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 VO2rest and VO2max tests prior to a series of 5-min bouts of treadmill walking at increasing speed while wearing an Apple Watch on both wrists and with oxygen consumption measured continuously. Five-min exercise bouts were added until the Apple Watch advanced the green ‘exercise’ ring by 5 min (defined as the treadmill inflection speed). Validity was examined using a one-sample t-test, with inter- and intra-device reliability reported as the standardized typical error and intraclass correlation.
Results: The mean (SD) %VO2R at the treadmill inflection speed was 30 (7) % for both Apple Watches. There was a large underestimation of moderate-intensity exercise (left: mean difference -10% [95%CI: -14 to -7], d = -1.4; right: -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 over-estimation of moderate-intensity exercise minutes completed throughout the day.