Mikao Usui the developer of the Usui Reiki system is said to be related to the famous samurai Tsunetane Chiba from the 1500s. The Chiba clan was once an influential Samurai family in Japan during this time.
At the age of 12 Mikao Usui began to study the martial arts of Aiki Jutsu and an early form of Daito Ryu a form of jujutsu. He attained a high level of proficiency in both weaponry and grappling.
He was said to be associated with well know martial artists of his time. Some of them were Gichin Funakoshi the founder of Shotakan karate, Morihei Ueshiba the founder of Aikido and Jigoro Kano the founder of Judo. It is said that the ranking system of Reiki was based on the ranking system set up by Jigoro Kano for his judo students.
The first four levels of Reiki were combined into one called Shoden. It would have included the martial art rankings of rokyu, gokyu, yonkyu and sankyu. Equivalent to mid and upper level rankings of beginning students.
The second level of Reiki was split into two parts called Okuden Zenki (1st part) and Okuden Kuki (2nd part). These would have been equivalent to the martial arts ranking of nikyu, ikkyu, shodan and nidan which are upper level rankings of beginning students and the first two black belt ranks.
The master level of Reiki is called Shinpiden and was equivalent to sandan which is the third degree black belt rank.
Of course in Reiki as in the martial arts one was to achieve a level of proficiency and time in that rank before moving on to an advanced rank. It was expected that one should practice diligently in the martial arts and the same can be said for Reiki practitioners.
It is my belief that all martial artists should learn Reiki as a wonderful adjunct to their martial arts training.