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 that difference is your CSS time 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.
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