The grey zone is where seasons quietly go to die

Training that is too hard to be recovery and too easy to be a session eats the week and gives very little back.

The session that ruins people is not the brutal one. It is the moderately uncomfortable one, done four or five times a week, at the pace where you can still hold a conversation if the sentences are short.

Coaches call it the grey zone and the polarised training argument is largely about how much time to spend there. The observation that started it was simple. When researchers went and recorded what elite endurance athletes actually did, rather than what their plans said, the distribution turned out to be lopsided. Something like three quarters or more of sessions were genuinely easy, well below the first threshold, and most of the rest were properly hard. The middle was thin.

The mechanism people propose is about the cost of the stimulus rather than its size. Easy work builds aerobic machinery at almost no recovery cost, so you can do a great deal of it. Hard work drives the adaptations that easy work cannot reach, and you can only afford two or three doses a week. Moderate work sits in the worst place. It produces a fraction of the high intensity adaptation while consuming a large share of the recovery budget, so an athlete who lives there ends up too tired to do the hard sessions properly and too busy to do the volume.

I think the argument is right and I also think it gets preached badly. Telling an ambitious runner that eighty percent of her running should be conversational produces one of two outcomes. Either she does not believe you, or she believes you, runs everything at a crawl, and discovers eight weeks later that she has lost her ability to hold a hard sustained pace because she never practised it.

There is also a real objection about the middle distance events themselves. Threshold work has a long record of producing results, especially in athletes with fewer training hours available, and the polarised distribution was observed in professionals doing twenty five hours a week. If you have six hours, the calculus changes, and the case for spending some of them at threshold gets much stronger.

So the useful version of the rule is not a percentage. It is a question. For each session in your week, can you say what it is for, and could it be replaced by something either easier or harder without losing anything.

If the honest answer is no, you have found the session to delete.