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Fitness trackers often rely on heart rate, GPS, and possibly type of activity to estimate VO2 max. But, what if the user switches between an easier means of exercise and a harder one?

For example, what if a road bike rider switches from slick tires to mountain bike tires on her route? She is likely to go slower even with the same heart rate pattern as before, so it would seem that she has decreased her VO2 max somewhat.

Does a fitness tracker have any way of telling that she's just as fit as before?

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No, the fitness tracker cannot tell that exercise has changed. This will result in a skewed value.

Fitness trackers usually look at your age, weight, sex, heart rate, overall fitness level, and effort to determine a VO2max estimate. The way that effort is calculated is by examining your heart rate compared to the exercise type. So, if you tell the fitness tracker you're walking and your heart rate goes up because you're actually running, your effort goes up, and your VO2max would go down (GPS mitigates this to an extent; a better example might be walking vs walking in snow). Garmin themselves gives three good scenarios to estimate your VO2max and they are fairly rigid (i.e. strictly walking/running/cycling, at a steady pace, for greater than 10 minutes).

Additionally:

"Cycling VO2 Max Tip: If you are having difficulty obtaining a cycling VO2 Max Estimate, try riding a course where you can maintain a steady effort. You can also try using an indoor trainer."

Aside from that, a lot of VO2max computation and fitness tracker coding is proprietary. It is good to keep in mind that these are estimates regardless of the price of the fitness tracker.

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  • I agree. The fitness tracker must assign a certain "hardness" to each kind of exercise, so that it can distinguish between 1 km running and 1 km cycling, for example. – Kolbe Dec 13 '20 at 16:23

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