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by Michael Botyarov

May 7, 2021

This training methods helps you track your progress and ensure you’re completing workouts with a purpose

Aside for a love of being in the pool, many swimmers love to race. In order to race well, it’s important to consider the benefits of interval training as a means to track progress and ensure you’re completing workouts with a purpose. A simple way to incorporate a purpose into a workout is to use sets that include critical swim speed intervals.

Overview of CSS

Critical swim speed can be described as the threshold pace at which you can swim without getting overly tired, otherwise referred to as your aerobic swimming threshold. The faster you swim, the more oxygen your muscles use, which in turn makes you feel more tired because of a buildup of lactic acid in those muscles. After a hard race or a tough sprint set in a workout, you feel exhausted, perhaps even experience muscle soreness, all because of lactic acid.

By swimming slightly under race pace, also known as your aerobic threshold, you’ll be able to suppress the feeling of exhaustion while sustaining that pace. Doing CSS intervals will help you increase your endurance by slowly increasing your aerobic swimming threshold. Come race day, you’ll be able to swim faster since you have trained your body to hold a faster speed and use oxygen in your muscles in a more efficient way.

Calculating CSS

The great part of training using CSS intervals is that you’ll have a unique interval tailored to your swimming ability. In order to figure out what your CSS interval is, you’ll need to do a 400 (-yard or -meter) and a 200 (-yard or -meter) time trial. The key here is not to go all out but to go at a sustainable pace, slightly above moderate. You should do the 200 shortly after the 400, perhaps after a short active recovery set, such as light kicking. Once you have your 400 and 200 times, use an online calculator to show your tailored interval per 100 (yards or meters). Below is an example.

400-yard time: 5:05

200-yard time: 2:25

CSS interval: 1:20 per 100 yards

Once you have this interval, you’ll be able to use it for other distances beyond just a 100. For example, in a set of 3 x 500s, using the above interval, you would try to complete each 500 within 6:40 (1:20 multiplied by 5).

Incorporating CSS

You did the CSS interval tests, got your interval using an online calculator, now what? It’s time to put your interval into your swim workouts. A great way to swim CSS intervals is building sets of 100s up to 10 x 100s CSS by increasing by 1 x 100 each week, resulting in a 10-week training program.

It’s important to remember that rest time is included in the interval, meaning in the above example, if you finish a 100 in 1:15, you have five seconds before the next 100. Then, if you finish the next 100 in 1:16, you have four seconds prior to the next 100. Once you build up to 10 x 100s, you can recalculate your CSS interval by doing the same test. Over time, you’ll notice an improvement in your swimming ability.

Key Takeaways

Overall, CSS intervals are a great way to train efficiently for many races. Since the interval is calculated in terms of 100 yards or meters, the immediate benefit to your 200 is obvious: Swimming a set of 10 x 100s on your CSS interval will allow you to maintain a faster pace when you race a 200.

CSS intervals are also beneficial if you enjoy distance races, such as a 400/500 or 1500/1650. Since the 100 (-yard or -meter) interval is scalable, you can multiply to get a target for hitting your desired longer distance. Regardless of the distance, give CSS intervals a try if you’re interested in improving your swimming ability prior to your next big race.


  • Technique and Training