Swimming CSS (Critical Swimming Speed) Test: How to Calculate with 400m and 200m

Introduction

In swim training, how do you determine the pace that sits “right on the aerobic-anaerobic boundary”? Running has lactate threshold pace, cycling has power meters, and the most commonly used corresponding metric in swimming is CSS (Critical Swim Speed). CSS was proposed by British sports scientist Costill and colleagues, representing the highest swimming speed a swimmer can theoretically sustain for a “very long time” without accumulating excessive lactate. This article will introduce CSS testing methods and workout applications in the context of Taiwan’s pool environment.

What is CSS?

CSS is essentially the swimming version of “anaerobic threshold speed,” similar to the FTP (Functional Threshold Pace) concept in running. When you swim at CSS pace, your body relies heavily on aerobic metabolism, and lactate production rate balances with clearance rate, allowing you to sustain the effort for theoretically over 30–60 minutes.

Primary applications of CSS:

• As a benchmark pace for interval training (e.g., CSS+5 sec/100m as T1 training intensity)
• Assessing aerobic fitness improvements (retest CSS monthly for comparison)
• Setting race pacing strategies (target speed for 800m and 1500m events)

CSS Testing Method: 400m + 200m Timed Swim

Pre-Test Preparation

1. Choose a pool: A standard 25m or 50m pool works; it’s recommended to use the pool you normally train in
2. Warm up thoroughly: Swim 400–600m easy warm-up before the test, plus 2×50m gradual-pace swims to activate muscles
3. Rest adequately: Avoid high-intensity training the day before the test to ensure your body is in good condition
4. Prepare tools: Waterproof timer, pull buoy (optional), record sheet or swim watch

Test Protocol

First item: 400m all-out timed swim

• Swim 400m all-out and record the total time in seconds (T400)
• Recommended: 16 lengths in a 25m pool, 8 lengths in a 50m pool
• Keep turns fluid without taking extra rest

Rest: Take a full 10-minute rest after the 400m test (easy swimming allowed)

Second item: 200m all-out timed swim

• After adequate rest, swim 200m all-out again and record the total time in seconds (T200)
• Both tests must be completed on the same day

CSS Calculation Formula

$$CSS（sec/100m）= \frac{T400 - T200}{400 - 200} \times 100$$

That is:
$$CSS = \frac{T400 - T200}{2}$$

Calculation example:

Training Applications of CSS

Once CSS is calculated, you can set intensity zones according to different training goals:

Sample Workouts (using CSS = 125 sec/100m as an example)

T-zone threshold workout:

• Warm-up: 400m easy swim
• Main set: 4×400m, target time 8 min 20 sec per repeat (CSS pace), 45 sec rest between repeats
• Cool-down: 200m easy swim

I-zone VO2max workout:

• Warm-up: 400m
• Main set: 8×100m, target time 1 min 55 sec per repeat (CSS - 10 sec), 20 sec rest between repeats
• Cool-down: 200m

Retesting and Progress Tracking

It’s recommended to retest CSS every 4–6 weeks to track fitness improvements. Most amateur swimmers can improve CSS by 3–8 sec/100m after 8 weeks of regular training. In Taiwan, you can participate in timed events organized by county/city swimming associations or organize your own testing sessions, making CSS the most objective fitness indicator.

Practical Recommendations

1. Both tests must be all-out efforts: CSS is only meaningful if you give maximum effort in both tests.
2. Test in the same pool consistently: Differences in water temperature, currents, and length accuracy across pools can affect results.
3. Record the date and results of each test: Build your personal CSS improvement curve.
4. Use CSS to replace “feel” when deciding pace: Avoid charging by feel in every session; let numbers guide training quality.
5. Beginners should build a 6–8 week foundation before testing: CSS testing requires a certain level of swimming technique; otherwise, technical breakdown will distort the results.

Conclusion

CSS is currently the easiest self-administered fitness assessment tool in swimming science—just two timed swims give you a precise training benchmark. For amateur athletes accustomed to training in Taiwan’s pools, regularly testing CSS and incorporating it into workouts gives every session a clear goal and direction, greatly improving training efficiency and long-term progress.

Related Reading

• Swimming CSS Critical Swim Speed Testing and Training Applications: Find Your Aerobic Threshold Pace
• Building Swimming Aerobic Base: CSS Critical Speed Training Methods
• Swimming Lactate Threshold Training: CSS Calculation and Training Applications
• 8-Week Swimming CSS Pace Training: Master the Core Workouts for Critical Speed