Load & readiness

Critical speed

The fastest pace you can sustain in a steady state, a physiological threshold anchor.

Peer-reviewed

The critical power/speed concept is peer-reviewed, but the 2-parameter model is a simplification and the estimate is only as good as the spread and honesty of the efforts it is fit to.

Your Critical speed is the fastest pace you can hold in a metabolic steady state. It is a strong physiological threshold anchor: run above it and fatigue accumulates until you have to stop, hold at or below it and you can keep going far longer.

The idea in one line

Across a set of hard, all-out efforts, the distance you cover is a straight line in time. The slope of that line is your critical speed, and the line does not pass through zero: it starts a fixed distance ahead, the extra ground you can cover above critical speed before you are spent.

distance = CS × t + D^′

CS is critical speed in metres per second (the slope). D^′ is your finite anaerobic reserve in metres (the intercept): a fixed distance you can spend above critical speed before exhaustion.

Fitting the line

We take your best efforts from about 2 to 20 minutes and fit a line by linear regression. Efforts shorter than 2 minutes or longer than 20 break the model's assumptions and bias the slope, so we leave them out. We also report how tight the fit is: a wide confidence interval means low confidence in the number.

Two maximal efforts fix a straight line. Its slope is critical speed (about 4.35 m/s), and it meets the distance axis at D^′ (about 130 m).

Worked example

Two maximal efforts. Effort A: 1000 m in 200 s. Effort B: 3000 m in 660 s. Fit distance = CS × time + D^′ through the two points:

Critical speed (slope): (3000 − 1000) ÷ (660 − 200) = 2000 ÷ 460 = 4.35 m/s
Anaerobic reserve (intercept): 1000 − 4.35 × 200 = 1000 − 870 = 130 m

So CS is about 4.35 m/s (about 3:50 per km) and D^′ is about 130 m. Two efforts fix the line exactly; in practice we fit across several and report how tight the fit is.

What it is not

The 2-parameter model is a simplification. It treats D^′ as a fixed reserve and critical speed as a hard line, which real physiology only approximates. The estimate is only as good as the efforts it is fit to: it needs genuinely all-out efforts spread across a good range of durations, and any effort outside roughly 2 to 20 minutes will bias the slope. Read it as the critical power/speed concept applied to your own honest efforts, a peer-reviewed model rather than a lab measurement.