They conflate methodology with biology

Deep Dive of VO2 Max Science, published on Sean's Substack The Performance Pulse.

I recently found this article on my laptop that I was writing about a year ago in response to people heavily criticizing VO₂max, so figured I would post it now. Since VO₂max has boomed in popularity and public awareness recently, that naturally leads some people to try and dampen the hype for whatever reason.

The criticisms, as I remember it, rested on two main points. First, large-scale mortality studies almost often use imperfect VO₂max estimates from either submaximal exercise, or “performance” metrics like time-to-exhaustion. Both do not directly measure O2 uptake. In the latter case, performance is thus the longevity metric, not VO₂max. Second, many studies that do directly measure it don’t observe or confirm a plateau in VO2, suggesting maximal capacity was not actually reached. It follows that no longevity/health study has ever actually measured VO₂max and it is a useless metric with no supporting data (being somewhat facetious here, but analogous claims have been made a vocal few).

The arguments are based on cherry-picked data and semantics, and in my opinion, not understanding the physiology. They conflate a measurement problem with an issue about the biology.

VO₂ Max Estimates

The first point is conceptually true since submaximal exercise estimates do create noisier data and extrapolating submaximal heart or rate or VO₂ to a predicted maximum is not quite the same thing. It’s done because it is safer and more feasible in large populations. However, you could easily argue this gives more credence to the relevance of VO₂max, not less. Despite the noise in the data, an undeniable signal remains for mortality risk, metabolic syndrome, respiratory diseases, cancer, dementia, depression and anxiety, and so on. This even holds true for non-exercise estimates which are even noisier. I find this similar to studies on blood pressure, where the data on cardiovascular risk is unambiguously strong despite likely lots of variability in how manual blood pressure is measured between sites.

The point is that the physiologic signals behind these metrics is so important, even if your measurement method isn’t perfect, you still get valuable information. It’s not that the metric itself has no utility.

Then there’s the performance argument, in this context defined as time-to-exhaustion during an incremental exercise test. In these studies, like the classic work by Steven Blair, they run people through a VO₂max test protocol but without measuring oxygen. Start out with a walk, increase speed on the treadmill every 1-2 minutes, and go until you can’t anymore. Endurance treadmill time (ETT) is used as the cardiorespiratory fitness metric, or metabolic equivalents using published tables that say “this speed is equivalent metabolically to a VO₂ of X.” For instance, 6 mph is roughly 10 metabolic equivalents, or a VO₂ of 35 ml/kg/min.

Saying performance is better than VO₂max is a circular argument. They are interchangeable! The consumption of oxygen permits you to reach higher speeds. Just try holding your breath and see how far you’ll make it. Before using this in a study, they confirmed the positive relationship between speed and VO₂, in the case of the Blair study showing maximal test speed or ETT accounted for 84% of the variance in VO₂max. So, it’s not identical, but 84% of the way there is a good enough signal for health outcomes because VO₂max is so powerful.

Aerobic performance and VO₂max are interchangeable in the context of the performance of an incremental exercise test. I’m not saying performance in general (5k time, marathon time, etc.) and VO₂max are interchangeable. It is a big predictor of actual real world endurance performance, but not the only one.

Work rate (speed, power) is the primary driver of increased VO₂ because faster speeds require more intense muscle contractions, which requires accelerated breakdown and utilization of cellular energy, which requires more oxygen to provide/replenish that energy. Therefore, you can’t pit VO₂max against aerobic performance – consuming more oxygen is fundamentally what allows you run faster! The way to resolve this pointless debate is to search for a human species who can run faster (aerobically) without consuming more oxygen.

To be clear I think performance in general is an incredible longevity metric. I wrote a blog about this exact topic a year or so ago. It’s the ultimate expression of integrated physiology and the decline in performance over time closely mirrors that of molecular aging.

You can also often get very accurate VO₂max estimates from performance data, particularly for events whose duration sits squarely in 5-10 minute range and thus heavily dependent on VO₂max, like 2000 meter rowing, 3000 meter running, 1-2 mile running, 5 minute all-out power on the bike, etc. But that doesn’t mean performance is “better” than VO₂max. VO₂max is a physiological variable that underlies performance and health.

The one issue with defining performance as the length of a treadmill test (in units of time or maximal speed) is that the result is completely dependent on the protocol, and it is only comparable between people if there was 1 universal way to measure VO₂max (there is not). Take the below image as an example. This is 1 person doing 3 different VO₂max protocols. VO₂max is the same in all 3, but their maximal power output (analogous to maximal speed) ranged from 265 to 375 Watts. By using VO₂max as the outcome, the information stays constant. By using “performance” it is quite varied. These are measurement problems, not biological ones. VO₂max is biological, agnostic to the test protocol, and highly predictive of health and disease.

The VO₂ Plateau

The second point assumes that a plateau in VO₂ at the end of a VO₂max test is a de facto requirement to confirm true maximal capacity has been reached. It is not. This is another methodological issue, but critics might state that because a plateau wasn’t confirmed then VO₂max wasn’t actually measured. The VO₂ plateau phenomenon is when you increase speed/power during a test from one minute to the next but VO₂ stops increasing as it had been. It flattens out. You are asking more of the body by increasing the intensity of muscle contractions, but have reached a limit. The heart has reached its pumping capacity, no more oxygen can be extracted or diffused from blood-to-muscle, etc. You’ve hit your limit and will soon be unable to maintain the increasing pace. See below for an example of a plateau in oxygen uptake.

Example of a plateau in oxygen uptake.

The plateau phenomenon was developed from discontinuous exercise tests in a study by Henry Taylor where young men visited a research lab over several days, running at a constant speed and incline until exhaustion. They returned each day to run at an increasing incline (+2.5% per day) until they quit, then coming back until VO₂ failed to increase more than 150 ml/min (it plateaued). Why 150 ml/min? On average, VO2 increased by 300 ml/min each time the incline increased by 2.5%, so Taylor figured taking half of that is a good criterion for defining a plateau. The exact rationale was that if VO₂ failed to increase more than 150 ml/min with increasing work rate “there is small chance of making an error in deciding that the maximal oxygen intake had been reached.”

For one, the 150 number is clearly arbitrary. Two, that only holds true if you do VO₂max testing exactly like that: a constant speed of 7mph + 2.5% incline on consecutive days. Running at 9mph + 2.5%, or 7mph + 4%, or cycling with +30-watt increments every minute, or using continuous single-day incremental tests all induce different oxygen costs per change in intensity (i.e., it will not be +300 ml/min per stage in all tests and thus an absolute 150 ml/min criterion is invalid). Applying the 150 ml/min criterion for all tests is like putting a size 14 shoe on a size 9 foot.

Further, obtaining a plateau is influenced by many factors independent of you achieving actual maximal capacity. These include your anaerobic capacity as an athlete, the speed of your VO₂ kinetics, lactate threshold, and potentially motivation or pain tolerance. If you’re interested in reading further, there’s an excellent review titled The Oxygen Uptake Plateau—A Critical Review of the Frequently Misunderstood Phenomenon, as well as Lessons from history for VO2max and the VO2 plateau, part 1, 1920 – 1961.

Example of how having a low (top) or high (bottom) anaerobic capacity can influence the observation of a plteau.

One important one is simply how many times you measure a breath during the test (the sampling rate). A shorter sampling rate increases the likelihood of observing a plateau. One group suggests you should use 50 seconds to reduce false positives. In the below figure, a plateau was identified in 100% of subjects using a breath-by-breath or 15-second sampling rate, 57% using a 30-second sampling rate, and 8% using 60-seconds. From experience, most labs and published studies use 30 seconds for their VO₂ analysis and don’t pay much mind to sampling rate or even do a plateau analysis. Again, a methodological issue, not a biological one.

I recently wrote about why VO₂ Max matters from a purely biological perspective. Put all the testing issues aside, what does having a high VO₂max actually mean and why is that relevant to health? If you’re curious I’d encourage you to give it a read because the fundamentals are important when talking about VO₂ Max, a topic that can sometimes get corrupted on social media.