Produced from the factory with a lot of HP and not too much attention paid to handlling, concentrating on 1/4 mile times instead. A 70 chevelle is a muscle car, a 70 Ferrari, while probably faster, is not.
Original muscle cars=those involved in the HP wars of the 60's, 70's, whether in true two door fashion or even the High HP Sedans built with performance in mind, tho to a purist not sure if that qualifies either.
My first, and last run from the law involved a heavily modified 70 Impala 454 sedan, it wasn't mine, it was the cops, I lost and spent my first and last night in jail at 16,,,,Too bad they didn't have Vmaxes then..

dads old 75' GMC service truck loaded up with tool boxes and ladders just couldn't cut the mustard although she gave it her best....Man was he pissed...
Dig in to it a little deepr and there is actually a definition of the differnce between hot rods and street rods when entering vehicles in shows and what not, at least there used be anyway, don't know about now, hot rods being older and rounder 1920's-1950's stuff and street rods being newer and more modern tho not neccesarily built from original muscle cars.
Modern Muscle cars= anything with a V8 and a crap load of HP, the G8, qualifies with flying colors, along with the 565HP Caddies, GTOs, Dodge Chargers and the like, along with of course the modern day versions of the original muscle cars.
I think without a doubt the modern musclecars are way the **** faster than the originals in every respect, and handle well to boot...my 70 Ram Air IV Trans Am, advertised 365 HP, and would barely cut a mid 13 quarter, my wifes bone stock 350Z beat that time.
Forget the late 70's through 80's "muscle car" crap, they killed the compression and hadn't figured out how to make HP without smog yet.........i don't think a 15 second Camaro or Mustang can qualify as a muscle car, not out of the box anyway...although they had potential, had a 78 Camaro that would run low mid 12's and still take me to work every day, but inspection was hell living in Harris county.