Currently submitted to: JMIR mHealth and uHealth
Date Submitted: Aug 13, 2026
Open Peer Review Period: Aug 17, 2026 - Oct 12, 2026
(closed for review but you can still tweet)
NOTE: This is an unreviewed Preprint
Warning: This is a unreviewed preprint (What is a preprint?). Readers are warned that the document has not been peer-reviewed by expert/patient reviewers or an academic editor, may contain misleading claims, and is likely to undergo changes before final publication, if accepted, or may have been rejected/withdrawn (a note "no longer under consideration" will appear above).
Peer review me: Readers with interest and expertise are encouraged to sign up as peer-reviewer, if the paper is within an open peer-review period (in this case, a "Peer Review Me" button to sign up as reviewer is displayed above). All preprints currently open for review are listed here. Outside of the formal open peer-review period we encourage you to tweet about the preprint.
Citation: Please cite this preprint only for review purposes or for grant applications and CVs (if you are the author).
Final version: If our system detects a final peer-reviewed "version of record" (VoR) published in any journal, a link to that VoR will appear below. Readers are then encourage to cite the VoR instead of this preprint.
Settings: If you are the author, you can login and change the preprint display settings, but the preprint URL/DOI is supposed to be stable and citable, so it should not be removed once posted.
Submit: To post your own preprint, simply submit to any JMIR journal, and choose the appropriate settings to expose your submitted version as preprint.
Warning: This is an author submission that is not peer-reviewed or edited. Preprints - unless they show as "accepted" - should not be relied on to guide clinical practice or health-related behavior and should not be reported in news media as established information.
Moderate-to-Vigorous Physical Activity Is Not a Unitary Construct: Comparing MVPA based on Physiological Effort and Mechanical Movement Using Fitbit and ActiGraph
ABSTRACT
Background:
Fitness trackers like those offered by Fitbit and research devices like the ActiGraph GT3X+ are both used to measure "moderate-to-vigorous physical activity" (MVPA), the kind of activity health guidelines recommend getting 150 minutes of per week. But these devices don't measure the same thing, even though they use similar words and labels. ActiGraph estimates activity based on how much a person physically moves with everyone held to the same standard of MVPA. Fitbit largely estimates activity based on heart rate, which reflects how hard your body is working relative to your own personal range. One asks "how much did you move?" The other asks "how hard is your body working?" These aren't always the same.
Objective:
To test whether a consumer wearable (Fitbit) and a research-grade accelerometer (ActiGraph) measure a common latent construct or instead operationalize fundamentally distinct constructs of physiological effort–based intensity versus mechanically derived movement to assess MVPA, intensity. We further assess how this construct divergence affects estimates of MVPA accumulation under free-living conditions.
Methods:
Adults (N=356; mean age 47.1 years; BMI 27.5 kg/m²) participating in the YourMove trial (NCT05598996) wore a Fitbit Versa and an ActiGraph GT3X+ three times for one week each. Analyses were restricted to matched wear time, with Fitbit Intensity and Active Zone Minutes compared with Freedson based cutpoints of Actigraph data. Agreement was assessed using confusion matrices, Cohen’s kappa, and Bland–Altman analyses. Subgroup analyses examined differences by sex, age, Body Mass Index (BMI), and fitness.
Results:
Overall agreement was high (~0.94), but agreement beyond chance was low (κ=0.18–0.30), indicating limited concordance in classifying higher-intensities. Both Fitbit metrics systematically estimated higher MVPA with greater differences observed at higher activity levels. Agreement on weekly guideline attainment was moderate and depended on the Fitbit-derived reference metric. Discrepancies increased in males, older adults, and individuals with higher BMI.
Conclusions:
Fitbit and accelerometer-derived estimates of MVPA are not directly interchangeable in free-living conditions. Observed differences reflect systematic divergence in how physical activity intensity is operationalized. These construct-level differences have meaningful implications for estimates of guideline attainment and subgroup comparisons, underscoring the need to align metric selection with the intended interpretive and analytic purpose of the measurement. Clinical Trial: Clinicaltrials.gov NCT05598996
Citation
Request queued. Please wait while the file is being generated. It may take some time.
Copyright
© The authors. All rights reserved. This is a privileged document currently under peer-review/community review (or an accepted/rejected manuscript). Authors have provided JMIR Publications with an exclusive license to publish this preprint on it's website for review and ahead-of-print citation purposes only. While the final peer-reviewed paper may be licensed under a cc-by license on publication, at this stage authors and publisher expressively prohibit redistribution of this draft paper other than for review purposes.