Two of the earliest innovators in this regard were Ji Han-Jae and Kim Moo-Hong, both of whom were exposed to what were thought to be indigenous Korean kicking arts. They combined these forms together with the yu sool concepts for striking taught to them by Choi and during a period of 8 months training together in 1961 finalized the kicking curriculum which would be used by the Korea Hapkido Association for many years to come. Other influences also were exerted on the kicking techniques of important hapkido teachers. Kwon Tae-Man (권태만) initially studied under Ji Han-Jae before immigrating to southern California in the United States. Han Bong-soo studied under "Gwonbeop" () and Shūdōkan karate from Yoon Byung-In (윤병인), whose students were influential in the later forming of Kong Soo Do and Taekwondo styles, specifically the Chang Moo Kwan and Jidokwan. He, like Kim Moo-Hong, also trained briefly in the Korean art of Taekkyon under Lee Bok-Yong (이복용). Many other teachers like Myung Kwang-Sik (명광식), Jeong Kee-Tae (정기태), Lim Hyun-Soo (임현수), and many others trained in tang soo do and kong soo do, Shotokan and Shūdōkan karate based systems which predated and influenced the forming of first tae soo do and later modern taekwondo styles. Kim Sang-Cook states that while many of the original yu kwon sool students were exposed to many different contemporary Korean arts the Chung Do Kwan was of particular importance in the transition from the original jujutsu based form to what we know today as modern hapkido. Most forms of hapkido include a series of double kicks used to promote balance, coordination and muscular control. After these kicks are mastered using one foot kick the student moves on to jumping versions using alternate kicking legs. Kim Chong Sung (김종성, Jang Mu Won Hapkido Founder), was one of the oldest living active hapkido instructors, maintains that the source of these kicking methods is from the indigenous Korean kicking art of Taekkyon.