Measuring Moderate-Intensity Exercise with the Apple Watch: Validation Study
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Abstract
BACKGROUND Moderate fitness levels and habitual exercise have a protective effect for cardiovascular disease, stroke, type 2 diabetes, and all-cause mortality. The Apple Watch displays exercise completed at an intensity of a brisk walk or above using a green “exercise” ring. However, it is unknown if the exercise ring accurately represents an exercise intensity comparable to that defined as moderate-intensity. In order for health professionals to prescribe exercise intensity with confidence, consumer wearable devices need to be accurate and precise if they are to be used as part of a personalized medicine approach to disease management. OBJECTIVE The aim of this study was to examine the validity and reliability of the Apple Watch for measuring moderate-intensity exercise, as defined as 40-59% oxygen consumption reserve (VO2R). METHODS Twenty recreationally active participants completed resting oxygen consumption (VO2rest) and maximal oxygen consumption (VO2 max) tests prior to a series of 5-minute bouts of treadmill walking at increasing speed while wearing an Apple Watch on both wrists, and with oxygen consumption measured continuously. Five-minute exercise bouts were added until the Apple Watch advanced the green “exercise” ring by 5 minutes (defined as the treadmill inflection speed). Validity was examined using a one-sample t-test, with interdevice and intradevice reliability reported as the standardized typical error and intraclass correlation. RESULTS The mean %VO2R at the treadmill inflection speed was 30% (SD 7) for both Apple Watches. There was a large underestimation of moderate-intensity exercise (left hand: mean difference = -10% [95% CI -14 to -7], d=-1.4; right hand: mean difference = -10% [95% CI -13 to -7], d=-1.5) when compared to the criterion of 40% VO2R. Standardized typical errors for %VO2R at the treadmill inflection speed were small to moderate, with intraclass correlations higher within trials compared to between trials. CONCLUSIONS The Apple Watch threshold for moderate-intensity exercise was lower than the criterion, which would lead to an overestimation of moderate-intensity exercise minutes completed throughout the day.
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In recent studies involving predominantly steady-state aerobic activities, wrist-worn activity monitors have been shown to have reasonable accuracy in heart rate estimation (approximately 5% error) but a poor estimate of energy expenditure, where the error was found to be closer to approximately 30% with a negative bias.
“In recent studies involving predominantly steady-state aerobic activities, wrist-worn activity monitors have been shown to have reasonable accuracy in HR estimation (approximately 5% error) but a poor estimate of EE, where the error was found to be closer to approximately 30% with a negative bias [16,17].”
From Accuracy of Wrist-Worn Activity Monitors During Common Daily Physical Activities and Types of Structured Exercise: Evaluation Study (2018) · cites this paper as [17]
The Discussion (Principal Findings; page not available) says the Apple Watch does not substantially overestimate or underestimate heart rate, supporting the qualitative heart-rate point. It gives no approximately 5% heart-rate error or approximately 30% negatively biased energy-expenditure error. The Results report a 10-percentage-point underestimation of the moderate-intensity exercise threshold, which is a different measure. (LLM, conf=0.83)
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- Authors
- Abt, G., Bray, J., & Benson, A. C.
- Affiliations
- University of Hull (GB)
- Swinburne University of Technology (AU)
- Corresponding author
- Abt, Grant
- Venue
- JMIR Cardio
- Publisher
- JMIR Publications Inc.
- Published
- 2018-02-28
- Volume
- 2
- Issue
- 1
- Pages
- e6
- ISSN
- 2561-1011 (ISSN-L)
- PMC
- PMC6834212
- OpenAlex
- W2789999811
- Cited by
- 21
- References
- 27
- Citations by year
- 2026: 1 · 2025: 2 · 2024: 5 · 2023: 2 · 2022: 6 · 2021: 1 · 2020: 2 · 2019: 1 · 2018: 1
- FWCI
- 0.86
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Licence
- cc-by
- Locations
- JMIR Cardio (publishedVersion, cc-by)
- Europe PMC (PubMed Central) (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- Repository@Hull (Worktribe) (University of Hull) (submittedVersion, cc-by)
- Swinburne figshare (Swinburne University of Technology) (submittedVersion)
- Funders
- British Heart Foundation
- Topics
- Health Sciences › Medicine › Physiology › Physical Activity and Health
- Health Sciences › Medicine › Complementary and alternative medicine › Cardiovascular and exercise physiology
- Health Sciences › Health Professions › General Health Professions › Mobile Health and mHealth Applications
- Keywords
- Apple Watch, moderate-intensity exercise, exercise intensity monitoring, wearable devices, VO2max, treadmill walking, device validation, reliability statistics, exercise prescription, personalized medicine
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