Complex sentence: After studying at Kokushikan University, he became a kenshusei (or junior instructor) with the Japan Karate Association and began his career as a competitive "karateka" and, during the period 1974-1984, achieved substantial success both in kumite and kata.
Split into simpler sentences: After studying at Kokushikan University, he became a kenshusei (or junior instructor) with the Japan Karate Association and began his career as a competitive "karateka". In the period 1974-1984, he achieved substantial success both in kumite and kata.