[Amit_S]

Hmm...yeah I guess you are right. Machines are made for exercising individual bodyparts.
Arnold had a similar line of thinking - he always wanted to isolate and innervate a muscle as much as possible.

But here's my 2 cents:
Why would you want to isolate?
I don't like isolation exercises too much. They do have their uses for bringing up lagging bodyparts for people with asymmetrical development, rehab, prehab and for bodybuilders.
But for regular people who don't have glaring asymmetries, who don't have the desire to be pro athletes it would be much better to stick to exercises which don't isolate but rather integrate. Use exercises which would involve as many muscles as possible.

Meaning you could do leg extensions and as you said try to feel it only in you quads, but doing squats will be a better investment. (and you'll feel it in your calves, quads, hams, glutes, lower back ,abs) And of course- even better would be car pushing or pulling/pushing a sled. You'll feel car pushing in your entire body from the tips of your fingers to your feet. Also car pushing is not as technically demanding as squatting.