Critical Speed is a modelled running speed that estimates the maximal metabolic steady state: the highest intensity at which the body can remain close to a physiological steady state for a prolonged period. Above Critical Speed, fatigue-related disturbances accumulate more quickly, and the time you can sustain the effort becomes limited.
The term sounds technical, but the idea is straightforward. The faster you run, the less time you can sustain that speed. By using maximal performances over different distances or durations, you can model the relationship between distance and time and estimate the speed that sits at the boundary between more sustainable and rapidly fatiguing exercise.
Critical Speed is not simply your 5K pace, 10K pace, threshold pace or marathon pace. It is calculated from more than one performance, which is useful because any single race or test can be influenced by pacing, weather, terrain, fatigue and execution.
The exact testing and modelling method is important. A 2026 scoping review of Critical Speed in running found that there is still no single agreed best protocol for measuring Critical Speed and D′. That is a useful reminder not to treat one calculation method as universally correct.
Critical Speed is often discussed with a second value called D-prime, written D′. D′ represents a finite distance capacity available above Critical Speed. In practical terms, it helps describe how much work a runner can do above that boundary before exhaustion.
You do not need to understand the mathematics to see the coaching value. Multiple performances can give a more complete picture of current running fitness than one race result alone. That is one reason a single unusually good or bad run should be treated as information rather than a final verdict on your fitness.
For field testing, the performances need to be hard enough to represent what you can actually do and different enough in duration to define the relationship properly. SmartRunning uses a sequence of tests to capture performance across different distances rather than relying on one result.
That information can then help individualise training intensity. Two runners with the same marathon goal do not necessarily have the same current fitness, the same strengths or the same appropriate training paces. Goal pace tells you where you want to go. Current fitness helps determine where training should start.
This fits with the way we think about marathon readiness. Readiness is not a single pass-or-fail test. It is an ongoing assessment of whether your training is moving in the right direction for race day.
Critical Speed should not be treated as a magic number. Training history, recent workload, recovery, injury status and the way you are responding to sessions still matter. Training load also changes the meaning of any pace because the same workout can create very different demands depending on what came before it.
The useful part of Critical Speed is not having another number on your watch. It is having a repeatable way to estimate current performance capacity, compare meaningful changes over time and use those changes to make training decisions that are more specific to the runner.