VO2 max Part 1: What it is and why it matters

NOTE: Full citations and articles should be up shortly.

VO2 Max* is the maximal amount of oxygen you can breathe into your lungs, transport and then actually uptake and use by your muscles; it is typically expressed as ml per kg of mass (or muscle mass) per minute. No other physiologic test measures carries with it more misunderstanding, mystique, and perhaps value than VO2 Max. I suppose that’s a bit mellow dramatic, but in terms of testing, its one of the first tests we learn as undergraduates, and it’s the one that is most often cited in the sports-fitness media as the measure for performance. As many articles point out, VO2 Max is not the best performance predictor, yet many coaches continue to publish workouts purporting to increase VO2 Max. The purpose of this article is not merely put to rest long-held myths, but to explain what VO2 Max represents, and how it really does influence performance.

VO2 Max: Why engine size matters

The most common analogy made regarding VO2 Max is that of the car engine. While a high VO2 Max is indicative that your engine is big, it often oversimplifies the interaction it has with actual endurance performance. As defined earlier, VO2 Max is the maximum volume of oxygen that you can deliver and use. It is impacted by several variables, including:

  • Cardiac Output (L/min), how much blood your heart can pump in a minute, is possibly the biggest influencer of VO2 Max because it determines how much blood volume gets delivered to the muscles. Average values max out at about 25 L/min, while the very best can reach 40 L/min.
  • Oxygen extraction by the muscles (aka, A-VO2), is how much oxygen you can actually ring out of the blood when it gets to the muscle. While some elite athletes may have an advantage here, oxygen extraction usually is not a limiting factor in VO2 Max or performance except for altitude. In fact, the drop in performance at altitude is large related to association-dissociation curve discussed in my recent altitude article; the lower the difference in pressure between the blood and muscle, the less oxygen you can off-load.
  • O2 carrying capacity is the key variable that is manipulated by blood doping. Simply put, if cardiac output is capped and you cannot deliver more volume, you can only pack in more red blood cells (ie, increase density) within a given volume. The more blood bound O2 that gets to the muscle, the more that gets into the muscle to the mitochondria.
  • Mitochondrial capacity is the ultimately the determiner of endurance on many levels. The more mitochondria you have packed in your muscles, the better you will be at producing ATP aerobically. Assuming you can deliver the raw material (oxygen), then your ATP will produce the ATP. How fast they can do this individually depends on the fuel source, carbs are faster hands down, but if you have more, you get more ATP.

The figure below depicts the above interaction between the variables just outlined. The obvious conclusion that must be drawn is that if you deliver and use more O2 you will go faster, because you’re producing more ATP. This too is a bit too simplistic but helps to highlight the limitations in testing and drawing conclusions from a VO2 Max test. Dr. Tim Noakes (2008) has noted several issues with not only the concept, but also the testing methodology for VO2 Max. While beyond the scope of this article, many of the issues will permeate the remainder of my interpretation and validity of VO2 Max for athletes. Needless to say, it is fair to say that a large oxygen delivery is needed for success in endurance (activities last ~3 min or more). How large is open to debate if we compare max values some of true greats of the sport. Based on the available literature, 70 ml/kg/min seems to be the bottom of the range for male runners and cyclists, and 55 ml/kg/min for women. However, as I’ll discuss below, several factors can influence ultimate performance, especially among individuals within 10% of each other.


VO2 Max testing has become a brain dead test

Taking inspiration from Tim Noakes’ Brainless Model paper, I want to highlight why most VO2 Max testing methods provide you with little useful information about your ability level or how to improve your performance. Again drawing from Noakes, the traditional 8-12 min incremental test (see Part II, see footnote 1) artificially and rapidly preventing the brain from pacing appropriately. In other words, the brain can, and does influence resources in the body to produce as even a pace as possible. I believe Noakes is correct in criticizing the fact that humans simply do not start exercising very easily then continue to ramp up the pace until they collapse; this would be a poor way to run a race! Thus, you arbitrarily, and volitionally exhaust yourself, also called quitting. The test no longer becomes a purely physiologic measure. While I do not believe this is itself invalidates the test or eliminates a useful measure of fitness (see Part II, see footnote 2 on VO2 Max and health), it does undermine its ubiquitous use as a performance measure.

At the heart of the problem is how VO2 Max is defined and how it then defines exercise intensity. The idea that we consider VO2 Max MAX, then express workloads above it as supramaximal seems asinine on its face; MAX is MAX. Certainly, denoting maximum aerobic capacity as VO2 Max is acceptable, if it is specifically describing that capacity. However, as any good exercise science student will tell you, not all the ATP produced at VO2 Max are produced aerobically, as a small portion are derived via glycolysis (see Part II, footnote 3, When is Max, Max?) Thus, the oxygen consumption may indeed be maximum, but the pace at which it is achieve is likely higher than true VO2 Max pace. The implications here should be obvious; exercise output using traditional procedures overestimates workload at VO2 Max. This matters if your tester plans to prescribe workloads from that test.

Many may be wondering what all this means if you’re just looking get tested to see what your max is. The answer is simple: nothing really. If all you care about is getting tested and then sitting around the water cooler and discussing who has the bigger MAX, then email me and I’ll gladly take your money! However, if you are really interested in optimizing your performance, then VO2 Max is worth about as much as the paper your results are printed on. Any test facility that tells you otherwise is either unqualified and does not know any better (we have a our share in Richmond, VA) or they are lying to you. Regardless of whether you aim to improve your max, knowing it won’t influence your ultimate performance. As a screening tool it’s unnecessary, and as a predictor, it’s all but useless.

In Part II of this article, I will discuss why VO2 Max is not all that meaningful, whether you should test, and how you should do it. Also included are the additional side topics noted by the footnotes.

* VO2 Max literally means volume of oxygen (O2) used per unit of time, in this case 1 min. If you want to spot someone who is clueless about physiology look for the use of VO max, without the 2. Is this a pet peeve? Yes, and a good one. If you cannot use the physiological terminology correctly, how on earth can you be trusted to understand it? Also,  VO2 Max is often used interchangeably with   VO2 Max, which is not technically correct. In general, actual max must meet a specific number of defined criteria, whereas peak is the highest value attained. They are often the same or similar, but peak cannot be considered a true max.

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