Mongrel

From MangaHelpers:
Growing up, Wataru and his younger brother have both done gymnastics, much like their parents. At first, Wataru, being the older brother, could out-perform his younger sibling in the sport, but everyone quickly came to notice the large gap in talent between the two brothers. Now, his younger brother has been nominated to represent Japan in the Olympics, while Wataru lives his day-to-day life not knowing what to strive for. Wataru began attending a kick boxing gym, initially to just relieve stress. With the help of his gym mates and rivals, he begins to understand that hard work can surpass talent in the realm of kick boxing.