An Examination of Body Mass Index Influence on Activity Tracker Accuracy

Abstract

A variety of activity trackers with evolving technology are commercially available, yet it remains uncertain the influence body size has on these devices’ accuracy (as previous activity trackers have been), which would compromise their generalizability. PURPOSE: To examine the accuracy of consumer-grade activity trackers in quantifying steps and moderate-vigorous physical activity (MVPA) engagement in free-living conditions in a diverse sample of body mass index (BMI) categories. METHODS: Sixty individuals (21.1 ± 1.5 years, 2.3 ± 0.2 ft stride length) across three BMI categories (“Normal” [n=25]: 20-24.9 kg/m2; “Overweight” [n=22]: 25-29.9 kg/m2; “Obese” [n=13]: ≥30 kg/m2) wore four activity trackers during one 24-hour day (wear time 12.7 ± 1.9 hours). On the non-dominant side of the body, the Fitbit Charge HR (Charge) and Jawbone UP3 (UP3) were worn on the wrist and the Fitbit One (ONE) on the waist, with the NL1000 activity tracker (NL; serving as the criterion device) worn on the dominant side of the waist. Mixed between-within ANOVA analyses were performed to examine differences in steps and MVPA for the activity trackers across three BMI categories. RESULTS: There were no differences in steps or MVPA for the individual devices across BMI categories. In the Normal group, the UP3 (6667 ± 3366 steps) and ONE (7400 ± 8135 steps) underestimated steps (p=.000), compared to the NL (8135 ± 3562 steps). Similarly, in the Overweight group, the UP3 (8799 ± 3986 steps) and ONE (9019 ± 3841 steps) underestimated steps (NL 9312 ± 3986 steps, both p<.05). In the Obese group the Charge overestimated steps (8847 ± 4168, p<.05) and the ONE underestimated steps (7726 ± 3791, p<.05), compared to the NL (8224 ± 4264 steps). The UP3 overestimated MVPA in the Normal group (67.7 ± 44.3 minutes), compared to the NL (27.6 ± 16.3, p=.000). In both the Overweight and Obese groups, the UP3 overestimated MVPA (Overweight: 81.5 ± 44.3 minutes, Obese: 77.5 ± 39.4 minutes, both p=.000), whereas the ONE underestimated MVPA (Overweight: 25.9 ± 26.5 minutes, Obese: 19.5 ± 25.3 minutes, both p<.05), compared to the NL (Overweight: 36.6 ± 19.2 minutes, Obese: 31.6 ± 27.3 minutes). CONCLUSION: Within BMI groups, The UP3 underestimated steps and overestimated MVPA, the ONE underestimated steps and MVPA, whereas the Charge performed relatively well for both measures.