Swim Threshold Pace (CSS): What It Is and How to Test It

Short answer: critical swim speed (CSS) is the fastest pace per 100 metres you can keep up for a long, hard swim before fatigue starts to snowball. It is the swim version of threshold. You find it with two timed swims in a pool, a 400 m and a 200 m, and one line of arithmetic. Once you have it, every swim set has a target pace instead of a guess.

Many people swim every session at the same "somewhat hard" pace. CSS lets you make the easy swims easier, the hard swims properly hard, and gives you a starting point for race pacing.

Which test should I do?

Do the 400/200 test in a pool. It needs nothing but a pace clock or a watch, and it is the two-swim method tested in the study that introduced it (1).

• Warm up well, swim 400 m as fast as you can evenly hold, rest fully, then swim 200 m flat out.
• Your CSS pace per 100 m is simply (400 m time − 200 m time) ÷ 2.

Example: 400 m in 7:00 and 200 m in 3:20. The difference is 3:40, or 220 seconds. Half of that is 110 seconds, so your CSS is 1:50 per 100 m.

If you are newer to swimming, the main gain is simply having a number, so your easy swims stop drifting into hard ones. If you are experienced, the gain is precision: pacing threshold sets to a narrow range, and seeing whether a training block moved the number (as a rule of thumb, a change of only a second or two between tests may just be noise). How much you can expect to improve depends on where you start; newer swimmers often see bigger changes between tests.

Test in the same pool, at the same time of day, after the same warm-up, so results are comparable.

What CSS actually is, in plain words

If you swim a short distance flat out, you can go fast. Double the distance and your average pace drops. Time yourself over a few distances, each as fast as you can hold, and plot distance against time: the points fall close to a straight line. The slope of that line is CSS, which researchers defined as the speed you could in theory hold for a very long time without exhausting yourself (2).

That last part is what makes CSS useful. Swim clearly slower than CSS, by several per cent, and a long effort feels hard but stable. Swim faster and the effort climbs rep by rep until you have to back off. CSS sits at or just above that dividing line, which is why it works as an anchor for training paces, like FTP (the highest power you can hold for about an hour) on the bike or threshold pace on the run.

One honest caveat: later research found that CSS is a little faster than the fastest pace at which blood lactate stays steady over a 30-minute swim, by roughly 3–6% according to one review (3, 4). Treat CSS as a hard pace, not a cruising pace.

How do I turn CSS into training paces?

Swim zones are set as a percentage of your CSS pace, the same way bike zones come from FTP. "10% slower" means 10% more time per 100 m. Z1 to Z5 number the zones from easiest to hardest, and VO2max is the very hard effort where your body uses oxygen at its top rate. These are the ranges Tito uses, shown for a swimmer with a CSS of 1:50 per 100 m. If you don't know your CSS yet, use the effort column.

Zone	Pace vs CSS	Example (CSS 1:50)	How it should feel
Easy / endurance (Z1–2)	10–30% slower	2:01–2:23 per 100 m	Relaxed; long, smooth strokes; you could keep going
Tempo (Z3)	3–10% slower	1:53–2:01	Steady and focused, breathing harder but rhythmic
Threshold / CSS (Z4)	2% faster to 3% slower	1:48–1:53	Hard but controlled; breathing is heavy, the effort creeps up rep by rep, and the last rep is tough but still on pace
VO2max (Z5)	2–10% faster	1:39–1:48	Very hard; you need the rest between reps

The threshold zone is deliberately narrow: at a CSS of 1:50 it spans only about five seconds per 100 m, so a couple of seconds either way changes the session.

How to use CSS in a real training week

Here is an example for a triathlete swimming three times a week, about 2,000–3,000 m per session.

Session 1: CSS set (the key swim). After a 400–600 m warm-up, swim 8–12 × 100 m at CSS pace with 15–20 seconds' rest. Later in the season, stretch the reps: 5 × 200 m or 3 × 400 m at CSS with 30 seconds' rest.

Session 2: endurance and technique. Mostly easy-zone swimming: drills, then longer swims of 300–500 m at a pace where you could chat at the wall. Easy should genuinely feel easy. If your "easy" 100s are within a few seconds of CSS, slow down.

Session 3: race-specific. Longer steady swims at tempo pace, such as 3 × 500 m, practising sighting (lifting your eyes to check your line) every 6–8 strokes. In summer, swim this one in open water, but never alone: join a supervised session or swim with a partner, wear a bright cap and a tow float (a small inflatable buoy that trails behind you), and stay close to shore until you know the water.

If you swim twice a week, keep the CSS set and the endurance-and-technique swim, and add a few minutes of sighting practice to the endurance swim.

When a CSS set fails. If you cannot hold the pace past the fourth or fifth rep, finish the set at the pace you can hold. If it keeps happening with normal rest and sleep, your CSS is probably set too fast, so retest. If the set feels comfortable for several weeks, you have probably got fitter: retest then too.

As a rough guide for a triathlete, keep one hard session (the CSS set) and at most one steady tempo session a week, with everything else easy.

The 400/200 test, step by step

Use a 25 m or 50 m pool, push off the wall at the start, and count turns as normal.

1. Warm up (about 15 minutes). 400 m easy, then 4 × 50 m building from easy to fast, with 20 seconds' rest after each.
2. Rest 3–5 minutes.
3. Swim the 400 m. Aim for even pacing: go out too hard and the last 100 m will cost you more than you gained. Record the time.
4. Rest fully, about 10 minutes, mostly easy swimming or floating. A short rest makes the 200 m slower, which makes your CSS pace look faster than it really is, so don't rush this.
5. Swim the 200 m flat out. Record the time.
6. Cool down with 200–400 m easy.
7. Calculate CSS as shown above, with both times in seconds.

As a rule of thumb, do the test when you are reasonably fresh, not the day after a hard session, and retest every six to eight weeks. Skip the test if you're ill or unusually tired.

Stop and get out if you feel chest pain or pressure, dizziness, or breathlessness out of proportion to the effort, and see a doctor before testing again. If you're returning after a long break or have a heart condition, check with your doctor before doing maximal efforts.

Check that the result makes sense. Your CSS pace should be a little slower than your average 100 m pace in the 400 m. In the example, the 400 m averaged 1:45 per 100 m, so a CSS of 1:50 is a little slower, as it should be. If it comes out faster, the 200 m probably wasn't truly flat out, or the rest before it was too short. Redo the test on another day.

What if…

…I can't swim 400 m continuously yet? Build up first. Until then, pace sets by effort using the table above, and choose a stroke you can hold for a whole test. The test only works with front crawl you can sustain.

…I only swim in open water? Test in a pool anyway. Open-water GPS distances are rough, and currents and sighting change the pace.

…the number seems too fast to hold for a long swim? That is expected. CSS often lands a little above true steady state (3, 4), and the test distances you choose change the number (4). Treat it as threshold, not race pace for a long event.

How does CSS relate to open-water racing?

Your pool CSS is the starting point for race pacing, not the answer. Expect open-water pace without a wetsuit to be somewhat slower than in the pool. You lose the push-off from every wall, you have to lift your head to sight, and you may swim further than the course distance.

A review of 17 studies found wetsuits improved swimming performance by up to 11%, mainly through extra buoyancy and less drag. The gain varied with swimming ability and wetsuit model, and familiarity may matter too. Most studies were short or middle-distance pool swims, so the authors called the size of the improvement questionable (5).

As a coaching rule of thumb, then:

• Sprint: about CSS effort, hard but controlled.
• Olympic: CSS to a few seconds per 100 m slower; roughly a CSS set with the rests taken out.
• Half Ironman (70.3): roughly 3–6% slower than CSS (about 1:53–1:57 for a 1:50 CSS); strong, with rhythmic breathing.
• Full Ironman: roughly 6–10% slower (about 1:57–2:01); a pace you could hold for well over an hour.

These paces assume pool-like conditions. Open water without a wetsuit is usually a little slower, and a wetsuit may make you faster at the same effort.

In open water, race by effort and breathing rather than your watch. The aim is to reach transition without a pounding heart rate. Test this in training: a 1,500–2,000 m open-water swim at your planned race effort, in the wetsuit you will race in, tells you more than any formula. The same open-water safety rules apply.

The science behind it

This section is optional reading for anyone who wants the evidence.

CSS comes from the critical power concept used in cycling. In the original swimming study, nine trained college swimmers swam to exhaustion at six different speeds in a flume (a pool with a current to swim against). Distance against time fell on an almost perfect straight line, and its slope (the critical speed) correlated strongly with 400 m freestyle performance and with lab measures of threshold (2). A year later, the same group showed that just two all-out swims, 200 m and 400 m, gave a critical speed that correlated strongly with the speed at a blood lactate of 4 mmol/L. Lactate is a substance muscles produce faster during hard work, and it builds up in the blood above a certain pace. In a steady-state test, it stayed level at 100% of critical speed and rose steadily at 102% (1). That was a study of eight swimmers.

Later work was less tidy. A study of eight well-trained swimmers measured maximal lactate steady state directly, the fastest pace at which blood lactate stays flat over 30 minutes. CSS came out faster than it, and the two were too far apart to use interchangeably, although they were strongly correlated (3). A review of the research put critical speed roughly 3–6% faster than maximal lactate steady state, depending on which test distances are used, and noted that children and younger swimmers responded differently to adults (4).

The main limitation: almost all of this research used young competitive pool swimmers, often only eight or nine at a time, and few studies looked at adult triathletes. Your own repeated tests and how sets feel are a better guide than any single published percentage.

On how to distribute training, the evidence in swimmers is mixed. A systematic review found that elite swimmers' intensity mix depends on their race distance: sprinters followed polarised (mostly easy plus some very hard) or threshold patterns, middle-distance swimmers threshold or pyramidal ones, and long-distance swimmers mainly pyramidal (mostly easy, a fair amount of moderate, a little very hard) (6). In one crossover study of 22 elite junior swimmers, six weeks of polarised training gave a small extra improvement in 100 m time and less fatigue than threshold-heavy training (7). Those were young elite pool swimmers tested over 100 m, so treat it as a hint rather than a rule for an age-group triathlete.

References

1. Wakayoshi K, Yoshida T, Udo M, et al. Does critical swimming velocity represent exercise intensity at maximal lactate steady state? Eur J Appl Physiol Occup Physiol. 1993. doi:10.1007/BF00863406
2. Wakayoshi K, Ikuta K, Yoshida T, et al. Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer. Eur J Appl Physiol Occup Physiol. 1992. doi:10.1007/BF00717953
3. Dekerle J, Pelayo P, Clipet B, et al. Critical swimming speed does not represent the speed at maximal lactate steady state. Int J Sports Med. 2005. doi:10.1055/s-2004-821227
4. Toubekis AG, Tokmakidis SP. Metabolic responses at various intensities relative to critical swimming velocity. J Strength Cond Res. 2013. doi:10.1519/JSC.0b013e31828dde1e
5. Quagliarotti C, Cortesi M, Gatta G, et al. Wetsuit use during open water swimming. Does it "suit" everybody? A narrative review. Int J Sports Physiol Perform. 2021. doi:10.1123/ijspp.2020-0808
6. González-Ravé JM, Hermosilla F, González-Mohíno F, et al. Training intensity distribution, training volume, and periodization models in elite swimmers: a systematic review. Int J Sports Physiol Perform. 2021. doi:10.1123/ijspp.2020-0906
7. Pla R, Le Meur Y, Aubry A, et al. Effects of a 6-week period of polarized or threshold training on performance and fatigue in elite swimmers. Int J Sports Physiol Perform. 2019. doi:10.1123/ijspp.2018-0179