March 04, 2024

Top Line

Would you only see a doctor once a year to assess how your personal health is progressing? I sure wouldn’t. We should always be aware of our health, and thankfully today we have convenient ways to stay in touch with it. Wearable fitness devices have become increasingly popular in recent years, and can range anywhere from watches to straps to rings. It is important to understand how well these devices are measuring what they say they are measuring, and how accurate, reliable, and specific they are to the data we want.

Popularized fitness devices like Fitbit and Apple Watch can also provide users with ways to track, compete, and monitor adaptation and response to their workouts over time. Out of all the metrics that these devices are able to read, the most important one is your VO2 max - a measurement of how much oxygen your body can uptake and utilize during exercise. Not only is it important to use for training, it can also be used to prognose and diagnose diseases in clinical, nutritional, epidemiological, and rehabilitation settings. 

The question we aim to answer in this article is how well you can rely on the most popular wearable fitness devices in the industry to accurately assess the progression or regression of your cardiorespiratory and total fitness.

How do fitness trackers measure VO2 max scores?

The inner surface of these wearable devices have sensors that use infrared light to capture when your heart is contracting and relaxing based on the change in density of hemoglobin throughout the blood through a process called photoplethysmography (6).

The blood data from this process (density, volume, and velocity) is then plugged into a complex formula that combines this data and prior user data to make an estimate of your VO2 max (6).

Why it Matters

Your VO2 max should be measured every six months to one year, as it is heavily correlated with longevity, all cause mortality risk, and even cancer related deaths (6). True VO2 max tests can be expensive, hard to access, and not practical for people with preexisting conditions, the elderly, and anyone who does not have an easily accessible center. Therefore, it is important to investigate how fitness wearable devices measure and estimate VO2 max using sub-maximal exercise protocols. Here is the standardized equation that is commonly used to estimate this data.


Wearable devices measure VO2 primarily through heart rate variability. Heart rate variability is directly correlated to how well your body can pump oxygenated blood to your heart and muscles during maximally strenuous exercise (heart rate max), and how well your body can help it get back to “rest and digest” (resting heart rate). Your VO2 max is a measurement of how your body goes from one extreme to the other, so this is why heart rate variability is such a good metric to use. Having an understanding of how to increase your heart rate (HR) max, resting HR, and efficiency in a variety of heart rate zones will make sure that you don’t plateau in your training, whether it is your aerobic base or anaerobic power.

Comparing Devices


Fitbit’s most popular device, the Fitbit Charge, is able to predict your VO2 max and display the outcome through a user-friendly “cardio fitness score”. It does this by continuously tracking the relationship between your running/walking pace and how your heart rate elevates and recovers over time. It also takes into account age, sex, weight, and other personal information in its measurement (4).


On the other hand, the Fitbit Sense is able to track your SPO2 (blood oxygen levels), blood temperature, as well as metrics to monitor sleep and stress. These metrics are given to you in your cardio fitness score, however they do not directly impact your VO2 in the system.

If Fitbit wanted to make their measurements more accurate, they would design a way that will allow your SPO2 measurement to directly affect the reading of your VO2. The reason why this would make Fitbit’s measurement more accurate is because your VO2 max measures how your body is able to oxygenate blood and send it to the heart and muscles to push your body. SPO2 is a measurement of how saturated your blood is with oxygen, so this would add to the accuracy of a user's understanding of their fitness level.

A study was conducted to understand the accuracy of Fitbit’s VO2 max prediction. It was compared to the gold-standard VO2 max test and a non-exercise prediction equation.

The 16-week study found that when comparing a true VO2 max test, Fitbit’s measurement, and the heart rate prediction equation, the Fitbit was ~3 ml/kg/min off from the true value rather than the prediction equation ~1 ml/kg/min off from the true value. It tended to overestimate (4).

The study additionally pointed out that the accuracy of the Fitbit to correctly detect exercise HR at increased intensities was the limiting factor. Because the non-exercise equation was more accurate than the Fitbit’s measurement (which uses exercise data to make its calculation), a Fitbit does not seem to be a worthwhile investment. A device that is able to detect HR accurately at higher percentages of the maximum HR is crucial, as VO2 max is measured at maximum intensity.

Apple Watch

The apple watch predicts your VO2 max by collecting user data after walking, running, or hiking outdoors on relatively flat ground with adequate GPS, heart rate signal quality, and exertion (an approximate increase of 30 percent of the range from resting heart rate to max) (1).


Motion can affect the heart rate sensor, which is why rhythmic movements like running or cycling give better results than irregular movements like tennis or boxing. This is one of the limitations, as some people may not have the ability or data to do these activities consistently,  skewing potential results (1).

These estimates of VO2 max are also based on submaximal predictions (linear estimation of how your heart rate will climb during a less intense workout) of VO2 max rather than doing a complete VO2 test to exhaustion. As such, users don’t need to achieve peak heart rate to receive an estimate, however, a notion of peak heart rate is needed. 

Apple conducted a study where participants completed a true VO2 max test and a submaximal VO2 test. The submaximal test value was projected using predicted VO2 from user data combined with heart rate variability. It found a difference of less than ~1 ml/kg/min when compared to the baseline VO2 testing. It tended to underestimate (1).

Apple’s technology had the ability to effectively detect heart rates at extreme and moderate exercise intensities (1). It’s algorithm was even accurate in comparison with the true value when tracking walks, runs, and hikes that were less than 10 minutes in duration (1). Almost everyone can participate in that, meaning the Apple Watch seems to be a very reliable, accessible, and time efficient method to test your VO2 max.


In terms of understanding performance, predicting health complications, and increasing longevity, VO2 max is a vital measurement to know. Apple Watch and Fitbit are relatively cheap, accessible, and accurate ways to stay on track with this metric and never miss a beat (ha get it).

What is important to understand is that these devices directly make their predictions by using heart rate data, and the relationship between heart rate and VO2 is different for everyone. Someone’s lung capacity (ability of the lungs to take air and use it in the body) could be much different than their cardiac output (ability of the heart to pump oxygenated blood throughout the body).

Nature vs Nurture

When looking to understand overall health, it is important to understand the role that genetics have in your VO2 max. Genetics can provide insight on the ability to improve your VO2 max, as well as what type of training (aerobic or anaerobic) will have more of an impact. 

A study looked at the VO2 max values in twins and concluded that genetic factors explained for 72-74% of the similarity in VO2 max and even when “sport participation” was factored in, genetic factors still explained more (3).

However, putting genetics aside, it should be known that being able to improve or maintain your VO2 max is strongly associated with decreased mortality. Every 1 ml/kg/min increase in VO2 max decreases risk of death by 9%, and every decrease can lead up to a 27% higher risk (1).

One study has proven that even high-intensity aerobic interval training (HIIT) resulted in significantly increased VO2max compared with long slow distance activities. The study pointed out that as long as a high enough percentage of your max HR was reached (~85%) VO2 will improve. 


This is especially important for athletes or anyone who struggles to improve their VO2 max. We all can make use of different training modalities and energy systems to reach an intensity that will increase our VO2 max (your genetics aren’t an excuse). More information on your VO2 max’s ability to impact athletic performance can be found in this article.

Bottom Line

In summary, wearable devices have proven to be more accurate than a submaximal test or prediction equation because they constantly take into account the progression of user data that is measured day to day by the device. Fortunately, the way these devices extract and analyze data is only going to improve year by year. This means the gap in accuracy between true testing and wearable device prediction will continue to decrease.

At the moment though, these devices monitor VO2 better than any other metric (ie. calories burned, respiration rate, etc.) which is fitting as it is the single most important metric in understanding your health. So, if you’re looking to make a worthwhile investment, get a wearable fitness device you’ll get an understanding of how your health adjusts and improves regularly.


  1. Apple. (n.d.). Using Apple Watch to estimate cardio fitness with vo2 Max. Retrieved August 5, 2022, from
  2. Admin. “How Does Apple Health Calculate vo2max?” Wearable Guides, Optimist Minds, 22 June 2021, 
  3. Fagard, R., et al. “Heritability of Aerobic Power and Anaerobic Energy Generation during Exercise.” Journal of Applied Physiology, vol. 70, no. 1, 1991, pp. 357–362., 
  4. Freeberg, K. A., Baughman, B. R., Vickey, T., Sullivan, J. A., & Sawyer, B. J. (2019). Assessing the ability of the fitbit charge 2 to accurately predict VO2max. MHealth5, 39–39.
  5. Ramjee, A. (n.d.). How to train with a genetically low vo2max. DNAfit Blog. Retrieved August 4, 2022, from
  6. Smith, N. (2022, May 28). Low VO2 MAX score on fitness tracker? here's what you should know. MyHealthyApple. Retrieved August 4, 2022, from 


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