A muscle car, by the strictest definition, is an intermediate sized, performance oriented model, powered by a large V8 engine, at an affordable price. Most of these models were based on "regular" production vehicles. These vehicles are generally not considered muscle cars, even when equipped with large V8s. If there was a high performance version available, it gets the credit, and not the vehicle that it was based on.
Examples: Buick GS, Chevrolete Chevelle SS, Dodge Charger R/T, Ford Torino/Cobra, Plymouth GTX, Plymouth Road Runner, Oldsmobile 442, Pontiac GTO