I just thought I would share this with all of you, since there is alot of new and less experienced trainees on this site. I wish I had access to this kind of information when I started to lift.
It is from Charles Poliquin and the full article can be found here.
What Does Soreness Really Mean?
Q: Do I have to get sore in the gym to be making progress?
A: For hypertrophy, I'd say yes. Hypertrophy is a biological adaptation to a stressor, and the stressor here is micro-tears in the muscle.
So I'd agree that this is true, but that's one of the reasons you have to change your training routine every six workouts -- to get that new soreness. But do you need to be sore all the time? No, but you should certainly be sore 48 hours after you initiate a new program. If not, then you probably did a Richard Simmons training routine.
Now, when an athlete is peaking at the end of his training phase, then he doesn't want to get sore. But when you're trying to build muscle, then yes, you should be sore to some degree after the first two workouts. (You shouldn't however strive to get sore so badly that you'd need an Advil sandwich to get out of bed.) The next four workouts you adapt, and then by the sixth workout you're ready for new soreness from doing something else.
The rule is, the program is only as good as the time it takes you to adapt to it. The changes in the program don't have to be dramatic. For example, you could do shoulder-width stance back squats for six workouts then go to front squats with the heels elevated and you'll be sore.