Ken Zen Ichi Nyo

One difficulty for us Westerners who use an alphabetical system of transcription is that the a great number of Characters (Kanji) used in written Japanese sound the same. Hence the numerous misinterpretations and conflicts between self-appointed experts... I would like to address here the different forms of  "Ken Zen Ichi Nyo" often heard in Martial Art schools. 

  

剣 禅 一 如

The Sword and Zen are one.

Yagyu Munenori is credited with this formula in his work Heiho Kadensho. The meaning here is that the study of swordsmanship (Kendo or Kenjutsu) cannot be separated from the practice of Zen  meditation. In other words, do not even think you can become good at Swordsmanship if you don't Practice Zen. Munenori - a friend of Zen Master Takuan - considers that the practice Zen is a necessary condition to be successful in the practice of the arts of the Sword.

I have not found any indication that he would consider the other way around - that the practice of the sword could be a good way to improve one's Zen practice.

拳 禅 一 如

The Fist and Zen are one.

This formula is pronounced exactly in the same way as the previous one, however, here the Kanji 剣 (Sword), has been replaced y the Kanji  拳 (Fist). This formula is generally used in Karate Dojo.

A similar expression is “Ken Shin Ichi Nyo”.

剣 心 一 如

The Sword and Mind are one.

It is very close to Ken Zen Ichi Nyo (The Sword and Zen are one) because the Kanji 心 means Mind and Zen is interested in experiencing the universal mind. 

Why are the Chinese and Japanese using the same character for 2 concepts apparently so different as Mind and Heart ?

In fact, these concepts are not so different. Traditional Chinese Medicine considers the Heart as the seat of the Mind. And even in our Western languages, the word Heart refers not only to the blood pumping organ , but also as being involved with such mental activities as learning ( I learned it by heart), to emotions (My heart fills with joy), or will power (a strong-hearted person).

Numerous Martial Arts masters pointed out that the practice of Zen was beneficial to the practice of their Arts. 

There are fewer sources about the other way around :  how the practice of Martial Arts could be beneficial to the practice of Zen.

Therapeutic techniques such as the Naikan or Kikai Tanden described by Zen Master Hakuin are related to Qi Gong exercises (Some of them attributed to the first Chan Patriarch Bodhidharma.) developed around the 6th century in China. 

We are in good company...