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Training for Swimming - Maximise Your Speed With CSS / Threshold Work

what sort of fitness training should you do to reach peak fitness?

Related link: Read about our philosophy on fitness training vs. technique work in 10 Contentious Issues In Swimming.

This article is appropriate for Intermediate and Advanced swimmers.


Sustain your threshold (CSS) pace and your fitness will fly upwards.

Your swimming week

Swim Smooth believe that every session you do should have a balance of 3 elements - technique, fitness and open water specific training. If you are swimming three times a week we suggest you hit this sort of routine:

1x Technique orientated session

1x Endurance biased session with longer steady paced swim sets

1x Quality session working on your threshold speed

Each of those sessions will contain a little of the 3 elements technique/fitness/open water skills, but the amount of each will vary between the sessions.

Worried about getting this right? You need one of our training plans to follow!

Your Quality swim Sets

For distance swimmers - including open water athletes and triathletes - one physiological factor is all important: your lactate threshold. If you can improve your lactate threshold speed your race speeds will improve. Your ability to sprint or work anaerobically above threshold is largely irrelevant in distance swimming and triathlon. So is your ability to lift heavy weights.


For most swimmers, the change to CSS sets will mean more swimming and less recovery.

Here's the secret: To improve your lactate threshold you want to do your quality swim sets at your current threshold pace or just below it. Many athletes make the mistake of training above lactate threshold in short sharp swim sets - that isn't nearly as effective. We'll explain more about getting that right below.

Lactate Threshold, Threshold and CSS

In the training and coaching world we often shorten 'lactate threshold' to just 'threshold' - it means the same thing. In a laboratory we'd measure your threshold by taking small samples of your blood as you exercise at increasing intensities and look for the characteristic kick up in blood lactate when you reach your threshold swim speed. Blood tests like that are expensive and tricky to perform in a wet environment, fortunately there is a better way to find your threshold speed. Enter CSS.

CSS is an acronym for Critical Swim Speed. It's an approximation of your lactate threshold speed and you can find it by doing a couple of swimming tests (no blood involved - just a stopwatch!). It's not precisely the same as lactate threshold but it will be within a couple of seconds per 100m, which is plenty accurate enough to guide your training.

Test To Find CSS

The CSS test involves two timetrial swims - a 400m and a 200m. Before attempting these swims perform a thorough warmup and a small build set to get you used to swimming fast.

Do the 400m timetrial first, it's less likely to effect the 200m than the other way around. Recover completely between each timetrial with some easy swimming. Perform both timetrials from a push off from the wall, not a dive.

Try and pace the trials as evenly as possible, don't start too fast and slow down. If you're not sure get someone to take your 100m splits - they can be very revealing.

Calculate your Critical Swim Speed (think threshold speed) using the calculator:

If you're good at maths and want to know the CSS calculation, it is:

CSS (m/sec) = (400 - 200) / (T400 - T200)

Where T400 and T200 are your 400 and 200m times in seconds.

We then convert your speed from m/sec into time per 100m.

CSS Calculator:

Pool Length:

400m timetrial:

min sec

200m timetrial:

min sec


CSS Pace:

Tempo Trainer Pro Setting:
(use mode 2 to give one beep per length)

Note: A previous version of the CSS test used 400m and 50m timetrials, you will find references to this test around the internet. We recommend you use this 400m and 200m test as it gives more accurate results for most swimmers.

How To Train Using CSS

Now you know your CSS speed / 100m you can use it to set a pace for your quality swim sets. Remember, target CSS speed to develop your lactate threshold and become faster.

Here are some examples - these are challenging sets you might do once a week for the main set in your quality swim.

You swim 7:30 or slower for 400m:

6x200m with 20sec recovery

or

3x400m with 45sec recovery

or

4x(200m then 100m) with 10sec recovery

or

12x100m with 10sec recovery

You swim 5:45-7:30 for 400m:

8x200m with 20sec recovery

or

4x400m with 40sec recovery

or

5x(200m then 100m) with 10sec recovery

or

15x100m with 10sec recovery

You swim sub 5:45 for 400m:

10x200m with 20sec recovery

or

5x400m with 40sec recovery

or

18x100m with 10sec recovery

or

3x600m with 60sec recovery

These are tough sets with each of the repetitions targeting your CSS pace. You can shorten them if you want a slightly easier session.

The key point with all these sets is sustained speed with short recoveries. These are just examples, make up your own sessions to keep things interesting.

Compared to how you normally train you might find the pace slightly slower but the recoveries much shorter. It's a different sort of challenge.

CSS feels easy for the first few hundred metres but creeps up on you as the sets go on. The word 'relentless' probably describes it best!


Learn to use the pace clock at your pool to keep an eye on your splits and to time your recoveries.

The Importance of Pacing

When you swim CSS sets it's important to pace things well. If you start too fast and then slow down you won't get the same training benefit.

Try and swim each repetition at the same pace. Very often this will mean the first few hundred metres feels fairly steady - good pacing technique is an important skill to learn and is one of the key differences between amateur and elite swimmers.

The Mental Side

If you are used to doing swim sets with more recovery, a CSS session can look pretty nasty when you write it down! You've got to sustain a fast pace for quite a while and you don't get much recovery time between swims.

Most of this is just a mental thing because you're not used to swimming like that. 4x400 fast sounds tough but the running equivalent is approx 4x1 mile, in terms of time. Runners, doesn't sound so tough?

We recommend you don't think too much, just do it! It sounds much harder than it is.

What if it really is too hard, or it feels too easy?

The CSS test isn't foolproof.

If it feels too easy to complete the example sessions above then it is too easy. Increase the pace slightly until it feels hard but do-able. Just make sure you sustain the same pace throughout - don't fade.


CSS training could be the key to you moving up the field.

Alternatively, if it really is too hard and you can't complete a decent set then swallow your pride and back off the pace a couple of seconds per 100m. 2 secs/100m doesn't sound like much but it can make all the difference. This is because CSS pace really is a fundamental *threshold* - it's impossible to hold a faster pace continuously.

Retesting Your CSS

We'd normally suggest repeating the 400+200m CSS test every 4 to 6 weeks. If things are going well then test every 4 weeks because your fitness can improve quite fast and you'll need to increase the pace of your CSS sets.

For most triathletes we suggest training hard for 3 weeks and then taking an easier recovery week. The perfect time to repeat the CSS test is at the end of the recovery week.

Don't I Need to Swim Faster Than My Threshold To Improve It?

Believe it or not, no you don't. That line of thinking comes from "I'll train at this faster pace and my body will get used to it". That logic is flawed because the body doesn't work like that. When you train faster than threshold you end up splitting the train effect into your anaerobic system too - which you don't need much when you race. And you give your body a much greater recovery task after the session, which means it has less energy left over to make the fitness adaptations you are looking for. The result is that training above threshold gives you less adaptation of your threshold, not more.


Lane 4 is free - what are you standing there for slacker?

Not convinced? Compare it to weight training. Say you wanted to increase your maximum bench press. You can currently lift 50kg and want to get to 60kg. You can't go and lift 60kg - you're not strong enough. But by doing sets at 45-50kg your body adapts and improves. To stress a body system and prompt your body to adapt you just need to approach your current limit.

Still not convinced? Take the challenge! Try it for 4 weeks, swimming one CSS session per week. Over 4 weeks you'll notice your speeds improve as the CSS training starts to work its magic.

Should I Never Swim Above Threshold Then?

You certainly can include some anaerobic work and short sprints in your training - they're good for your swimming technique in themselves: they give you a feel of the water at higher speeds and trains your nervous system to the higher forces. What we're suggesting is you shift your big training sets away from anaerobic swimming to CSS swimming. In practise this means slowing the pace a touch and shortening the recoveries - making the speed more sustained.

So, still include some anaerobic and sprint work but reduce it and give it less importance in your swimming.

Related Links

If you are technically minded, check out the articles on this subject by Training Peaks. If you train on the bike with a power meter and use Training Peak's WKO, they show you how to use your swimming CSS as an input to calculate your swimming training stress score. Neat.

Swim Smooth Terms and Conditions

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