Consistent CSS swim training leads to measurable improvements.

Hey swimmers, as a coach, one of the most common questions I get is:

How much can I improve in a given time frame?

Whether you're changing your stroke technique or starting a dedicated training program, swimmers always want to know if the time they invest will produce real results.

This is of course totally understandable. However, when I immediately respond with the questions:

“Where are you at right now with your speed? What has your training been like the last couple of months? What are you planning to do to improve?”

The answers are usually very vague at best.

You’d never enter a progress meeting at work without being able to answer with a similar objective overview, and whilst you might not view your swim training as important as a board meeting, being able to answer these questions accurately should be both very simple and will pay big dividends over just answering, “I don’t think I’m getting any better”.

At Swim Smooth, we’ve been championing Critical Swim Speed (CSS) training for over 20 years because we’ve seen first-hand how effective it is in both tracking and improving performance. It’s one of the most effective methods we’ve seen for boosting your swim prowess, and the results we’ve witnessed in our squads around the world speak for themselves.

What is Critical Swim Speed (CSS)?

Simply put, CSS is the fastest pace you can swim over a long distance without hitting exhaustion. It’s an indicator of your endurance, measured as your threshold pace over a 1,500m swim. By training at your CSS pace, you’re focusing on efficiency, pacing, and endurance, which helps you swim faster for longer. The best part? If you improve your CSS pace, you improve your swimming performance, no matter your starting point.

We calculate CSS by performing two time trials: a 400m swim and a 200m swim. These times are then used to calculate your CSS pace per 100m. From there, you can structure your training around that pace.

The goal of the 10-Week CSS Development Program is to progressively reduce that CSS time by 5% over the course of the program—helping you swim faster and more efficiently.

Getting a Weird CSS Result You Don’t ‘Agree’ With?

If you’ve already had a fiddle around with a CSS test, you might be curious why your CSS pace might slow down even as your 200m time gets faster? Have you ‘cracked the code’ as how to game the CSS calculator to giving you a ‘better’ result? This article from 2014 explains how the CSS test reveals whether you're more suited to sprinting or distance swimming, based on the drop-off between your 200m and 400m times and is well worth a read if you’re comparing your results with a swim buddy especially.

For reference, and to avoid falling down the rabbit hole of never being satisfied or believing in the result the CSS test gives you, Swim smooth has baked in the ability to Tweak your CSS pace systematically in response to your training which we will discuss a little later on.

The Original Case Study: How The Perth Squad Did It

Way back in 2012, Paul in Perth tested this 10-week program with 128 swimmers from the Perth squad. These swimmers, ranging in abilities from 12 to 20 minutes per kilometre, were asked to swim at their calculated CSS pace in one session per week for 10 weeks. Each week, their CSS pace was made 0.5% faster to progressively challenge them and build endurance.

By the end of the 10 weeks, 47 out of 57 swimmers who retested improved their CSS pace, with an average improvement of 2.59%. Some saw incredible results, improving by 13.62%! Others saw smaller gains, but all reported feeling more in control of their pacing and able to maintain their technique over longer distances.

The takeaway from this case study was simple: consistent CSS training leads to measurable improvements.

How You Can Follow the Program in 2025

While the original case study produced great results, we’ve since developed new tools within the Swim Smooth GURU to make following this program even easier and more effective. Here’s how you can use the GURU to get the most out of your 10-week challenge:

1. Start with the CSS Calculator in Week 1

Before you dive in, use the CSS Calculator within the GURU to determine your baseline pace. Perform the 400m and 200m time trials and plug your times into the calculator. This will give you your CSS pace per 100m, which will act as your benchmark throughout the program.

2. Use the Ten Week Challenge Tab to Adjust Weekly

Once your CSS is set, head over to the Ten Week Challenge tab in the GURU (regular swimmers and athletes of mine will get the GURU Routine membership free of charge).

Here, you’ll find recommendations on how to adjust your CSS pace week by week. If you’re aiming for proactive improvements, these suggested increments will help you aim for that 5% reduction over the 10 weeks.

But remember—many swimmers hit a plateau around week 6 or 7, so don’t worry if your improvements start to slow down.

Alternatively, you could use the CSS Tweaker in the GURU to make smaller, more reactive adjustments based on how you feel each session. This might give you slower improvements, but you’ll feel more in control of your progression. Using the Tweaker is also a good way to ‘funnel down’ on your true CSS pace following a CSS Test that you might be dubious about against your recent pace work.

3. Commit to One CSS Session Per Week

To see results, you need to stay consistent.

Pick a new CSS session per week from the GURU’s extensive library of workouts. It doesn’t really matter which one you choose—what’s most important is that you stick to it. You could even use the Session Roulette feature to randomly select a workout if you’re feeling adventurous. The key here is commitment—once a week, every week.

4. Add Two or More Additional Sessions

While your CSS session is the cornerstone of this program, we recommend adding two more sessions to your weekly routine. You can choose a Technique/Endurance session and a Red Mist or Pink Mist session for balance. These sessions will help improve your technique, stamina, and pacing ability, which in turn will enhance your CSS performance.

Ideally, space these sessions out with 36-48 hours between them to give your body enough recovery time.

5. Track Your Progress with a Garmin or an Apple Watch

If you’re using a Garmin or Apple Watch, sync your device with the GURU to track your session splits against your CSS targets. This will not only help you monitor improvements week by week but will also give you insights into your fitness and fatigue balance over the 10 weeks. Tracking your progress is an excellent way to stay motivated and keep yourself accountable.

6. Let Us Know How You Get On!

At the end of the program, let us know how you did! Whether you followed the proactive weekly improvement method or the reactive CSS Tweaker approach, we’d love to hear your results. And it's a great opportunity to join our Friday night squad where we swim a CSS session every week!