What exactly do fitness guides mean when they give a relative V?O?max?
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 V?O?max [...]
Here, the author seems to recommend a workout that keeps a specific quantity in a certain range relative to an individual's V?O?max 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 V?O?max? 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 V?O? 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:
If the considered quantity, X, really was the momentary V?O? value, then it should by definition not be possible to achieve an X beyond the V?O?max value (= beyond 100 percent), right? Nevertheless, such X values seem to be possible.
In this figure, there is a V?O?max 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, V?O?max 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:
What exactly is the quantity (X) that is expressed relative to a person's V?O?max value?
What does it mean to “attain 100 percent of your V?O?max” in practice?
Why is V?O?max also referred to aerobic fitness if it is achieved at such high heart rates?
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Yes, V?O? 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, V?O?max 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 V?O?max.
Of course, it should be unsurprising that exercise at V?O?max is, by its very definition, maximally aerobic.
I hope that answers your questions.
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