I've never run with a heart rate monitor or anything like that, but I do feel a certain internal "governor" when riding, especially riding fast with others. I can do it for a while, but there comes a point where it's just time to back off. I've heard that a key element of bicycle racing is dealing with the pain of exertion. I guess when I feel that pain beyond a certain threshold, I back off. I suspect the harm comes from habitually pushing beyond that point.
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I stop for people / whose right of way I honor / but not for no one.
Originally Posted by
bragi 
"However, it's never a good idea to overgeneralize."