Comparison of free-living physical activity data obtained from a Fitbit Zip, the Apple iPhone Health app and a modified Bouchard Activity Record.

There were strong correlations between the physical activity data measured by Fitbit Zips and iPhones, but the iPhone Health app significantly underestimated the number of steps per day taken and the distance travelled when compared to the Fitbit Zip. SO WHAT?: Understanding the comparability of accelerometer devices provides useful information for future pragmatic physical activity measurement.