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Published on 27.02.17 in Vol 5, No 2 (2017): February

This paper is in the following e-collection/theme issue:

    Letter to the Editor

    Critique of: “Physical Activity Assessment Between Consumer- and Research-Grade Accelerometers: A Comparative Study in Free-Living Conditions”

    1Faculty of Sport Sciences, PROFITH “PROmoting FITness and Health through physical activity” research group. Department of Physical Education and Sports, University of Granada, Granada, Spain

    2Karolinska Institutet, Department of Biosciences and Nutrition, Karolinska Institutet, Huddinge, Sweden

    Corresponding Author:

    Jairo H Migueles, B Sc, M.Res.

    Faculty of Sport Sciences

    PROFITH “PROmoting FITness and Health through physical activity” research group. Department of Physical Education and Sports

    University of Granada

    Carretera de Alfacar s/n

    Granada, 18071


    Phone: 34 958 24 43 53

    Fax:34 958 24 43 53


    In a recent issue in this Journal, Dominick et al., compared the outcome of a consumer-grade accelerometer against a research-grade accelerometer [1]. More specifically, they compared the Fitbit Flex (Charge and Surge) placed on the wrist against the GT3X (ActiGraph, Pensacola, USA, FL) placed on the hip. The authors observed large differences between methods, i.e. “Fitbit significantly overestimated METs for average daily activity, for overall minutes of reported exercise bouts, and for walking and run or sports exercises (all P-values <.001); and for average daily activity, Fitbit significantly underestimated the proportion of time in sedentary and light intensity by 20% and 34%, respectively, and overestimated time by 3% in both moderate and vigorous intensity (all P-values <.001)”.

    We find a major problem in the design of the present study, with potential to largely affects its results and interpretation. The authors aimed to compare activity measured by two different devices. However, these two devices were attached to two completely different locations, i.e. wrist (Fitbit) vs. hip (GT3X). As a consequence, the differences observed in this study could actually be due to the different locations rather than the real differences between devices. It is well known that the same accelerometer when attached to the wrist register markedly more accelerations than when attached to the hip [2–4]. As expected, the authors observed a higher level of activity in the wrist-accelerometer than in the hip-accelerometer. If the authors wanted to compare a consumer-accelerometer with a research-accelerometer, which is a very interesting research question, they should have placed both devices (Fitbit and GT3X) on the same wrist. Large-scale studies such as the National Health Examination Survey, NHANES, are placing the GT3X accelerometer on the wrist. There are now available cut-points to classify accelerations from GT3X attached to the wrist into time spent in different intensities of physical activity [2,3], so it would have been fully correct methodologically to attach both devices to the wrist. The authors acknowledge as a limitation that accelerometers were placed in different locations. However, there is no explanation as to why they did so. Unfortunately, we will only be able to know how comparable these two accelerometers are when a future study places both of them on the same location.

    Conflicts of Interest

    None declared.


    1. Dominick GM, Winfree KN, Pohlig RT, Papas MA. Physical Activity Assessment Between Consumer- and Research-Grade Accelerometers: A Comparative Study in Free-Living Conditions. JMIR Mhealth Uhealth 2016;4(3):1-11. [CrossRef] [Medline]
    2. Hjorth MF, Chaput JP, Damsgaard CT, Dalskov SM, Michaelsen KF, Tetens IEA. Measure of sleep and physical activity by a single accelerometer: Can a waist-worn Actigraph adequately measure sleep in children? Sleep Biol Rhythms 2012;10:328-335. [CrossRef] [Medline]
    3. Dieu O, Mikulovic J, Fardy PS, Bui-Xuan G, Béghin L, Vanhelst J. Physical activity using wrist-worn accelerometers: comparison of dominant and non-dominant wrist.. Clin Physiol Funct Imaging 2016:n/a-n/a. [CrossRef] [Medline]
    4. Hildebrand M, VAN Hees Vincent T, Hansen BH, Ekelund U. Age group comparability of raw accelerometer output from wrist- and hip-worn monitors. Med Sci Sports Exerc 2014 Sep;46(9):1816-1824. [CrossRef] [Medline]
    5. Chandler JL, Brazendale K, Beets MW, Mealing BA. Classification of physical activity intensities using a wrist-worn accelerometer in 8-12-year-old children. Pediatr Obes 2015 [FREE Full text]

    Edited by G Eysenbach; submitted 21.10.16; peer-reviewed by G Dominick, M Joselli; accepted 26.01.17; published 27.02.17

    ©Jairo H Migueles, Cristina Cadenas-Sanchez, Francisco B. Ortega. Originally published in JMIR Mhealth and Uhealth (, 27.02.2017.

    This is an open-access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work, first published in JMIR mhealth and uhealth, is properly cited. The complete bibliographic information, a link to the original publication on, as well as this copyright and license information must be included.