I had heard that when this show was in production, David Carradine had absolutely no training in any style of martial arts. It was only later in his life after the show that he really studied Tai Chi and Kung Fu.
As I understand it, when the show started he had no training, but as the show was produced he insisted on learning himself and performed most of the stunts himself; because, he felt the audience wouldn't buy into its authenticity as much and he was right.
David's always been pretty upfront about not having much in the way of actual Martial Arts training. At the start of the series he didn't have any training. Shortly after the series started he started to do more and more of his stunts. By the third season he started to actually study martial arts. Throughout the years after the series he has studied off and on but never consistantly enough to become any type of true martial arts expert.