I remember a magazine article a few years ago that accidentally made a great definition of the term muscle car.
They pit a '70 440 4-speed Charger against a new one. The old Charger had non power brakes or steering.
They said something to the effect of (in reference to the slalom course) "it was a real workout manipulating the car through the course fighting the heavy duty clutch and lack of power brakes while muscling the steering wheel left to right and slaming the pistol grip into gear. Something about the shifter commands the control of the car. They were called muscle cars because they required and built serious muscle to drive them."