Assessing the ability of the Fitbit Charge 2 to accurately predict VO_2max

The aim of this study was to assess the ability of the Fitbit Charge 2 (FBC2) to accurately estimate VO_2max in comparison to both the gold standard VO_2max test and a non-exercise VO_2max prediction equation.

Thirty healthy subjects (17 men, 13 women) between the ages of 18 and 35 (age =21.7±3.1 years) were given a FBC2 to wear for seven days and followed instructions on how to obtain a cardio fitness score (CFS). VO_2max was measured with an incremental test on the treadmill followed by a verification phase. VO_2max was predicted via a non-exercise prediction model (N-Ex) using self-reported physical activity level.

Measured VO_2max was significantly lower than FBC2 predicted CFS (VO_2max =49.91±6.83; CFS =52.53±8.43, P=0.03). N-Ex prediction was significantly lower than CFS but not significantly lower than measured VO_2max (N-Ex =48.79±6.32; CFS vs. N-Ex: P=0.01; VO_2max vs. N-Ex: P=0.54). Relationships between both VO_2max vs. CFS and VO_2max vs. N-Ex were good (ICC: VO_2max vs. CFS=0.87, VO_2max vs. N-Ex =0.87); Bland-Altman analysis indicated consistency of CFS measurement and lack of bias. The coefficient of variation (CV) and mean absolute percent error (MAPE) were greater with CFS than N-Ex (CV: CFS =6.5%±4.1%, N-Ex =5.6%±3.6%; MAPE: CFS =10.2%±6.7%, N-Ex =7.8%±5.0%). Heart rate (HR) estimated by the FBC2 was lower than estimated (Est) HR for pace based on HR extrapolation (FBC2 =155±18 bpm, Est =183±15 bpm, P<0.001). The difference in CFS and VO_2max was inversely correlated with the difference in FBC2 HR and Estimated HR (r =-0.45, P<0.001).

The FBC2 shows consistent, unbiased measurement of CFS while overestimating VO_2max in healthy men and women. The non-exercise VO_2max prediction equation provides a similar, slightly more accurate, VO_2max prediction than the CFS without the need for an exercise test or purchase of a Fitbit.