Ki is the Japanese term equivalent to the Chinese qi notion. The Chinese metaphysical principle (qi) was introduced to Japan in the Nara period (710-794 AD) and the Heian period (794-1185 AD). The ki concept was rapidly assimilated into the Shintoist beliefs regarding the nature. On this basis many works show the connection between nature and the spirit.
The ki energy idea underwent dramatic changes at the end of the Heian period, when the samurai class appeared, flourishing in the middle of the 19th century.
Ki became an integral part of warrior discipline, courage, willpower and physical power. It was also crucial in the conservation of energy and even of the prolongation of breath; essential elements on which the life and death of the samurai depended. In nature, the ki energy is regarded as being both a principle of unification between the individual and the universe and an expression of the power of the respiratory breath. Obtaining any victory depended on the harmonious and rapid manipulation of the ki energy. These samurai principles have been transposed today into the martial arts.