Ashi no Ura no Iris

Gaku loves to run more than anyone he knows, and because of that he always strove to be as fast as he could possibly be. He keeps on running as earnestly as always despite the fact that most students make fun of the running method he uses to compensate for his short stature. As the pain and humiliation starts to wane his motivation, he promises to himself that the next event will be his last.

(Source: Anilist)