The Fist that Calls the Rainbow

(from amazon.jp and wikipedia.ja):

Volume 1: A karate manga made by Ikki Kajiwara and Jiro Tsunoda, just before "Karate Master" (空手バカ一代, Karate Baka Ichidai). It was serialized in the "Adventure King" (Boken-oh) magazine. Kajiwara chose Tsunoda because he wanted to shine a light on him. Tsunoda was known to be in a slump at the time.

Makihiko Kasuga has transferred to a junior high school in a newly developed residential area in the Tokyo metropolitan region, and although his studies are the best in his class, he is very poor in athletics.

On the other hand, his classmate Ryuhei Akagaki is an all-around sportsman, but he is not very good at studying. Kasuga was envious of Akagaki.

When Kasuga hears that Akagaki's athletic prowess is the result of his karate training, he decides to join the same dojo as Akagaki. However, he is not a good athlete by nature, so his training does not go very well.