How to Measure Your Swim Threshold in Triathlon Training

Most technically savvy triathletes are familiar with terms like “functional threshold power” on the bike or “lactate threshold for their run.” These are measurements of your pace based on your sustained threshold ability for a given amount of time; usually one hour. In other words, what is the maximum pace you can hold for an all out one-hour effort? However, few triathletes know their functional threshold in the swim or even know how to obtain it.

Knowing your threshold ability in all disciplines is essential. In essence, there are two types of triathlon training: aerobic and anaerobic. Any kind of effortful training below your threshold is aerobic. Anything above is anaerobic.

Now in actuality, triathlon training zones are much more complicated than this, but for the sake of simplicity you can see why knowledge of your threshold is important. It’s the baseline upon which your training will be dependent. Sometimes it’s good to do aerobic swim training and sometimes it’s good to do anaerobic swim training. Just how aerobic or anaerobic will depend on your threshold ability.

We can obtain a pretty good idea of your bike and run functional thresholds through time trials and the aid of data devices like heart rate monitors and power meters. However, what’s the best way to obtain your functional threshold in the triathlon swim?

This is where something known as “Critical Swim Speed” (CSS) comes into play. Companies like SwimSmooth (a pioneer of the CSS) and TriDot Triathlon Training are implementing the Critical Swim Speed evaluation in order to accurately measure you swimming functional (or even lactate) threshold.

The CSS is an approximation of your threshold based on the completion of two intelligently prescribed swim sets. No devices or lactate blood work is required. If done correctly, your lactate threshold should be within a few seconds of your actual pace per 100 meters.

The test is made up of two time-trial swims: a 400 meter and a 200 meter.  It’s best to complete the 400 before the 200 in order to get more accurate results. Be sure to recover completely between each test.  It’s also important to pace the trials as evenly as possible. You can use the CSS Calculator on SwimSmooth’s website.  By using your 400 and 200 assessed times you can easily calculate your Critical Swim Speed pace. This will act as your functional or lactate threshold speed per 100 meter.

Once you have this swimming baseline, a host of workout opportunities arise. TriDot Triathlon Training uses your Critical Swim Speed to create intelligent workouts with the sole purpose of making this threshold pace faster. This might mean repetitions at or near your swim speed with minimal rest. It also gives you an ideal approximation of just how fast your aerobic and anaerobic swim sets should be in order to both generate the most quality results and the fastest recovery.


TriDot Triathlon Training uses your Critical Swim Speed, a meaningful assessment of swimming threshold, to provide smarter workouts and pacing intended to make you faster.


Do you know your Critical Swim Speed? If not, how have you paced your swimming in the past?

JARED MILAM is a former professional triathlete, TriDot coach, and former member of the Tri4Him Pro Team. He has 17 years of competitive running experience and 12 years of competitive triathlon experience with a half Iron PR of 3:59 and a full Iron PR of 8:30. Coaching under the TriDot system since 2011, Jared loves working with aspiring triathletes of all ages and performance levels.

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