The reason why everyone in the Armstrong family is so crazy strong is because one of their great ancestors was the very first successful human chimera. He’d been infused with a Mantis Shrimp and was a womanizing flirt.
The Armstrongs are, in fact, the descendants of King Arturia Pendragon (from Fate/Stay Night). This is clear in their shared dramatic fighting style and signature hair curl, which has evolved from being handed down through generations!