www.alphadictionary.com

I am surprised I did not know the root of this word before!
"Mouse"
and I learned of it in a magazine I get for my 10 yr old granddaughter!
"muscle (n.)
late 14c., from Middle French muscle "muscle, sinew" (14c.) and directly from Latin musculus "a muscle," literally "little mouse," diminutive of mus "mouse" (see mouse (n.)).

So called because the shape and movement of some muscles (notably biceps) were thought to resemble mice."

cute, huh?

I learned this when I learned the Russian for muscle. The same root is in the last name of the hero of Dostoevsky's "The Idiot," Lev Myshkin. His first and last names can be taken as meaning Lion Mouse.

I learned the etymology of muscle when I was, as was your granddaughter, ten years old. I read it in a children's magazine.