Finding the Threshold
Critical Swim Speed (CSS) is the single most important metric for endurance swimmers and triathletes. Conceptually, CSS is your absolute "Aerobic Threshold" — the maximum speed you can sustain continuously for roughly 30 to 40 minutes before the buildup of lactic acid forces you to slow down.
Before the invention of the CSS test, finding this threshold required expensive blood-lactate testing in a laboratory.
The Two-Distance Test
The CSS concept was adapted for swimming in the 1990s. It mathematically isolates your aerobic engine by stripping away your anaerobic sprinting ability. To calculate it, you must perform two separate, maximum-effort time trials on the same day: a 400m swim and a 200m swim, separated by full recovery.
The Formula
The genius of the formula is that it subtracts the 200m sprint data out of the 400m endurance data, leaving only the pure, sustainable speed.
How to Use CSS in Training
Once you know your CSS pace (e.g., 1:40/100m), you have the blueprint for your entire season.
- Endurance Days: Swim long, continuous distances at CSS + 5 seconds (1:45/100m).
- Threshold Days: Swim short, grueling intervals exactly at your CSS pace.
- Sprint Days: Swim 50m repeats at CSS - 10 seconds (1:30/100m).