According to
howstuffworks.com, the origin of this myth dates to early experiments on brain function in the 19th century. Researchers simulated different parts of brains to see what happened, and noted that simulating certain parts would elicit a response from certain muscles. Experimenting this way, they figured out what about 10% of the brain was involved in.
If this is true, then it appears that about 10% of the brain is involved in motor function directly, the rest it took us longer to figure out. They have a source for this little fact, but the link is broken, so I've no idea how reliable the information is.