What Is Critical Swim Speed (CSS)?
Critical Swim Speed (CSS) is regarded as the fastest pace you can sustain continuously without hitting the wall of exhaustion. The swimming equivalent to lactate threshold—the point where your body starts producing lactic acid faster than it can clear it. Training at or just below this speed helps you push your limits without burning out.
Why Does CSS Matter?
If you’re into distance swimming or triathlon, understanding your CSS is a game-changer.
You can use your CSS to work out swim paces so you can:
- Boost Your Aerobic Capacity: Training at CSS strengthens your ability to use oxygen efficiently, which means better endurance.
- Sharpen Your Training Effectiveness: Knowing your CSS gives you a personalized pace for workouts, making your sessions more effective.
- Improves Pace Perception: Regular pace training improves your perception of speed, helping you stick to the right speed on race day.
- Elevates Performance: As you raise your CSS, your overall speed and endurance for longer events naturally improve.
How Do You Find Your CSS?
To find out your CSS there is a simple test. Here’s a straightforward method:
- Warm-Up: Start by swimming easy for 10-15 minutes, including a few short efforts to get your body fully ready.
- 400-Meter Time Trial: Swim 400 meters as fast as you can at a steady pace, noting down the time.
- Recover: Swim gently for 5–10 minutes to shake off the effort and recover fully.
- 200-Meter Time Trial: Now, swim 200 meters as fast as you can, noting down the time.
- Calculate CSS: Your CSS pace (in seconds per 100m) is your 400m time (in seconds), minus your 200m time (in seconds), divided by 2.
- You can always use our calculator HERE
How to Use CSS in Training
Once you know your CSS, you can use it with the following training zones to plan your training. Our CSS calculator also gives you these zones based on your results.
- Zone 1: 77-87% of your CSS
- Zone 2: 87-94% of your CSS
- Zone 3: 95-98% of your CSS
- Zone 4: 99-104% of your CSS
- Zone 5: >105% of your CSS
The Ups and Downs of CSS Testing
Like any tool, CSS testing has its pros and cons in making you a faster swimmer. Here’s a quick look:
Pros:
- Requires minimal equipment—just a pool and a stopwatch.
- Easy to perform and repeat.
- Provides a practical, race-specific training focus.
- Tailored to you—it’s your unique speed metric.
Cons:
- Results depend on your effort on the day, which can vary.
- It may overestimate your true threshold speed causing you to work too hard in training and not give you the results you want.
- Best suited for distance events—less relevant for sprinters.
- Two data points aren’t perfect and can lead to inaccuracies.
My Thoughts on CSS
Critical Swim Speed is a powerful tool when used correctly for making your training more effective, especially for distance events. It helps you find that sweet spot between pushing hard and staying sustainable. But CSS isn’t always completely accurate. There are other tests that could be more beneficial to you, check out this post on swimming test sets.
Remember all swim training works best when combined with technique improvements, strength training, and personalized coaching.
So, don’t just chase a number. Use CSS to guide your progress, and remember, it’s all about becoming a stronger, more efficient swimmer—on your own terms.
