How it works
distance = CS · time + D′ (CS = (d₂−d₁)/(t₂−t₁), D′ = d₁ − CS·t₁)
Over the range of a few minutes to about an hour, the distance you can cover in a maximal effort is almost a straight line against time: distance = critical speed × time + D′. The slope of that line is your critical speed (CS) — the speed you could in theory hold indefinitely — and the intercept is D′, a fixed distance reserve you can spend running faster than CS. Two test efforts pin down the line: CS is the extra distance divided by the extra time between them, and D′ is what is left once CS is accounted for. For example, 1500 m in 5:00 and 3000 m in 11:00 give CS = 1500 m ÷ 360 s = 4.17 m/s (4:00 per km) and D′ = 250 m. The closer your two efforts bracket the range you care about, the better the estimate.
Sources
- Critical power / critical speed model Monod, H., & Scherrer, J. (1965). “The work capacity of a synergic muscular group.” Ergonomics 8, 329–338 — the linear work–time (distance–time) model behind critical speed.
- Critical speed in running Critical speed is the running analogue of critical power: the asymptote of the distance–time relationship, closely tied to the maximal metabolic steady state (≈ threshold).
- D' (anaerobic reserve) D′ is the finite distance that can be covered above critical speed, reflecting anaerobic work capacity.
FAQ
What is critical speed?
The fastest pace you can sustain without fatigue accelerating — essentially your aerobic threshold expressed as a speed. Running faster than critical speed draws on a finite reserve (D′) and cannot be held for long; running at or below it is far more sustainable.
How do I test my critical speed?
Run two maximal time trials of different distances on the same day or within a few days, fully rested — for example 1200–1500 m and 2400–3000 m. Enter the distance and time of each. Pacing both as hard, even efforts gives the most reliable critical speed.
What is D′?
D′ (“D prime”) is your anaerobic distance reserve — the extra distance you can cover running above critical speed before that reserve is depleted. A larger D′ means more capacity for surges and finishing kicks; it is rebuilt by easing back below critical speed.
How is critical speed different from VDOT or threshold pace?
They are closely related. Critical speed is a direct, test-based estimate of your sustainable threshold, while VDOT and Daniels paces come from a single race and a physiological model. Critical speed often lands near threshold (T) pace; comparing them is a useful cross-check.
Why must the two efforts be different lengths?
The model fits a line through two points, so the efforts need different times to define a slope. Using two distances that are too similar makes the estimate unstable — pick a clearly shorter and a clearly longer effort for a robust result.
How should I use critical speed in training?
It anchors threshold and tempo work: sustained runs at or just below critical speed develop your steady-state ceiling, while short intervals above it train your D′. Re-test occasionally, as critical speed rises with aerobic fitness.
Critical speed and D′ are estimates from a two-point model and depend on running both tests at a true maximal, even effort. Treat them as training anchors, not exact physiological constants. General information for training, not medical advice.