I would agree with Ro- the top of the foot is not the best part to hit with. I would attribute this to Bruce developing his kicking from the Korean style he picked up from Chuck Norris. You see lots of instep roundhouse kicks in TKD sparring- where power isn’t as emphasized, and foot pads may be worn. This might also come from the study of savate, where shoes are worn (especially the roundhouse using the toe or ball of the foot- the fouette). Although Bruce studied muay thai, I read that his info came from a book called Asian Fighting arts by Don Draeger, not an in depth analysis. If he lived, I’m sure he would have looked at it more.
As I see it, if you have that kind of power to snap a neck by kicking the arm, you’d probably be better off targeting the legs. A variation I use sometimes is to kick the leg with a standard shin kick, and if they block by raising the leg, you turn the foot to hit wiht the ball of the foot sideways, usually still hitting the thigh. If you had the freakish power to cause whiplash in the neck, by using standard and the varied version of the leg kick, it would be hard to go wrong.
