Let’s start with the question we’re trying to answer:
What does it mean to be in an Aerobic, Anaerobic, or Balanced physiological state?
This isn’t a label for the sake of categorization. Understanding an athlete’s physiological state helps us answer a much more important question:
What kind of training will give this athlete the greatest return on their training investment right now?
Almost any endurance training will "work" to some degree. The real question isn’t whether training works - it’s which training works best, given the athlete’s current physiology.
Why Physiological State Matters
Whenever we’re coaching an athlete, our job is not simply to make them fitter in some abstract sense. Our job is to apply stress in a way that produces the most efficient adaptation possible.
That means understanding what the athlete actually needs.
Physiological state gives us that context. It helps us identify whether an athlete would benefit most from:
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Building aerobic durability
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Raising their sustainable speed or power
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Improving their ability to access and utilize what they already have
Without this context, training becomes a scattershot approach. With it, training becomes intentional.
Where Physiological State Comes From
At QT2, we determine physiological state using the same testing that gives us Critical Power (CP) on the bike and Critical Speed (CS) on the run.
From short, medium, and longer efforts, we calculate:
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Critical Power or Critical Speed (the athlete’s sustainable ceiling)
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A proxy for power or speed at VO₂ Max (their high-end capacity)
The key relationship we care about is the ratio between critical intensity and VO₂ Max intensity.
That ratio tells us something fundamental about how the athlete’s physiology is currently organized.
The Athletic Architecture Model
To make sense of this, it helps to use a visual model - what we refer to as Athletic Architecture, originally developed by Dr. Phil Skiba.
Think of the athlete as a house:
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The foundation is aerobic fitness and durability - how much work the athlete can support over time.
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The ceiling is Critical Power or Critical Speed - the athlete’s sustainable speed or power potential.
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The roof is power or speed at VO₂ max - their upper limit.
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The little guy inside the house isn’t the athlete themselves, but their ability to use their fitness in a race‑specific way.
For endurance athletes, the most important metric defining speed potential is Critical Power or Critical Speed. Durability - the size of the foundation - determines how long that potential can be expressed.
The Attic: Where Physiological State Lives
Physiological state is defined by what we call the attic space of the house.
That attic is the space between:
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Critical Power / Critical Speed
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Power or speed at VO₂ Max
In other words, it’s the gap between what you can sustain and your absolute upper limit.
The size of that attic tells us whether an athlete's physiology is in a Balanced-, Anaerobic-, or Aerobic-state.
The Balanced Physiological State
When an athlete’s Critical Power or Critical Speed sits at roughly 85-88% of their power or speed at VO₂ Max, we consider them to be in a Balanced physiological state.
This is generally a healthy place to be.
The attic space is proportional. The relationship between sustainable intensity and high‑end capacity makes sense. From a training perspective, this gives us flexibility.
Athletes in a Balanced physiological state tend to respond well to:
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Base/Durability training blocks
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Specificity training blocks
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Hybrid training approaches
The goal here isn’t to change the relationship - it is to move the entire structure upward. We want Critical Power, Critical Speed, and power/speed at VO₂ Max to improve together, maintaining physiological balance between upper-end and sustainable efforts.
The Anaerobic Physiological State
When Critical Power or Critical Speed occurs at less than ~85% of power or speed at VO₂ Max, the athlete is in an Anaerobic physiological state.
In house terms, the attic is too large.
This doesn’t mean the athlete lacks top‑end ability - quite the opposite. It means they have more high‑end capacity than they can sustainably access.
In this case, the biggest return on training investment comes from raising Critical Power or Critical Speed relative to power or speed at VO₂ Max.
We do this by:
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Emphasizing sub‑threshold and just‑above‑threshold work
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Improving aerobic support and durability
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“Closing the attic” by lifting the ceiling, not lowering the roof
The aim is to make more of the athlete’s available capacity usable for longer durations.
The Aerobic Physiological State
When Critical Power or Critical Speed sits at greater than ~88% of power or speed at VO₂ Max, the athlete is in an Aerobic physiological state.
Here, the attic space is very small - almost like a crawl space.
In some cases, progress is limited because Critical Power or Critical Speed is already very close to the athlete’s high‑end capacity. To create further movement, we may need to address VO₂ Max itself.
This is where a VO₂ Max block can make sense - not as a default, but as a targeted intervention.
There are also situations where we accept that Critical Power or Critical Speed may not move much in the short term. In those cases, the focus shifts toward sub-threshold durability:
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Extending time‑to‑exhaustion (TTE)
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Improving the athlete’s ability to use their sustainable intensity for longer
The goal isn’t always to raise the ceiling - sometimes it’s to strengthen the foundation.
Why This Framework Matters
Classifying an athlete's physiological state as Aerobic, Anaerobic, or Balanced isn’t about boxing them in.
It’s about understanding where the biggest physiological opportunity exists right now.
This framework helps us:
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Choose the right block at the right time
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Avoid chasing adaptations that aren’t available
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Maximize return on training investment
Physiology doesn’t dictate everything - but it sets the boundaries within which smart decisions live.
Inside QT2.0, this is one of the ways we move from “training hard” to training intentionally.
If better understanding your physiological state could help you as an athlete, speak with a QT2 Coach, HERE.
