Critical Speed
Critical Speed (CS) is the fastest pace a runner can sustain without continuous lactate accumulation. Learn how critical speed differs from threshold and how to use it for training.
Quick Answer
Critical Speed — (CS) is the highest running velocity that can be maintained in a "metabolic steady state"—where oxygen supply meets demand and lactate doesn't continuously rise. It's similar to lactate threshold pace but derived differently, using time trials rather than blood tests.
What is Critical Speed?
Critical Speed represents the boundary between sustainable and unsustainable running intensities. Below CS, you can run for extended periods. Above CS, fatigue accumulates until you must slow or stop.
Key concepts:
| Term | Definition |
|---|---|
| Critical Speed (CS) | Sustainable steady-state velocity |
| D' (D-prime) | Anaerobic running capacity above CS |
Think of CS as your "fuel efficiency speed" and D' as your "turbo boost tank."
Critical Speed vs Related Metrics
| Metric | What It Measures | Typical Relationship |
|---|---|---|
| Critical Speed | Sustainable velocity | ~90-95% of 5K pace |
| Lactate Threshold | Blood lactate turnpoint | Similar to CS |
| FTP (cycling) | Threshold power | Cycling equivalent |
| VDOT | Race-derived fitness | Different calculation |
CS and lactate threshold often align closely, but CS is measured from performance data rather than blood lactate.
Read our comparison: VDOT vs Critical Speed.
How Critical Speed is Calculated
The Critical Speed Model
CS is derived from time-trial performances at two or more distances:
Formula:
Distance = CS × Time + D'
Where:
- CS = Critical Speed (m/s)
- D' = Anaerobic distance capacity (meters)
- Time = Duration of effort
Example Calculation
Using 3:00 (1000m) and 15:00 (3000m) time trials:
- Plot distance vs time
- Find the linear relationship
- Slope = Critical Speed
- Y-intercept = D'
Result: If CS = 4.5 m/s = 5:56/mile pace
Testing Protocol
Best tests for calculating CS:
| Option | Tests | Recovery |
|---|---|---|
| Standard | 3-min and 10-min all-out | 48 hours apart |
| Alternative | 1200m and 2400m time trials | 48 hours apart |
| Comprehensive | 800m, 1600m, 3200m | 48 hours each |
Using Critical Speed for Training
Pace Zones Based on CS
| Zone | % of CS | Purpose |
|---|---|---|
| Recovery | < 75% | Easy runs |
| Endurance | 75-85% | Base building |
| Tempo | 85-95% | Threshold development |
| Critical | 95-100% | CS improvement |
| VO2 Max | 100-110% | Aerobic capacity |
Training Benefits
Understanding CS helps with:
| Application | How CS Helps |
|---|---|
| Race pacing | Set realistic targets based on CS |
| Interval design | Match work intervals to energy systems |
| Progress tracking | Monitor CS changes over time |
| Recovery | Stay below CS on easy days |
D' (D-Prime) Explained
D' represents your anaerobic running capacity—the "extra" distance you can run above CS before exhausting anaerobic reserves.
Typical D' values:
- Untrained: 100-200 meters
- Recreational: 200-300 meters
- Trained: 300-400 meters
- Elite: 400-500+ meters
Using D' in Racing
Time above CS = D' / (Speed - CS)
Example: With D' of 300m and CS of 4.5 m/s, running at 5.0 m/s:
Time above CS = 300 / (5.0 - 4.5) = 600 seconds = 10 minutes
This predicts how long you can sustain speeds above your critical speed.
Critical Speed for Race Prediction
CS predicts race performances across distances:
| Race | Typical Pace Relative to CS |
|---|---|
| 800m | 105-110% of CS |
| 1500m/Mile | 103-107% of CS |
| 5K | 100-103% of CS |
| 10K | 95-100% of CS |
| Half Marathon | 88-93% of CS |
| Marathon | 82-88% of CS |
Longer races require pacing well below CS to avoid D' depletion.
Improving Critical Speed
1. Threshold Training
Work at or near CS:
- 20-40 min continuous at 95-100% CS
- Cruise intervals: 4-6 × 5-8 min with 1 min recovery
2. VO2 Max Intervals
Develop the aerobic ceiling:
- 5-6 × 3 min at 105-110% CS
- Equal work-rest ratios
3. Endurance Volume
Build the aerobic base:
- Long runs at 70-80% CS
- Easy runs at < 75% CS
4. D' Development
For shorter race performance:
- 600-800m repeats at 10K pace
- Sprint intervals for neuromuscular development
Common Questions
Is Critical Speed the same as Lactate Threshold?
They're closely related but measured differently:
- CS: From time trial performances
- LT: From blood lactate testing
For most runners, they correlate within 2-3% of each other.
How often should I retest Critical Speed?
Every 6-8 weeks during training, or after:
- Completing a training block
- Racing at key distances
- Returning from injury
Can I use Critical Speed for marathon pacing?
Yes, but marathon pace is well below CS (typically 82-88%). CS helps establish your aerobic limits, then apply appropriate discount for marathon distance.
What affects Critical Speed?
Primary factors:
- Training volume and consistency
- VO2 Max development
- Lactate threshold training
- Running economy
- Recovery and freshness