4×800m CSS Threshold Blocks: Pool Endurance Swim Session

Session Overview

This 4×800m CSS threshold session targets lactate endurance — the ability to sustain race pace over distance. Each 800m block is swum at your Critical Swim Speed (CSS) pace, the speed you can sustain across a triathlon swim without accumulating fatigue. This is a key session for Olympic and 70.3 athletes in the 8–12 weeks before their A-race.

What You’ll Need

  • A 25m or 50m pool
  • Pace clock or GPS swim watch for interval timing
  • CSS pace already established (run a 400m TT and 200m TT to calculate)
  • Pull buoy (optional — for the warm-up only)

Warm-Up (10 minutes)

Swim 400m easy as mixed strokes — 100m freestyle, 100m backstroke or catch-up drill, 100m freestyle, 100m pull buoy. Keep the effort at RPE 2–3. Follow with 4×50m building from easy to CSS pace on 15 seconds rest.

Main Set

4×800m at CSS pace with 60 seconds rest between each. Hold your CSS target pace as consistently as possible across all four blocks. If your pace drops significantly in the third or fourth rep, you’ve gone too hard — ease off slightly next time.

  • Rep 1: 800m at CSS pace — start controlled, find your rhythm
  • Rep 2: 800m at CSS pace — aim to match Rep 1 split
  • Rep 3: 800m at CSS pace — this is where the session earns its adaptation
  • Rep 4: 800m at CSS pace — push to hold your split; a small negative split here is excellent
  • Rest: 60 seconds between each 800m block

Cool-Down (5 minutes)

300m easy backstroke or very slow freestyle. Focus on long, relaxed strokes and bringing your heart rate down before leaving the pool.

Coaching Notes

  • CSS pace is calculated from a 400m TT minus your 200m TT split — use a pace calculator if unsure of your exact target
  • Don’t go out too hard on Rep 1. The session is designed to accumulate stress across four reps, not blow up on the first
  • To make it easier: shorten each rep to 600m or extend rest to 90 seconds
  • To make it harder: reduce rest to 45 seconds or add a fifth 800m rep
  • RPE target: 7–8 out of 10 across the main set. If you can hold a full conversation, you’re too slow

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.