Anyone who "takes the knee" or kneels infront of someone.

having a motorcycle so leaned over in a turn your knees scrape the pavement

A person that rides a motorcycle, particularly on a race track, that is able to turn in such a way, and at such a velocity, at such a low height, that their knee drags against the ground.

People who use telemark skis in which the heel is free from the binding as opposed to downhill skiing with which the heel is locked into the binding. Thus, these types of skiers have to drag there knee on the ground to make the tight turns which come so easily to the downhill skier.