Beginners, Podcast, Swimming, Training

Training Zones part 1: Swimming | EP#27

 June 1, 2017

By  Mikael Eriksson

Training Zones part 1: Swimming | EP#27

Training zones. You know of them, but do you know just how important it is that you use them? And most importantly, how to go about using them in swimming, biking and running?

In part 1 of this episode series on training zones, we cover how to use training zones in swimming.

In this Episode you'll learn about:

  • How to determine training zones in swimming
  • How to use training zones in different types of workouts.
  • Is heart rate a valuable metric in swim training or should you just use pace?


Purpose of training zones (as it relates also to swimming)

  • In general, it is to train or swim at the right intensity at the right time in order to make the training as effective as possible.​
  • Just training and going out hard every single time will not make you progress at an ideal rate.
  • Plodding at a very leisurely pace will not develop you much either. You will have a very limited progress compared to being strategic about what intensities you train at.
  • So zones are all about training intensities. Intensity can be measured in a few different ways depending on the sport.
  • Essentially, when you’re supposed to go easy, you don’t go moderate. When you’re supposed to go hard, you don’t go moderate. These are the most common mistakes. Triathletes go moderate all the time, never going really easy or hard enough according to the purpose of the workout.
  • Training zones will help you do this because you will have something to stick to, see if you’re on track, and get instant feedback.

Key takeaway

You need to know the purpose of why you swim at a certain intensity or effort level at a certain time. Swim at the right intensity at the right time to make your progress in the sport the fastest possible.

Typical architecture of a swimming session

1. Warm up

02:42 -

  • For example a 10 minute easy swim at an intensity that is easy. It might include some technical works and some drills.​

2. Build set

02:54 -

  • ​After the warm up, there might be a slightly faster set of moderate intensity swimming. But nothing too much, just building to that main set.
  • Depending on the length of the swim session and the ability of the swimmer, there might be a pre-main set or a build set forming a bridge between the warm up and the main set. This is where you go into that moderate swimming and build the intensity by adding something fast (moderate plus to fast minus) depending on what the upcoming main set has in store for you.
  • This is usually how I do a build set. It gets me fully warmed up and increases my heart rate for me to be ready for the main set:
    • 4 x 50 meters "build" (accelerations) as:
      • 1st 12.5 meters - easy pace
      • 2nd 12.5 meters - moderate minus
      • 3rd 12.5 meters - moderate plus
      • 4th 12.5 meters - hard
    • 15 second rest after each 50 m build
    • Repeat 4 times
  • The total duration of the build set is usually 5-10 minutes in an hour long swim session.

3. Main set

04:25 -

  • ​Typical main sets are either threshold sets, endurance sets, or speed sets.
  • In endurance sets, the intensity is moderate in order to be able to perform the entire set at the prescribed pace. 
  • The intervals will be long relative to your abilities, so towards the end it will feel hard.
  • In threshold sets the pace is moderate plus or hard minus, which is something that you can sustain for 1500 meters in a race.
  • Speed sets are short but intense intervals at a pace that is hard plus.
  • The main set is the bulk of the swim session and is usually around 30-40 minutes, sometimes more, in an hour long swim session.

5. Cool down

  • 5-10 minutes.​

Keep in mind

If you swim all the parts of a swimming session at the same time intensity, you will not be getting all of the benefits that you could be getting.

For example, if you start too hard in the warm up it will consume an unnecessary amount of energy that you will need in your upcoming threshold set (for example). This may lead to you not completing the set at the right intensity.

These kinds of things will happen if you don’t use specific training zones, or in the case of swimming, pace prescriptions.

How intensity is measured in swim training

06:58 -

  • The single correct answer is pace although there are heart rate monitors for swimming.
  • Most of the training that we do in swimming is interval based. Many of the intervals are 3 minutes or less and heart rate takes at least 3 minutes to stabilize. So we really can’t monitor heart rate in real-time in swimming.
  • Comparing to cycling, if you are a cyclist using a power meter, then you don’t do your sessions based on heart rate because power is the output that is produced. Heart rate represents the internal workload, which is important, but is affected by other factors that are less controllable like caffeine and quality of sleep.
  • So with pace, we need to hit the intervals in specific times. For example, if the main set is 15 x 100 m and you are supposed to do them at threshold pace, let's say 1:45 per 100m. Then that is what you are supposed to be following even if you have a heart rate monitor.
  • Data is good. So by all means, use a heart monitor. But don’t use this to guide your swimming workouts because you won’t be training specifically enough.
  • All multi-sport watches these days have great pace functions that can be easily displayed. But don’t let this derail you from your focus on your actual swimming.
  • You can also use a pace clock at the wall of the swimming pool which most pools have.

How to determine training zones based on pace
and Critical Swim Speed (CSS)

10:54 -

  • CSS is essentially threshold swimming pace.​
  • Determining your CSS is done through a CSS test that is a 400 meter and 200 meter time trial performed in the same session.
    • Do a thorough warm up and a build set.
    • Go into the 400 meter time trial and record your time.
    • Take a 10-15 minute active recovery rest to keep yourself warm and loose.
    • Do the 200 meter time trial and record your time.
    • Then use the CSS calculator to calculate CSS pace. This takes into account your pace for the 400 meter and the 200 meter and how much quicker you are in the 200 meter.
    • The CSS pace is the pace that you should be able to sustain for 1500 meters, so Olympic distance race pace, or even 1900 meters or half distance race pace.

How I structure swim training

  • I'm a big proponent of the Swimsmooth philosophy developed by Paul Newsome. This school of thought on swimming has a lot of focus on threshold training, where you work hard, but can still maintain good form and technique, so you work that at the same time.
  • Depending on your ability, if you’re a fairly decent swimmer, your main sets will be 1500 - 2000 meters all at CSS pace with short recoveries. For example:
    • 20 x 100m @ CSS pace, 15 s recovery 
    • 10 x 200m @ CSS pace, 20 s recovery 
    • 5 x 400m @ CSS pace​, 30 s recovery 
  • ​If you're an advanced swimmer, your main set length can be longer still, up to 2500-3000 m.
  • You will also have some endurance training sessions with paces up to 10 seconds per 100 meters slower than CSS depending on the set and the interval length. For example:
    • 3 x 1000m @ CSS+10s per 100 meters / 2-3 min recovery
    • 8 x 400m @ CSS+5s per 100 meters / 1 min recovery

    Or descending workouts like:

    • 1000m @ CSS+10s, 2 min recovery
      800m @ CSS+8s per 100m, 1:30 recovery
      600m @ CSS+6s per 100m, 1:00 recovery
      400m @ CSS+4s per 100m​, 30 s recovery
      200m @ CSS+2s per 100m
  • Finally, you'll have some speed sessions which are fast and don't need to be based on pace. Just go by feel, try hitting 95% of all out effort. Not 100% because chances are you are just going to be fighting the water and going slower compared to being slightly more controlled.
  • One guideline for how fast this pace would be: 10 seconds per 100 meter faster than CSS pace.

Key takeaway

For swimming, you don’t really need training zones but you should be doing your training based around precisely prescribed paces.

An alternative system: Joe Friel’s training zones​

18:43 -

  • You can calculate your swim training zones using Joe Friel's system directly in Training Peaks
  • You can also have a look in his book: Your Best Triathlon
  • His swimming zones are based around a 1000 meter time trial. For example:
    • You swim your 1000 m time trial in 19 min, which is 1:54 min per 100 meters
    • The lowest intensity zone (Z1) is active recovery, which would be 2:21 min plus (slower than 2:21 per 100 m)
    • The second zone (Z2) is aerobic endurance, which would be 2:13 - 2:20 per 100 m, or at least 20 seconds slower than time trial pace
    • Tempo is the third zone (Z3). This is a moderate intensity and would be 2:04 - 2:12, up to 10 seconds slower than time trial pace
    • Your threshold zone (Z4) would be from tempo through 1:53 pace. Remember, we swam the 1000 meter time trial in 1:54 pace. So your time trial pace is the high end of the threshold zone.
    • Aerobic capacity or VO2max (Z5) is 1:44 - 1:52. Up to 10 seconds faster than TT. This zone would be used in speed sessions.
    • Anaerobic capacity would be faster still. Most age-group triathletes can’t hold anaerobic capacity pace for longer than 25 meters or a couple of 50 m repeats at most, and this is not a zone that triathletes should spend a lot of time in either.


  • Training zones for swimming are based around paces, in particular the Critical Swim Speed (CSS), which can be calculated from a 400 and 200 meter time trial.
  • Much of the training will be based exactly on that CSS pace. This type of training is like threshold training in swimming.
  • Some will be endurance-based training, at paces up to 10 seconds per 100 m slower than that CSS pace for repeats of e.g. 1000, 800, or 600 meters.
  • Some sessions will be speed sessions where swim clearly faster than CSS-pace. However, these sessions are done based on perceived exertion, not necessarily trying to hit exact paces.

Links and resources

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Mikael Eriksson

I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal.

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  • Really enjoyed this article. Very clear and informative. Just need to put it into practice now and use my garmin and pace clock instead of just winging it.

  • Unfortunately this CSS calculator is no longer available for setting zones. It will tell you your CSS speed, but the zones are hidden behind a paywall.

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