CSS and Interval Training

I have always viewed myself as one-paced in the swimming pool.  I trundle along for miles and miles at around 1:40 per 100 m pace, but aside from the occasional time trial (https://swimsequence.wordpress.com/2013/01/31/january/) I rarely churn the water up unduly.

However, a week ago I decided to finally take the plunge, and dip my toe into the scary waters of interval training.  Before starting any intervals, it is important to understand what your CSS is.  CSS stands for Critical Swim Speed, and is the speed that corresponds closely to your lactate threshold.  It is this speed that you are supposed to try and maintain during a CSS interval session.

You get chapter and verse on CSS if you go to the SwimSmooth website:


There are detailed instructions on there about how to measure your CSS, but essentially you do the following:

(1) Warm up – I did 24 lengths at my usual trundling speed – about 1:40/100 m

(2) Rest for a couple of minutes

(4) 400 m as fast as you can – 5:43 – About 1:26/100 m pace

(5) Rest for a couple of minutes

(6) 200 m as fast as you can – 2:44 – About 1:22/100 m pace

(7) 16 lengths Warm Down


You then take the timings for the 400 and the 200, and put them in to a formula on the Swimsmooth website, or on their free Swimulator App.  This spits out your CSS speed, in my case 1:30/100 m.


The day after measuring my CSS, I did my first interval session, starting off nice and easy with the following set:

600 m W/U Easy

10 x 100 @ CSS pace, on 1:40

400 m W/D Easy

This is what the pace for this set came out as:







Whoa there!

Looks like I have a very poor idea of what swimming at my CSS actually feels like, as I was going way too fast, especially at the beginning.  There’s actually quite a big difference between 90 and 86 (well, 1 second a length, obviously), and you are not really supposed to be swimming below your CSS.  This is probably the reason I felt pretty tired.  I was working pretty hard.

Next time I do this I will try and dial in the pace to closer to to 90s, and not beat myself up too much if I go a second or two over once or twice.  I will also be aiming to take the number of reps out to 20.

It is also important to understand the pace you are going to swim at for longer sessions.  I have the following approximate paces at the moment:

(1) Hard 5 K session – average of CSS + 5 seconds

(2) Hard 10 K session – average of CSS + 9 seconds

(3) Easy 10 K session – average of CSS + 11 seconds

I have rarely swum much past 10 K in the pool.  It just gets too boring and time consuming.  But it is something I am planning on doing soon, hopefully starting with a 4 hour session on Monday (it’s a public holiday in the UK).

I will probably try and dial in at CSS + 13 seconds for a 13 K or so swim.

What might my pace be in the channel?  Well I guess it all depends.  So far my open water swim speed seems quite similar to my pool speed, but in the open water so many more variables are in play, the temperature, the waves, the wind, current to name just a few.  I have seen people suggest that your CSS speed + 20 seconds might be about right for the 35 km or so of the channel, but is that fast enough to stop you getting cold.

I guess I have the next year and a bit to find out more about how this plays out.  Jesus Green Lido opens soon, and it will be great to get into some cold water…….


2 thoughts on “CSS and Interval Training

  1. I have the same problem keeping to a pace. I swim once a week by myself, which I call my long day, as I can do a good 2-3.5 hours, depending upon when I can get off work.

    Anyway, I have been doing CSS for some time now, but I still could not pace myself properly. I finally bought one of those beeper things from Finis a couple months ago. Swim Smooth actually has an ipod app that you can punch your 400 and 200 times into and it’ll tell you what to set your beeper to. I now do a couple long sets (2000s or 2500s) on my long days with the beeper under my cap. I still find it difficult to swim so slowly (I’m now prepping for a really long marathon so I’m swimming CSS + 10sec) and constantly beat the beep and have to slow down. But, my CSS is improving (was 1:43 a couple months ago and now it’s 1:41), so I must be doing something right, right? 😉

  2. @myerstyson, I had a tempo trainer a while ago, but it leaked and died after a couple of months. I might invest in another one day and see if it does any better. Takes the guesswork out of it IYKWIM. Even pacing is super important for long distance, and it is easy to swim the first mile or so too quickly!

    Cheers, and thanks for reading!


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