4x600m CSS Endurance Swim: Threshold Pace Development Session

Session Overview

This 4x600m threshold session develops your lactate threshold in the water using Critical Swim Speed (CSS) — the sustainable pace between aerobic and anaerobic effort. Holding 600m reps at or near CSS builds the kind of steady-state swim endurance that separates solid triathlon swimmers from those who blow up in the opening minutes. Ideal for Olympic and 70.3 athletes building their swim fitness.

What You’ll Need

  • 25m or 50m pool
  • Swim watch or pace clock
  • Knowledge of your CSS pace (calculate from a 400m and 200m time trial if unsure)
  • Goggles and swim cap

Warm-Up (10 minutes)

400m easy freestyle to loosen up. Follow with 4x50m descending — each 50m slightly faster than the previous, finishing the last one at CSS effort. 10 seconds rest between each. This warms your body up to the threshold intensity you’ll hold through the main set.

Main Set

4x600m at CSS pace. Your CSS pace is typically 2-5 seconds per 100m slower than your best 400m pace.

  • Rep 1: 600m at CSS pace. 60 seconds rest.
  • Rep 2: 600m at CSS pace. 60 seconds rest.
  • Rep 3: 600m at CSS pace. 60 seconds rest.
  • Rep 4: 600m at CSS pace or 2 seconds per 100m faster if you feel strong — a strong negative split on the final rep is the goal.

Total main set distance: 2,400m. Total session distance: approximately 2,900m.

Cool-Down (8 minutes)

200m very easy backstroke followed by 100m slow freestyle focusing on long strokes and full rotation. Allow your breathing to return to normal before exiting the pool. This is not a session to skip the cool-down — your muscles will thank you on your next run.

Coaching Notes

  • If you don’t know your CSS pace, estimate it as 5 seconds per 100m slower than your best 200m pace. A proper CSS test (400m + 200m TT) is worth doing before starting a structured swim programme.
  • Consistency across the four reps is the goal. If your pace drops significantly on reps 3 or 4, your CSS estimate may be too fast or your aerobic base needs more work before attempting this session.
  • To make it easier: increase rest to 90 seconds between reps and target a pace 3-4 seconds per 100m slower than CSS.
  • To make it harder: reduce rest to 45 seconds and attempt the final rep 3 seconds per 100m faster than CSS.

Training at your own risk. The information provided is for general guidance only and is not a substitute for professional medical advice. Please consult a doctor before starting any new exercise programme, especially if you have any pre-existing health conditions.