Everybody trains for the way up. Nobody has ever trained for the way down.

Going up, your muscles shorten to lift you. It's hard, you breathe like a horse, and the tissue is doing the job it finds most natural. That's why you can be genuinely unfit and still get up a mountain on stubbornness.

Going down, the same muscles have to lengthen while resisting — braking you, step after step, for hours. That contraction is where actual muscle damage comes from, and the numbers are not subtle. A review of the downhill running literature reports immediate strength losses in the knee extensors of roughly 14 to 55 per cent, creatine kinase — a marker of muscle damage — elevated around 420 per cent at twenty-four hours, and recovery taking four to five days.

Now here's the part that matters, and it's the most useful sentence on this page: prior exposure protects you. The people who take the most damage from a descent are consistently the ones least accustomed to descending. Your body will absorb this if you let it rehearse. It just has never once been asked to.

Which is why the standard advice — build up your cardio, get some miles in — reliably produces someone who is superbly prepared for the first half of the day. We were all taught to train the climb. Trails go up on the way out and everyone's watch measures elevation gained, and somewhere in there the entire industry forgot that you have to come back.

The practical version is in the hiking conditioning guide, with the winter equivalent in snowshoe preparation. And if the season after this one involves a chairlift, it's the same knee and the same conversation six weeks earlier — that's over here.

A 60-minute assessment first — the knee's history, any surgery, what you've already tried, and the actual hikes you want back. Then we test what it currently accepts going down: single leg, controlled, under load, repeatedly. That last word is the one that finds things. Plenty of knees pass a step-down. Fewer pass the fortieth.

From there, one-on-one sessions of 45 to 60 minutes with me at the studio at 1800 Glenarm Place. We build braking capacity progressively, in doses your knee will accept, and then we build the endurance of it — because the problem was never the first descent of the day.

I've covered the hiking version of this on 9NEWS as their fitness expert, and I've been doing this in Colorado since 2001. If your case isn't a fit for what I do, I'll tell you in the first half hour and point you at whoever is.

So half the day is a lifting job and half the day is a braking job, and we all trained one of them. Written down like that it sounds like something you'd work out in an afternoon, which is roughly the reaction I had when it was finally explained to me, some years into a career built on knowing things about legs.

Anyway. You know it now. Good luck un-knowing it at four in the afternoon on the last mile down.