I stand by what I said. Perhaps I didnt say it right, so let me clarify. You can put your body through a new stress, but It doesn't mean your muscles will grow. First off, you need food (well duh).
You can keep your muscle ALWAYS guessing by always putting them through something different, but obviously, if the nutrients aren't there, growth wont happen. There's also progress. Lets take legs for example. And lets say my leg workout consisted of squats, leg press, leg extensions and leg curls. Suppose Im a guy that goes by soreness as my gauge of success.
After about 6 weeks of doing this program, I no longer get sore. So now i include 2 rest pauses after squats, triple drop sets for leg press, and negatives for both leg extensions and curls. The next day I'm VERY sore. However, if I keep pushing the envelop with such intensive methods just to be sore the next day, with no regards to improving, then again, my growth potential wont be as high as it could.
Again, what Im saying is that soreness means nothing without adequate food and progression. And in order to progress, you cant change things on a frequent basis, which means you'd have to stick to something for quite some time. And if one has to stick to something for a long time, then eventually that person wouldn't get sore from his exercises.