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AnswersWhat is Critical Swim Speed?
Critical Swim Speed, or CSS, is roughly the fastest pace a swimmer can sustain without rapidly tiring — the swimming equivalent of threshold. It is calculated from two timed swims, usually a 400 m and a 200 m: CSS pace per 100 m is the difference between the two times divided by two. A swimmer who does 400 m in 7:30 and 200 m in 3:32 has a CSS of 1:59 per 100 m.
CSS from two swims
| Test swims | Critical Swim Speed |
|---|---|
| 400 m in 7:30, 200 m in 3:32 | CSS 1:59 per 100 m |
| 400 m in 6:30, 200 m in 3:00 | CSS 1:45 per 100 m |
| 400 m in 8:30, 200 m in 4:02 | CSS 2:14 per 100 m |
| 400 m in 5:30, 200 m in 2:32 | CSS 1:29 per 100 m |
Training paces off a CSS of 1:59
| Recovery | 2:09/100 m — Easy enough to think about your stroke rather than your lungs. |
|---|---|
| Endurance | 2:04/100 m — Steady. You could keep going for a long time. |
| CSS | 1:59/100 m — Threshold. Hard but repeatable across a whole set. |
| VO₂max | 1:56/100 m — Faster than threshold, in short reps with real rest. |
| Speed | 1:53/100 m — Short and quick. Technique first, effort second. |
Swim the two tests in the same session, 400 m first, with a full easy recovery between. Pace the 400 evenly — a fast first 100 gives a falsely slow result. For most amateur swimmers, technique limits speed more than fitness does, so CSS sets useful training paces but will not on its own tell you why you are slow.
Common questions
How often should I retest CSS?
Every four to six weeks. It moves quickly for improving swimmers.
Is CSS the same in open water?
Open-water pace is slower, because there are no walls and you have to sight. Use CSS to set pool sessions and effort in open water.
Why does technique matter so much?
Water punishes drag far more than air. A swimmer whose hips sink can gain more from fixing that than from months of harder sets.
Sources
- Wakayoshi, K. et al. (1992). Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer. European Journal of Applied Physiology.