Let’s start with the question.
What is Critical Power?
Critical Power represents the boundary between physiological stability and physiological instability. It is the threshold between intensity you can sustain...and intensity that will eventually force you to stop.
If you’ve heard the term FTP - Functional Threshold Power - described as the highest power you can hold for roughly 60 minutes, you’ve already brushed up against this concept. In many ways, FTP was our attempt to approximate what Critical Power actually measures.
Typically speaking, Critical Power is an intensity that can be sustained for somewhere between 30 and 70 minutes, depending on the athlete and their durability.
But what really matters isn’t the clock.
What matters is where the body transitions from stable to unstable.
The Stable-Unstable Divide
Below Critical Power, the body can reach a steady state. Energy production, oxygen delivery, and waste removal balance out. The effort is demanding, but sustainable.
Above Critical Power, that balance breaks. Lactate accumulates more rapidly, reliance on anaerobic energy systems increases, and fatigue builds in a way that has a clear expiration date.
In simplified terms:
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Below CP → primarily aerobic in nature
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Above CP → increasingly anaerobic contribution
Critical Power is the dividing line.
The Power Duration Curve
Every cyclist has a Power Duration Curve.
For short durations, you can produce very high power. As duration increases, the maximal power you can sustain decreases. The curve slopes downward and gradually approaches a limit.
That limit - the line the curve approaches but never quite reaches - is known as the asymptote.
That asymptote is your Critical Power.
The area above that line represents something else important: W′ (W prime) - your anaerobic capacity. This is the finite amount of work you can perform above Critical Power before failure.
In other words:
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Critical Power defines sustainable intensity
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W′ defines how much work you can do above it
The Mathematical Problem
The Power Duration Curve is an exponential decay curve. And exponential decay curves are not easy to model cleanly with real-world training data.
That makes identifying the exact asymptote - the true Critical Power - difficult if we try to model the curve directly.
Fortunately, there’s a workaround.
The Elegant Solution: Linear Regression
Dr. Phil Skiba demonstrated that if we change our perspective slightly, the problem becomes much simpler.
Instead of plotting power against time, we calculate total work:
Work = Average Power × Duration
When we plot:
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Duration (x-axis)
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Total Work (y-axis)
The relationship becomes linear.
And when something becomes linear, we can use good old-fashioned algebra:
y = mx + b
By testing three maximal efforts - typically something along the lines of:
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~3 minutes (short)
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~6 minutes (moderate)
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~12 minutes (long)
-we generate three data points.
Through those three points, there exists a line of best fit.
Using linear regression:
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The slope (m) of that line represents Critical Power
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The y-intercept (b) represents W′, the athlete’s anaerobic capacity
What once required complex curve modeling now becomes accessible and practical.
Input three maximal efforts into a calculator, apply linear regression, and you have:
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Critical Power
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W′
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Training zones derived from them
No lab required.
From Critical Power to Training Zones
Once Critical Power is established, training zones stop being arbitrary percentages.
We gain a clear reference point that allows us to map intensity across energy systems.
Relative to Critical Power, we can estimate:
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Where LT1 typically occurs
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Where Type I fiber dominance gives way to greater Type IIA - Oxidative contribution
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Where crossover into more glycolytic Type IIA - Glycolytic fibers begins
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How much W′ an athlete has available for supra-threshold work
Instead of guessing at where aerobic ends and anaerobic begins, we anchor training zones to a physiologically meaningful boundary.
That precision changes how we prescribe work, evaluate response, and progress an athlete over time.
Why Critical Power Matters
Without Critical Power, training decisions can drift.
Athletes often ride slightly too hard on aerobic days, not quite hard enough on quality days, and burn through anaerobic capacity without realizing it.
With Critical Power:
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Easy work remains truly aerobic
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Threshold work sits where it is intended to sit
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Supra-threshold work draws intentionally from W′
Training doesn’t become easier.
It becomes more deliberate.
Critical Power isn’t just a number on a chart, or easily defined as peak 60-minute wattage (P60).
It is the anchor point that allows endurance training to move from approximation to intention - and it is a foundational metric inside QT2.0.
