What is a good critical speed for runners?

CS varies by fitness. Recreational runners often have CS around 3.0-3.5 m/s (roughly 8:00-7:37 per mile). Competitive runners may exceed 4.0 m/s (under 6:42 per mile). Compare against your own progress, not population averages.

How is critical speed different from critical power?

Critical speed is the running equivalent of critical power (CP) from cycling. Both use the hyperbolic speed/power-duration relationship. CS applies the same two-component model to running velocity instead of wattage.

How often should I retest critical speed?

Retest every 8-12 weeks during a training block, or after a race that represents a true best effort. CS shifts with aerobic fitness improvements and can drop during detraining.

Can I use race times to estimate CS?

Yes. Recent 5K and 10K race times work well if they were maximal efforts on reasonably flat courses. Avoid using paced or tactical races where you held back.

What is D-prime in running?

D-prime is the finite distance you can run above critical speed before exhaustion. Typical values range from 150-300 meters for recreational runners and 200-400 meters for trained athletes.

What Is Critical Speed? The Runner's Guide to CS and D-Prime

Quick Answer

Critical speed (CS) is the boundary between sustainable heavy-intensity running and unsustainable severe-intensity running. Below CS, you can hold a steady pace for a long time. Above CS, time to exhaustion is limited by how much you exceed CS and how much anaerobic reserve (D') you have left.

In plain terms

CS is the fastest pace you could hold for roughly 30-60 minutes in a fresh state. It sits near, but is not identical to, lactate threshold and marathon pace for many trained runners.

The Two-Component Model: CS and D'

Critical speed theory uses two parameters. CS represents aerobic sustainable speed. D' (D-prime) represents a finite anaerobic work capacity above CS, measured in meters. Think of D' as a fuel tank you draw from whenever you run faster than CS.

Distance-time relationship

t = D' / (S - CS)

Where t is time to exhaustion at constant speed S (when S > CS). From multiple time trials, plot distance versus time: CS is the slope and D' is the y-intercept in the linear model d = CS·t + D'.

How to Estimate Critical Speed

Two-point method

Use two time trials at different distances (e.g., 3K and 5K, or 1500m and 3K). Plot distance vs time; CS is the slope and D' is the y-intercept of the linear relationship.

Three-point method

Add a third effort (e.g., 1 mile, 3K, and 5K) for better accuracy. More data points reduce error from pacing mistakes or incomplete effort on a single trial.

Protocol	Distances	Accuracy	Best for
Two-point	3K + 5K	Moderate	Quick field estimate
Three-point	1 mi + 3K + 5K	Higher	Serious training planning
Race-based	Recent race results	Variable	Runners who race frequently

CS vs Other Thresholds

Metric	What it measures	Typical duration
Critical speed	Heavy-severe intensity boundary (model)	Often near 30-60 min effort
Lactate threshold (LT2)	Blood lactate accumulation	45-75 min
VDOT T pace	Daniels threshold training pace	20-40 min intervals
Marathon pace	Race-specific sustainable pace	2-5 hours

Training Applications

Intervals above CS

VO2max and speed sessions should exceed CS. The further above CS and the longer the rep, the faster you deplete D'. Recovery between reps allows partial D' restoration.

Tempo at or near CS

Sustained efforts at 95-100% of CS build threshold fitness. Typical format: 20-30 minutes at CS pace or 3-4 x 8 minutes with short recovery.

Race pacing

5K race pace is often 105-115% of CS. 10K is near 100-105%. Half marathon is 90-95%. Marathon is 80-88% of CS for well-trained runners.

Limitations

CS estimates depend on time-trial quality. Pacing errors, heat, hills, or incomplete effort skew results. CS is a model, not a direct physiological measurement like blood lactate. Use it alongside other metrics for training decisions.

Calculate Your Critical Speed

Enter two or three recent time trials to estimate CS and D' with the Critical Speed Calculator.

Critical Speed Calculator