This is a title scene from a movie (The Naked Weapon). It has a young muscle girl, between thirteen to fifteen years old, working as a street fighter on whom people wager their money. She is shown to be kicking a boy in a one-to-one fight with the audience surrounding them and cheering them up. The young girl strikes at the boy with considerable power.
More young girls in the scene below from the same movie. They are professionally trained to be fighters and assassins. The young girls have started going through severe commando-like training at such a tender age. They practice martial arts, pull heavy objects, and carry strong arms on their backs. Really, it is rare to see young girls with muscles in mainstream movies. This movie has considered this aspect to some extent.