Back to Blog

Critical Swim Speed Training Explained

Image of Alexandra Petala
Alexandra Petala

Critical Swim Speed

Critical Swim Speed (CSS) is an important metric to understand in the world of swimming. It measures the fastest speed a swimmer can sustain for a given distance without getting tired. Coaches mainly use CSS to set training zones and predict race times. In this blog post, we will discuss what CSS is, how to measure it, why it’s important in training, and what’s possible with tech automation.

What is Critical Swim Speed (CSS)?

Critical Swim Speed, as mentioned above, is the fastest speed a swimmer can maintain without getting fatigued.  It is impacted by the swimmer's level of fitness, stroke efficiency, and drag. 

In detail, CSS is higher than the speed at Lactate Threshold and Maximal Lactate Steady State (MLSS) but lower than the velocity at VO2max. Swimming at 2–4% below CSS enables swimmers to maintain a steady-lactate concentration in their bodies. While lactate is a normal by-product of aerobic metabolism, too much lactate can lead to muscle fatigue. This is because the body can only clear lactate from the bloodstream at a certain rate. If the lactate is produced faster than it can be cleared, it will build up in the bloodstream and cause fatigue. 

Critical Swim Speed Graph

How is Critical Swim Speed (CSS) Calculated?

There are a few different ways to measure Critical Swim Speed. Unlike lactate testing, it doesn’t require specialized equipment or drawing blood. The most popular measurement for CSS is to swim a 200m and a 400m time trial and look at the difference between the times.

 CSS [in secs/100m] = (T400 - T200)/2

T400 is the time the swimmer takes to complete the 400m time trial in seconds, and T200 is the time taken to complete the 200m time trial in seconds.

New call-to-action

Why is Critical Swim Speed training important?

Critical Swim Speed training, like Heart Rate, provides coaches with practical, inexpensive and non-invasive ways to estimate exercise intensity and determine the pace.

Including Critical Swim Speed training in your workouts can improve your swimmers’ speed and pace judgement and teach them how to move through the water with less effort and at greater speeds.

What are the benefits of training at or near CSS?

Training at or near your CSS has a few key benefits.

  1. First, it helps improve the athletes' anaerobic threshold, which is the point at which their body starts to produce lactic acid.
  2. Second, it helps train their muscles to utilize oxygen better, which can lead to improved endurance.
  3. Finally, it can help them focus on their training so that they're always working at an intensity that's beneficial for them.

Moreover, CSS, is a pace that pushes swimmers to swim hard enough to develop their aerobic capacity but not too hard that they’ll require days to recover. The best part of training using CSS is that you have a unique interval tailored to each of your swimmer’s abilities.

However, one thing to note is that although test sets are excellent in helping you define different training intensities, they also heavily rely on the athletes' ability or willingness to apply their maximum effort that day. Not to mention they require manual work, time and effort to map out. So, how can tech automation help alleviate some of these pain points? With AI-driven CSS and automated training zones!

Automated Training Zones 


TritonWear has trained an advanced AI system that estimates a swimmer’s CSS from the swimming they have done in their past practices. AI-driven CSS allows us to track how CSS changes over time (7-30-90 days) while maintaining an up-to-date estimate of how fast a swimmer is going relative to their usual speed.

Critical Swim Speed is an easily measured swimming performance indicator that needs no specialized equipment, invasive blood testing, or interpretation by trained specialists. It is accessible to swimmers of all levels and more relatable than lactate levels. With CSS, swimmers of all ages understand how to maintain faster speeds over long distances as their skills and fitness increase.  While lactate testing still has a place in individual zoning for elite teams, by basing scoring and zoning on CSS, TritonWear makes tracking physiological adaptations easy. 

How will TritonWear map out training zones using CSS?

Our training zones are the result of extensive research into the top training zone methodologies from around the world, like France, British Swimming, USA Swimming, Australia Swimming, Urbanchek, etc. 

How did we do it?

We started by plotting them all against the lactate threshold because it was the only parameter that all of these systems could agree upon, and then explored the science behind each system's construction (and any problems they had) with breakdowns in order to respect cross-discipline differences. 

The shocking finding was how different each methodology is regarding zoning. These differences can quickly affect a swimmer's ability to adapt to changing training environments and new coaches (i.e. going to college). Especially if the coach uses different methodologies (which is likely given how many options there are). 

Finally, we took the best elements from our findings to create a new, universal zoning system. You’ll find this in our new app.

Top training zone methodologies from around the world.

Chart_CSSThe new TritonWear universal zoning system.


In our research, we discovered that Critical Swim Speed could be used to estimate the speed corresponding to lactate threshold, lactate turn point, and maximum aerobic uptake. This paper found that CSS zoning was generally more accurate than heart rate speed zoning (beats below max).

Although lactate testing to determine Lactate Threshold (LT) and Lactate Turning Point (LTP) / Maximal Lactate Steady State (MLSS) is still the gold standard for creating speed zones, it is also quite invasive and costly. Moreover, it can’t be run at every session. TritonWear updates CSS estimates after every tracked pool session, making zoning that much easier.

If you see yourself as a coach with a growth mindset, or someone who is ready to take coaching to the next level, grab this playbook; it's step-by-step guidance to help you integrate data into your training.

Next Generation Swim Coaching eBook


Related Posts

Evaluate The Training Plan And Hit Your Goals in Swimming

Image of Alexandra Petala
Alexandra Petala

The training plans and swimming goals are all set at the start of the swim season, and both coaches...

Read more

Megan Oesting On Making Swim Training Engaging Using Tech

Image of Alexandra Petala
Alexandra Petala

Megan Oesting, Head Coach of Bend Swim Club, ASCA Board member and TritonWear Elite Advisory Board...

Read more