What defines MUSCLE in muscle car ???

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What defines MUSCLE ???

Is it because it has a V8 ,a 6 pot or even a 4 pot ?
There are some tuff factory V8s of course, but there a some tuff factory sixes, and lets not forget the cosworth sierra
Maybe it has to be quick down the 1/4 mile, maybe good top end,maybe just that G force into the seat when the load pedal is pushed

Alot of us are ol school and cherise our ol V8s, they have that ol persona of muscle
But compared to a new high tech 6/4 pot, they will get beaten to 100, beaten down the quarter (generally ,lets not forget the GXL 351 1/4 mile times, you could near run faster )
I know its not the same, but im not asking whats better to drive or gets more attention
What defines MUSCLE in muscle cars ???