How to Find & Train Your Critical Swim Speed

What is CSS pace, and why should you care?

Bike and run fitness will not make you a faster swimmer. Swimming asks for something more on top of a strong engine: the ability to hold your technique together as you fatigue. CSS pace is how you train that ability.

Cycling and running are mostly a test of cardiovascular fitness. Swimming is that same fitness, combined with how well you can keep your position and propulsion intact as the effort builds. Technique will carry you for four or five minutes on its own. After that, fitness takes over, and technique alone will not save you.

Efficiency is the whole point

A pool swimmer's job ends the moment they touch the wall. Whatever the effort cost them, the race is over as soon as they touch the wall. A triathlete's race has barely started when they climb out of the water. Swim hard like a swimmer and you will feel it on the bike, and feel it again on the run.

For an AG Triathlete, efficiency is king. The goal is to get through the swim as fast as possible while building as little fatigue as you can. The most effective way to do that is to swim your intervals at your threshold pace, known as CSS, or Critical Swim Speed.

Picture it like climbing Everest

Every climber going up Everest starts at Base Camp and then works their way up. Past a certain altitude, the air gets so thin that the body can only survive there for a limited window before you have to descend back down. Climbers call that part of the climb the Death Zone.

Your swim pace works the same way. Below a certain pace, you can hold effort almost indefinitely: your breathing is under control and your stroke stays intact. But, push past a specific point and you will find you own Death Zone. If you try to stay at that fast pace for too long and you will find yourself gasping, with heavy arms and flailing legs, and form that gets worse and worse. The pace at which you start to fall apart is your threshold, and CSS is simply how you measure where it sits.

Two problems we can solve with CSS Pace Sets

This is the single most common message I receive from AG triathletes learning to swim well. Why it happens is fairly simple: you are swimming above your threshold pace, sending yourself into the Death Zone, and your body cannot deliver oxygen fast enough to match the effort. If you bring the pace back below the line, continuous laps will come easily.

This issue tends to shows up after a year or two of consistent swimming. When I see this issue in an AG Triathlete, almost always, they are swimming every set at the same comfortable pace, with no structured intervals or target times. The body only adapts when it is asked to work beyond its current comfort zone, a principle known as progressive overload. Threshold pace training, tested and adjusted every four to eight weeks, is what forces that adaptation without turning every session into an all-out effort.

Why threshold pace training works

It builds fatigue you can tolerate. Threshold work is hard enough to build real swim fitness, without leaving you too cooked for the bike and run sessions still ahead that week.

It is built for the distance you actually race. CSS is calibrated for endurance swimming, not sprinting. It will not turn you into a 50m specialist, but it is a great tool for getting faster over triathlon race distances.

It teaches you to pace. Training against a known numbers builds real pacing awareness, instead of starting fast and unravelling by the back half of the swim.

How to find your number

There are a few ways to establish a threshold pace, and most land in a similar place. The version below is the Critical Swim Speed Test.

1. Warm up properly. This is not a set to walk into cold.

2. Swim a 400 time trial off a push, not a dive, and record your time.

3. Rest until you are genuinely, fully recovered.

4. Swim a 200 time trial, same rules, and record your time.