Hip extention on Thai style kicking is very important. Its interesting to see how many Westeners have a problem with it and cannot kick the same way Thais do (for better or worse) I have had problems too. The Thais I have trained with tend to have a far greater reach and power then anyone else I've seen.

Sakmonkol (great fighter and kicker for sure) kicks a little differently than most Thais. He seems to get less hip into it. Maybe its a style thing but I would like to know how tight his hip flexors are.

Back pain is uncommon in unindustrilised countries and almost everyone in the Western World will experiance some at some point in their lives. I believe this is largely to do with sitting for long periods of time-one of the major results being tight hip flexors (protruding belly and forward head posture as well) -all of which conventional ab traning usually promotes.