Let’s look at a typical aerobic swim set: 10 x 100’s on :20 rest.

On the surface, a perfectly good workout. However, that rest interval doesn’t take how hard you’re working into consideration. Meaning, you could swim 100 yards / meters with good effort at the start. But regardless of how fast you swim each 100, you’re going to get :20 rest. So in theory you can put forth less effort and get the same amount of rest. The aerobic benefits could easily be lost near the end of the set as fatigue sets in and you begin to slow down.

To get the most out of a swim set, you need to “work for your rest”. If a set is designed to work your aerobic energy system then you need to keep your aerobic energy system working. A “hard” send-off requires you to put the effort in. More importantly, if you want to start working your energy systems appropriately you need to know our base 100 time and structure workouts with the proper amount of intensity and rest to ensure the proper usage of the energy system in question.

So, looking at that same workout, what if it were written with the assumption that it takes you 1:30 to swim 100 yards / meters - your base 100 time:

10 x 100 on the 1:50

Now you have to maintain that pace and effort to get your :20 rest. If you slow down, you get less rest. If you swim faster, you get more. The bottom line is that now all of your workouts can be written with specificity to be aerobic, anaerobic, VO2, whatever is desired. You just have to know how to structure workouts to focus on specific energy systems. So, how do you determine your base 100 time to get things started? Do the following workout:

Warm up - If you require a longer warm up, swim a little extra.
250 easy swim
250 pull

4 x 50 on 1:00 or :15 rest.
4 x 100 on 2:00 or :30 rest. Descend 1-4 (meaning, swim faster on each 100)

Main set / test set:

100 @ 90% effort. Yep, you’re going to go fairly hard on this one
90 seconds recovery.
400 for time. Shoot for 90+% effort. Please note, you have to swim the 400 as hard as you can swim a 400. Don’t go all out in the first 100 and crash and burn! Instead, keep in mind that the “entire” 400 is as hard as you can go. So you do have to pace yourself. This effort will hurt a little bit.

Record your times. You’ll use this info to structure all future swim workouts. Dividing your 400 time by 4 will give you your base 100 times for upcoming workouts. Simply convert your 400 time to seconds, then divide by 4, and then convert back to minutes.

So, for the aerobic set we first mentioned, a good aerobic workout will give you around :15 - :20 rest. So assuming you have a base 100 time of 1:30 (a 6:00 400 time if you’re wondering) we’d write the workout as follows:

10 x 100 @ 1:50

You get a little more rest if you swim faster than your base. A little less if you swim a little slower. But most importantly at the end of the set you will have gotten absolutely everything out of that aerobic workout.

Another example, as you start to alter your future swim workouts, send-offs can be looked as as follows:

If your base-100 time was 1:30, and you want to do some aerobic work with strong 50’s on :30 rest, and 100’s on :30 rest, you’d have:

10 x 50 strong @ 1:15 - and 10 x 100 @ 2:00

Taking it further, 6 x 200 @ Base + :30, with a base 100 time of 1:30, then your send-off is 3:30. Hint, the rest isn’t doubled for the 200! The rest interval is added at the end of adding your base 100 times together.