The reference method: measuring, not estimating
A cardiopulmonary exercise test measures VO2 max rather than inferring it. The participant exercises to exhaustion on a treadmill or cycle ergometer under a graded protocol while expired air is analysed breath by breath, so oxygen uptake is read directly as the workload rises and then plateaus.
This is the method behind the reference standards themselves. The FRIEND Registry standards used in ACSM's Guidelines come from directly measured maximal treadmill tests, which is why comparing a watch estimate against those percentiles carries the watch's error with it.
Field tests: an equation instead of a gas analyser
Field tests replace the analyser with a regression equation fitted against laboratory measurements in some original study population. They are cheap, need no equipment, and inherit both the accuracy and the limitations of the population they were fitted on.
The Cooper 12-minute run, published by Kenneth Cooper in JAMA in 1968, uses the distance covered in a maximal 12-minute effort: VO2 max = (distance in metres − 504.9) ÷ 44.73. The Rockport 1-mile walk, from Kline and colleagues in 1987, uses walking time and post-exercise heart rate together with sex, age and body weight, which makes it suitable for people for whom a maximal run is not appropriate.
| Method | What it needs | Effort required | What limits it |
|---|---|---|---|
| Cardiopulmonary exercise test | Laboratory, gas analysis, supervision | Maximal | Cost and access |
| Cooper 12-minute run | A measured track and a stopwatch | Maximal | Pacing, weather, terrain; fitted on young fit adults |
| Rockport 1-mile walk | A measured mile and a heart-rate reading | Brisk walk | Heart-rate accuracy; walking must be brisk and steady |
| Step tests | A step and a timer | Submaximal | Step height and cadence must match the protocol |
| Smartwatch estimate | The device and usually a chest strap | Submaximal runs | Algorithm assumptions; accuracy varies with fitness level |
What smartwatch estimates are worth
A 2025 qualitative systematic review screened 252 records and included 13 studies of wearable VO2 max estimation, most of them examining Garmin watches paired with a chest-strap heart-rate sensor and the Firstbeat algorithm. In every case the comparison was against a laboratory graded exercise test, while the device estimate came from submaximal outdoor running.
Seven of the 13 studies judged the wearable valid or acceptable. One reported a mean absolute percentage error below 5%, and four reported 5% to 10%. At the other end, one study found systematic underestimation with a mean difference of −4.51 ml/kg/min and a mean absolute percentage error of 15.78%.
The reviewers noted that validity was demonstrated in healthy untrained adults, recreational athletes and team-sport professionals, while usefulness in elite endurance sport remained questionable. A practical consequence follows: an error of a few ml/kg/min is easily enough to move a reading across a percentile class boundary, so a watch figure is better used to track your own trend than to claim a class.
Comparing results without fooling yourself
Because each method carries its own error, the comparisons that mean something are the ones that hold the method constant. Two Cooper tests a season apart on the same track are informative; a Cooper test compared with last year's watch estimate is mostly measuring the difference between two methods.
- Hold the method constant when tracking change over time.
- Hold the conditions constant too — wind, heat, terrain and pacing all move a field-test result.
- Record which scale you read the label from, since category names differ between published charts.
- Treat an estimate as a band rather than a point, and expect boundary cases to flip between attempts.
- Maximal tests are demanding: they are inappropriate for some people, and medical advice comes first.
Frequently asked questions
What is the most accurate way to measure VO2 max?
A cardiopulmonary exercise test in a laboratory, in which expired air is analysed while the participant exercises to exhaustion under a graded protocol. It measures oxygen uptake directly rather than inferring it from an equation, and it is the method behind the reference standards that classification charts are built on.
How accurate is the VO2 max on my smartwatch?
In a 2025 systematic review of 13 studies, seven found wearables valid or acceptable against a laboratory test. One study reported a mean absolute percentage error below 5% and four reported 5% to 10%, while one found underestimation with a mean difference of −4.51 ml/kg/min and an error of 15.78%. Accuracy varied with fitness level, and usefulness in elite endurance athletes was questionable.
Is the Cooper test or the Rockport walk test better?
They suit different people. The Cooper 12-minute run requires a maximal effort and was fitted on young, fit adults, so it is best for people already accustomed to hard running. The Rockport 1-mile walk needs only a brisk walk plus a heart-rate reading and includes age, sex and body weight in its equation, which makes it more appropriate for people for whom a maximal run is unsuitable.
Why does my VO2 max change between tests?
Field tests infer VO2 max from performance, so anything that affects performance affects the estimate. Pacing, wind, heat, terrain, footwear, sleep and how recently you trained all shift the result, and heart-rate-based tests add the error of the heart-rate reading itself. Matching conditions between tests is what makes two results comparable.
Can I compare a watch estimate against an ACSM percentile chart?
You can, but carry the caveat with it. The percentile standards come from directly measured laboratory tests, so comparing an estimate against them adds the estimate's error to the comparison. Since a few ml/kg/min can move a reading across a class boundary, a watch figure is better suited to tracking your own trend than to settling which class you are in.
References
- Cooper KH. A means of assessing maximal oxygen intake: correlation between field and treadmill testing. JAMA 1968; 203(3): 201–204.
- Kline GM, Porcari JP, Hintermeister R, et al. Estimation of VO2max from a one-mile track walk, gender, age, and body weight. Medicine & Science in Sports & Exercise 1987; 19(3): 253–259.
- Železnik Mežan L. Accuracy of wearables for determining the maximal oxygen uptake and lactate threshold: a qualitative systematic review. Frontiers in Sports and Active Living 2025; 7: 1707991.
- Kaminsky LA, Arena R, Myers J, et al. Updated reference standards for cardiorespiratory fitness measured with cardiopulmonary exercise testing: data from the Fitness Registry and the Importance of Exercise National Database (FRIEND). Mayo Clinic Proceedings 2022; 97(2): 285–293.
- American College of Sports Medicine. ACSM's Guidelines for Exercise Testing and Prescription, 12th edition. Wolters Kluwer, 2024.