CSS Main Set: 12 × 100m at Critical Swim Speed

Session Overview

This 60-minute pool session is built around a 12 × 100m main set at your Critical Swim Speed (CSS) — the pace you could theoretically hold for a 1500m time trial. Repeating 100m intervals at this pace is one of the most effective methods for improving your triathlon swim split, building lactate threshold and aerobic efficiency simultaneously.

What You’ll Need

  • Access to a 25m or 50m pool
  • Pace clock or watch with lap function
  • Optional: swim paddles and pull buoy for warm-up drills

How to Calculate Your CSS Pace

If you haven’t done a CSS test recently: swim a 400m time trial, rest 5–10 minutes, then swim a 200m time trial. CSS pace per 100m = (400m time − 200m time) ÷ 2. For example: 400m in 7:00, 200m in 3:20 = CSS of 3:40 ÷ 2 = 1:50 per 100m.

Warm-Up (12 minutes)

400m easy front crawl (mix 50m catch-up drill + 50m full stroke each length pair). Then 4 x 50m build (easy → moderate → fast → easy) with 20 seconds rest between. Focus on high elbow catch and long reach at the entry.

Main Set

12 × 100m at CSS pace — leave on a fixed interval. Your rest period = (your CSS time + 10–15 seconds). So if your CSS is 1:50/100m, leave every 2:00–2:05.

  • Reps 1–4: Focus on holding exact CSS pace. Don’t go too fast early.
  • Reps 5–8: This is where the session gets honest. Maintain form: keep your hips up, kick consistently, don’t breathe every stroke.
  • Reps 9–12: Fight for every rep. If form breaks down, it’s more beneficial to slow 2 seconds than to thrash and reinforce poor mechanics.

Cool-Down (8 minutes)

200m easy backstroke, then 200m easy front crawl focusing purely on breathing rhythm (every 3 strokes). Finish with 2 minutes of floating and stretching.

Coaching Notes

  • CSS is a moving target. Retest every 4–6 weeks and adjust your pace accordingly.
  • If you can complete all 12 reps comfortably with 10 seconds rest, reduce rest to 5 seconds or increase pace by 2 seconds per 100m.
  • The first rep always feels easy. The challenge is consistent splits from rep 1 to rep 12.
  • RPE target: 7/10 throughout — controlled effort with some discomfort in the final third.

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.