5x500m CSS+5: Aerobic Threshold Pool Session

Session Overview

This 5x500m CSS+5 session trains you in the aerobic threshold zone — the sustainable pace that sits just below your critical swim speed (CSS). Swimming five 500m repetitions at CSS+5 seconds per 100m builds the prolonged endurance needed for Olympic (1500m) and 70.3 (1900m) race swims without the lactic debt of full CSS pace work. It is the ideal mid-distance swim build session for intermediate triathletes.

What You’ll Need

Warm-Up (12 minutes)

Swim 400m easy freestyle at Zone 1. Then 4 x 50m as 25m drill (choice) / 25m freestyle. Finish with 4 x 50m descending — each slightly faster than the last, ending at CSS pace. Rest 20 seconds between each 50m.

Main Set

Swim 5 x 500m at CSS+5 (five seconds per 100m slower than your CSS pace). Take 60 seconds rest between each repetition. For a swimmer with a CSS of 1:45/100m, this means targeting 1:50 per 100m — approximately 9:10-9:30 per 500m rep.

  • Rep 1: Settle into rhythm — this should feel comfortably manageable
  • Reps 2-3: Maintain consistent 100m splits, checking the pace clock each length
  • Rep 4: The effort begins to accumulate — focus on long strokes and relaxed breathing
  • Rep 5: Final rep — hold the pace to the wall with no acceleration and no fading

Cool-Down (8 minutes)

Swim 300m easy, mixing backstroke and very easy freestyle. Focus on elongated strokes and full shoulder rotation. Shoulder stretches poolside for five minutes after finishing.

Coaching Notes

  • Total session volume: approximately 3200m including warm-up and cool-down
  • To find your CSS: CSS = (400m time in seconds − 200m time in seconds) ÷ 200, expressed as seconds per metre
  • RPE for each 500m rep: 6/10 — this should not feel like a race effort
  • The 60-second rest is intentional — shorter rest increases the aerobic demand
  • Progression: once five reps feel comfortable, reduce rest to 45 seconds per rep

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.