JMIR Publications

ABSTRACT

Background:

Determining maximum oxygen uptake (VO2max) is essential for evaluating cardiorespiratory fitness. While laboratory-based testing is considered the gold standard, sports watches or fitness trackers offer a convenient alternative. However, despite the high number of wrist-worn devices, there is a lack of scientific validation for VO2max estimation outside the laboratory setting.

Objective:

The aim of the study was to compare the Apple Watch Series 7’s performance against the gold standard in VO2max estimation and Apple’s own validation findings.

Methods:

A total of 19 participants (7 female, 12 male), aged 18 to 63 years (mean age: 28.42 ± 11.43) were included in the validation study. VO2max for all participants was determined in a controlled laboratory environment using a metabolic gas analyzer. Thereby, they completed a graded exercise test on a cycle ergometer until reaching subjective exhaustion. This value was then compared with the estimated VO2max value from the Apple Watch, which was calculated after wearing the watch for at least two consecutive days and measured directly after a running test.

Results:

The measured VO2max (45.88 ± 9.42 mL/kg/min) in the laboratory setting was significantly higher than the predicted VO2max (41.37 ± 6.5 mL/kg/min) from the Apple Watch (t(18) = 2.51, p < .01) with a medium effect size (Hedge’s g = 0.53). The Bland-Altman analysis revealed a good overall agreement between both measurements. However, the Intraclass Correlation Coefficient ICC(2,1) = 0.47 (95% CI: 0.06 to 0.75) indicated poor reliability. The Mean Absolute Percentage Error (MAPE) between the predicted and the actual VO2max was 15.79%, while the Root Mean Square Error (RMSE) was 8.85 mL/kg/min. The analysis further revealed higher accuracy when focusing on participants with good fitness levels (MAPE: 14.59%; RMSE: 7.22 ml/kg/min; ICC(2,1) = 0.60, 95% CI: 0.09 to 0.87).

Conclusions:

Similar to other smartwatches, the Apple Watch also overestimates or underestimates the VO2max in individuals with poor or excellent fitness levels, respectively. Assessing the accuracy and reliability of the Apple Watch's VO2max estimation is crucial for determining its suitability as an alternative to laboratory testing. The findings of this study will apprise researchers, physical training professionals, and end-users of wearable technology, thereby enhancing the knowledge base and practical application of such devices in assessing cardiorespiratory fitness parameters.