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Background  Physical activity tracking wearable devices have emerged as an increasingly popular method for consumers to assess their daily activity and calories expended. However, whether these wearable devices are valid at different levels of exercise intensity is unknown.    Objective  The objective of this study was to examine heart rate (HR) and energy expenditure (EE) validity of 3 popular wrist-worn activity monitors at different exercise intensities.    Methods  A total of 62 participants (females: 58%, 36/62; nonwhite: 47% [13/62 Hispanic, 8/62 Asian, 7/62 black/ African American, 1/62 other]) wore the Apple Watch, Fitbit Charge HR, and Garmin Forerunner 225. Validity was assessed using 2 criterion devices: HR chest strap and a metabolic cart. Participants completed a 10-minute seated baseline assessment; separate 4-minute stages of light-, moderate-, and vigorous-intensity treadmill exercises; and a 10-minute seated recovery period. Data from devices were compared with each criterion via two-way repeated-measures analysis of variance and Bland-Altman analysis. Differences are expressed in mean absolute percentage error (MAPE).    Results  For the Apple Watch, HR MAPE was between 1.14% and 6.70%. HR was not significantly different at the start ( P =.78), during baseline ( P =.76), or vigorous intensity ( P =.84); lower HR readings were measured during light intensity ( P =.03), moderate intensity ( P =.001), and recovery ( P =.004). EE MAPE was between 14.07% and 210.84%. The device measured higher EE at all stages ( P <.01). For the Fitbit device, the HR MAPE was between 2.38% and 16.99%. HR was not significantly different at the start ( P =.67) or during moderate intensity ( P =.34); lower HR readings were measured during baseline, vigorous intensity, and recovery ( P <.001) and higher HR during light intensity ( P <.001). EE MAPE was between 16.85% and 84.98%. The device measured higher EE at baseline ( P =.003), light intensity ( P <.001), and moderate intensity ( P =.001). EE was not significantly different at vigorous ( P =.70) or recovery ( P =.10). For Garmin Forerunner 225, HR MAPE was between 7.87% and 24.38%. HR was not significantly different at vigorous intensity ( P =.35). The device measured higher HR readings at start, baseline, light intensity, moderate intensity ( P <.001), and recovery ( P =.04). EE MAPE was between 30.77% and 155.05%. The device measured higher EE at all stages ( P <.001).    Conclusions  This study provides one of the first validation assessments for the Fitbit Charge HR, Apple Watch, and Garmin Forerunner 225. An advantage and novel approach of the study is the examination of HR and EE at specific physical activity intensities. Establishing validity of wearable devices is of particular interest as these devices are being used in weight loss interventions and could impact findings. Future research should investigate why differences between exercise intensities and the devices exist.

Estimating Accuracy at Exercise Intensities: A Comparative Study of Self-Monitoring Heart Rate and Physical Activity Wearable Devices