I was sitting in class the other day talking with a substitute teacher. He was an eccentric man, so it didn't surprise me, even coming from the short, stout, bald man with gray hair as it did, when he struck up an unusual conversation with the kid next to me, who happens to play baseball. He began with:
"Have you ever heard of a curveball muscle?"
"No, a curveball muscle. Have you ever heard of one?"
The teacher proceeded to unbutton his cuff and roll up the sleeve on his right arm. As he awkwardly bent his wrist back towards his bicep, he pointed out the bulge on his forearm.
"After forty years, it's still there. Ask your pitcher. I bet he has one."
It was altogether unnecessary for the teacher to point out the muscle, taking into account its relatively enormous size. The bulge hinted at a bicep muscle on his forearm. It was simply incredible.
As it turns out, throwing a curveball does indeed develop a certain muscle along the ulna. The flexor carpi ulnaris is the prime mover muscle in this specific pitch.
When I, and a few surrounding students were done staring at his arm, the teacher rolled his sleeve back down, buttoned his cuff, and continued to begin a lovely discussion on quality vampire movies...
WebBall Bullpen. November 4, 2003. April 7, 2004. http://www.webball.com/bullpen/screwball_2.html