Female VO2max on Garmin: Why the Number Lies and What to Do

Does Garmin underestimate VO2max in women? The female accuracy problem explained

You know how to run well. A week of difficult training and the Garmin VO2max number climbs. Then it drops in the cold, or after a hard block, or for no obvious reason at all. If a lab test produces an entirely different figure, the question is why.

VO2max estimates from wrist devices are based on algorithms built on exercise physiology research that, for many years, predominantly used male participants. That imbalance matters because the accuracy differences are real and can affect how female runners interpret their fitness data.

The algorithm’s blind spot

Wearables do not measure VO2max directly. Instead, the Firstbeat algorithm that powers Garmin’s estimate analyses the relationship between heart rate and running pace during qualifying sessions. It selects 20-30-second windows where heart rate is above approximately 70 per cent of maximum and movement is steady, then extrapolates from those submaximal data points to estimate oxygen uptake at full effort.

The problem starts with the training data behind the algorithm. According to a validation study by Wiecha et al. (2023) involving 5,260 endurance athletes, 84.76 per cent were men, reflecting a well-documented historical pattern of underrepresentation of women in exercise physiology research.

Why female physiology can confuse the algorithm

Women tend to have lower average VO2max values than men due to differences in haemoglobin concentration, body composition, and cardiovascular physiology. Beyond that, factors such as stroke volume, heart rate response, and hormonal fluctuations across the menstrual cycle may affect the physiological signals the algorithm uses to estimate aerobic capacity. The relationship between pace, heart rate, and fitness may therefore be more variable than the prediction model assumes. However, direct evidence linking menstrual cycle phase to Garmin VO2max error is limited, and the wording should remain cautious.

The directional bias

Research has identified sex differences in the direction of VO2max miscalculation. A 2017 study using the Garmin Forerunner found that the predicted VO2max underestimated values for women and overestimated them for men. Polar’s V800 showed the opposite pattern: overestimation for women, underestimation for men. Different algorithms, samples, and testing conditions can all produce directional differences, and the magnitude and consistency of any sex-specific bias vary across devices and generations.

The heart rate measurement problem

Behind the algorithm lies the optical heart rate sensor. Optical sensors measure changes in blood flow under the skin using light. An American College of Cardiology press release summarising research presented at ACC.17 reported wrist device error margins ranging approximately ±15 to ±34 beats per minute under the specific devices and conditions tested. Firstbeat’s own validation reports approximately 5 per cent mean absolute error when paired with a chest strap; wrist-only heart rate raises that to 5-8 ml·kg⁻¹·min⁻¹.

What the validation studies actually show

In a study of the Garmin Forerunner 245 by Engel et al. (2025), the smartwatch underestimated VO2max on average across athletes, with mean differences of -4.73 ml·min⁻¹·kg⁻¹ and -4.05 ml·min⁻¹·kg⁻¹ for the first and second qualifying runs. Subgroup analysis revealed substantially better accuracy in moderately trained athletes (mean absolute percentage error of 4.1-2.8 per cent) than in highly trained athletes, where the watch underestimated by 6.3 ml·min⁻¹·kg⁻¹ with a mean absolute percentage error of 10.4-9.4 per cent.

Practical guidance: how to use your number

• Treat it as a trend, not a fact. Consistent increases over weeks and months indicate aerobic improvement. A sustained drop signals something has changed.
• Use a chest strap for important qualifying sessions. Firstbeat’s validation places accuracy within approximately 5 per cent of laboratory spirometry under controlled conditions when using chest-strap heart rate.
• Set your actual maximum heart rate. Garmin defaults to the 220 minus age formula, which is typically wrong by 10-15 beats for trained athletes. A 15 bpm error in maximum heart rate produces a 7-9 per cent error in the VO2max output.
• Understand device-specific patterns. If your device consistently underestimates your race performance or perceived fitness, note the gap.
• Get lab tested for consequential decisions. If VO2max will inform a significant training or racing decision, laboratory respiratory gas analysis is the only reliable figure.

Quick answers

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.

This article is part of the site’s female athlete tech coverage. For tested watch recommendations by case size, aesthetics, and female health tracking, see the best sports watch for women. For the full range of wearables, see the best wearables for female endurance athletes.

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Last Updated on 14 August 2026 by the5krunner