What is the Agreement Between Two Generations of Commercial Accelerometer in a Free-Living Environment for Young to Middle-Aged Adults?

Background
Rapid change in the commercial market can threaten consistency of activity data comparisons as devices are superseded. Purpose: To determine the level of agreement between two generations of Fitbit™ device for step count and activity level in a free-living environment.

Methods
Thirty-seven healthy participants (17 women, 20 men; M ± SD: age 34 ± 8 y; body mass index 25.4 ± 3.9 kg/m2) wore a Fitbit Flex™ and Flex 2™ on their non-dominant wrist over two weeks in a free-living environment. A waist-mounted ActiGraph GT3X+ was also worn to provide a comparison of step count data obtained against a commercial device.

Results
Comparison of step count between two generations of Fitbit™ device (Mean Absolute Percentage Error, 12%; Standard Error of Mean, 102.58 steps/d (p = .039); ICC = 0.955) showed closer inter-device agreement than comparison of step count data between commercial (Fitbit™) and research (ActiGraph GT3X+) grades of device (Mean Absolute Percentage Error, 31%; Standard Error of Mean, 124.6 steps/d (p < .001); ICC = 0.915). Statistically significant differences were identified for the Standard Error of Mean between generations of Fitbit™ device (p = .039) and grades of device (p < .001). A comparison of ‘fairly’ and ‘very’ active minutes showed no statistically significant difference between generations of Fitbit™ (p = .980); Mean Absolute Percentage Error, 38%; ICC = 0.908. The number of days of data captured for step count was comparable between to the two grades of device.

Conclusion
Users should be aware of potential variations in data estimates from different generations of Fitbit™ device, with step count data providing a more consistent comparison metric.