Critical Speed: The Training Metric Serious Runners Use

VDOT is the most popular metric in distance running. Heart rate zones are the most widely discussed. But among exercise physiologists who study competitive performance, a different metric commands growing respect: Critical Speed.

Originally described by Monod and Scherrer in 1965 and extensively developed by Andrew Jones, Mark Burnley, and Anni Vanhatalo over the following decades, Critical Speed defines a fundamental boundary in human exercise physiology — one that separates effort you can sustain indefinitely from effort that exhausts you.

The Two-Parameter Model

Critical Speed ($CS$) is derived from a simple observation: when you plot the distance a runner can cover against the time they can sustain a given speed, a hyperbola emerges. The asymptote of that hyperbola — the speed at which the curve flattens — is $CS$.

Mathematically, for two race time-distance pairs $(d_1, t_1)$ and $(d_2, t_2)$:

$CS = \frac{d_2 - d_1}{t_2 - t_1}$

This calculates the slope of the distance-time relationship — the rate at which extra distance is covered over extra time. Over multiple efforts, this converges on the maximal sustainable speed.

The second parameter, $D'$ (D-prime), is the curvature constant — the finite pool of anaerobic work that exists above $CS$.

$D' = d_1 - CS \times t_1$

What $D'$ Actually Means

$D'$ is not simply "glycogen" or "lactate tolerance." It is a composite of several physiological systems: anaerobic glycolysis capacity, the oxygen stores bound to haemoglobin and myoglobin, and the fast component of VO₂ kinetics. Think of $D'$ as a finite battery that charges slowly below $CS$ and drains rapidly above it.

The two-parameter CS model predicts time at velocity $v > CS$ as:

$t = \frac{D'}{v - CS}$

This hyperbolic relationship is why you cannot sprint a marathon. The moment you exceed $CS$, every second above that speed draws from a fixed account. Push hard enough for long enough, and you reach zero. Reaching zero is what marathon runners call "hitting the wall" — it is a literal physiological depletion event.

Comparing $CS$ to VDOT Zones

Critical Speed sits between the VDOT Threshold (T) and Interval (I) zones — roughly at 95–100% of Threshold pace for most trained runners. But it is not identical to either.

* Indefinite in the sense that below CS, $D'$ does not deplete. In practice, glycogen and muscle damage impose limits at durations beyond ~2 hours.

The critical distinction: below $CS$, metabolic steady state is achievable — lactate, oxygen uptake, and phosphocreatine stabilise. Above $CS$, they do not. This is why $CS$ is described as a "second threshold" in the research literature.

How to Calculate Your $CS$ From Race Data

You need at least two race results at significantly different distances. Three or more gives a better estimate through least-squares regression.

Two-race example:

• 3,000 m in 11:30 (690 s)
• 10,000 m in 43:00 (2,580 s)

$CS = \frac{10000 - 3000}{2580 - 690} = \frac{7000}{1890} \approx 3.70 \text{ m/s} = \text{4:31/km}$

$D' = 3000 - 3.70 \times 690 = 3000 - 2553 = 447 \text{ m}$

Regression method (three or more races): Fit a linear model to the distance-time data. The slope is $CS$; the y-intercept is $D'$. This reduces the impact of any single outlier performance.

The training pace calculator uses both two-race and regression methods. Select the Critical Speed model and enter multiple race times to get your CS estimate.

Training at and Around $CS$

$CS$ has a specific training implication: efforts that repeatedly take $D'$ to near-depletion and then allow recovery drive the largest increases in $CS$. This is the physiological rationale for the 3-minute hard effort interval format popularised by Jones and Vanhatalo's 2010 study.

Practical session formats that overload $D'$ and drive $CS$ upward:

Session format 1 — The 3-minute effort 6–10 × 3 minutes at current I-pace (approximately 110% of $CS$) with 1:30 recovery jog. The short recovery means $D'$ never fully recharges, creating progressive overload.

Session format 2 — Extended sub-CS tempo A 25–40 minute continuous effort at exactly $CS$ pace — just below the threshold. This is one of the most productive single sessions a runner can do, but it requires careful pacing.

Session format 3 — CS-pace cruise intervals 5 × 8 minutes at $CS$ with 2-minute jog. Analogous to threshold cruise intervals but at slightly higher intensity.

Why Most Runners Have Never Heard of $CS$

Critical Speed requires two or more race results to calculate — and those results need to be recent, well-paced, and from meaningfully different distances. Many recreational runners have only one recent race on record. The VDOT model only needs one. This is why VDOT is ubiquitous and $CS$ is not.

For runners with data from the 1,500 m through to the 10,000 m (or half marathon), the CS model is consistently the most accurate predictor of performance at the longer end of that range. It also adapts faster than VDOT to recent fitness changes — because a recent change in 3,000 m time updates $CS$ directly, whereas a single VDOT number smooths across performances more slowly.

If you've been training seriously for more than a year and have multiple races on record, Critical Speed is the metric worth understanding. It describes not just how fast you can run, but the fundamental architecture of your endurance.