Cardio Fitness
Run hard over a known distance and Pelaris estimates your VO2max from that result using Daniels’ VDOT tables, ranks it against the FRIEND reference standards to show how you compare with adults your age and sex, then expresses the same number as a fitness age. All three are estimates. Here is exactly where each one comes from, and where each one stops.
In 1979, Jack Daniels and Jimmy Gilbert published Oxygen Power, a set of tables relating running velocity to the oxygen cost of holding it. Those regressions became VDOT, a single number that stands in for the aerobic fitness a given race performance implies, and they are carried forward today in the fourth edition of Daniels’ Running Formula. It is the pace-to-fitness model distance coaches have trusted for over forty years, and it is the one Pelaris uses.
The logic runs in one direction: a performance implies a fitness level. Hold 4:30 per kilometre for 10 kilometres and the oxygen cost of that pace, sustained for that long, tells you something reliable about the engine that produced it. Pelaris reads your strong recent efforts and returns the VO2max that pace implies. The same VDOT system also sets training paces in Daniels’ Running Formula, one of the running methodologies your program may be built on.
Which means the quality of the effort is everything. A model that turns pace into fitness has nothing to work with if the pace was never tested. A genuine race, a hard time trial, or a session you finished knowing there was very little left, those are what sharpen the estimate. A comfortable Sunday run tells it almost nothing, and it will not pretend otherwise.
A VO2max number on its own is hard to feel. Is 48 good? It depends entirely on who is asking and how old they are. So Pelaris reads your estimate against reference standards published by Kaminsky, Arena and Myers in Mayo Clinic Proceedings in 2015, drawn from the Fitness Registry and the Importance of Exercise National Database, known as FRIEND.
What makes FRIEND worth using is what sits underneath it: thousands of adults who were actually measured with cardiopulmonary exercise testing, split by age and sex. When Pelaris tells you that you are fitter than a given percentage of people your age, it is reading off that table rather than inventing a scale. Your percentile means something because the comparison group is real.
It also means something specific. The people in the registry were tested in exercise physiology labs and clinical facilities. That is a large and genuine sample, and it is not a census of everyone alive. Read your percentile as “compared with adults my age and sex in a large reference dataset”, which is precisely what it is, and precisely as much as it claims.
The idea comes from Bjarne Nes and colleagues, who mapped VO2peak across 4,637 adults in Norway’s HUNT study and showed something people find immediately intuitive: aerobic fitness can sit younger or older than a birthday suggests. Two 45-year-olds can be twenty years apart on the only measure that a training plan can actually move.
Pelaris applies that idea rather than reusing the HUNT model wholesale, and the distinction is worth stating plainly. We take your estimated VO2max and find the age at which it would sit at the median of the FRIEND reference standards. If your estimate matches the typical 32-year-old in that data, your fitness age reads 32. The concept is Nes’s, the reference curve is FRIEND’s, and that combination is ours.
The honest framing. Fitness age is a way of making one number feel human, and it is a good motivator because it moves when you train. It is not a biological age, it is not a measure of how well you are ageing, and it is not a statement about your health. It is your aerobic fitness, wearing a friendlier label.
A number you can trust is a number that tells you where it stops. Four things worth knowing before you read anything into yours.
The gold standard for VO2max is a graded exercise test in a lab, wearing a mask that measures the gas you breathe. Pelaris does not measure anything. It infers your aerobic fitness from the pace you held over a known distance, which is a well-established inference and still an inference.
A genuine race or a hard time trial on a flat, well-measured course gives the model its best shot. Hills, heat, wind, a short course or a steady social run will all pull the number around. Watch the trend across a training block rather than reacting to any single result.
Your percentile is read against the FRIEND reference standards, built from adults measured by cardiopulmonary exercise testing. That is a real, sizeable population and it is not the whole world. It is the honest comparison group, and it is worth knowing which group you are being compared with.
Pelaris uses these numbers for exactly one thing: showing you where your training has taken you. They are not a screening tool, not a diagnosis, and not a prediction about your heart or anything else. Health questions belong with a doctor, not with a training app.
Three sources carry everything above. Each was checked by hand against a live page. The full shelf of sixteen sits on the science hub.
Your VO2max estimate from a hard run comes straight from Daniels' VDOT tables, the pace-to-fitness model coaches have trusted for over forty years.
Daniels, J. Daniels' Running Formula, 4th edition. Champaign, IL: Human Kinetics, 2021. ISBN 9781718203662, 320 pp. Carries forward the VDOT tables and the velocity-to-oxygen-cost regressions first published in Daniels, J. & Gilbert, J. Oxygen Power: Performance Tables for Distance Runners, Tempe, AZ, 1979.
View the sourceThe "fitter than X% of people your age" line is read off real reference standards built from thousands of lab-tested adults, so your percentile actually means something.
Kaminsky, L.A., Arena, R. & Myers, J. "Reference Standards for Cardiorespiratory Fitness Measured With Cardiopulmonary Exercise Testing: Data From the Fitness Registry and the Importance of Exercise National Database." Mayo Clinic Proceedings, 2015;90(11):1515-1523. doi:10.1016/j.mayocp.2015.07.026
View the sourceFitness age is the idea that your VO2max can sit younger or older than your birthday suggests, first mapped across 4,637 adults in Norway's HUNT study.
Nes, B.M., Janszky, I., Vatten, L.J., Nilsen, T.I.L., Aspenes, S.T. & Wisløff, U. "Estimating VO2peak from a nonexercise prediction model: the HUNT Study, Norway." Medicine & Science in Sports & Exercise, 2011;43(11):2024-2030. doi:10.1249/MSS.0b013e31821d3f6f
View the sourceAccurate enough to train on, and not a lab measurement. An estimate read from a run depends entirely on the quality of the effort it read: a genuine race or hard time trial on a flat, well-measured course gives the model its best information, while an easy run, a hilly course or a hot day will pull the number around. The most useful way to read it is as a trend line across a training block rather than a precise value on any single day.
There is no single good number, and any chart that gives you one is hiding how much it varies by age and sex. Rather than quote a threshold, Pelaris shows where your estimate sits against the FRIEND reference standards, published by Kaminsky, Arena and Myers from adults measured with cardiopulmonary exercise testing. That gives you a percentile within your own age and sex group, which is far more useful than a number pulled from someone else’s table. The direction it moves over a season matters more than its value today.
Fitness age expresses your estimated VO2max as an age rather than a number. Pelaris finds the age at which your estimate would sit at the median of the FRIEND reference standards, so if your number matches the typical 32-year-old in that data, your fitness age reads 32. The idea comes from Nes and colleagues in Norway’s HUNT study, which mapped VO2peak across 4,637 adults and showed aerobic fitness can sit younger or older than a birthday suggests. It is a way of making a number feel human. It is not a biological age and it is not a health measure.
No app can measure it. Measuring VO2max requires a mask and a metabolic cart analysing the air you breathe during a graded test. What Pelaris does is estimate it from running performance using Daniels’ VDOT tables, the same pace-to-fitness model coaches have used for over forty years. Estimating and measuring are different jobs, and we label ours as the estimate it is.
Pelaris does not answer that and does not try. Your estimate describes aerobic fitness as your running suggests it, nothing more. Pelaris gives AI-assisted general training information, not medical advice, and any question about your health belongs with a doctor or a qualified health professional.
Because it is read from what you do, not from a profile you filled in once. A strong new effort gives the model better information than an older, weaker one, so the estimate moves. That responsiveness is the point: it is how the number stays honest about your training rather than drifting out of date.
Log a strong run and Pelaris does the rest. Your estimate, your percentile, and your fitness age update from what you actually did.
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Pelaris gives AI-assisted general training information. It is not medical advice, it does not diagnose anything, and it is not a substitute for a doctor or a qualified health professional.