to me, muscle cars were cars that either came from the factory w/ bad ass powertrains or cars that guys hooked up be just as good or better than the factory cars. it caould be a toss up as which car started the muscle car movement. people had been drag racing for yrs prior to the 409 impala or the GTO. the 409 impala was a couple of yrs before the GTO, but it was still a big heavy car and the GTO was an intermediate size car that was probably more of what a muscle car would have been seen as. but after that, it was all up hill till the early 70s. after that, the muscle car era was over.i believe vettes were muscle cars more when they were still all steel. i think the new vettes look mean but i just dont see them as muscle cars. the same can be said of then viper, mustang, camaro, firebird, gto, or charger.(the new ford gt is an ecxeption, to me it looks pretty much like the origianal) and noooooooo, the avanti was definitely not a muscle car, it does not have anything mean lookin about it at all, we just had one in our shop a couple a months ago and i got to look at it closely, and it had more of european look to it. now even though the pantera and the cobra were european bodies w/ ford engines, they looked and still do look mean, so i would put them in that same catagory. others probably wouldnt. the pony cars of that era were muscle cars as long as they had the powertrains to back it up. i dont know about the straight 8 or v12 and would probably say not just because for as long as ive been around these cars ive never came across anything like that. so to sum it all up, in my opinion, muscle carscame about in the ealry 60s and left in the early 70s. they may be imitated but never duplicated. its just that look that they have and that feeling that they make people feel when they are around them that gives a sense of raw power that was meant for one thing, and that was to go from point A to point B as fast as possible. "MUSCLE CARS TILL I DIE"