10x400m CSS Threshold Pool Session: Long-Course Endurance Builder

Session Overview

The 10x400m CSS threshold set is a high-volume aerobic swim session designed for triathletes targeting 70.3 or Ironman swims. Holding critical swim speed (CSS) pace across 10 repetitions of 400m builds the aerobic base and lactate tolerance required to sustain your race swim pace when fatigued. Total distance with warm-up and cool-down is approximately 5,000m — a genuine long-course stimulus in a single pool session.

What You’ll Need

• 25m or 50m pool (50m preferred — fewer turns for better pacing)
• Pace clock or GPS swim watch with lap function
• Pull buoy (optional — for the last 2-3 reps if form deteriorates)
• Your CSS pace calculated from a recent 400m/200m time trial

Finding Your CSS Pace

CSS pace is calculated from a 400m and 200m time trial, completed in the same session with 5 minutes rest between them. Formula: CSS = (400m time in seconds minus 200m time in seconds) divided by 200. The result is your pace per metre in seconds; multiply by 100 for pace per 100m. Example: 400m in 6:00 and 200m in 2:45 → (360-165)/200 = 0.975 sec/m = 1:37.5/100m CSS pace. This is the pace at which you can sustain indefinitely in aerobic terms — your aerobic ceiling.

Warm-Up (15 minutes)

800m easy mixed: 200m freestyle, 100m backstroke, 200m pull buoy easy, 100m kick only, 200m building effort to approximately 90% CSS pace. The final 200m should feel controlled but not comfortable — you are activating the aerobic system before the main set.

Main Set

10x400m at CSS pace with 30 seconds rest between each repetition. Target the same split on every rep — CSS is an aerobic pace and should feel sustainable from rep 1 through rep 10. If you finish rep 3 feeling like you can swim at this pace forever, you are below CSS and need to increase speed. If rep 5 feels genuinely hard, you are at CSS. If rep 7 requires significant effort to hold the pace, you may be slightly above CSS — that is acceptable for experienced swimmers.

• Reps 1-3: settle into your CSS pace and rhythm; do not go out fast
• Reps 4-7: the aerobic core of the session; focus on even splits and relaxed breathing
• Reps 8-10: the test of the session; hold pace when tired — this is where fitness is built

Cool-Down (10 minutes)

400m easy with alternating backstroke and freestyle lengths. Use the cool-down to flush lactate, focus on long strokes and allow heart rate to return below 120 bpm before exiting the pool.

Coaching Notes

• Use a pull buoy only on the final 2-3 reps if your kick is consuming too much energy late in the set — do not rely on it as a crutch from the start.
• A common mistake is starting too fast on rep 1. Treat each 400m as a separate time trial at exactly CSS pace, not faster.
• Easier scaling: reduce to 8x400m with 45 seconds rest. Harder scaling: reduce rest to 20 seconds or add a 400m paddles-only rep as rep 11.
• RPE should build from 6/10 early in the set to 7-8/10 by rep 9-10 — if you feel more than 8/10 at any point before rep 8, slow down.