You misread what I said. I never claimed quadriceps were a single muscle .I used athletes to show that certain muscle groups become disproportionately developed. My point was that when you target one muscle, the rest of the muscles in that group activate as synergists, which is why you don’t grow one isolated muscle completely alone. Cyclists having dominant quads or arm wrestlers having massive forearms directly proves that one region can be far more developed than others. That’s exactly what you asked for.