Critical speed and D′: finding your sustainable-pace boundary without a lab
Critical speed is not another heart-rate zone — it is the physiological line between “I can hold this” and “this is about to end”. Here is how to measure it with two efforts on the track, and what your D′ reserve means.
Every runner knows the sensation first-hand: there is a pace you can hold for a very long time, and a pace just a touch faster where the countdown starts almost immediately. Between them lies a narrow but fundamental boundary. Physiology has a name for it: critical speed (CS). And unlike heart-rate zones, you can measure it yourself on a track in a single evening.
What critical speed is
Formally, CS is the highest intensity at which metabolic processes still reach a steady state. Below CS, oxygen uptake, lactate and pH stabilise at a new level and work can continue for a long time. Above CS there is no stabilisation: the markers creep upwards to their limit, and failure becomes a matter of time rather than willpower.
The second parameter of the model is D′ (pronounced “D prime”): the finite distance you are able to cover above critical speed before fatigue forces you to stop. It helps to think of it as battery capacity: CS is the speed at which the battery is not being drained, and everything above that draws on D′. Surge on a climb or in a breakaway and you write off part of the reserve; drop back below CS and it starts to recharge.
This pair explains things heart rate cannot. Why two runners with the same 10 km time handle a broken pace so differently. Why one has a kick on the final lap and the other does not. Why tempo work run “just a bit faster than it should be” wrecks the session more than it seems.
How to measure it on the track
A recent systematic review of field tests (19 studies, selected from 450 papers) examined what actually works outside the lab. Two methods dominate.
Multiple time trials. You run 2–3 maximal efforts of different lengths, and CS and D′ are computed from the distance–time relationship. The working range is efforts lasting 3–12 minutes. Shorter than three minutes and the anaerobic contribution is too large, distorting the model; much longer and it is hard to hold a genuinely maximal effort.
The 3-minute all-out test (3MT). A single maximal effort of exactly 3 minutes: you start aggressively and simply refuse to let yourself back off. The average speed of the last 30 seconds is taken as CS, and the “surplus” above it as D′. In the review, test–retest coefficients for the 3MT exceeded 0.90, and the correlation between the field version and the treadmill was r = 0.92.
Practical pointers from the review:
- At least two efforts; three is more accurate.
- 30–60 minutes of rest between attempts is enough. That is important news: it used to be assumed that separate days were needed, and that alone made the test unworkable for amateurs.
- A flat track, no wind and the same shoes — otherwise you are measuring the weather, not yourself.
How to use it
The value of CS is that it is an anchor tied to your physiology, not to an age-based formula.
- Long tempo runs are best held slightly below CS. Straying a couple of seconds per kilometre turns steady work into D′ expenditure — and explains why an “easy tempo” sometimes leaves you wrecked.
- Intervals above CS are conveniently structured through D′: if the reserve is finite, the number and length of the reps stop being a matter of feel.
- Race pacing. At distances of roughly 3 to 15 km, race pace sits above CS, and the only question is how to spread D′ over the distance. Hence a practical rule: a hard start is the most expensive way to spend the battery.
- Fitness tracking. CS is responsive to training and can be tracked with a repeat test every 6–8 weeks — a fairer measure than chasing changes in VO₂max.
Limitations worth knowing
The model is elegant but not perfect, and the review says so directly.
- Field methods systematically underestimate D′ by about 16% compared with lab measurements. CS itself is more robust.
- In the field it is impossible to confirm that the effort really was maximal: there are no physiological markers to hand, and the model cannot tell whether you held back or emptied the tank.
- Study samples are heavily skewed towards men (207 men versus 66 women), and there are no dedicated protocols for women yet.
- Wind, surface and terrain break repeatability. Only tests run in similar conditions are worth comparing.
One more thing: CS is not a replacement for everything else, but an additional reference point. It sits well alongside thresholds and zones without cancelling them out.
Key points
- Critical speed is the boundary between sustainable work and inevitable failure; D′ is the finite reserve of work above that boundary.
- It is measured in the field: 2–3 time trials of 3–12 minutes or a 3-minute all-out test (reliability above 0.90).
- 30–60 minutes between attempts is enough — the test genuinely fits into a single session.
- Keep long tempo runs below CS; everything above it spends D′ — including a hard start and climbs.
- Field methods underestimate D′ by about 16%, are sensitive to wind and surface, and data on women is scarce.
- Retest every 6–8 weeks — it is a more responsive indicator of fitness than VO₂max.
Sources: “The Measurement and Application of Critical Speed and D′ in Running: A Scoping Review”, Sports Medicine, 2026. https://doi.org/10.1007/s40279-026-02410-x. “Field-based tests for determining critical speed among runners and its practical application: a systematic review”, 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC11933073/