The point of this threshold test is to determine your threshold pace regardless of of your pool is 25 yards, meters, or 50 meters. The workout is short including the warmup. The test protocol is:
Swim a 100 as fast as you can and time yourself
Take a full recovery (as long as you want while you swim an EZ 100, 150, or 200)
A fixed distance test ranging between 400 yards or meters if you’re a beginner, all the way up to 1500 yards or meters if you’re a fast advanced swimmer. A fixed distance swim rewards you for swimming harder because the harder (well, maybe I should say FASTER) you swim the sooner you’re done.
You might have some difficulty picking what distance to swim the first time you test. The goal is for the test to take between 15 minutes and 20 minutes, so you’ll need to have a general idea of your threshold pace to pick the correct distance.
The first time you test if you come in less than 15 minutes swim longer next time, and if you’re over 20 minutes either swim shorter next time, or perhaps your fitness will improve by the next time you test and you can swim the same distance again.
After the test you’ll be tired (you should be tired if you truly swam as hard as you possibly could for whatever distance you choose).
Take another full recovery, and then you’re going to swim another 100 as fast as you can.
An elite swimmer will hardly get slower on the 2nd 100 – in fact they might even get faster. An intermediate or beginner is unlikely to get faster, but hopefully your time is within about 5 seconds of your first 100. If your time is more than 10% slower than your first 100 it can mean you need to do more strength work outside of the pool, use paddles more often, or if you’re not using paddles yet, sprint even harder on the sprint sets.
The times on your 100 are NOT your threshold pace – they are a measure of how good you are at swimming. Because a 100 is fairly short the time on your 100 doesn’t depend that much on your aerobic energy system (well, unless your 100 time is over 3:00, but it won’t be over 3:00 for long if you keep training). If your 100 times keep getting faster but your threshold test pace stays the same it could mean you need to work much harder on the long workouts and threshold workouts. If your 100 times stay the same but your threshold pace gets faster it means you’re becoming more aerobically fit OR your technique has improved and you’re just not trying hard enough on your 100s.
If you do a threshold practice and make all the purple intervals “no problem” then the threshold pace you’re basing your intervals off is too slow and you need to pick a faster threshold pace or do the threshold test over again. You don’t have to do the threshold test every time you want to use a faster threshold pace. I’m not a fan of testing more than once every 2 months. I want my swimmers to respect the test and take it very seriously each time they do it. Testing too often will make it “just another workout” which it certainly is not.