When you run, excessive friction can be a sign of inefficiency since it isn’t helping you move forward, particularly if you are plowing forward into the ground. Rather than landing like an airplane, picture your foot contacting like a helicopter touching down. When you watch experienced barefoot runners in slow motion, it’s as if their foot pauses for a moment in the air prior to touching down gently (see video below). Why? Because if they plowed forward they’d tear the skin off of their feet.
This is something that I check too. Pete does his thing -- explaining efficiency in easily understood terms -- so, so well.
Nothing to add here except that I very much want to go to Hong Kong to run and also visit with inexpensive tailors now. Click through: it's really pretty.
A solid synopsis of an interesting experiment: have 75 middle-schoolers run 750 meters on a 150 meter track, then have half run the same run again and the other half run as far as they can for the same amount of time.
The take home message is that some rise and fall is necessary, even advantageous, when running.* So when someone tells you that a runner is efficient because their head stays perfectly still when running, maybe you should hand them your smartphone and tell them to play a little game.