Currently, LYNX offers “CSS development” swim sessions on Tuesdays and Thursdays.  This post will define CSS, explain why it might be relevant to you and include a sample CSS session at the end.

What does CSS stand for?

CSS stands for Critical Swim Speed. It is the 100 meter (or yard) pace you can sustain for a 1500m time trial. It is also an approximation of your lactate threshold speed or “threshold pace” for short.  One can think about threshold effort as swimming comfortably hard. For open water swimmers and triathletes competing in “distance events” (750m or longer), the goal is to improve your threshold pace so that you can swim faster, longer.

What does this mean to you?

To improve your threshold pace you will want to do your quality swim sets at your current threshold pace or slightly slower than it. For most swimmers, the shift to CSS sets means more swimming with less rest at the wall.  This can be a tricky change in mindset for athletes. Early in a session swimmers new to CSS sets will make comments like “I don’t feel like I’m working hard enough”.  But, then, midway through the set they are winded and wanting more rest…and that’s when the real work begins. To summarize: CSS development work is beneficial because it’s race pace specific training that doesn’t take you days to recover from.

How does one determine their CSS pace?

There are 2 fitness tests I utilize to determine CSS pace. One method is to have swimmers complete a 400m and 200m time trial where I record total times as well as the first 100m split of the 400. From this information, I can calculate a CSS pace. Also, by looking at the drop off in pace between the first 100m and average 100m pace held during the last 300m, I can provide feedback to the swimmer about how well they are currently trained for distance swimming. I can tell if they are a “diesel engine” who paces well or more of a sprinter who has some initial speed but then falls off. If an endurance triathlete’s data shows that they are more of a sprint machine versus diesel engine, I recommend more CSS / Endurance training to help them get faster over longer distances.

A second method to determine a person’s CSS pace is to do a 1500 time trial. The average 100m pace held during the test is the swimmer’s CSS pace. Although I will have the squad do a few 1500m time trials between now and May, typically the CSS testing conducted is the 400/200m time trial method. Both methods yield very similar results and both allow swimmers to monitor their progress as the season progresses.

How does one incorporate CSS training into their weekly schedule? 

To maximize improvement in your swimming you should get in a variety of swim sessions each week. They are all listed HERE.  If you are swimming 2x per week you might want to pick one technique oriented session for form, and one CSS session for improving threshold speed. If you are swimming 3x per week, in addition to the above sessions, you will want to add in an Endurance session with longer steady paced swims.

Sample CSS workout

The workout below is a great way to “check in on” or validate your current CSS pace. If you can complete the entire 1500m main-set while “staying on the beep”, then your defined CSS pace is reasonably accurate.  If you are ahead of the beep and then fall off, your CSS pace is actually slower than you set it to because you were not able to stay on the pace. If you are ahead of the beep with ease, then your CSS pace is likely faster than you thought.


  • 200 easy choice swim
  • 200 fins – kick on side, alternate sides by 25m
  • 200 pull and optional paddles breathing every 5 strokes
  • 200 swim, building pace from easy to hard over the 200

Main-set: on CSS pace / by 4 and entered into Mode 1.  Stay on the beep

  • 100 +1 beep recovery
  • 200 +1 beep recovery
  • 300 +1 beep recovery
  • 400 +1 beep recovery
  • 500 +1 beep recovery

Cool-down: 200 easy choice swim, include some backstroke

To set your pace:

  1. Turn on your tempo trainer (bottom right button)
  2. Select Mode 1 on your tempo trainer (hold top button until you see mode 1)
  3. Take your 100m CSS pace in seconds and divide it by 4 to get pace per 25m
  4. Enter it in mode 1 of your tempo trainer

Example calc.

  • If CSS pace is 2:00/100m
  • 120 seconds / 4 = 30:00 sec
  • In Mode 1, this swimmer would set their beeper to 30:00.

To start, push (don’t hold) the reset button (single top button) on the beeper. The goal is to swim at a pace where you will reach the end of the 25m pool as the tempo trainer beeps. If you are swimming at the right pace, you will touch the wall at the same time as the beep. This is called “staying on the beep”.  If you hear the beep after you turn, then you are “ahead of the beep”. And, If you hear the beep before you reach the wall, you are “behind the beep”.

Once you finish each interval (100 or 200 etc), you get a “1 beep recovery”. Assuming you stayed on the beep for each 25m, you will hit the wall on the beep. Then, you rest until you hear the next beep and off you go!

Assuming you have a valid CSS pace (that is essential), you can see how this type of training is excellent to help you learn how to pace. If you aren’t able to stay on the beep, then I recommend doing a CSS test session to get a more accurate CSS pace. Give it a go and see what you think!

Coach Mary