Validation of the Fitbit One for assessing physical activity

Accelerometer-based wearables can provide the user with real-time feedback through the device's interface and the mobile platforms, and this may help to encourage people to get and remain physically active. However, meaningful feedback can only be provided if such wearables validly measure physical activity levels. Very few studies have focussed on the minute-by-minute validity of wearables, which is essential for high quality real-time feedback. In a study just published online by the Journal Medicine & Science in Exercise & Sports we aimed aims to assess the validity of the Fitbit One compared to what is considered a 'gold standard' for assessing physical activity, i.e. the ActiGraph GT3x+. We looked at validity in measuring steps taken, and time spent in moderate, vigorous, and moderate-vigorous physical activity) among young adults using traditional time intervals (i.e., days) and smaller time intervals (i.e., minutes, hours).
Thirty-four healthy young adults participated and wore the ActiGraph GT3x+ and a Fitbit One for one week. As compared to ActiGraph the Fitbit One systematically overestimated physical activity for all aggregation levels: on average 0.82 steps per minute, 45 steps per hour, and 677 steps per day. Strong and significant associations were found between ActiGraph and Fitbit results for steps taken, and weaker but statistically significant associations were found for minutes spent in moderate, vigorous and moderate-vigorous physical activity for all time intervals.
We conclude that although the Fitbit One overestimates the step activity compared to the ActiGraph, it can be considered a valid device to assess step activity also for real-time minute-by-minute self-monitoring. However, agreement and correlation between ActiGraph and Fitbit One regarding time spent in moderate, vigorous and moderate-vigorous physical activity were lower.