The CSS Test Explained

If you don't know already, your CSS (Critical Swim Speed) is your threshold pace when swimming. It's roughly the pace you can swim a flat-out 1500m and is a single number which you can base all your training intensities from.

For instance in our squads in Perth, swimmers in lane 1 have a threshold pace around 1:50-2:00 /100m, lane 2 1:40-1:50 /100m, lane 3 1:25-1:40 /100m and lane 4 1:15-1:25 /100m. You'll find a similar split between the lanes in every Swim Smooth squad around the world.

CSS training: A huge range of speeds but one accurate way of training for everyone.

If you have trained with a power meter on the bike you probably know your FTP (Functional Threshold Power) and running you might be aware of your threshold pace (the pace you can sustain for a race around 10km to 10 miles in length). CSS, FTP and threshold running pace are the equivalent thing in all three sports. Know them and you can train different energy systems accurately and track your progress over time.

So how do you find your CSS pace? Swim a 1500m timetrial? Well you could... but it would be mentally challenging to do and actually there's another way of doing it that gives more insight into your fitness:

The Standard CSS Test

After a thorough warmup and after a few key drills, swim a 400m time-trial. Pace it well and remember this is a flat out effort, you shouldn't finish and think "maybe I could have gone harder"!

If at all possible, have a friend, coach or lifeguard time your first 100m split of the 400m. As we'll come to later this will give you real insight into your pacing abilities. Record your overall time and also your 100m split.

Spend a few minutes recovering with some easy laps of freestyle until you feel you are ready for part 2 - a 200m time-trial. Again, well paced and a good hard effort. Record your time.

Note: if you are in a yard pool, swim 400yd and 200yd time-trials instead.

Swim an easy cool down to recover from your efforts.

Calculating Your CSS Pace

Armed with your times, enter your numbers into the basic CSS calculator here:

Or for a much more thorough analysis (including pacing analysis and performance predictions over different distances) use the Guru (Pro subscription required):

The Guru gives you Swim Smooth's full analysis of your CSS test.

The Guru will use your 100m split from the 400m to assess your pacing abilities - if these are poor then it's critical you address them to make decent headway with your swimming.

Using Your CSS Pace

So how do you control your pace to swim at CSS? The secret is to use a Finis Tempo Trainer Pro set to beep at regular intervals such that you pace things out to be passing every 25m (or 25 yards) when it beeps. For instance, if your CSS pace is 2:00 /100m then set your beeper to 30 seconds. Get ahead of the beep every 25m and you are going to fast, behind the beep and you're travelling too slowly.

The key to CSS training is sustained speed with short recoveries. This is a little different to traditional master's swimming where the swims are faster but with much longer recoveries (sprint training).

An example CSS set is:

3x (100m + 200m + 300m) - all swum at CSS pace with 10 seconds rest after every swim

That's 1800m in total, suitable for swimmers with a CSS pace around 1:30 /100m.
If your CSS pace is around 1:20 /100m, design sets around 2000m in length.
If your CSS pace is around 1:45 /100m we suggest a set around 1600m
If your CSS pace is around 2:00 /100m try 1400m in total

The key to these sets is sustained speed with short recoveries between swims. This will effectively develop your "diesel engine".

Find more information about Tempo Trainers visit our shop here:

CSS Training And The Guru (this is where it gets really good)

With a Pro Guru subscription you can take CSS training to the next level:

- First up there's hundreds of fun and effective training sessions in the Guru for all ability levels. Each uses your CSS pace to set your intensity accurately.

- Whenever you use a Tempo Trainer, the Guru pops up and tells you what to program in (no maths required from you!) to accurately control your training intensity:

- Tell the Guru how your session went and it will automatically adjust your CSS pace up or down for next time. This means it's not necessary to perform the CSS test regularly as the Guru will actually track your CSS pace as you go along. We call this process "CSS tweaking", something that's unique to Swim Smooth:

In Summary

CSS isn't a concept that Swim Smooth invented but we like to think that we have popularised it for swimmers and developed a whole system of focused effective training around it to help you achieve your potential in the water.

In essence it:

- Provides focused training to improve your distance swimming.

- Is quicker to recover from than many other training sets.

- Allows you train scientifically using a Tempo Trainer Pro.

- Inherently develops your pacing skills.

No excuses now - get to work!

Swim Smooth!


Unknown said...

The Finis Tempo Trainer Pro is an incredibly poorly executed product. It never lasted me more than a few weeks, and I went through quite a few of them. At the price point it is selling at, this is outrageous.

I wish there was an alternative. Is there? Anyone?

Unknown said...

I've used you 'Goldilocks' set for occasional CSS training. Just to clarify, how exactly you suggest to split 1400-1600m CSS workouts for slower CSS speeds?

Anonymous said...

I'm a coach and over many years with Masters I've found the swimmers can never hit the CSS time calculated. I have to add 5 seconds to their calculated time for them to make say, 10x100, or any other set.

Adam Young said...

Hi Unknown1,

Was that recently? The Tempo Trainer Pro has been much more reliable since some updates a little while back.

Hi Unknown2,

For a shorter Golidlocks set, reduce the number of 100s but keep the 200/300/400. So it might be 2x100+200, 2x100+300, 2x100+400.

Hi Anonymous,

First up are you using the 400/200 test (as we recommend) or the old 400/50 protocol? Also, check the pacing of your swimmer's timetrials, for instance if they underperform in the 200 through poor pacing then the CSS pace will end up being too quick.

Hope that helps! Adam

Unknown said...

Hi Adam,

The most recently the Tempo Trainer died on me was in December last year.
After less than two weeks the display went weird, and in another week it was illegible.
That was it for me. I will not buy another one.


Anonymous said...

For those with a dodgy Tempo Trainer.... I've had lots of them. Some work and some bomb out, as you describe. As most of my swim squad are triathletes, they all have watches that can be set to beep like a TT. Even a cheap digital watch (cheaper than a TT), can do the same. It's a workaround.

Connie said...

If you have the tempo trainer on mode 1 to beep every 30 seconds for example , what button do you press to measure the ten second rest ?
If you wait for the next beep you would be getting thirty seconds rest, which defeats the purpose of CSS ?
I’m just trying to visualise how to easily do this and press a button through the swim cap, or take it out and put it back again in the ten seconds?

Adam Young said...

Hi P, did you buy it from us? Of course we would refund or exchange if that were to happen.

Hi Connie, just use the reset button on the top of the Tempo Trainer, so come into the wall on the beep (if you've kept with it), either count 10 seconds (or use the pace clock to time). Press the reset button and go. You should be able to do that through the swim cap fairly easily - you should be able to feel the bump on the edge of the tempo trainer and the press the reset button next to it.



Konrad said...

A basic question which you touch on Paul, could you elaborate? "The key to CSS training is sustained speed with short recoveries." Why? I know that interval training is generally considered to be a good thing. But for long distance training, why is it desirable to have short recoveries? I'm guessing that maybe it is because we all (regardless of fitness & ability) can sustain better form and technique if we have a short break to re-focus, rather than complete the continuous distance that we're training for - but I'm not sure. I've scoured your excellent book and web site for an explanation - but I've found nothing to explain why training with short recoveries works. I'm not being sceptical as I do believe you, I would just like to understand the physiology and psychology of why? Konrad Balcerak

Adam Young said...

Hi Konrad,

Yes that's right, short recoveries give you a quick physical and mental break which would otherwise be a very hard paced continuous swim. Plus you can also adjust the pace slightly, for instance you can spend some of the set slightly above CSS and some slightly below - giving different adaptations.

Hope that helps!


Stuart said...

Hi, hope you can answer a question from an old thread...

I love my finis tempo trainer, and I love css sessions (is that weird?).

My favourite session is 100,200,100,300,400,100,300,200,100, all on css pace. OOOOft, that mama bear (300) on the way back down is a killer.

I typically use my tempo trainer to set the rest period as well. So I always allow a single 'mode 2' interval for rest. I'm swimming 28s /25m on mode 2.

My question is, is this too much rest?

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