This blog is written by Tom Jenkinson, Coach at Envol, who are providing training support for UltraSwim 33.3 in 2024

If you’ve been swimming a while you have probably come across Critical Swim Speed (CSS). But what exactly is it, and how can you use it to improve your swim times?

What is CSS?

Critical Swim Speed (CSS) represents the highest intensity that is sustainable for a prolonged duration, without eliciting maximal oxygen uptake (V02max) it is just above the Lactate Threshold (LT2) and Maximal Lactate Steady State (MLSS) but lower than the velocity at VO2max.

In the below graph, Swimmer 3 is trained for events 400m or shorter (like the majority of swimmers) faster over shorter distances but can not sustain as high a percentage of that speed over longer distances. Swimmer 2 “appears” to be the slower swimmer, and in regular Masters training might even think they are – but in reality, she is the best endurance-trained and has the best CSS pace!

*CSS Modelled Distance Time Relationship. Graph provided by Tom Jenkinson*

How do I get a CSS value?

To get a CSS value you do two maximal time trials of 400m and 200m:

Begin with a 400m time trial, starting from a push (not a dive), and note down your time.
Take sufficient rest or recovery after the 400m trial to FULLY recover and be prepared for a maximum effort in a 200m time trial.
Swim a 200m time trial, starting from a push (not a dive), and record your time.

You then use a CSS calculator to get your CSS Pace. On this MyProCoach calculator, you can print out your zones. If you use TrainingPeaks (or similar) you can also enter this value as the Threshold Pace in Training Peaks on the desktop version Under Settings -> Zones. The app will work it out from your CSS value and they are set you will see them in the mobile app too.

For mathematicians, CSS is obtained from the modelling of the distance-time relationship using the two-parameter model and is the angular coefficient of the linear regression between the distance (x-axis) and the time (y-axis).

The zones we use are calculated as a percentage of Threshold Pace (CSS)

Zone 1 = 77 to 87%
Zone 2 = 87 to 94%
Zone 3 = 95 to 98%
Zone 4 = 99 to 104%
Zone 5 = >105%

You can think of these zones as five different “gears” and you should use these different gears during your swim training. See Swimming and the Principle of Least Action.

Can you give me a real example?

So for a swimmer with a CSS pace of 1m50s per 100m, the zones/gears would look like this:

*CSS Based Zoning System. Graph provided by Tom Jenkinson*

What is the benefit for my swim training?

If your ambition is to be a better distance swimmer, it is time to train like one!

Turning you into “a diesel engine”.

As a distance swimmer, aerobic capacity is key
Anaerobic speed work is a complement, not a replacement

Many swim systems use CSS but Swim Smooth (Paul Newsome) popularized CSS-based training with the use of Finis Tempo Trainers at or near your threshold (short rests), which we could also label sweet spot training.

This is different from traditional high-volume competitive swimming and master’s swimming programs.

A key factor in Paul’s philosophy was the amount of available training time for him and his athletes.

This means fewer total hours, but having a razor focus on doing a relatively high proportion of them at sweet spot intensity.

What is Sweet Spot intensity?

Sweet-Spot – where you are placing your body under sustained stress, without inducing high levels of fatigue… in other words the sweet spot between intensity and volume.

In the zoning system above this is focused on Zone 3 where…

You train specificity – neuromuscular adaptations and skill acquisition at this “race” pace.

On a cellular level, training at this intensity increases your mitochondria density – the mitochondria are the powerhouses of the cell and, by increasing your mitochondria density, you’re making your body able to create energy more easily.

If you’re limited on time and pool access, you will want to maximize what you do have. Sweet Spot offers the most bang for your buck in terms of training time.

Rather than slogging away for hours in the pool, sweet-spot training produces similar physiological adaptations and training effects from a much shorter session.

“You might not like it, but it is good for you!”

Example – What is the best way to pace this main set?

1800m as 2x Though the following (FS = Freestyle and RI = the Rest Interval in seconds):

1 x (25 FS) :10RI
1 x (50 FS) :10RI
1 x (75 FS) :10RI
1 x (100 FS) :10RI
1 x (125 FS) :10RI
1 x (150 FS) :10RI
1 x (175 FS) :10RI
1 x (200 FS) :10RI

Which option is best with the objective of optimal aerobic stimulus?

Go out at Z4/5 but then fade and do most of the duration at Z1/2 pace
Have a nice comfortable Z2 swim
Pace consistently at Z3 ≈CSS+2 to +5
Skip the rest and do two continuous 900m swims

Spoiler – Answer 3 is the correct one! But I see the other alternatives used all the time!

“If you are swimming less than 5x per week, having easy swims is a waste of time” Coach Joel Filliol

I have seen people using beepers (FINIS Tempo Trainers) to pace sets. How does this work?

There are two common pacing protocols CSS+X/100m and RM Sets:

CSS+X/100 is a STAY ON PACE protocol

CSS+0 would mean swim at your CSS pace/100
CSS+5 would mean swim at your CSS pace/100 +5s
Example; swimmer A has a CSS pace of 1:40/100 or 100 seconds
Take 100 and divide by 4 to give you 25 seconds
To pace at CSS+0/100 In mode 1 set the timer for 25s and then aim to hit the beep every 25m as you touch the wall
For every whole second (i.e., CSS+1) add 0.25 to the timer for a 25m pacing prompt.

RM (Red Mist Cycles) is a START-ON (start-time) protocol

Where you swim faster than the beep to earn rest
The same swimmer A takes her CSS+0 value and divides by 2 to give 50s (RM0)
RM5 would be start every interval on 55s
To start on RM5, in mode 2 set the timer on 55
Beet the beeper to gain rest and start on the next beep

But my CSS test results don’t improve?

See your CSS test result as a way to set effective training paces. Avoid seeing it as an objective measure of your potential swim performance, after all, we are only setting a 100m pacing metric.

Let me take a parallel from running. Imagine you have a threshold run pace of 4:00 minutes per kilometer. That gives you a pacing of 24 seconds per 100m. Let’s say you improve that to 23 seconds per 100m that might not seem much but it is a full 10s faster per kilometer or a new threshold pace of 3:50 minutes per kilometer. In terms of predicted run race times this makes a huge difference:

Because water is far denser than air paces and predicted race times across distances are even more compressed, therefore even a fraction of a second improvement that may well not be seen in the test, will manifest itself on race day. The most important thing is that you are training in the right pace zone to get optimal training stimulus.

The other thing to remember is that even if your pace at LT or VO2 doesn’t change significantly over a series of training blocks, what you are doing is training to hold a higher % of that pace for longer. That is what makes you a competitive distance swimmer!

Are there any downsides to CSS training?

Getting the pacing right on the time trials can be difficult for less experienced swimmers which can give inaccurate CSS values.

Not recovering enough and therefore getting a slower-than-possible time on the 200 would give you a faster predicted CSS. This is a common issue!

Swim training at a high % of threshold pace can be perceived as “stressful” and takes away the enjoyment of swimming for some people, whilst others like it. Remember CSS+0 is Zone 4, you can get the aerobic stimulus you need in Zone 3 without crushing yourself every session.

It requires discipline. Many people have difficulty staying on a pace that feels too easy at first. The first minute or so will feel “too” easy but stick to the pace anyway.

I would also argue that this kind of sweet spot training is more appropriate for more experienced swimmers (let’s say 1:45/100 or faster). It could be more beneficial for developing swimmers to train at slower paces where they can have more focus on developing their technique. Technique and swim skills can have a greater impact on these swimmers’ development than focusing on Z3 swim fitness.

How do you use it in your training?

I use threshold “CSS” pace and the Zoning system to ensure I train “where I am” at the right intensity for my current level of swim skill and fitness.

I retest approximately every 6 to 8 weeks.

I don’t swim every session based on pace – in many of my sessions, I just swim on the feel/perception of effort as per the zoning system.

I like to seed some CSS-paced sessions throughout my blocks to “keep me honest”.

I like to think in zones instead of a fixed pace. Zones give me a span that allows me some flexibility to adjust depending on how I am feeling that particular session, and to not beat myself up if I can’t meet a very precisely prescribed pace target in a given session.

I usually have a few long OW swim events (no gear) during the season, where these Z3 swimming sets become a prioritized aspect of my swim training especially when constrained on weekly volume.

For technique development, I incorporate fartlek multi-pace swimming e.g. X meters EASY + Y meters HARD (and vice versa) this has the intention of being able to focus on your technique during the easy duration and then developing your technique to be able to “hold on” and develop your swim skills to be able to swim faster.

Perceived exertion by zone; easy, steady, mod hard, and hard is used in many of my sessions as it is relevant across different sets with different combinations of gear in workouts that are intended to develop swimrun-related performance.

Final words from Coach Wayne Goldsmith

“Swimming fast is about the swimmer’s capacity to maintain maximum speed, outstanding technique, and brilliant skills. You don’t learn this by swimming a lot of laps at mediocre speeds with terrible technique and awful skills.”