BACKGROUND:  Cardiorespiratory fitness is indicated by maximum oxygen consumption (V̇O2max). Higher values of VO2max are inversely related to all-cause mortality and the risk of developing cardiovascular disease. Indirect calorimetry, the gold standard method of assessing VO2max, requires costly laboratory equipment, trained test administrators, and vigorous exercise on behalf of the participant. Thus, this assessment is out of reach for many. To improve the accessibility of this important health variable, methods of predicting V̇O2max are available in many wearable activity monitors. However, the predictions have not been rigorously externally validated. The purpose of this study was to compare V̇O2max estimates from personally owned consumer-grade activity monitors to V̇O2max measured by indirect calorimetry. METHODS: Healthy individuals between the ages of 18 and 35 years, who owned a consumer-grade activity monitor with a predicted VO2max were recruited for this study. Forty participants (23.4 ± 4.5 yrs) completed a V̇O2max test using a treadmill-based ramp protocol in the Exercise Science lab. The treadmill protocol included walking or running at a constant speed while the grade was increased by 1% at the end of every minute. Participants were encouraged to continue exercising until maximal exertion. V̇O2max was assessed with a metabolic gas analyzer (CardioCoach); oxygen consumption (ml·kg−1·min−1), rating of perceived exertion, and heart rate were recorded each minute. Paired samples t-tests were used to compare the monitor-predicted to measured V̇O2max for 1) all monitors grouped together, 2) Apple monitors, and 3) Garmin monitors. RESULTS: On average, when all monitors were grouped together, V̇O2max estimates were 7.2 ± 7.0 ml·kg−1·min−1 lower than measured V̇O2max (P<.001). Apple monitors estimated 8.3 ± 7.6 ml·kg−1·min−1 lower than measured V̇O2max (P<.001) and Garmin estimated 5.3 ± 6.8 ml·kg−1·min−1 lower than measured V̇O2max (P=.011). Conclusion: In this sample of ongoing research, V̇O2max predicted by consumer-grade activity monitors was significantly lower than lab-measured V̇O2max. This discrepancy between estimated and measured V̇O2max should be considered when assessing cardiorespiratory fitness using consumer-grade activity monitors.​

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