Well cherry has a point.. mostly when you say "american muscles" it's only refer to a "ford mustang"

Ni hao Adam, Muscle car is a term used to refer to a variety of high performance automobiles. Building on the American phenomenon and developing simultaneously in their own markets,they were called " American Muscles".The trend grew later in Australia , UK etc and elsewhere . Check this site : http://americanmuscle4u.com/