My friend pedals with his heels
One of my friends that I go riding with has a bad habit of putting his heels on his pedals instead of the balls of the feet. I keep asking him to do pedal properly but he just says that it feels more natural to him to do this. What kind of damage is this causing to his feet and legs, if any? I'd like to have a better argument to convince him other than saying, "Well, that ain't right!"