Fitness trackers and wearable fitness technology are a rising trend for both serious athletes and individuals who like to occasionally exercise. Both types are caring more about their health stats these days, and VO2 plays a big role in that department. VO2 Max has been used in high-end watches for a while now, but it has begun to go mainstream, being used on the latest popular fitness devices too. See Also: Check out the latest fitness tracker in a pair of glasses For example, the Fitbit Charge 2 uses VO2 Max to display its Cardio Fitness score. The higher end Garmin devices are using it too, such as the Fenix 3 HR. So, what is VO2 Max, and how does it measure overall fitness? Can a fitness tracker or smartwatch be trusted to give a correct reading? The maximum volume of O2 (oxygen) which is transferred to the blood, is what VO2 Max represents.