Accuracy of VO2 max Estimates From Apple Watch Series 10

Objective: max) estimates from Apple Watch Series 10 through comparison with indirect calorimetry, the criterion method. Participants and Methods: max estimate with Apple Watch Series 10. The agreement between Apple Watch and indirect calorimetry was assessed using Bland-Altman analysis, mean absolute error, and mean absolute percentage error. Data were collected between October 2024 and January 2025. Results: max overall, with wide limits of agreement indicating measurement variability (mean difference, -6.25 mL/kg per minute [95% CI, -8.28 to -4.23]; limits of agreement, -18.23 to 5.73). The mean absolute error was 6.79 mL/kg per minute (95% CI, 4.91-8.67), and the mean absolute percentage error was 13.2% (95% CI, 9.6-16.8). Based on reference standards from the Fitness Registry and the Importance of Exercise National Database, Apple Watch correctly classified the cardiorespiratory fitness percentile of 5 (14%) participants and classified a further 12 (34%) within 1 adjacent percentile band of their actual band. Conclusion: max estimates showed poor individual-level agreement with criterion measures, although our findings suggested utility for population-level assessment in research trials. Longitudinal validation of measurement reliability is needed to evaluate use as an alternative to conventional submaximal prediction methods.