Critical Speed

Run above Critical Speed and you start spending a finite, non-renewing reserve called anaerobic work capacity, usually written D′. It drains at a predictable rate, and when it is empty you slow down whether you want to or not. That is the last mile of a hard race, or the last rep of an interval session. Run below Critical Speed and the reserve is never touched, so the effort holds for a very long time. Other limiters do turn up eventually, glycogen and heat and your own head, but not that one.

The idea starts with Monod and Scherrer's critical power model (1965) for muscular work, and was later extended to running as critical speed. The standard two-parameter model fits distance = CS × time + D′ across at least two maximal efforts of different durations, ideally three or more, commonly between 3 and 12 minutes. Plot distance against time: CS is the slope, D′ is the intercept.

Critical Speed sits close to lactate threshold pace but is a physiologically distinct number. For many runners it lands slightly faster than LT2, somewhere around 30 to 60 minute race effort. How close the two sit depends on the individual, and on how fatigue-resistant their anaerobic system is.

Critical Speed gives interval training a ceiling to plan around. Staying under it through a long tempo keeps you off the D′ cliff edge. Intervals do the opposite: they spend into D′ deliberately, and count on the recovery to refill it before the next rep. It also anchors training zones better than max heart rate for runners whose heart rate is noisy from heat, caffeine or fatigue, since it comes straight out of pace and duration.

Fitting Critical Speed from a single time trial does not work, though people try it constantly. The model needs at least two efforts of different durations, because it is built on the relationship between distance and time pairs, and one pair has no relationship. Critical Speed is also not a race pace. Elite marathoners race below theirs for the full distance, since even a small draw on D′ compounds badly over two hours and more.