The passer is running backwards and saying, "so long fuckwad" while holding up two middle fingers.

It's the brain, not muscles. If you were to assemble 100 items, you'd get better at the end of it. It's your brain figuring out how to do the repetitive motions better and faster. It's tougher with sports because putts aren't always the same, throws aren't always the same. It's the things that don't change where you get the memory.