Allow me to say a few words in defense of the "Muscle Confusion" principle.
As background, I was into the bodybuilding scene back in the 1970's and 1980's - think "Pumping Iron" - as a wannabe and fanboy. Later, starting in 1990, I became part owner in 3 different Gold's Gyms. I also got certified as a Personal Trainer by the ACSM in the early 1990's.
The "Muscle Confusion" principle was one of many popularized by Joe Weider:http://www.getbig.com/boards/index.php?topic=425660.5;wap2
The one in question:
MUSCLE CONFUSION: Constantly change variables in your workout — number of sets, number of reps, exercise choice, order of exercises, length of your rest periods — to avoid getting in a rut and slowing growth.
Now, I'll stipulate that Weider's principles were not necessarily held out to be scientific - most were based on anecdote, though I'll venture a few have been validated scientifically.
Still, I think I can make a case for "Muscle Confusion":
1) As stated, doing the same exercise day after day can get very boring. Mixing it up can keep things fresh and improve program compliance.
2) Different exercises stress muscles in different ways, stimulating different fibers. Think of bicep curls - wrist position makes a difference. Hence, you can get slightly different results by using different bars or different machines, for example.
3) Similarly, joints, ligaments and tendons are stressed to various degrees by different exercises. By mixing up exercises, one may decrease the frequency of overuse injuries.
As an aside, one other idea that seemed promising - variable resistance - seemed like a good idea, but at least as of the early 1990's had little or no evidence of efficacy. I'm talking about Nautilus machines, Hammer Strength and virtually every machine incorporating a cam. They may feel
better, but as far as I know there's no real evidence they do