Critical swim speed (CSS)
Your sustainable threshold swim pace from two all-out test swims, plus training paces built around it.
Your test swims
Warm up well, then swim the long distance all-out. Rest fully (or swim it on another day), then swim the short one all-out. Use the same pool for both.
Your critical swim speed
Training paces Coaching tip
| Zone | Pace /100 m | Use for |
|---|---|---|
| Easy / recovery | 1:55–2:00 | Warm-up, drills, recovery |
| Endurance | 1:50–1:55 | Long aerobic sets, Ironman swim pace |
| Threshold (CSS) | 1:43–1:47 | Main sets like 10 × 100 m, 70.3 race effort |
| Fast | 1:39–1:42 | Short repeats with longer rest |
These zones are common coaching practice, not research findings. Open water is usually a bit slower than the pool. Wetsuits are usually a bit faster.
How it works
Critical speed is the slope of the line through distance and time for all-out swims; with two swims that's (long − short distance) ÷ (long − short time). It closely tracks 400 m race speed (Wakayoshi et al. 1992).
Using it
Retest every 6–8 weeks. CSS pace suits main sets like 10 × 100 m on short rest. The zone table is common coaching practice rather than research. More in the methodology.