Critical speed is the reference, not your best 100
Critical swim speed (CSS) is the highest speed you can hold in a sustained way without lactate accumulating out of control. It comes from two maximal tests in the same session, and the formula is a simple slope between two points:
That number, expressed as minutes per 100 metres, is what defines your training zones. Threshold pace is not "the pace that leaves me wrecked", but a specific one you could hold for 30 or 40 minutes, from which every other intensity is derived by percentages.
Which tests to do, and in what order
The classic protocol is 400 m flat out, 8 to 10 minutes of full recovery, then 200 m flat out. Both in the same pool and the same session, after a 10 to 15 minute warm-up. If you go out too hard in the 400 and sink in the last 100, your CSS will read high and the sets that follow will be unreachable: it is the most common mistake. Adding a 1000 m test gives a third point and makes the fit far more stable.
Reference levels, in minutes per 100 m
| Level | CSS (min/100 m) | 1500 m reference |
|---|---|---|
| Beginner | 2:15 – 2:00 | 34 – 30 min |
| Age group triathlete | 2:00 – 1:45 | 30 – 26 min |
| Competitive age grouper | 1:45 – 1:30 | 26 – 22 min |
| Club swimmer | 1:30 – 1:20 | 22 – 20 min |
| Elite | < 1:15 | < 18 min |
In open water everything slows down. The turns and wall push-offs disappear, worth around 3.5 %, and sighting, chop and temperature appear. A wetsuit gives back between 3 % and 8 %, and curiously it helps more the worse your position in the water is: if you float well, you gain less.