Skip to content

Critical Swim Speed Training Explained

Learn everything about Critical Swim Speed Training, the advanced training method that enables athletes to swim with less effort at greater speeds.

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. This blog post will discuss CSS, how to measure it, why it's essential 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 fatiguedIt 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 specific rate. If the lactate is produced faster than it can be removed, it will build up in the bloodstream and cause fatigue. 

Critical Swim Speed Graph

Why is Critical Swim Speed Training Essential?

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 bodies start producing lactic acid.
  2. Second, it helps train their muscles to utilize oxygen better, improving endurance.
  3. Finally, it can help them focus on their training and how different each gear they're shifting into feels to always work at the right intensity.

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.


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 used measurement to determine Critical Swim Speed Pace is to swim a 200m and a 400m time trial and look at the difference between the times.

 CSS Pace [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.

One thing to note is that although test sets have been the go-to method to help coaches and athletes define different training intensities, they also heavily rely on the athletes' ability or willingness to apply their maximum effort that day. They require manual work, time and effort to distribute intensity into different zones. This is what wearable technology like TritonWear help solve with AI-driven CSS and automated training zones.

AI-Driven Critical Swim Speed


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 improve. While lactate testing still has a place in individual zoning for elite teams, by basing scoring and zoning on CSS, TritonWear easily tracks physiological adaptations. 

TritonWear Training Zones

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

How did we do it?

We started by plotting them all against the lactate threshold because it was the only parameter all these systems could agree upon. Then, we explored the science behind each system's construction with breakdowns to respect cross-discipline differences. 

What we found was that each methodology was vastly different regarding zoning. These differences may quickly affect a swimmer's ability to adapt to changing training environments and new coaches (i.e. going to college). Especially considering how likely each coach is to follow a different zoning system.

Finally, we used our findings' best elements to create a new, universal zoning system. You'll find this in the TritonWear app.

Top training zone methodologies from around the world



TritonWear Zones (Infographic) (1)

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) 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 much easier.

If you see yourself as a coach with a growth mindset or someone 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.

Become a Next-Generation Coach. Download the Guide.


Alexandra Petala

Alexandra Petala is the Content Marketing Manager at TritonWear. Before joining TritonWear, Alexandra had created her own company delivering freelance services for lead generation. She also served as the Growth Manager at Just' Geter Done. A former swimming coach and Greek National Champion herself with over 20 years of experience in competitive swimming. Alexandra graduated from the Empire State University with a degree in Business Economics and Marketing.

Latest Articles

How to Stay the Course and Improve Your Swimming Technique?

How to Stay the Course and Improve Your Swimming Technique?

From setting achievable goals to consistent practice and visual feedback, this blog has everything you need to stay on track and improve yo...

How To Use Zone-Based Training in Swimming?

How To Use Zone-Based Training in Swimming?

Learn how to optimize your swim training and achieve your goals with zone-based training. Understand the benefits and key concepts of this ...

Using Energy, Distance and Rest to Unlock Intensity

Using Energy, Distance and Rest to Unlock Intensity

Intensity provides a detailed breakdown of athlete effort . Dive in to learn how energy system usage, distance, and rest impact training in...