The Definition of "MuscleCar"

Muscle is relative, I agree with that. I thought my 70 Challenger R/T 383, 4sp, 3.91 sure-grip was great. Had no power steering, brakes, or nothin'! I still remember the first time I drove it. (In 1978)
But, my neighbor had a 69 RR w/a six pack. When it was new, there was a problem w/ the end carbs. They would stick open, and when he jumped on the Schuylkill Expressway one time in Philly, he had to quick-think-what-to-do! How about sitting on the side of the highway, w/ a lift-off hood car, closing your six-pack carbs?! NICE! - That was muscle to be respected!
I bought my 87 5.0 Mustang new. In the late 80's, it was a rocket. It ran a best of 13.95 stock. There weren't many new cars that could do that then. They had been building junk since '72, w/ a few exceptions. Relatively, it was fast! I still own it. It'll run low 12's and I can drive it anywhere. But there's a lot faster stuff out there.
My newest will be my 71 Duster w/ a 360, 727 and a 4.10 gear. Will it be a muscle car? I'll find out!