title: Comparison of wrist-worn Fitbit Flex and waist-worn ActiGraph for measuring steps in free-living adults.
abstract: BACKGROUND
  Accelerometers are commonly used to assess physical activity. Consumer activity trackers have become increasingly popular today, such as the Fitbit. This study aimed to compare the average number of steps per day using the wrist-worn Fitbit Flex and waist-worn ActiGraph (wGT3X-BT) in free-living conditions.
  
  
  METHODS
  104 adult participants (n = 35 males; n = 69 females) were asked to wear a Fitbit Flex and an ActiGraph concurrently for 7 days. Daily step counts were used to classify inactive (<10,000 steps) and active (≥10,000 steps) days, which is one of the commonly used physical activity guidelines to maintain health. Proportion of agreement between physical activity categorizations from ActiGraph and Fitbit Flex was assessed. Statistical analyses included Spearman's rho, intraclass correlation (ICC), median absolute percentage error (MAPE), Kappa statistics, and Bland-Altman plots. Analyses were performed among all participants, by each step-defined daily physical activity category and gender.
  
  
  RESULTS
  The median average steps/day recorded by Fitbit Flex and ActiGraph were 10193 and 8812, respectively. Strong positive correlations and agreement were found for all participants, both genders, as well as daily physical activity categories (Spearman's rho: 0.76-0.91; ICC: 0.73-0.87). The MAPE was: 15.5% (95% confidence interval [CI]: 5.8-28.1%) for overall steps, 16.9% (6.8-30.3%) vs. 15.1% (4.5-27.3%) in males and females, and 20.4% (8.7-35.9%) vs. 9.6% (1.0-18.4%) during inactive days and active days. Bland-Altman plot indicated a median overestimation of 1300 steps/day by the Fitbit Flex in all participants. Fitbit Flex and ActiGraph respectively classified 51.5% and 37.5% of the days as active (Kappa: 0.66).
  
  
  CONCLUSIONS
  There were high correlations and agreement in steps between Fitbit Flex and ActiGraph. However, findings suggested discrepancies in steps between devices. This imposed a challenge that needs to be considered when using Fibit Flex in research and health promotion programs.
authors.affiliation: Saw Swee Hock School of Public Health, National University of Singapore, Singapore, Singapore.
authors.forename: Anne H Y
authors.identifier: http://orcid.org/0000-0002-6512-4683
authors.initials: AH
authors.lastname: Chu
authors.affiliation: Saw Swee Hock School of Public Health, National University of Singapore, Singapore, Singapore.
authors.forename: Sheryl H X
authors.initials: SH
authors.lastname: Ng
authors.affiliation: Department of Computer Science, School of Computing, National University of Singapore, Singapore, Singapore.
authors.forename: Mahsa
authors.initials: M
authors.lastname: Paknezhad
authors.affiliation: Department of Computer Science, School of Computing, National University of Singapore, Singapore, Singapore.
authors.forename: Alvaro
authors.initials: A
authors.lastname: Gauterin
authors.affiliation: Saw Swee Hock School of Public Health, National University of Singapore, Singapore, Singapore.|PAPRSB Institute of Health Sciences, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong, Brunei Darussalam.
authors.forename: David
authors.initials: D
authors.lastname: Koh
authors.affiliation: Department of Electrical Engineering and Computer Science, Lassonde School of Engineering, York University, Toronto, Ontario, Canada.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Brown
authors.affiliation: Saw Swee Hock School of Public Health, National University of Singapore, Singapore, Singapore.|Institute of Social Medicine, Epidemiology and Health Economics, Charité University Medical Centre Berlin, Berlin, Germany.
authors.forename: Falk
authors.initials: F
authors.lastname: Müller-Riemenschneider
mesh_terms: D061725:Accelerometry / Q000295:instrumentation / Q000592:standards*; D056044:Actigraphy / Q000295:instrumentation / Q000592:standards*; D000328:Adult; D000368:Aged; D004734:Energy Metabolism / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D018670:Monitoring, Ambulatory / Q000295:instrumentation / Q000592:standards*; D012420:Running / Q000502:physiology*; D016138:Walking / Q000502:physiology*; D014953:Wrist