Swim Testing – Critical Swim Speed

Most of our training sessions revolve around this magic speed called critical swim speed. What exactly is it and why should it affect your swim training so much?

What is Critical Swim Speed (CSS)?

Critical Swim Speed is the estimated pace you are capable of doing for 1500+m. It is considered to be your aerobic swimming threshold. It should be sustainable for a prolonged period of time. From a physiological point of view, it is a speed you can travel at where your lactate production is equal to your body's ability to dispose of lactate. As a distance swimmer, it is your body's capability of swimming at such a pace or intensity over a prolonged period of time that makes you a successful endurance swimmer. Therefore this is a system you need to stress to become better at it. 

How do I work out my own Critical Swim Speed?

From the information above it is clear that knowing your CSS is really important (critical some might say!). 

Your CSS is easy to calculate after completing a short test: a 400m best effort, 200m best effort and a 50m best effort. The 400m and 200m is used to calculate your CSS and the 50m shows us what your swim profile looks like. 

If you consider the fact that you will be swimming at an effort over both the 400m and the 200m swim time trials that is part aerobic and part anaerobic. We are only interested in seeing what you are capable of swimming aerobically as this is a sustainable effort. In essence we ignore the anaerobic part of your test.

Therefore, the assumption is that you are unable to swim the whole 200m anaerobically. (If you are stronger swimmer, it may be worth increasing the distances). Subtract the time taken for the 200m test from the time taken to complete the 400m test.

For example, if a swimmer can swim 6:00 for 400m, 2:50 for 200m, the resultant time for the remaining 200m is 3:10. This time is the time taken to aerobically swim 200m. 

If it is divided by two, then that is the pace per 100m (1:35/100m) to swim aerobically. This is the swimmer's CSS pace. NB it is slower than the best effort for the 400m (1:30/100m) – why? Because it is sustainable.

Calculate Your CSS:

400m Time (total number of seconds) =
200m time (total number of seconds) =
CSS/100m (seconds) =
CSS/25m (seconds) =
Your CSS/25m pace can be used with a Finis Tempo Trainer Pro set to the CSS/25m interval above to ensure you hold your CSS pace throughout the whole session. This leads to perfect pacing!

What is a swim profile?

Each person has a different speed profile. For example, sprinters are excellent over 50-100m, but can't hold that same speed over 400m whereas 10km swimmers can churn out the same pace for a very long time but they couldn't swim 100m as fast relatively! 

Fig. 1: Swimming profile of a national standard swimmer agains a top age group Ironman distance swimmer

As a sprinter, you have to be occupied by increasing the left hand side of the graph. However, as an endurance distance swimmer, you really want to increase the right hand side of the graph. and you don't really care if the graph becomes flatter because of it. Therefore, training should be partly focussed on the right hand side of the graph.