Critical Swim Speed (CSS) Test Session

About 1,800 m, 40 to 50 minutes. The session that gives every other pace on this list a number. Warm-up: 400 easy, then 4 x 50 building to fast, 15 s rest. Main: 400 as fast as you can hold. Swim 200 very easy, then 200 flat out. Cool-down: 400 easy, any stroke. Subtract the 200 time from the 400 time and halve it: that is your CSS pace per 100. A 7:00 400 and a 3:20 200 give 1:50. The 200 has to be a true all-out effort, or the gap shrinks and the pace comes out too fast. Repeat every six to eight weeks to see progress.