Swimming Tool
CSS Critical Swim Speed
Calculate Critical Swim Speed and training pace zones from two all-out tests.
Test Results
200m + 400m
m
m
CSS pace
01:50 /100m
Critical swim speed per 100m
Training Zones
| Zone | Pace | Purpose |
|---|---|---|
REC Recovery | 02:23-02:10/100m | Technique, recovery and relaxed long swimming |
EN1 Aerobic | 02:09-01:59/100m | Aerobic base and longer repeat sets |
EN2 Tempo | 01:58-01:53/100m | Steady endurance close to CSS |
CSS Threshold | 01:52-01:48/100m | Critical-speed and lactate steady-state work |
VO2 Above CSS | 01:47-01:41/100m | High oxygen uptake and short-rest intervals |
SPR Sprint | 01:40-01:28/100m | Speed and neuromuscular work |
Theory
CSS (Critical Swim Speed) = (long-distance time - short-distance time) / (long distance - short distance), commonly tested with 400m and 200m. Wakayoshi et al. (1992) validated that this two-point regression correlates strongly with the physiological lactate threshold, making it a more reliable, individualized basis for threshold main sets. Tests should be well-rested with consistent turn conditions so technique differences do not skew the result.
Practical: retest CSS every 4-6 weeks (it should improve as you train), and feed the new pace straight into the Set Builder to generate that week's main set.
Sources: Wakayoshi et al. 1992, Eur J Appl Physiol, Jones & Vanhatalo 2017, critical speed review