Threshold Swim: CSS Pace Work

Session Overview

This 45-minute threshold swim session targets your Critical Swim Speed (CSS) — the pace you can sustain for long efforts without accumulating excessive lactate. It’s designed for intermediate to advanced swimmers looking to improve race pace endurance and mental toughness in the water.

What You’ll Need

• Access to a 25m or 50m pool
• Swim watch or pace clock
• Pull buoy (optional)
• Fins (optional, for the warm-up drills)

Warm-Up (10 minutes)

300m easy freestyle — Focus on long, relaxed strokes. Breathe bilaterally every 3-5 strokes. RPE 3-4/10.

4 x 50m drill choice (15 seconds rest) — Choose drills that improve your catch and pull: catch-up drill, fingertip drag, or single-arm freestyle. These prepare your technique for the harder efforts ahead.

Main Set

The main set is built around your CSS pace. If you don’t know your CSS, use your comfortable Olympic-distance race pace, or aim for an effort you could hold for 15-20 minutes continuously (RPE 7-8/10).

• 6 x 200m at CSS pace — 30 seconds rest between reps. Hold the same pace for all six reps. If you start slowing after rep 4, your pace is too fast. Adjust accordingly.
• 4 x 100m at CSS pace — 20 seconds rest. These should feel controlled but challenging. Focus on maintaining stroke length and breathing rhythm.
• 4 x 50m at slightly faster than CSS — 15 seconds rest. Push the pace 3-5 seconds per 100m faster than CSS. This builds speed endurance and mental resilience.

Cool-Down (5 minutes)

200m easy choice stroke — Swim backstroke, breaststroke, or very easy freestyle. Let your heart rate drop and focus on controlled breathing. Finish with 30 seconds of static stretching for shoulders and lats.

Coaching Notes

• Pacing is everything — The goal is consistent splits across all reps. If you negative split (get faster), you started too slow. If you fade, you went out too hard.
• Common mistake — Don’t let your stroke rate spike. Hold 16-20 strokes per 25m. Increasing turnover without increasing distance per stroke wastes energy.
• Scaling down — Beginners can reduce volume: 4 x 200m, 3 x 100m, 3 x 50m. Keep the rest intervals the same.
• Scaling up — Advanced swimmers can increase the main set to 8 x 200m or reduce rest to 20 seconds on the 200m reps.
• Heart rate zones — You should be working in Zone 4 (threshold). If using a swim HR monitor, aim for 85-90% of max HR.