The term "MUSCLE CAR" was coined, (as I remember it,) by the automotive press in 1964, when the first GTO.'s were introduced. (I was 26 in 1964, when this all started...)
The writers described a muscle car as an intermediate body (there were no pony cars, yet... the Mustangs came a bit later in the year...) into which the factory had fitted a supposedly powerful, big block engine, ostensibly, giving the car "muscular" performance capabilities.
There were no Mopar "muscle cars" fitting this definition, although Ma built many, many cars that would handily outrun the purported "muscle cars" without half trying... the great bulk of the early muscle cars, the GTO's, the Olds 442's, Buick Gran Sports, Ford Fairlane GTA's, and big block Chevelles would only run in the low 14's at best, while the Mopar Super Stock 413's 426's and 426 Hemis were worlds faster.... but, they were not not true "muscle cars" because their engines were in full sized cars, not intermediates.
I guess the only "musclecars" back then, to come from Mopar were the big block (and Hemi) Barracudas and Darts of 1967, 1968 and '69. The Challengers and 1970-up Barracudas were not intermediates; they were ponycars (patterned after the Mustang; hence the name "ponycar.") In the area of performance, the so-called musclecars of the era were weak tea... A 340 Duster will open a can of whoopass on a whole lot of them (early GTO's included) but in the parlance of the automotive writers of the time, it is not really a musclecar because it doesn't have a big block engine, regardless of the fact that it may be faster... and, usually is.
So, the term "musclecar" in not about speed... it's just a way to classify a big block engine in an intermediate chassis. Has nothing at all to do with how fast (or, slow) it may be...
That's the best explanation I can think of... from someone who was an NHRA Area Tech Advisor at the time... and was paying attention.
It's really just a slang term with no roots in performance... just a big block intermediate.
Hope this helps:glasses7: