How to use Critical Swim Speed training

  • By Philip Mosley / TrainingPeaks
  • Published February 10, 2016

Critical Swim Speed (CSS) is the pace at which you’d currently swim a 1500 Time Trial (in yards or metres). There’s nothing complicated to worry about, but it’s a super-effective way to prepare for race distances of 400 and above. Here’s why CSS training is effective and how you can use it.

The Benefits of CSS Training
CSS is a pace that’s tough enough to develop your aerobic capacity but not so hard that it’ll take you days to recover. So you can improve your swim fitness and still have enough energy to go running or cycling (or swimming again).

CSS is a race-specific training pace. It may not make you the fastest 50 or 100 swimmer, but it will train you to sustain a moderately high speed for longer distances.

CSS training teaches you about pace awareness the hard way (which is usually the best way) Go off too fast and you’ll pay the price later. Ouch!

Thankfully you can test your current CSS pace without having to swim a solo 1500 Time Trial. See below for instructions.

Once you know your CSS pace, you can use a Finis Tempo Trainer Pro to help you train. It’s a small beeping device that attaches to your goggles. Dial in your CSS pace and it’ll happily beep every lap so that you can maintain perfect pace.

How To Test Your CSS
To test your current CSS pace you need to swim a 400 and 200 Time Trial within the same session (see Test Workout below). Ideally, get a friend or coach to time you and record your 100 splits and strokes per minute. Failing that, simply record the 400 and 200 times yourself. Once you’re done, enter your 400 and 200 times into this CSS CALCULATOR.

CSS Test Workout
Warm Up 300 easy freestyle as 200 fins as 50 choice drill, 50 freestyle – 4 x 50 freestyle (25 fast + 25 easy) +10 seconds rest then 4 x 100 freestyle (20 seconds rests) Do these 100’s at what you perceive to be the AVERAGE pace that you can sustain for a 400 Time Trial.

Main Set 400 time trial

Take 5 to 8 minutes easy swimming/stretching. Feel fully recovered, then:
200 time trial
Warm Down 100 easy choice of stroke

CSS Sample Workouts
Once you’ve worked out your CSS pace, the training possibilities are endless. You could start off by trying these four simple workouts. You can shorten them if you’re not quite ready for 2000 metres or yards yet. Don’t forget to include a warm up (400 to 800m/y) and warm down (100 to 200m/y) too.

1. 20×100 with 15 second recoveries. All at CSS
2. 10×200 with 20 second recoveries. All at CSS
3. 5×400 with 30 second recoveries. All at CSS
4. 3×600 with 45 second recoveries. All at CSS

Including CSS sets into your swim training can help improve your sustainable speed and enhance your pace judgement. In addition to CSS training, it’s also important to strike a balance between speed, threshold and endurance workouts in order to meet the needs of your target races. Technique work and open water training are equally important too! For more information about CSS training check out the SwimSmooth Training System.

About the author: Phil Mosley is a multiple top-ten Ironman finisher and five-time national UK age-group triathlon champion. He is the current Coaching Editor of Triathlon Plus magazine and in the UK and has written hundreds of expert features for titles including Runners World, Cycling Plus and Cycling Weekly. He has an honors degree in Sports Science and 10-years experience as a triathlon, running and cycling coach. His coaching company is called

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