What is Critical Power and Critical Speed?
Understanding Critical Power and Critical Speed: Your True Physiological Threshold
The science behind your body’s natural performance ceiling
Critical Power and Critical Speed represent something fundamental about how your body works during sustained effort. They’re not estimates or averages - they’re actual physiological markers that reveal where your metabolism can hold steady and where it begins to break down.
So what exactly are CP and CS, and why do they matter?
What Is Critical Power?
Critical Power (CP) represents the maximum power output an athlete can sustain before their physiology becomes unstable. This typically occurs between 30-70 minutes of sustained effort.
Think of it as your body’s natural ceiling – the point where you can no longer maintain steady-state metabolism. It’s not a single razor-thin line, but rather a neighborhood of intensity where your body transitions from stable to unstable.
For most athletes, Critical Power corresponds to an effort that can be maintained for roughly 25 to 40 minutes, depending on individual durability.
Why CP Matters
- Below CP: Your physiology can settle. Oxygen delivery, energy production, and waste removal reach equilibrium. You can stay here for extended periods.
- Above CP: Balance disappears. Lactate accumulates faster than it can be cleared, breathing intensifies, and fatigue compounds rapidly toward failure.
Critical Power is the boundary between sustainable and unsustainable effort.
What Is Critical Speed?
Critical Speed (CS) is the running equivalent of Critical Power.
It’s the maximum pace you can sustain before your running physiology becomes unstable. Just like CP for cycling, CS represents your true physiological threshold for running – typically sustainable for 30-70 minutes.
What Makes CS Special
Critical Speed is your actual physiological tipping point – the pace where your body’s steady-state metabolism begins to fail. This is YOUR unique threshold, derived from YOUR performance data.
CP vs FTP: Understanding the Difference
You may have heard of Functional Threshold Power (FTP). Here’s how it relates to Critical Power:
Functional Threshold Power (FTP)
- Defined as the highest power you can maintain for exactly 60 minutes
- A data point on the power-duration curve
- A practical reference point for training
Critical Power (CP)
- Represents the actual tipping point where your body’s physiology becomes unstable
- Your TRUE physiological threshold
- The dividing line between what you can sustain and what will eventually break you
Both metrics provide valuable information for structuring training.
The Math Behind It (Don’t Worry, It’s Simple)
Dr. Phil Skiba demonstrated that we can estimate Critical Power and Critical Speed using linear algebra – yes, the y = mx + b kind you learned in school.
How Testing Works
Athletes perform efforts at three different durations:
- Short effort: 75-90 seconds (e.g., 400 meters)
- Medium effort: 2.5-3.5 minutes (e.g., 800 meters)
- Longer effort: 10-15 minutes (e.g., 2 miles)
Each effort gives us a distance and a time. When we plot those points and apply linear regression, a line of best fit emerges.
- The slope of that line = Critical Speed or Critical Power
- The y-intercept = Your anaerobic capacity (how much work you can do above threshold before failure)
At Angela Naeth Coaching, we use calculators that apply this regression across multiple data points to determine your CP, CS, and training zones.
Why This Matters for Your Training
Once CP and CS are established, we can map muscle fiber and energy system utilization across different intensities.
Example: An athlete with Critical Speed of 7:18/mile
- Around 8:36/mile: Full utilization of Type I (slow-twitch) fibers – Zone 1
- Between 8:36 and 7:42/mile: Training Type I and oxidative Type IIA fibers – Tempo/Zone 2
- Faster than 7:42/mile: Recruiting additional glycolytic Type IIA fibers
- Above Critical Speed (faster than 7:18/mile): System becomes unstable as metabolically expensive fibers come online
Critical Speed allows us to understand which fibers we are stressing – and why – at different paces. EXAMPLE FOR RUN:
The 30-70 Minute Zone
Your Critical Power and Critical Speed typically occur in the 30-70 minute range of sustained effort.
This is the sweet spot where your body reaches its maximum sustainable output. Go above this intensity, and your physiology becomes unstable. Stay at or below, and you can maintain steady-state for extended periods.
What Happens When You Go Above CP/CS?
Once you exceed your Critical Power or Critical Speed, several things happen:
- Lactate accumulation accelerates
- Breathing becomes labored
- Muscle fatigue compounds rapidly
- You cannot sustain this intensity indefinitely
This is the unstable zone – you must manage it carefully.
Understanding this threshold is key to smart training and racing.
The Value of Knowing Your Numbers
Understanding your CP and CS provides clear boundaries for training intensity. You gain insight into:
- Where your aerobic system can function efficiently
- When you’re recruiting different muscle fiber types
- How to structure workouts for specific adaptations
- What pace or power you can sustain in competition
Training becomes more intentional when you understand these physiological markers.
Why Does This Matter?
As athletes and coaches, we’re constantly seeking ways to enhance performance and push boundaries.
Understanding CP and CS unlocks new levels of success in triathlon, cycling, and running by revealing the intricate dynamics between physiology, psychology, and power output.
These metrics provide a foundation for evidence-based training decisions.
Your Personal Threshold Matters
Here’s what makes CP and CS valuable: they are YOUR unique physiological markers.
They are derived from YOUR actual performance data, not:
- Someone else’s power or pace
- Generic formulas
- Population averages
Once you know YOUR numbers, you can create personalized training zones tailored to YOUR physiology. This precision helps guide training decisions.
ECHO 1-1 Coaching:
At Angela Naeth Coaching ECHO 1-1 Coaching:
- Test your Critical Power and Critical Speed
- Profile your physiological strengths and weaknesses
- Train based on your unique numbers
- Adapt and Progress Training on specific training
- Retest every 8-12 weeks and build every training session around YOU.
This is ECHO Coaching: Evidence-based. Individualized. Proven.
Questions? info@angelanaethcoaching.com