It is only bad for your knees if you don't know how to run downhills and don't train for it. Running injuries come from overload or overuse (or bad health). Bo

It is only bad for your knees if you don't know how to run downhills and don't train for it. Running injuries come from overload or overuse (or bad health). Both causes can be prevented by being conscious of what causes overload and overuse and then training in a way that will prevent it. If all you ever do is run flat asphalt and then you suddenly decide to run hills then yes, the downhill can be bad for your knees -- but that's just because you stupidly decided to introduce a new load your joints and tendons are not adapted to. Same happens whenever you change technique or have some kind of imbalance -- your body will adjust by moving loads from one place to another but your brain may not be smart enough to realise the new place may not have had enough time to adjust to it. Anybody can write a book, I guess. But don't take any running books as gospel. Nobody really understands everything about running (well... maybe Phil Maffetone does). I run with severed ACL in one of my knees and so out of necessity I have to be much more knowledgeable to be able to keep running safely and keep my legs in good shape.