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Critical swim speed (CSS): what it is, how it is measured and where it stops being valid

Swimming7 min read
On this page
  1. What it is
  2. What the reviews say
  3. Checks on the data before using a result
  4. CSS calculator (two trials)
  5. What we do not do
  6. What we know and what we do not
  7. How we reviewed it
  8. Sources

What it is

The idea comes from Wakayoshi and colleagues (1992): plot the distance swum against the time taken over several maximal trials and the points sit almost on a line. The slope of that line is the critical speed.1 With two trials, for example 200 m and 400 m, the formula is simple: CSS = (D2 − D1) / (T2 − T1), in metres per second.

What the reviews say

A review of 41 studies found 16 different protocols for calculating CSS. In young national-level swimmers the 200 m and 400 m pair was repeatable (typical error up to 0.04 m/s, variability below 4%), but the authors consider it a crude method suited to in-season checks. Better precision needs at least three trials. Distances of 50 and 100 m are excluded and the suggested trial duration is 2 to 15 minutes. Triathletes and amateurs are not covered by that review.2 Another study of swimmers concludes that the D′ parameter (the distance capacity above CSS) is not reliable.3

Checks on the data before using a result

These are arithmetic and consistency checks, not physiological thresholds:

  • The long trial must take more time than the short one, over a longer distance. Otherwise the denominator makes no sense.
  • CSS should come out below the mean speed of both trials. That only happens if the long trial is swum at a lower mean speed than the short one. If CSS comes out higher, the times are not coherent: recheck the figures or repeat the test.
  • A positive slope is not enough. A number can come out of the formula with wrong times or trials done under different conditions (pool, start, form on the day).
  • Trials must be done the same way (pool, type of start, rest between trials), because the protocol changes the result.

CSS calculator (two trials)

Enter two maximal trials swum the same way (same pool, same start). The calculation runs in your browser: nothing is sent or saved. The result is an indicative estimate from the two-point method, not a threshold measurement, and it produces no zones.

What we do not do

We do not publish swim zones: they need a source valid for your group and a test of your own. The calculator above stops at CSS and refuses incoherent data instead of returning a number.

What we know and what we do not

We know how it is calculated, that the result depends on the protocol, and that the 200 m and 400 m pair is repeatable in a group of young competitive swimmers. We do not know how well it holds for amateurs and triathletes, or whether zones built on CSS improve training in those groups.

How we reviewed it

Full text of a scoping review (41 studies) and a preprint; abstract only for the original 1992 study.

Limits. Result depends on the protocol; repeatability shown in young national-level swimmers; triathletes and amateurs not covered.

Sources

  1. Wakayoshi, Yoshida, Udo, Kasai, Moritani, Mutoh, Miyashita. A simple method for determining critical speed as swimming fatigue threshold in competitive swimming. International Journal of Sports Medicine, 1992. PubMed (abstract only)
  2. Petrigna, Karsten, Delextrat, Pajaujiene, Mani, Paoli, Palma, Bianco. An updated methodology to estimate critical velocity in front crawl swimming: A scoping review. Science & Sports, 2022. DOI (full text, author version)
  3. Scott, Burden, Dekerle. Stroke-Specific Swimming Critical Speed Testing: Balancing Feasibility and Scientific Rigour. Journal of Human Kinetics, 2023. DOI (preprint read)

Sources rechecked on 28 October 2026.

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