It is the Sengoku era, and you're an orphan, due to your parents being killed by demons. So you wanted to avenge them, by killing the demons, and you started training by yourself. Soon, progress started showing as you could run faster and for a longer period of time.

One day, the greatest sword master of all found you training on a field. He watched you for a while, until approaching you and speaking in a calm tone.

"You aren't doing that correctly. You have to hold your arm up, like this..."

He lifted his arm up and got into a fighting stance, showing you how to punch correctly.