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Take the following quote from Jack Daniels – Daniels' Running Formula (2014) as an example:

[...] E running is typically performed at about 59 to 74 percent of V̇O₂max [...]

Here, the author seems to recommend a workout that keeps a specific quantity in a certain range relative to an individual's V̇O₂max value. As far as I can tell, similar statements can also be found in many other running or fitness guides.

However, I don't quite understand what this quantity actually refers to. What should I try to keep between 59 and 74 percent of my V̇O₂max? For the sake of brevity, let's refer to this mysterious quantity as X in the following. At first, I thought that X was the instantaneous V̇O₂ during training, i.e., the weight-normalized volume rate of oxygen that is consumed during training expressed in ml/(min*kg). The following points made me skeptical, though:

  1. If the considered quantity, X, really was the momentary V̇O₂ value, then it should by definition not be possible to achieve an X beyond the V̇O₂max value (= beyond 100 percent), right? Nevertheless, such X values seem to be possible.
  2. In this figure, there is a V̇O₂max region that is mapped to heart rates between 90 and 100 percent of a person's maximum heart rate, which should be way beyond the aerobic region. Nevertheless, V̇O₂max is also referred to as aerobic fitness, which is a bit confusing and makes me think that there is significantly more to this value than it seems at first.

It would be great if someone could help me clean up this confusion. To be more specific:

  1. What exactly is the quantity (X) that is expressed relative to a person's V̇O₂max value?
  2. What does it mean to “attain 100 percent of your V̇O₂max” in practice?
  3. Why is V̇O₂max also referred to aerobic fitness if it is achieved at such high heart rates?
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Yes, V̇O₂ is “the weight-normalized volume rate of oxygen that is consumed during training,” expressed in ml/min/kg, but more fundamentally, it is a metric for the time-rate of energy production. Thus, V̇O₂max is the maximum aerobic power capacity for an individual, and the quantity X is a power output expressed as a percentage of that maximum, or alternatively the heart rate with which it coincides.

It is important to remember that bioenergetic systems operate as a continuum. Exercise at any intensity is neither perfectly aerobic nor perfectly anaerobic. This is why it is possible to speak of values of X that are greater than 100%, which refer to total power output relative to aerobic power output—the difference being from anaerobic pathways.

The common categorisation of exercises' being either ‘aerobic’ or ‘anaerobic’ is both misleading and erroneous. All exercise is both aerobic and anaerobic; it is just a matter of degree. And that degree is also very widely misunderstood. In a 50- to 60-second maximal effort—think, an elite 400-metre sprint—roughly half of the energy produced comes from aerobic pathways. By the time we are performing a 3-minute maximal effort, that figure is around 95%! This is why the common multi-stage aerobic capacity test, like the sub-maximal test administered in gymnasiums, uses 3-minute intervals; the test depends on the knowledge that the vast majority of the power is produced aerobically. And it is also the reason that HIIT protocol effectively trains V̇O₂max.

Of course, it should be unsurprising that exercise at V̇O₂max is, by its very definition, maximally aerobic.

I hope that answers your questions.

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    That makes sense, thank you very much! To make sure that I understand you correctly: Is it accurate to say that V̇O₂max describes the maximum power that can theoretically be delivered via aerobic processes, and “attaining V̇O₂max during training” means that aerobic and anaerobic processes combined deliver power equivalent to this value? If it is: What do you mean by “maximally aerobic” in the second last paragraph? This in turn sounds like training at V̇O₂max is totally unrelated to anaerobic processes. – bosonic Jun 3 at 23:38
  • @bosonic: Excellent questions. Yes, V̇O₂max should be thought of as an indicator of the maximum power that can theoretically be delivered via aerobic processes, but the quoted %V̇O₂max is a percentage of the heart rate at which V̇O₂max is theoretically achieved. Thus, at any given %VO₂max, our total power output is greater than our aerobic power output. It can be estimated to occur at the point that we can only maintain our work for 8 minutes. – POD Jun 4 at 0:52
  • @bosonic: Note that I have corrected the post above to clarify the point. – POD Jun 4 at 1:38
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    Alright, that clears up most of my confusion. Just on thing: The quote I included in my initial question actually goes like this: “Although E running is typically performed at about 59 to 74 percent of V̇O₂max or about 65 to 78 percent of maximum heart rate, at times you may feel more comfortable going a littler faster (or slower).” Is is possible that the author does actually use X to refer to the momentary power or intensity? At times, he also mentions %V̇O₂max values greater than 100, but a heart rate beyond the maximum heart rate is probably not achievable, right? – bosonic Jun 4 at 7:04
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    @bosonic: Yes, it does sound as though the author is referring to momentary aerobic power, in that case, and he must also be referring to power in order to speak of percentages above 100. – POD Jun 4 at 7:49

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