What is critical speed?
Critical speed (CS) is the running equivalent of cycling’s critical power. It marks the boundary between the heavy exercise domain, where your body can reach a physiological steady state, and the severe domain, where oxygen uptake and fatigue keep rising until you have to stop. In practice it is a pace you could race for roughly 30–40 minutes, close to what many coaches call threshold.
D′ (“D-prime”) is the fixed amount of distance you can run above critical speed before exhaustion — a kind of anaerobic battery. Runners with a big D′ tend to have strong kicks and handle surges well.
How critical speed is calculated
For efforts lasting roughly 2 to 15 minutes, the distance you can cover is almost a straight line when plotted against time. The slope of that line is your critical speed and the intercept is D′:
with two efforts: CS = (d₂ − d₁) ÷ (t₂ − t₁), D′ = d₁ − CS × t₁
Worked example
1200 m in 4:20 (260 s) and 3600 m in 14:10 (850 s): CS = (3600 − 1200) ÷ (850 − 260) = 2400 ÷ 590 = 4.07 m/s, a pace of 4:06 per km. D′ = 1200 − 4.07 × 260 ≈ 142 m.
Good test protocols
| Protocol | Efforts | Notes |
|---|---|---|
| Distance-based | 1200 m + 3600 m | Easy to run on a track; can be done on separate days or with 30 minutes of recovery. |
| Time-based | 3 min + 12 min all-out | Popular with GPS watches and treadmills; record distance covered. |
| Race-based | Recent mile + 5K | Uses genuine all-out efforts; keep them within a few weeks of each other. |
| Three-point | 800 m + 1600 m + 3200 m | A third effort shows how well the straight-line model fits you. |
Using CS in training
Below CS
Threshold sessions — e.g. 3–5 × 8–10 minutes at 92–97 % of CS speed — build the pace you can sustain without drifting into the severe domain.
At CS
Short blocks right at CS are hard but repeatable; many coaches keep them to 20–30 minutes of total work.
Above CS
VO2max intervals spend D′ on purpose. With a known D′ you can estimate how long a rep at a given pace should take you to near-exhaustion.
Racing
In races longer than about 40 minutes you run below CS; shorter races use part of D′ — ideally saved for the finish.
Model limits. The linear model works best for efforts between about 2 and 15–20 minutes. Very short sprints or very long efforts do not fit the straight line well, so avoid them as inputs.
Compare CS with your threshold pace from the VDOT calculator, predict race times with the race time predictor, and set heart rate targets with the heart rate zone calculator.
- Jones AM, Vanhatalo A (2017). The ‘critical power’ concept: applications to sports performance with a focus on intermittent high-intensity exercise. Sports Med 47(Suppl 1):65–78.
- Poole DC, Burnley M, Vanhatalo A, Rossiter HB, Jones AM (2016). Critical power: an important fatigue threshold in exercise physiology. Med Sci Sports Exerc 48(11):2320–2334.
Last reviewed: September 2026 · Health disclaimer