Kamiizumi Nobutsuna served as lord of Kamiizumi Castle in Kozuke Province under Nagano Narimasa, bravely resisting Takeda Shingen's invasion. After his lord's fall, he embarked on a sword pilgrimage and perfected the Shinkage-ryu. He invented the fukuro-shinai for safe practice and advocated transforming swordsmanship from the killing sword to the life-giving sword. Through his disciple Yagyu Muneyoshi, his art was transmitted as the Yagyu Shinkage-ryu, making him the most influential Sword Saint in Japanese martial arts history.

In the 1560s, Takeda Shingen invaded Kozuke Province and destroyed the Nagano clan, whom Kamiizumi Nobutsuna served. Having lost his lord, Nobutsuna left Kamiizumi Castle to travel and train as a warrior, later perfecting the Shinkage-ryu style of swordsmanship and becoming renowned as a sword saint. Shingen's invasion thus proved a turning point in the castle lord's fate.