to −4.47) | <.001 | |
| H | 228 | .81 (0.65-0.88) | −5.20 (−6.45 to −3.94) | <.001 | |
| I | 248 | .40 (0-0.68) | −12.3 (−13.6 to −11.1) | <.001 | |
| J | 73 | .93 (0.88-0.95) | −0.92 (−1.69 to −0.14) | .02 | |
aICCs derived using two-way mixed effects model for absolute agreement.
bStatistics derived in paired t tests.
cCalculated as percent of maximal heart rate (220 bpm—age in years) for discreet 1-min epochs drawing from the average 10-s Polar measure and age of the respective participant.