Swim Smooth: training zones based on CSS
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Why CSS instead of heart rate
For the author, heart rate is hard to measure in the pool (straps shift, the wrist is hard to read) and very individual. CSS is specific to swimming, measurable with a stopwatch and responds to training.1
The zones the source makes visible
| Zone | Pace per 100 m | Stated use |
|---|---|---|
| Easy | CSS + 8–10 s | Technique and recovery, relaxed form |
| Steady | CSS + 6 s | Aerobic conditioning |
| Threshold (hard) | CSS | Work at CSS pace |
The other zones are in the paid table and are not reproduced.1
Illustrative example
With a CSS of 1:40 per 100 m: steady = 1:46, easy = 1:48–1:50, threshold = 1:40. Numbers chosen to show the calculation, not a prescription.
Testing and progression according to the source
The source suggests retesting every 6–8 weeks, and gives as an estimate an improvement of 0.3–0.5 s per 100 m per week, with a plateau after 8–12 weeks. These are the author’s estimates, not study results: they should not be read as a promise.1
Limits
- Swim Smooth’s CSS (400 m + 200 m, 1500 m pace) is theirs; the scientific definition uses the distance-time slope. See CSS and critical speed.
- Commercial source, partly paywalled: the full zone table and the sample sets are not reproduced.
- No comparison with other zone systems was searched.
Related tool: two-trial CSS calculator. It estimates CSS from the times of two maximal trials; it does not calculate the Swim Smooth zones.
Sources
- Paul Newsome, Understanding Swim Training Zones for Endurance Swimming, Swim Smooth, 24 July 2025. blog.swimsmooth.com. Public part read.
Source read on 29 October 2026.