The word "Kosen" is an abbreviation of Koto Senmon Gakko (literally "higher speciality school") and referred to Japanese colleges of technology that cater to students aged 15-20. The Kosen schools began holding competitions in 1898, and began inter-collegiate tournaments called the Kosen Taikai from 1914-1944. Largely, the matches held at these events followed rules set by the Kodokan (pre-1925) with input from the Dai Nippon Butokukai. Some major differences to other competitions were the fact that competitors could pull guard and go right into ground work, and were still allowed to perform some techniques that had since been banned in normal Kodokan contests. Naturally, this allowed for competitors to develop a sophisticated grappling repertoire.
Kano himself disapproved of these rules and was quoted in 1926 as saying that Kosen Judo contributed to create judoka more proficient at winning sport matches at the cost of being less skilled in self defense. Interestingly, this is one of biggest criticisms of modern BJJ by the martial arts community today. Ultimately, the Kosen school system was abolished in 1950 and the first iteration of Kosen Judo ceased to exist.
Today the terms Nanatei Judo and Kosen Judo have become interchangeable in the common vernacular, but since the Kosen school system no longer exists and the "Seven Imperial Universities" are no longer imperial, for the purpose of this article I will use Nanatei to describe the college training program and Kosen to describe the training methodology focusing on an expanded grappling component. By its very name, Nanatei Judo cannot be used to describe the training offered outside the 7 Universities (Tokyo, Kyoto, Tohoku, Kyushu, Hokkaido, Osaka and Nagoya) no matter how similar the training is.
One of the most famous judoka promoting Kosen Judo was Kimura Masahiko, famous among both judoka and BJJ practitioners for his often unmatched skill level. He became the youngest 4th Dan in Kodokan history at age 15, after only 6 years of training, and the youngest 5th Dan just three years later, ultimately reaching 7th Dan by age 30. Outside of Japan, Kimura is most well-known for having defeated Helio Gracie using gyaku ude garami, a technique that now bears his name in BJJ circles. The Japanese documentary below by Kimura demonstrating various ne waza is a great explanation and demonstration of Kosen Judo.
It's not uncommon for instructors and practitioners to put clarifiers in front of their art to describe what they do, and frankly it's important to be honest about what you teach. While it's all technically Judo and traces back to the teachings of Kano Jigoro, there are actually 3 distinct styles of Judo training in Japan alone:
- Kodokan Judo - The complete art encompassing atemi waza, nage waza and katame waza
- Kosen Judo - Judo training focusing on an expanded ne waza curriculum
- Olympic Judo - Modern sport focusing solely on nage waza
Of course, these are not independent systems of each other - just different styles of the same art separated by teaching methods. Instead of simply saying Judo, which could mean just about anything grappling related, being specific by adding either Kodokan, Kosen or Olympic lets people know what exactly you teach. Saying "Kodokan Judo" does not mean you are licensed by the Kodokan any more than saying "Olympic Judo" means you went to the Olympics.