A muscle car is an automobile with a high horse power engine, modest weight, capable of producing high levels of acceleration.
It is distinguished from sports cars, which were customarily considered smaller, two-seat cars, or GTs, two-seat or 2+2 cars intended for high-speed touring and possibly road racing.
The GTO wasn't the epitome of a Muscle Car. With 400 HP the car was relatively small with a BIG engine. In the 60s a Muscle car formula involved taking a normal looking car and putting a BIG engine in it and saying "GO!!!!" That's exactly what the GTO did.