Although there are many principles to learn when training, the confusion principle is the single most important principle in fitness, with the exception, perhaps, of intensity level. Adaptation is the real process that's taking place when we train the body for fitness and optimal health. The human body has to adapt, scientifically. It has no choice. We have a choice but our body simply does not.
Give the body the right treatment and every body will adapt. However, if we continue to do the same things, the body has little need to adapt at all. If you're exercising to maintain your current level of fitness, then the "same old workout" may do. But, if you're exercising to produce change in your body, then CHANGE is what is needed. You could add more resistance, more cardio, alternate cardio w/ resistance, more repetitions, the list goes on and on. It's o.k. to be creative but it's not o.k. not to challenge yourself.
The human body needs about 3 to 4 weeks to really adapt to a new set of "rules". Therefor, one only needs to alter their routine about that often. Changing things up more often than that may backfire because not only has the body not even had time to adapt to the current exercises, but it is also too easy to fall into the trap of actually doing the same things over and over, causing the body to only become accustomed to the current pattern of what may seem like continual changes. Given a nice foundation of 3 weeks of the same program of exercises will allow you to make superb progress in your current fitness level but more importantly will give room to completely revamp the routine 3 weeks out and "shock" your system all over again.
