Although the martial arts term ‘Ninja’ is a distinctly ‘Japanese pronunciation – the two ideograms used to express this concept are of Chinese origin – namely ‘忍者’ (Ren Zhe). Whether this concept originally spread from China as a martial arts principle – or was distinctly developed in Japan - is open to debate. Certainly, the ‘Ninja’ of medieval Japan occupied entire clan-systems which ‘mirrored’ perfectly their Samurai equivalents with the only difference being that the Samurai clans were socially accepted and the ‘Ninja’ clans were clandestine and considered ‘illegal’. The ‘Ninja’ communities were made-up of the peasantry and any outcast members of the nobility and criminal fraternity, etc. Although the ‘Ninja’ communities were hidden from open view, they were disciplined, followed strict codes of conduct and were dedicated to perfecting many different martial skills designed to ‘counter’ or ‘negate’ every martial advantage the Samurai believed they possessed. In-short, the ‘Ninja’ communities represented a ‘different’ but related blue-print for Japanese feudal society – perhaps one that was internally democratic and fairer than its Samurai alternative, as women were considered ‘equal to men – and practiced martial techniques designed by women for women to use on the battlefield or during ‘assassinations’ - a key skill of the ‘Ninja’ warrior.  ​

The character ‘忍’ (ren3) is comprised of a contracted version of the lower particle ‘心’ (xin1) - which translates as ‘mind’ and ‘heart’ - and the upper particle ‘刃’ (ren4) - which represents a ‘bladed weapon’ such as a ‘knife’ or ‘broad-sword’, The Japanese version of this ideogram appears to have a handle affixed to a blade – a blade said to be covered in ‘blood’: ​

When combined together, the ideogram ‘忍’ (ren3) suggests a situation where the human mind (and body) is said to be highly skilled swordsmanship – together with ‘tolerating’ the ‘lose’ of a certain amount of one’s own blood – as well as spilling that of the opponent. The training in this martial art is arduous and painful to experience – but this is the path that must be ‘endured’ if mastery is to be achieved. Whereas the ideogram ‘者’ (zhe3) is comprised of the lower particle ‘白’ (bai2) which carries the meaning of the colour ‘White’, whilst the upper particle is ‘耂’ (lao3) and refers to a ‘an old man who is bent-over and has long hair’ - usually implying ‘acquired wisdom overtime’. Therefore, ‘者’ (zhe3) appears to mean a ‘body of expert knowledge acquired by an individual over a long period of study’. The combined term of ‘忍者’ (Ren Zhe) - or ‘Ninja’ - refers to the concept of an ‘accumulated body of knowledge and martial arts skill and acquired by an extraordinary person overtime’. Or, an ‘acquired body of knowledge and martial arts skill that transforms an ordinary person into an extraordinary person’.  ​

Related Terms: 

忍術 (Ninjutsu) - ‘Ren Shu’ = ‘Endurance Art’ 
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忍法 (Ninpo) - ‘Ren Fa’ = ‘Endurance Law’ 

Ninjutsu originally derived from an indigenous, traditional Japanese fighting technique known as the ‘Thorn Kill Art’ (刺杀术 - Ci Sha Shu) - perhaps implying the ability to ‘assassinate an opponent using a poisoned-dart'. Later, this art absorbed several Chinese cultural influences such as the ‘Art of War’ by Sunzi", and the martial principles contained within the ‘Six Secret Teachings’, etc. There is a legend that a Chinese Buddhist monk travelled to Japan early-on, and brought various Tantric Buddhist and Traditional Chinese Medicine techniques (perhaps around the 8th century CE) which were combined with Japanese Shintoism. This mixture of Chinese and Japanese martial elements was integrated to finally form ‘Ninjutsu’. The techniques of ‘evasion’ and ‘invisibility’ in were emphasised in the city – whilst ‘ambushes’ and the ability to suddenly ‘disappear’ was perfected in the mountainous areas.  

Chinese Language Reference: 
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https://baike.baidu.com/item/忍术/381309 

Every genuine martial arts style or system contains a rich mosaic of principles and techniques that at the most brutal assist in the surviving of the combat experience, and at the most sublime provide a psychological and physical pathway of self-development and self-transcendence. The best styles of Okinawan and Japanese karate confirm to his blue-print, and I would say that Wado-Kai, Shukokai and Goju Ryu are prime examples of the best that this genre has to offer. All these styles (two Japanese; Wado-Kai and Shukokai – and one Okinawan; Goju Ryu) have their historical roots within Chinese martial arts which probably are linked to the Fujian province of Southern China. Indeed, Wado-Kai and Shukokai derive from the Okinawan karate of Master Gichin Funakoshi – the Okinawan martial artist accredited with transmitting his style of Chinese martial arts to Mainland Japan.  I practiced these styles only as an accident of fate – they happened to be prevalent in the areas I lived in to pursue my education throughout the 1980s. No one at the time (not even my close friends) knew of my Hakka Chinese gongfu background. I saw no reason to explain this, an they saw my familiarity with certain techniques as me just being talented in these areas (I was viciously fast and very good with kicking and moving, as in the traditional Northern martial arts in China, a student perfects kicking first – whilst due to cultural differences in the West – it is punching that is perfected first with kicking usually appearing uncomfortable). On the other hand, as I entered my late teens, Master Chan Tin Sang (1924-1993) directed me to develop my punching to a very high degree. This is where Wado-Kai (correct alignment) and Shukokai (correct hip and shoulder twist) assisted my development tremendously! Goju Ryu, on the other hand, has a special place in my heart. Its all-round body-conditioning reminded me of our own Hakka Chinese gongfu style which developed tremendous power, endurance and strength! The bones certainly hardened, whilst the mind became calm and broadly aware. Master Chan Tin Sang was an extraordinary teacher as he entrusted me to enter the training halls of other styles, dedicate myself to their ways, learn them fully and then integrate these lessons and skills into the Ch’an Dao System. This could happen as each karate style brought out a particular aspect of our own fighting system. I have the utmost respect for all legitimate styles of martial arts training and see no reason why there cannot be a positive cross-training that enhances rather than diminishes. As the US at the time was emphasising Japanese martial culture throughout the West as a means to side-line and obscure Chinese martial culture (due to the Cold War), European cuntries with no historical connection with Japan suddenly found themselves hosting (usually at their own expense) Japanese ‘ambassadors’ of various karate styles and associations. This arrangement all came crashing down when President Carter established full diplomatic relations with Mainland China in 1979, and Westerners started travelling en masse to China to train in gongfu – and Chinese teachers came to the West to teach – although the effects of this change would take around a decade to fully manifest itself. In the meantime, I trained in Japanese karate at the tail-end of this time period and consider it a very interesting and unique experience.  ​

Starting from around 16 years of age with a ruck sack of around 15Ibs - 20Ibs - a practitioner should start running over various types of terrain, and for different distances at various times of the year. The point is to 'sweat' and a number of clothing layers should be worn. This practice is a combination of weight-lifting and running. As the body (and mind) strengthens, the weight should be slowly increased. As a grown man, the weight should reach 56Ibs (perhaps 40Ibs for a woman) - but temporary increases up to 76Ibs can be used for short periods. Distances should vary from 1.5 miles to 5 miles on paved roads, but it is also good to run on grass and occasionally on sand. Much longer distances can be used if running is replaced with hiking. If a practitioner is only just beginning to get fit, or is recovering from injury or illness, always start by walking over a certain distance. It is a good idea to lift free weights for a time prior to ruck sack running so as to build the bones, muscles and joints, and to practice ordinary running (and periodic sprinting) to condition the cardiovascular system. For many, just 'wearing' the iron vest for any extended period of time is very difficult to do, and creates substantial muscular pain in the neck, shoulders and back, as well as in the knees, ankles and various parts of the feet. This is normal and will pass. Just as the iron vest compresses the body - the structures of the body 'resist' and generate an outward counter-pressure - which generates a substantial repelling force. This is how the body withstands and rejects any incoming power. Furthermore, carrying this weight forward generates tremendous 'forward' momentum which is very difficult for an opponent to 'stop', and which has the effect of 'uprooting'. All the blood vessels are opened and strengthened through a stimulated circulation, the bones are hardened and the both strengthened and relaxed in a coordinated and dynamic manner. The mind becomes calm and all expansive as the qi (vital force), jing (essential nature) and shen (all embracing empty mind) are thoroughly cultivated.