When it comes to muscle growth.
In the Holy City, it is unlikely to find another person on par with Winry.
She has limitless potential for muscle fiber growth.
It seems that in Winry's genetic system, there are no gene sequences limiting the growth of her muscle fibers, which allows for her muscle density to increase indefinitely.
Accordingly...
Winry must have a body capable of controlling muscle tissue that's dozens, if not more, times greater than a normal person's.
At the same time, she needs a powerful circulatory system to pump enough oxygen to each muscle cell.
Like a complementing system, Winry was born possessing a perfect body for muscle growth.
Starting at the age of five, under the standard requirements of an apprentice, she completed various blacksmithing tasks in the Underground Furnace... It seemed like training in the art of forging, but in reality, it was honing her body.
A decade or so later, Winry's body had been honed into a "human weapon".
