What is the relationship between chi gung and the internal martial arts (tai chi, hsing-i, and ba gua)?

The three internal martial arts of tai chi, hsing-i, and ba guaⁱ are all based on the nei gungⁱ system of chi development. They combine the most effective fighting techniques of ancient China and fuse them with the nei gung system of internal power development. This combination produces two seemingly unrelated results: superior competitive athletic and fighting skills and superior health. These arts are also, at high levels, complete spiritual development systems. Tai chi is the best known of the internal martial arts. Today, most people practice tai chi for health only and never learn its self-defense aspects.