Most wearable devices do not directly measure VO₂ max. Instead, they estimate it using data such as heart rate, pace, and personal information, which means the number on a smartwatch may not be as precise as many people assume. Understanding how these estimates work helps explain both the value and the limitations of wearable fitness technology.
Your Wearable Says Your VO₂ Max Is Improving—But How Much Should You Trust the Number?
A runner finishes a workout feeling strong. The pace felt comfortable. Recovery between intervals seemed faster than usual. There is a sense of accomplishment after the effort.
Then comes the moment many fitness enthusiasts know well: the glance at the watch.
The screen displays a VO₂ max score, and suddenly that number becomes the focus. If it has increased, the workout feels validated. If it has dropped, frustration can quickly replace confidence.
As wearable fitness technology becomes more advanced, millions of people now have access to performance metrics that once belonged almost exclusively to sports science laboratories. Among the most popular is VO₂ max, a measurement widely associated with cardiovascular fitness and endurance performance.
Yet there is one important detail many users never realize.
The number displayed on a smartwatch is often not a direct measurement at all.
Understanding what that number actually represents—and what it does not—can help people make better decisions about their training while avoiding unnecessary confusion along the way.
In 'The VO₂ Accuracy of Wearables | Peter Attia', the discussion dives into the importance of accurate VO₂ measurements from fitness devices, exploring key insights that sparked deeper analysis on our end.
The Excitement of a New Fitness Score—and the Assumption Behind It
Fitness trackers have transformed the way people view exercise.
Years ago, many individuals judged a workout based on how they felt. Today, watches and wearable devices can report heart rate, calories burned, recovery scores, sleep quality, stress levels, training readiness, and VO₂ max estimates within seconds.
This constant flow of information can be motivating. Numbers create a sense of progress. They provide feedback and make fitness goals feel measurable.
VO₂ max has become one of the most respected numbers among endurance athletes because it is often associated with aerobic capacity and cardiovascular fitness. A higher score is generally linked to a greater ability to perform sustained exercise.
As a result, many users naturally assume that if a watch displays a VO₂ max value, the device must be directly measuring oxygen use inside the body.
That assumption is understandable.
The reality, however, is far more complicated.
What VO₂ Max Really Measures Inside the Human Body
VO₂ max refers to the maximum amount of oxygen the body can take in, transport, and utilize during intense exercise.
When a person exercises, oxygen travels from the lungs into the bloodstream and is delivered to working muscles. Those muscles then use oxygen to produce energy that powers movement.
The more efficiently this process works, the greater the body's aerobic capacity. That relationship has been the focus of exercise scientists for decades.
Researchers who study endurance performance have spent decades trying to better understand this relationship.
Dr. Benjamin Levine, Professor of Internal Medicine at the University of Texas Southwestern Medical Center and Director of the Institute for Exercise and Environmental Medicine, has dedicated much of his career to studying cardiovascular fitness and human performance.
His research has contributed to a growing body of evidence showing that cardiorespiratory fitness is strongly associated with long-term health outcomes.
From that perspective, VO₂ max represents far more than a performance number. It reflects how effectively the heart, lungs, blood vessels, and muscles work together to deliver and use oxygen during exercise.
This is why VO₂ max has long been considered an important indicator of endurance performance.
Runners, cyclists, swimmers, rowers, and other endurance athletes often pay close attention to it because it reflects how effectively the cardiovascular and respiratory systems support sustained effort.
In laboratory settings, VO₂ max is measured through specialized testing. Participants exercise on equipment such as treadmills or stationary bikes while wearing masks that analyze the oxygen they inhale and the carbon dioxide they exhale.
This process allows researchers and exercise physiologists to directly measure gas exchange and calculate oxygen consumption with a high degree of accuracy.
For decades, this type of testing has been considered the gold standard.
Some of the most important signs of fitness happen in places a watch can never fully see.
The Hidden Reality: Your Watch Isn't Measuring Oxygen at All
This is where many people are surprised.
Most wearable devices do not directly measure oxygen consumption during exercise. Instead, they estimate VO₂ max using algorithms.
Those algorithms analyze information such as:
Heart rate
Running pace
Cycling power output
Age
Weight
Sex
Activity history
Using these variables, the device attempts to predict what a person's VO₂ max might be.
That distinction is important because prediction and measurement are not the same thing. Dr. Peter Attia, a physician, longevity researcher, and author of the bestselling book Outlive, has frequently discussed the strengths and limitations of wearable fitness technology.
Through his work examining the science of aging, performance, and metabolic health, he has noted that most wearable devices estimate VO₂ max using algorithms rather than directly measuring oxygen consumption.
While those estimates can be useful for identifying trends, they are best viewed as informed approximations rather than precise physiological measurements.
Exercise and longevity experts have pointed out that estimating VO₂ max without directly measuring gas exchange introduces limitations from the very beginning.
If oxygen consumption is not being measured directly, the final estimate depends heavily on the quality of the data and assumptions built into the algorithm.
That does not mean the estimate lacks value. It simply means it should be viewed as a guide rather than a precise measurement.
Many users are surprised to learn that one of their favorite fitness metrics is actually generated behind the scenes through mathematical modeling.
Numbers often feel precise, even when they are built on assumptions.
Why Two People Can Get Very Different Numbers From Similar Workouts
Even the best algorithm depends on accurate data.
One challenge is that most wearables rely heavily on wrist-based optical heart rate sensors. These sensors use light to estimate blood flow beneath the skin and calculate heart rate.
While the technology has improved significantly over the years, it is not perfect.
Several factors can influence accuracy:
Watch placement
Strap tightness
Arm movement
Skin characteristics
Exercise intensity
Environmental conditions
A watch that shifts slightly during a workout may collect different information than one that remains securely in place.
High-intensity intervals can also create situations where wrist sensors struggle to keep up with rapid heart rate changes.
Because heart rate is often one of the most important variables used to estimate VO₂ max, even small inaccuracies can affect the final calculation.
Sports technology specialists frequently note that wearable devices can be directionally useful for tracking trends over time. They may identify general trends over time.
However, when people focus on small changes of only a few percentage points, sensor limitations become more significant.
Every fitness score begins with data, and every piece of data has limits.
When Good Data Leads to Bad Decisions
Data can be incredibly useful.
Problems arise when people assume every number is perfectly accurate.
Imagine an athlete who has been training consistently for several months. Endurance has improved. Recovery feels easier. Workouts that once seemed difficult now feel manageable.
Then the watch reports a lower VO₂ max estimate.
Suddenly, confidence drops.
The athlete begins questioning whether progress is actually happening.
Situations like this are more common than many people realize.
Some experts have noted that certain training styles can influence wearable estimates. For example, athletes who spend a large amount of time performing lower-intensity aerobic training may sometimes see lower estimated values despite making meaningful fitness gains.
This does not necessarily mean the person's actual fitness has declined.
Instead, it may reflect the way the algorithm interprets available workout data.
The opposite can happen as well. An unusually favorable estimate may encourage someone to train harder than their current fitness level supports.
In both situations, the danger lies in allowing a single metric to override other important signs of progress.
Performance improvements, recovery quality, consistency, energy levels, and real-world results often tell a more complete story than any one number alone.
Progress is not always reflected immediately by a single metric.
The Smarter Way to Use Wearable Technology
The goal is not to ignore wearable data but to use it wisely.
Many coaches and performance professionals encourage athletes to view wearable metrics as tools rather than verdicts.
A VO₂ max estimate can provide useful information when viewed over long periods of time. Consistent upward or downward trends may reveal meaningful changes in fitness.
Many sports scientists encourage athletes to focus less on individual readings and more on long-term patterns. Dr. Shona Halson, a professor and sports scientist whose research focuses on athlete monitoring, recovery, training load, and wearable technology, has spent years studying how performance data can support training decisions.
Her work suggests that a single workout or unexpected reading rarely tells the entire story.
Meaningful trends typically emerge over weeks and months, providing a clearer picture of fitness progress than any one data point alone.
At the same time, those trends work best when combined with other indicators, including:
Workout performance
Race results
Recovery metrics
Training logs
Personal observations
Overall energy levels
For individuals who want the most accurate assessment possible, laboratory testing remains an option. While not necessary for most recreational exercisers, direct testing can provide valuable insight for serious athletes or those seeking precise performance data.
The strongest fitness gains still come from showing up consistently, putting in the work, and allowing progress to build over time.
The Future of Fitness Tracking May Be More Accurate Than Ever
Wearable technology continues to evolve at an impressive pace.
Advances in sensor design, machine learning, artificial intelligence, and data analysis are helping devices become more sophisticated each year.
Future wearables may incorporate larger amounts of personalized information, allowing algorithms to better understand individual physiology rather than relying primarily on population averages.
Improved sensors could also reduce some of the measurement challenges that affect current devices.
As technology advances, fitness trackers may become increasingly capable of providing insights that are both more detailed and more accurate.
Yet even as these innovations emerge, one principle will likely remain true.
Technology works best when people understand both its strengths and its limitations.
The real value of technology may not be in telling people exactly how fit they are, but in helping them stay engaged long enough to become fitter.
Looking Beyond the Number
VO₂ max remains one of the most respected measurements in endurance fitness, and wearable technology has made it more accessible than ever before.
The estimates provided by modern devices can offer useful guidance, but they work best when viewed alongside real-world performance, recovery, and long-term training consistency.
Because at the end of the day, fitness is not defined by a single number on a screen.
It is reflected in stronger workouts, improved endurance, better health, and the confidence that comes from continuing to move forward.
Looking for more fitness inspiration and practical movement guidance? Explore Fitness Focus — and discover additional wellness, lifestyle, and community stories on Sacramento Living Well.
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Authored by the Sacramento Living Well Editorial Team — a publication of DSA Digital Media, dedicated to highlighting wellness, local living, and inspiring community stories throughout Greater Sacramento.
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