Swimming at threshold pace is high-intensity training used almost universally by swimmers and triathletes to develop aerobic endurance. During a threshold workout, you typically swim intervals of up to 400 yards or meters at threshold pace, with relatively short rests between the work intervals.
Your swimming threshold is an even pace that you can maintain for about 3-4 minutes, and it challenges your ability to maintain stroke efficiency while pushing your physical abilities to the brink. When you exceed this threshold for any length of time, you will need to slow down or stop completely to recover. For competent and reasonably fit swimmers, a threshold-level effort correlates nicely with both your maximal oxygen consumption (VO2max) and your lactate threshold, which are two gold standards for aerobic endurance development.
Note that threshold pace is not your sprint pace for a swim of 200 yards or less, which will be faster, and it’s not your best race pace, which will be somewhat slower (unless your race is less than 400 yards).
It’s important to know what your threshold pace is, because by training properly – and precisely – at that pace, you can usually improve it dramatically, together with your targeted race pace and your overall fitness level.
A good tool for estimating your threshold pace is the CSS (critical swim speed) Test, which was developed in the 1980s by Coach Ernie Maglischo. The test consists of two maximum-effort time trials, one long and one short, usually 400 and 200 yards or meters (the unit of measure doesn’t matter – so long as your test and use of the calculated CSS is consistent).
After swimming the two tests (long test first – with complete rest before both tests), enter your times in a CSS calculator (see the CSS Test Workout). This calculates your CSS, which is a velocity expressed as yards (or meters) per second, which is somewhat slower than your short test velocity but somewhat faster than your long test. The calculation removes your short test time and distance from the long test time and distance, and calculates the velocity for the remaining time and distance as your CSS. This calculation was chosen because for reasonably fit college-age competitive swimmers, it correlated very nicely with most swimmers’ lactate threshold pace. Aside: At that time it was fairly common to draw blood multiple times during a workout to test blood lactate levels. This test was developed not only because drawing blood was cumbersome, disruptive, and unpleasant (and everybody hated it!), but with the arrival of AIDS on the scene, the era of widespread testing of blood during workouts pretty much came to an end. (It is still done in some high-end performance labs.)
From your CSS you can calculate threshold training times for various distances – in our case since the pool is most often set up for 25-yard lanes, we set threshold training times for 25-yard lengths and 50-yard laps. We then use those with a Tempo Trainer Pro (or other suitable timing device) to swim every length or lap at threshold pace during a threshold set. A timing device such as the Tempo Trainer Pro that provides feedback every 25 or 50 yards is invaluable for keeping us at threshold pace – without these tools we invariably start too fast and finish too slow, or we crash and burn before completing the set.
Based on your CSS, you can also estimate your endurance training pace, which will be 3-5 seconds per one-hundred slower than your threshold training pace, and is the pace you will use for your long, continuous swims and perhaps other longer-distance or recovery sets.
If you swim regularly at pools configured with either different lane lengths or units of measure, you will need to know your CSS for each differently configured pool.
You might repeat a CSS test every 4-6 weeks, or you can adjust your CSS manually from time to time, based on your performance of CSS based threshold workouts.
Below are links to a CSS test workout and sample CSS based threshold workouts:
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