Building an Aerobic Base in Swimming: The CSS Critical Swim Speed Training Method

Introduction

In running training, “lactate threshold pace” is the gold standard for aerobic training; in swimming training, the corresponding concept is called CSS (Critical Swim Speed). CSS represents the fastest pace a swimmer can theoretically sustain continuously without accumulating lactate. Training at CSS pace effectively improves aerobic metabolic efficiency and is one of the most scientific methods for building a swimming endurance base.

What is CSS

The concept of CSS originates from research by sports scientist Costill in the 1990s, and was later popularized by swimming training systems such as Swim Smooth. The theoretical basis of CSS is that training at the critical point between aerobic and anaerobic metabolism simultaneously improves the efficiency of both systems, providing optimal training stimulus near the “aerobic threshold.”

From a practical standpoint, CSS is the pace at which you swim in a “somewhat breathless but sustainable” state—faster than easy swimming, slower than all-out sprinting. It roughly corresponds to 400–1500m race pace, or T1 lactate threshold intensity.

How to Calculate Your CSS

Standard Testing Method (Swim Smooth Version)

1. Warm up thoroughly (300–400m easy swimming)
2. Swim an all-out 400m, record the time (T400)
3. Rest for 10 minutes
4. Swim an all-out 200m, record the time (T200)
5. Calculation formula: CSS (pace per 100m) = (T400 - T200) / 2

Example:

• T400 = 6 minutes 40 seconds (400 seconds)
• T200 = 3 minutes 00 seconds (180 seconds)
• CSS = (400 - 180) / 2 = 110 seconds = 1 minute 50 seconds / 100m

CSS Training Session Design

Training Zone Settings

• CSS+5 seconds: Easier aerobic training, suitable for high-volume, low-intensity days
• CSS pace: Core aerobic threshold training, 2–3 times per week
• CSS-5 seconds: Light lactate endurance, used during race season

Weekly Session Example (4-Week Base Building Phase)

Monday (Aerobic Base):

• Warm-up 300m
• Main set: 6 × 100m @ CSS+5 seconds, 20 seconds rest
• Cool-down 200m
• Total distance: approximately 1800m

Wednesday (Critical Speed):

• Warm-up 400m
• Main set: 4 × 200m @ CSS pace, 30 seconds rest
• Supplementary: 4 × 50m @ CSS-5 seconds, 60 seconds rest
• Cool-down 200m
• Total distance: approximately 2200m

Saturday (Extended Aerobic):

• Warm-up 500m
• Main set: 2 × 800m @ CSS+5 seconds, 60 seconds rest
• Cool-down 300m
• Total distance: approximately 2400m

Common Mistakes and Corrections

• Excessive fatigue after testing: The CSS test is a high-intensity test; schedule an easy swim or rest the day after testing, and do not follow it with high-intensity training
• Rest intervals too short: Rest interval design in CSS training is important—too short turns it into anaerobic training, while too long loses the aerobic stimulus effect; strictly follow the rest intervals in the session plan
• CSS underestimated: If the swimmer is overly nervous or has technical issues before the test, the measured CSS will be conservative, resulting in insufficient training intensity; retest every 4–6 weeks

Practical Recommendations

1. Retest CSS every 6 weeks to track progress; if CSS improves by more than 3 seconds, it indicates a substantial improvement in aerobic capacity
2. CSS training is best performed without excessive fatigue; schedule it on a moderate-intensity day within the training cycle, not the day after a high-intensity session
3. Use a pool pace clock (or Garmin swim watch) to accurately record each interval time and confirm whether the pace reaches the target zone
4. CSS is only one metric and cannot replace comprehensive training design; the weekly schedule should still include easy swimming, anaerobic sprints, and technique work

Conclusion

CSS transforms “feeling somewhat tired but still holding on” from a subjective sensation into a measurable, objective pace metric. Designing aerobic training sessions around CSS ensures that every workout is performed within the most effective physiological zone, making it a scientific training tool worth adopting from amateur swimmers to competitive athletes.

Related Reading

• Swimming Lactate Threshold Training: CSS Calculation and Training Application
• Swimming CSS (Critical Swim Speed) Test: Calculation Method for 400m and 200m
• 8-Week Swimming CSS Pace Training: Mastering the Core Critical Speed Sessions
• Swimming Interval Training: Calculation and Training Application of CSS (Critical Swim Speed)