In a study of 30 participants, Apple Watch VO₂ max estimates were on average 6 mL/kg/min lower than values measured with the gold-standard treadmill test. The results varied widely between individuals, sometimes underestimating or overestimating VO₂ max by a large margin. Overall, the watch showed a mean error of 13%, indicating that it may not provide a precise measurement for clinical or performance purposes.