All of modern exercise theory is based off of a study done in the 1930s by Selye. People have since expanded on the basic theory, and has two main factors that govern the body's ability to adapt:
- Stress. The body needs enough stress to disrupt homeostasis (forcing a supercompensation) that you desire.
- Recovery. The body needs enough rest and resources to perform the supercompensation.
As the body becomes more adapted to doing a certain kind of work, it is normal for progress to slow down. A beginner can make adapt every session, but after about 3-9 months (depending on prior training, genetics, and size) recovery starts taking longer. The typical 3 day a week work sets is too much work to compensate in time for. The trainee is then intermediate.
In particular, for intermediate trainees there needs to be some variation in intensity and in work volume to facilitate active recovery and continue progress. Additionally, the more advanced a trainee becomes, the more prone to over-training they can get because they are becoming much closer to their genetic potential. A common strategy that is used by intermediate lifters is now know as the "Texas Method". The work looks similar to this:
- Monday is a heavy day. 5x5 at your current 5RM.
- Wednesday is a light day. 3x5 or 3x3 at 70-80% of Monday's weight.
- Friday you go for a personal record. For lifts like squats you go for 5lbs more than Monday's lift. For lifts like a bench press you alternate between 1RM (RM= rep max), 2RM, or 3RM.
This provides enough variation for many intermediate lifters to keep maintaining progress. If your goal is hypertrophy, you will still apply the same principles, but you will be using different rep ranges for your sets.
I highly recommend getting the book "Practical Programming for Strength Training" by Rippetoe and Kilgore, as it goes into much more detail than I have room for here. It's a very balanced look at weight training, and does not presuppose you are only doing power lifting or any one specific focus.