Broken 400 CSS Swim: High-Speed Short-Rest Speed Work

Session Overview

The broken 400 CSS session is a short, sharp speed workout designed to build your critical swim speed through high-quality 100m repetitions at threshold pace with minimal rest. Ideal for advanced swimmers looking to raise their CSS and sharpen race-pace fitness, you’ll complete 8 x 100m at CSS with just 15 seconds between reps.

What You’ll Need

• 25m or 50m pool with pace clock or swim watch
• Your current CSS pace (400m and 200m TT protocol)
• Optional: swim paddles for the second round to add upper-body load

Warm-Up (10 minutes)

400m easy continuous freestyle at RPE 3–4/10. Include 4 x 25m single-arm drill mid-way to activate your catch, then finish the final 100m with bilateral breathing focus. Your HR should be at 60–65% max before the main set begins.

Main Set

Two rounds of 4 x 100m at CSS pace with 15 seconds passive rest. The compressed recovery mirrors open water and race conditions. Focus on holding consistent split times — don’t go out too fast in Round 1 at the expense of Round 2.

• Round 1: 4 x 100m at CSS pace, 15 seconds rest between each rep
• 200m easy swim between rounds (active recovery, RPE 3/10)
• Round 2: 4 x 100m at CSS pace, 15 seconds rest between each rep

Cool-Down (5 minutes)

200m very easy (RPE 2/10), alternating backstroke and freestyle to loosen the shoulders. Finish with 4 x 25m kick-only to release the hips and lower back.

Coaching Notes

• Find your CSS pace using the standard 400m/200m TT — it’s roughly the pace you could hold for 30 minutes of sustained swimming
• Don’t start Round 1 faster than CSS — consistent splits beat a dying fade through Round 2
• If you can’t hold CSS with 15 seconds rest, extend recovery to 20–25 seconds and rebuild from there
• RPE should be 7–8/10 on each rep — tough but controlled breathing throughout