Critical swim speed (CSS)
Critical swim speed (CSS) is the fastest pace a swimmer can hold continuously without tiring rapidly — the swimming equivalent of threshold. It is usually expressed as a pace per 100 metres and estimated from two time trials of different distances.
How to calculate CSS
After a good warm-up, swim 400 m as fast as you can evenly, recover, then swim 200 m all out. CSS pace per 100 m is the difference between the two times divided by two.
For example: 400 m in 6:40 (400 seconds) and 200 m in 3:10 (190 seconds). The difference is 210 seconds for 200 metres, so your CSS pace is 105 seconds — 1:45 per 100 m.
How to train with CSS
Threshold sets are swum at or just under CSS pace with short rests, such as 10 × 100 m at CSS with 10–15 seconds' rest. Easy aerobic swimming sits about 10–15 seconds per 100 m slower than CSS. Retest every 6–8 weeks; as CSS drops, your sets get faster with it.
In The Next Race
The Next Race's benchmark week includes a CSS test, and your swim threshold pace sets the swim pace zones the app uses to estimate each swim.