80/20 Polarized Training

If you had to remember just one endurance training principle, it would be this: the vast majority of your volume should be easy. This is the heart of polarized training, popularized by Stephen Seiler's research.

What is 80/20?

The idea: split your training into ~80% low intensity (base endurance, zones 1-2) and ~20% high intensity (threshold and above, zones 4-5). Avoid zone 3 as much as possible, that "no man's land": too hard to recover properly, not hard enough to create real adaptation.

Why does it work?

• Easy volume builds base endurance (capillarization, mitochondria, efficiency) without generating too much fatigue → you can do a lot of it.
• The 20% intense, well-placed, push your engine (VO2max, threshold) when you're fresh.
• The trap of "moderately hard all the time": it fatigues you greatly for little gains, and leads to overtraining.

How easy is "easy"?

Really easy: you should be able to hold a full conversation. If you're breathless, you're too high. It's counter-intuitive — many amateurs find it "too slow" — but that's precisely what builds the engine.

How to apply it concretely

On a week of 5 sessions, it typically gives: 4 easy sessions + 1 quality session (intervals, threshold). On 6 sessions: 4-5 easy + 1-2 intense. The zones are based on your thresholds (HR threshold, bike FTP, threshold pace) — which is why knowing your benchmarks matters.

The most common mistake

Turning every run into a little race. Result: you're always slightly tired, you plateau, and you end up getting injured or losing motivation. Slowing down your easy sessions is often the change that unlocks progress.

A coach who doses it for you

Sports Coach AI automatically applies this polarization: it calculates your zones, keeps your easy sessions truly easy, and places intensity at the right time based on your freshness (TSB) and how you feel.