Critical Swim Speed (CSS)
Critical Swim Speed (CSS) is the swim pace you can hold in a long, controlled effort, usually expressed in seconds per 100 metres. For swimming it is what threshold pace is for running and FTP for cycling: the anchor under your zones and your load calculation.
What is CSS?
Swimming steers badly on heart rate. In the water your heart rate runs lower than on land, a watch measures poorly at the wrist during a stroke, and reading it while swimming is hardly possible. Pace per 100 metres is therefore the practical steering signal, and CSS is the reference point that pace relates to.
Below your CSS the effort stays stable and holdable for a long time; above it you draw on a limited reserve and exhaustion is a matter of time. Like its running counterpart Critical Speed, it is a boundary that comes out of a calculation model, not a pace you literally sustain forever. One important detail: CSS mainly says something within your own technique. A more efficient stroke lowers your CSS time without your fitness having changed: in swimming, technique gains are often the fastest gains.
How do you determine your CSS?
The classic test is simple: in the pool, swim a maximal 400 metres and, after ample rest, a maximal 200 metres. The time difference over the extra 200 metres gives your pace around threshold. Put simply: subtract the 200 time from the 400 time and divide that difference by two, and you have your CSS time per 100 metres. Example: 6:40 for the 400 and 3:10 for the 200 gives a difference of 3:30, so a CSS of 1:45 per 100 metres. Swim both evenly; starting the 400 too fast distorts the result.
You do not strictly have to test: the same boundary can be estimated from several maximal or near-maximal distances in your normal training. Two conditions always apply. Do it in the pool, because that is where the distance is exact; GPS distances in open water are too noisy from current, sighting and losing signal underwater. And recalibrate periodically: a good technique block, or a winter of consistent lane swimming, shifts your CSS noticeably.
How Trimetra personalises this
Trimetra estimates your CSS continuously from your own swim logs across different distances, so a separate 400/200 test is not compulsory. Only pool sessions feed that estimate; open-water swimming counts towards your load as normal but keeps the calibration clean. If the estimate differs meaningfully from the anchor you currently use, you get a proposal. You decide. Alongside your longer-term CSS the app also looks at a more recent window, so you can see which way your swim form is moving.
Frequently asked questions
How do you do a CSS test?
Swim a maximal 400 metres in the pool and, after ample rest, a maximal 200 metres, both as evenly as possible. Subtract the 200 time from the 400 time and divide that difference by two: that is your CSS time per 100 metres. For example, 6:40 and 3:10 give a difference of 3:30 and a CSS of 1:45 per 100 metres. Starting the 400 too fast distorts the result.
Why doesn’t open water count for your CSS?
Because GPS distances in open water are too noisy: current, sighting and signal loss underwater make the measured distance unreliable. In the pool the distance is exact, which is why a CSS determination belongs there.
How do you get faster at your CSS?
In swimming, technique gains are often the fastest gains: a more efficient stroke lowers your CSS time without your fitness needing to change. Beyond that, a winter of consistent pool sessions or a good technique block shifts your CSS noticeably, so recalibrate periodically.
Trimetra puts swimming, cycling and running in one plan that adapts every day to your training and recovery. Trimetra for triathletes on the App Store
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