Commercially available activity trackers are increasingly popular for the general public in measuring physical activity. Daily energy expenditure (EE) is one of the primary measures provided to the public by these devices. Whether these devices can accurately estimate EE is not clear.
PURPOSE: To compare the EE values obtained from two commercially available devices (Fitbit Charge and Fitbit One) to that measured by the Actigraph GT3X and indirect calorimetry (IC).
METHODS: Seventeen men (n = 4) and women (n = 13) (Age: Mean ± SD = 27.8 ± 9.0; BMI = 24.7 ± 4.2 kg/m2) completed a submaximal treadmill exercise trial while wearing a Fitbit Charge, a Fitbit One, and an Actigraph GT3X. Oxygen consumption and EE (in kcals) was measured via breath-by-breath IC system. The exercise trial consisted of four, 5-minute stages (2.0 mph/0% grade; 2.0 mph/5%; 3.0 mph/0%; 3.0 mph/5%). To approximate the EE reported by the Fitbit devices, an estimated resting metabolic rate was added to the EE reported by the Actigraph. Repeated measures ANOVA was used to asses the main effects for measurement device and stage.
RESULTS: There was a significant main effect for measurement device (P < 0.01) and stage (P < 0.01). Post-hoc analysis determined that the Fitbit Charge (P < 0.01), the Fitbit One (P < 0.01), and the Actigraph (P < 0.01), all differed from IC in measuring EE. The Fitbit Charge (P < 0.01) overestimated EE across all stages (25.3 ± 4.7, 31.0 ± 3.7, 33.0 ± 5.8 and 34.2 ± 6.1 vs 13.2 ± 2.3, 18.8 ± 3.0, 20.5 ± 2.8 and 26.9 ± 5.0 for Fitbit One and IC, respectively). The Fitbit One (14.4 ± 2.7, 15.3 ± 2.7, 19.9 ± 3.4 and 21.1 ± 4.6; P < 0.01) and the Actigraph (6.7 ± 3.8, 7.4 ± 25.4 ± 8.5 and 26.4 ± 7.9; P < 0.01) underestimated EE compared to IC across all stages. Significant interactions between IC and the Fitbit One (P < 0.01), and IC and the Actigraph (P < 0.01), however, indicate that they are less sensitive to grade changes than IC.
CONCLUSION: Results show that the wrist-worn Fitbit Charge overestimates EE, which may negatively impact the fitness goals of the wearer, particularly as it relates to energy balance and weight loss. The hip worn devices appear to be closer to IC in EE measures, but do not accurately reflect changes in treadmill grade, impacting those who use this exercise mode.