Here's the thing, you don't/can't "isolate" development of a muscle, especially a big one
Working out has systemic (anabolic) effects
Working out has systemic (anabolic) effects
This is not true at all. You may be confusing it with the assertion that you can't isolate individual heads of a particular muscle.
Yes, training has some systemic effect through hormonal activity, but muscle anabolism is very highly specific to local stimulus. Otherwise we'd all be just doing curls and watch our legs grow.
For a comically extreme example of localized functional muscle hypertrophy, search for the armwrestler Matthias "Hellboy" Schlitte :)
But you can bet Matthias Schilitte doesn't stand all day training only said muscles (also he has a genetic problem)