Validity of Smartphones and Activity Trackers to Measure Steps in a Free-living Setting over Three Consecutive Days.

OBJECTIVE: This study assessed the validity of a consumer activity wristband, a smartphone, and a research-grade accelerometer to measure steps in a free-living setting. APPROACH: Thirty healthy adults were equipped with two Garmin Vivofit (non-dominant wrist), one iPhone SE (right pants pocket), three ActiGraph wGT3X+ (two on the hip, one on the non-dominant wrist), and one StepWatch (right ankle) and instructed to wear the devices continuously during a 3-day monitoring period. All activities of daily living were recorded in 15-minute intervals in a diary. The StepWatch served as the criterion method and validity was evaluated by comparing each device with the criterion measure using mean absolute percentage errors (MAPE). MAIN RESULTS: The MAPE for the total step count during the 3-day monitoring period was high with a general underestimation of steps by all devices of >20% compared to the criterion measure. The wrist-worn ActiGraph markedly overestimated steps during predominantly low active (public transport or driving, and standing) or even inactive (sitting and lying) activities of daily living. SIGNIFICANCE: The overall high MAPE of the devices compared to the StepWatch during step-based activities, likely caused by inaccuracies during short and intermittent bouts of activity, may limit their validity in a free-living setting.