Why does my Garmin VO2max go up and down week to week?

The Firstbeat algorithm reads the relationship between heart rate and running pace during qualifying sessions. Heat, fatigue, poor sleep, a loose strap, and wrist sensor dropout all elevate heart rate relative to pace, which the algorithm reads as reduced fitness. A single qualifying run with a chest strap in consistent conditions is more informative than several wrist-only readings taken across varying weather.

Does Garmin overestimate or underestimate VO2max?

It depends on the athlete and the device generation. Engel et al. (2025) found the Garmin Forerunner 245 underestimated VO2max on average, with the gap widest in highly trained athletes. A 2017 study found Garmin underestimated VO2max in women and overestimated in men, while Polar showed the opposite pattern on its V800. For most recreational female runners, Garmin tends to underestimate slightly. For highly trained athletes, the underestimation can be material enough to affect pacing and training zone decisions.

Is Garmin VO2max less accurate for women?

Validation studies show directional biases that differ between device brands. Some studies find Garmin underestimates VO2max in women while overestimating in men; Polar has shown the opposite pattern on certain devices. The algorithms were developed on datasets that were heavily male-skewed, which may contribute to sex-specific error patterns. The size and consistency of the effect vary by device generation and athlete fitness level.

Does using a chest strap improve Garmin VO2max accuracy?

Yes, meaningfully. Firstbeat’s own validation reports a mean absolute error of approximately 5 per cent when paired with a chest strap. Wrist-only heart rate raises that error to 5-8 ml·kg⁻¹·min⁻¹.

How important is setting the correct maximum heart rate in Garmin?

Significant. Firstbeat’s data show that a 15 bpm error in maximum heart rate results in a 7-9 per cent error in VO2max. The default 220 minus age formula is typically off by 10-15 beats per minute for trained athletes. Setting a measured maximum heart rate from a ramp test or a verified hard effort materially improves the estimate.

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