How it works
CSS = (D₂ − D₁) ÷ (t₂ − t₁) → pace per 100 m = 100 ÷ CSS
Over the few-minute range of a 200 m and a 400 m swim, the distance you can cover at a maximal, even effort is almost a straight line against time: distance = CSS × time + c. The slope of that line is your critical swim speed — the speed you could in theory hold for a long time before fatigue forces a slowdown. Two test swims pin the line down: CSS is the extra distance between them divided by the extra time. With the standard test, D₂ − D₁ = 400 − 200 = 200 m, so CSS = 200 ÷ (t₄₀₀ − t₂₀₀) metres per second, and pace per 100 m is simply 100 ÷ CSS. For example, 400 m in 6:00 (360 s) and 200 m in 2:50 (170 s) give CSS = 200 ÷ 190 = 1.053 m/s, which is 95 s — about 1:35 per 100 m. Training a few seconds slower than CSS builds aerobic endurance; holding CSS itself is your threshold work.
Sources
- Critical swim speed (original method) Ginn, E. M. (1993). The application of the critical power test to swimming and the determination of critical swim speed. National Sports Research Centre, Australian Sports Commission — CSS from a 400 m and a 200 m time trial.
- Validation against blood-lactate threshold Wakayoshi, K., Ikuta, K., Yoshida, T., et al. (1992). “Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer.” European Journal of Applied Physiology 64, 153–157.
- Critical power / critical speed model Monod, H., & Scherrer, J. (1965). “The work capacity of a synergic muscular group.” Ergonomics 8, 329–338 — the linear distance–time model CSS applies to swimming.
FAQ
What is critical swim speed (CSS)?
CSS is the fastest pace you can sustain in the water while staying mostly aerobic — your swimming threshold. It is expressed as a time per 100 m and is the reference pace most pool sets are built around. Swimming faster than CSS taps a finite reserve and cannot be held for long.
How do I test my CSS?
Swim a maximal, evenly paced 400 m and a maximal 200 m, fully rested, with enough recovery between them (or on separate days). Enter both times. CSS is the 200 m difference in distance divided by the difference in time, so honest, even pacing on both swims gives the most reliable result.
Why a 400 m and a 200 m time trial?
They are the distances in Ginn’s original CSS protocol and bracket the threshold range well. The 200 m difference between them makes the arithmetic clean: CSS = 200 m ÷ (400 m time − 200 m time). You can use other distances, but 400/200 is the established standard.
What are CSS+2s and CSS+5s?
They are training paces a few seconds per 100 m slower than CSS. CSS itself is threshold/race effort; CSS+2s is for aerobic endurance reps; CSS+5s is easy or recovery swimming. Working at the right offset keeps each session at its intended intensity.
How is CSS different from running critical speed?
They use the same two-point distance–time model — CSS is critical speed applied to swimming. The difference is the units: swimmers track pace per 100 m rather than per mile or kilometre. If you also run, comparing the two is a useful cross-check of your aerobic ceiling.
How often should I retest my CSS?
Every 4–6 weeks of consistent swimming, or after a training block, since CSS rises as your aerobic fitness improves. Re-testing keeps your pace targets honest — training to an out-of-date CSS leaves your sets either too easy or too hard.
CSS is an estimate from a two-point model and depends on swimming both time trials at a true maximal, even effort. Treat it as a training anchor, not an exact physiological constant. General information for training, not medical advice.